ALBERT

Description: The high elevation of the Southern Puna Plateau, the widespread melting of its crust, the gap in intermediate depth seismicity and the recent eruptions of ignimbrite complexes can be explained by delamination of the lithospheric mantle beneath it. To test this hypothesis, an array consisting of 73 broad band and short period seismic stations was deployed in the region for a period of two years starting in 2007. We inverted the data using the two plane wave approach and obtained 1D and 3D Rayleigh wave phase velocities. Our dispersion curve shows that at short periods (〈70 s) the phase velocities are slightly higher than those of the Tibetan plateau and lower than those of the Anatolian plateau. At periods of 100-140 s we observe a low velocity zone that might be remnant hot asthenosphere below a flat slab (7-10 Ma). We estimate the average continental lithosphere thickness for the region to be between 100 and 130 km. Our three dimensional Rayleigh wave phase velocities show a high velocity anomaly at low frequencies (0.007, 0.008 and 0.009 Hz) slightly to the north of Cerro Galan. This would be consistent with the hypothesis of delamination in which a piece of lithosphere has detached and caused upwelling of hot asthenosphere which in turn caused widespread alkaline-collision related volcanism. This interpretation is also corroborated by our shear wave velocity model where a high velocity anomaly beneath the northern edge of Cerro Galan at 130 km depth is interpreted as the delaminated block on top of the subducting Nazca slab.

Description: Chlorine isotope compositions of high-pressure (~2.3 GPa) serpentinite, rodingite, and hydrothermally altered oceanic crust (AOC) differ significantly from high- and ultrahigh-pressure (〉3.2 GPa) metasedimentary rocks in the Aosta region, Italy. Texturally early serpentinites, rodingites, and AOC have bulk δ 37 Cl values indistinguishable from those of modern seafloor analogues (δ 37 Cl = -1.0 to +1.0‰). In contrast, serpentinites and AOC samples that recrystallized during exhumation have low δ 37 Cl values (-2.7 to -0.5‰); 37 Cl depletion correlates with progressive changes in bulk chemistry. HP/UHP metasediments have low δ 37 Cl values (median = -2.5‰) that differ statistically from modern marine sediments (median = -0.6‰). Cl in metasedimentary rocks is concentrated in texturally early minerals, indicating modification of seafloor compositions early in the subduction history. The data constrain fluid sources during both subduction- and exhumation-related phases of fluid-rock interaction: (1) Marine sediments at the top of the downgoing plate likely interacted with isotopically light pore fluids from the accretionary wedge in the early stages of subduction. (2) No pervasive interaction with externally derived fluid occurred during subsequent subduction to the maximum depths of burial. (3) Localized mixing between serpentinites and fluids released by previously isotopically modified metasediments occurred during exhumation in the subduction channel. Most samples, however, preserved protolith signatures during subduction to near-arc depths.

Description: We have measured Ni, Ca, and Mn in olivine phenocrysts from volcanoes in the Galápagos Archipelago to infer the mantle source lithologies. Results show that peridotite is the dominant source lithology for Fernandina, Floreana, Genovesa, Wolf Island, and Darwin Island. These volcanoes largely characterize the PLUME, WD, FLO and DUM Nd, Sr, and Pb isotopic endmembers of Harpp and White (2001). Volcan Wolf, Alcedo, Marchena, and Cerro Azul, also produced from the melting of peridotite sources, have isotopic compositions that can be defined by mixing of the 4 isotopic endmembers. Our analysis suggests that peridotite was present in the sources of the volcanoes covered in this study and therefore is the dominant source lithology of the Galápagos plume. Pyroxenite melting is generally focused in two isotopically distinct domains: Roca Redonda, Volcan Ecuador, and Sierra Negra in the enriched western part of the archipelago, and Santiago, Santa Cruz, and Santa Fe in the depleted east. One implication of this finding is that the Western and Eastern Pyroxenite Domains represent two separate bodies of recycled crust within the Galápagos mantle plume. Furthermore, both isotopically enriched and depleted domains of the archipelago were generated from mixtures of peridotite and pyroxenite. This suggests that there is no relationship between the source lithology of the Galápagos plume and its isotopic characteristics. The identification of peridotite source melting in volcanoes with isotopic characteristics that have been attributed to recycled crust points to the importance of mixing in OIB genesis, consistent with studies in the Canary Islands.

Description: Paleomagnetic analysis and radiocarbon dating of an expanded Holocene deep-sea sediment sequence recovered by Integrated Ocean Drilling Program (IODP) Expedition 303 from Labrador Sea Site U1305 (Lat: 57°28.5 N, Long. 48°31.8 W, water depth 3459 m) provides insights into mechanisms that drive both paleomagnetic secular variation (PSV) and magnetization acquisition in deep-sea sediments. Seventeen radiocarbon dates on planktonic foraminifera define postglacial (c. 8 ka) sedimentation rates as ranging from 35 to 〉 90 cm/kyr. Alternating field (AF) demagnetization of u-channel samples show that these homogeneous sediments preserve a strong, stable, and consistently well-defined component magnetization. Normalized remanence records pass reliability criteria for relative paleointensity (RPI) estimates. Assuming that the age of magnetization is most accurately defined by well dated PSV records with the highest sedimentation rates, allows us to estimate and correct for temporal offsets at Site U1305 interpreted to result from post-depositional remanence acquisition at a depth of ~ 20 cm. Comparisons indicate that the northern North Atlantic PSV and RPI records are more consistent with European than North American records, and the evolution of virtual geomagnetic poles (VGP) are temporally and longitudinally similar to global reconstructions, though with much larger latitudinal variation. The largest deviations from a geocentric axial dipole (GAD) are observed during times of the highest intensities, in contrast to the usual assumption. These observations are consistent with the idea that PSV in the North Atlantic and elsewhere during the Holocene results from temporal oscillations of high latitude flux concentrations at a few recurrent locations.

Description: The conditions permitting mantle serpentinization during continental rifting are explored within 2D thermotectonostratigraphic basin models, which track the rheological evolution of the continental crust, account for sediment blanketing effects, and allow for kinetically controlled mantle serpentinization processes. The basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur [ Perez-Gussinye and Reston , 2001]. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures. Sediment supply has therefore, through the temperature-dependence of the viscous flow laws, strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high. In a case study for the Norwegian margin we test whether the inner lower crustal bodies (LCB) imaged beneath the Møre and Vøring margin could be serpentinized mantle. Multiple 2D transects have been reconstructed through the 3D data set by Scheck-Wenderoth and Maystrenko [2011]. We find that serpentinization reactions are possible and likely during the Jurassic rift phase. Predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may be partially serpentinized mantle.

Description: The origin of the Bermuda swell and volcanism remains enigmatic. The lack of an associated time-progressive hotspot track and absence of present-day volcanic activity make it difficult to reconcile with a deep mantle plume model. We analyze shear wave splitting measurements to estimate mantle flow direction and receiver function stacks to place constraints on the mantle transition zone thermal structure. *KS phases exhibit well-resolved null arrivals (no splitting) beneath the swell over a range of back azimuths. We find that the 410 and 660 km discontinuities are 49 ± 5 km and 19 ± 5 km deeper than the global average, respectively, leading to a transition zone thickness that is 27 ± 4 km thinner than average. Together, an apparently isotropic upper mantle and a thinned mantle transition zone suggest that mantle flow is primarily vertical beneath the swell, consistent with the presence of hot, buoyant mantle at depth.

Description: We present an improved density model and a new structural map of the Neapolitan Yellow Tuff caldera, the active portion of the nested Campi Flegrei caldera. The model was built using a new 3D inversion of the available high-precision gravity data, and a new digital terrain and marine model. The inversion procedure, based on a variable-depth lumped assembling of the subsurface gravity distribution via cell aggregation, gives better defined insights into the internal caldera architecture, that well agree with the available geological, geophysical and geochemical data. The adopted 3D gravity method is highly efficient for characterizing the shallow caldera structure (down to 3 km depth) and defining features related to regional or volcano tectonic lineaments and dynamics. In particular, the resulting density distribution highlights a pronounced density low in correspondence of the central portion of the caldera with a detail not available till now. The joint interpretation of the available data, suggests a subsurface structural setting that supports a piecemeal collapse of the caldera, and allows the identification of its headwall. Positive gravity anomalies localize dense intrusions (presently covered by late volcanic deposits) along the caldera marginal faults, and the main structural lineaments both bordering the resurgent block and cutting the caldera floor. These results allow us to both refine the current geological-structural framework and propose a new structural map that highlights the caldera boundary and its internal setting. This map is useful to interpret the phenomena occurring during unrest, and to improve both short- and long-term volcanic hazards assessment.

Description: Catastrophic collapses of submarine volcanoes have the potential to generate major tsunami, threatening many coastal populations. Recognizing the difficulties surrounding anticipations of these events, quantitative assessment of collapse-prone regions based on detailed morphological, geological and geophysical mapping can still provide important information about the hazards associated with these collapses. Rumble III is one of the shallowest, and largest, submarine volcanoes found along the Kermadec arc, and is both volcanically and hydrothermally active. Previous surveys have delineated major collapse features at Rumble III; based on time-lapse bathymetry, dramatic changes in the volcano morphology have been shown to have occurred over the interval 2007 to 2009. Furthermore, this volcano is located just ˜300 km from the east coast of the North Island of New Zealand. Here, we present a geophysical model for Rumble III, that provides the locations and sizes of potential weak regions of this volcano. Shipborne and near-seafloor geological and geophysical data collected by the AUV Sentry are used to determine the subsurface distribution of weak and unstable volcanic rocks. The resulting model provides evidence for potentially unstable areas located in the Southeastern flank of this volcano which should be included in future hazard predictions.

Description: Broadband seismic experiments over the last two decades have produced dense data coverage across Tibet. Yet, the mechanism of the India-Asia lithospheric convergence beneath it remains a puzzle, with even its basic features debated and with very different end-member models advocated today. We measured highly accurate Rayleigh- and Love-wave phase-velocity curves in broad period ranges (up to 5-200 s) for a few tens of pairs and groups of stations across Tibet, combining, in each case, hundreds to thousands of inter-station measurements made with cross-correlation and waveform-inversion methods. Robust shear-velocity profiles were then determined by extensive series of non-linear inversions of the data, designed to constrain the depth-dependent ranges of isotropic-average shear speeds and radial anisotropy. Temperature anomalies in the upper mantle were estimated from shear velocities using accurate petro-physical relationships. Our results reveal strong heterogeneity in the upper mantle beneath Tibet. Very large high-velocity anomalies in the upper mantle are consistent with the presence of underthrust (beneath southwestern Tibet) and subducted (beneath central and eastern Tibet) Indian lithosphere. The lithosphere. In contrast to the Indian lithosphere, Tibetan lithosphere and asthenosphere display low to normal shear speeds; Tibetan lithosphere is thus warm and thin. Radial anisotropy in the upper mantle is weak in central and strong in northeastern Tibet, possibly reflecting asthenospheric flow above the subducting Indian lithospheric slab.

Description: The Pacific Northwest (PNW) has experienced voluminous intraplate volcanism over the past ~17 Ma, beginning with the Steens/Columbia River flood basalts and continuing with the still-ongoing volcanism in the High Lava Plains (HLP) and eastern Snake River Plain (SRP). Here we present two complementary datasets (SKS splitting and Rayleigh wave phase velocity anisotropy) that place constraints on the anisotropic structure of the upper mantle beneath the HLP and SRP regions. Beneath the HLP, SKS phases reveal dominantly E-W fast splitting directions and large (up to ~2.7 sec) delay times, with pronounced lateral variations in δ t . Lateral and depth variability in the strength of anisotropy beneath the HLP is also evident from Rayleigh wave dispersion. Beneath the SRP, SKS splitting delay times are much smaller (~0.5 sec) and surface wave observations suggest a region of upper mantle anisotropy (~50-150 km depth) with a geometry that deviates significantly from the generally plate motion parallel fast directions observed just outside of the SRP. Beneath the HLP, the geometry of the anomalously strong anisotropy is similar to the anisotropy in the deeper parts of the upper mantle, resulting in constructive interference and large SKS splitting delay times. Beneath the SRP, the geometry of the anomalous anisotropic region in the shallow mantle is different, resulting in destructive interference and reduced SKS splitting delay times. We discuss several possible explanations for these observations, including variations in olivine lattice preferred orientation (LPO) strength, transitions in olivine fabric type, and a contribution from aligned partial melt.

Description: The bulk composition of the silicate portion of the Earth (BSE) has long been assumed to be tied to chondrites, in which refractory, lithophile elements like Sm and Nd exist in chondritic relative abundances. However, the 142 Nd/ 144 Nd ratios of modern terrestrial samples are 18±5 ppm higher than the ordinary-chondrite reservoir, and this challenges the traditional BSE model. Here we investigate a hypothesis that this terrestrial 142 Nd excess is related to a Sm/Nd ratio 6% higher than chondritic. This Sm/Nd ratio yields a superchondritic 143 Nd/ 144 Nd (~0.5130) similar to that identified in the highest 3 He/ 4 He mantle reservoir, and we argue that this reservoir represents the BSE composition for lithophile elements. We develop a compositional model for BSE in which the elevated Sm/Nd requires a shift of 143 Nd/ 144 Nd from 0.51263 (chondritic) to 0.51300. The new BSE composition is depleted in highly incompatible elements, including K, relative to the chondrite-based BSE, and offers a solution the “missing” 40 Ar paradox. This BSE compositional model requires that 〉83% of the mantle is depleted to form continental crust. It also implies a ~30% reduction in BSE U, Th and K, and therefore in the current rate of radiogenic heating and, thus, a proportional increase in the heat flow delivered to surface by plate tectonics. We explore thermal history models including effects related to a newly recognized evolution in the style of plate tectonics over Earth history: The lower radiogenic heat production may delay the onset of core convection and dynamo action to as late as 3.5 Gyr.

Description: The internal geological structure of the Northeast German Basin (NEGB) is affected by intense salt diapirism and by the presence of several stratified aquifer complexes of regional relevance. The shallow Quaternary to late Tertiary freshwater aquifer is separated from the underlying Mesozoic saline aquifers by an embedded Tertiary clay enriched aquitard (Rupelian Aquitard). An important feature of this aquitard is that hydraulic connections between the upper and lower aquifers do exist in areas where the Rupelian Aquitard is missing (hydrogeological windows). Three-dimensional thermohaline numerical simulations are carried out to investigate the effects of such hydrogeological windows in the Rupelian Aquitard on the resulting groundwater, temperature and salinity distributions. Numerical results suggest that hydrogeological windows act as preferential domains of hydraulic interconnectivity between the different aquifers at depth, and enable vigorous heat and mass transport which causes a mixing of warm and saline groundwater with cold and less saline groundwater within both aquifers. In areas where the Rupelian Aquitard confines the Mesozoic aquifer, dissolved solutes from major salt structures are transported laterally giving rise to plumes of variable salinity content ranging from few hundreds of meters to several tens of kilometers. Furthermore, destabilizing thermal buoyancy forces may overwhelm counteracting stabilizing salinity induced forces offside of salt domes. This may result in buoyant upward groundwater flow transporting heat and mass to shallower levels within the same Mesozoic Aquifer.

Description: Long continuous seismic data recorded at five broadband seismic stations during 2006 at Campi Flegrei caldera have been analyzed. Introducing a coarse-grained method, we evaluate the time evolution of amplitude and polarization of the seismic noise in the frequency band common to Long-Period events. The series are modulated on tidal time scales: the root-mean square is basically dominated by solar contribution, while the azimuth of the polarization vector shows lunar diurnal and semidiurnal constituents. In addition, we find that in the frequency band common to Long-Period events the azimuths are polarized towards a specific area, suggesting that these persistent oscillations can be induced by the activity of the shallow geothermal reservoir.

Description: We report on the mineralogical assemblages found in the hyper-alkaline springs hosted on Liguria and Oman ophiolites based on exhaustive XRD and SEM analyses. In Liguria, hyper-alkaline springs produce a thin brownish calcite precipitate that covers the bedrock due to the concomitant atmospheric CO 2 uptake and neutralization of the hyper-alkaline waters. No brucite and portlandite minerals are observed. The discharge of alkaline waters in Oman ophiolite forms white-orange precipitates. Calcium carbonate minerals (calcite and/or aragonite) are the most abundant and ubiquitous precipitates and are produced by the same mechanism as in Liguria. This process is observed as a thin surface crust made of rhombohedral calcite. Morphological features of aragonite vary from needle-, bouquet-, dumbbell-, spheroidal-like habitus according to the origin of carbon, the temperature and the ionic composition of the hyper-alkaline springs, and the biochemical and organic compounds. Brucite is observed both at hyper-alkaline springs located at the thrust plane and at the paleo-Moho. The varying mixing proportions between the surface run-off waters and the hyper-alkaline ones control brucite precipitation. The Layered Double Hydroxide minerals occur solely in vicinity of hyper-alkaline springs emerging within the bedded gabbros. Finally, the dominant mineralogical associations we found in Oman (Ca-bearing carbonates and brucite) in a serpentinizing environment driven by the meteoric waters are surprisingly the same as those observed at the Lost City hydrothermal site in a totally marine environment.

Description: A computer program (PBUQ) that uses Monte Carlo simulations to propagate uncertainty through regression equations and the equation for the paleosol carbonate CO 2 paleobarometer is presented. PBUQ includes options for all of the common approaches to determining values for input variables and incorporates several recent advancements relevant to determining values for soil-respired CO 2 concentrations, δ 13 C values of respired CO 2 , δ 13 C values of atmospheric CO 2 and temperatures of soil carbonate formation. PBUQ is intended to improve confidence in paleoatmospheric CO 2 research by helping researchers draw statistically significant conclusions. PBUQ can also be used to attribute and partition error among various sources and thereby advance this technique. Sensitivity analysis indicates that S(z) is the largest source of uncertainty for most paleosols and that uncertainty is minimized for soils in which CO 2 is an evenly balanced mixture between soil-derived and atmospheric components. Evenly balanced mixtures are most likely for paleosols formed in deserts and for weakly-developed paleosols. Development of proxies for soil-respired CO 2 concentrations and δ 13 C values of soil-respired CO 2 specifically for such soils is perhaps the most crucial next step for improving this technique. Currently, calcic paleosols are best used to test the significance of trends and/or differences among time slices in paleoatmospheric CO 2 concentration. Application to quantifying Earth System Sensitivity will require large scale averaging of determinations from individual paleosols and/or reduced uncertainty associated with input variables.

Description: Long-lived detachment faults are now known to be important in tectonic evolution of slow-spreading mid-ocean ridges, and there is increasing evidence that fluid flow plays a critical role in development of detachment systems. Here we document a new manifestation of low-temperature hydrothermal venting associated with the detachment fault that formed Kane Megamullion ~3.3-2.1 m.y. ago in the western rift-valley wall of the Mid-Atlantic Ridge. Hydrothermal effects on the detachment surface include 1) cemented mounds of igneous rock and chalk debris containing hydrothermal Mn oxides and Fe oxyhydroxides, and 2) layered deposits of similar Fe-Mn minerals ± interbedded chalks. Mounds are roughly conical, ~1-10 meters high, and contain primarily basalts with lesser gabbro, serpentinite, and polymict breccia. The layered Fe-Mn-rich sediments are flat-bedded to contorted and locally are buckled into low-relief linear or polygonal ridges. We propose that the mounds formed where hydrothermal fluids discharged through the detachment hanging wall near the active fault trace. Hydrothermal precipitates cemented hanging-wall debris and welded it to the footwall, and this debris persisted as mounds as the footwall was exhumed and surrounding unconsolidated material sloughed off the sloping detachment surface. Some of the layered Fe-Mn-rich deposits may have precipitated from fluids discharging from the hanging-wall vents, but they also precipitated from low-temperature fluids venting from the exposed footwall through overlying chalks. Observed natural disturbance and abnormally thin hydrogenous Fe-Mn crusts on some contorted, hydrothermal Fe-Mn-rich chalks on ~2.7 Ma crust suggest diffuse venting that is geologically recent. Results of this study imply that there are significant fluid pathways through all parts of detachment systems and that low-temperature venting through fractured detachment footwalls may continue for several million years off-axis.

Description: P and S relative arrival time residuals from teleseismic earthquakes recorded on over 60 temporary AfricaArray broadband seismic stations deployed in Uganda, Tanzania and Zambia between 2007 and 2011 have been inverted, together with relative arrival time residuals from earthquakes recorded by previous deployments, for a tomographic image of mantle wave speed variations extending to a depth of 1200 km beneath eastern Africa. The image shows a low wave speed anomaly (LWA) well developed at shallow depths (100-200 km) beneath the Eastern and Western branches of the Cenozoic East African rift system and northwestern Zambia, and a fast wave speed anomaly at depths ≤ 350 km beneath the central and northern parts of the East African Plateau and the eastern and central parts of Zambia. At depths ≥350 km the LWA is most prominent under the central and southern parts of the East African Plateau and dips to the southwest beneath northern Zambia, extending to a depth of at least 900 km. The amplitude of the LWA is consistent with a ~150-300 K thermal perturbation, and its depth extent indicates that the African superplume, originally identified as a lower mantle anomaly, is likely a whole mantle structure. A superplume extending from the core-mantle boundary to the surface implies an origin for the Cenozoic extension, volcanism and plateau uplift in eastern Africa rooted in the dynamics of the lower mantle.

Description: In the Garibaldi Belt, the northern segment of the Cascade arc, basalts at Bridge River Cones, Salal Glacier, and Mt. Meager (BSM volcanic centers) are alkalic, atypical for an arc setting. Subduction signatures are negligible or absent from primitive alkalic basalts from Salal Glacier and Bridge River, while altered oceanic crust may have contributed a minimal amount of fluid at Mt. Meager. More evolved BSM basalts display trace element signatures considered typical of arc lavas, but this is a consequence of deep crustal assimilation rather than primary input from the subducted slab. Primary BSM basalts represent 3-8% melts that segregated from enriched garnet lherzolite at significantly higher temperatures and pressures (70-105 km) than calc-alkaline Cascade arc basalts. The BSM mantle source is significantly more incompatible element-enriched than the depleted mantle tapped by calc-alkaline Cascade arc basalts. The BSM basalts are also isotopically distinct from calc-alkaline Cascade arc basalts, more similar to MORB and intraplate basalts of the NE Pacific and NW North America. The relatively deep, hot, and geochemically distinct mantle source for BSM basalts is consistent with upwelling asthenosphere. The BSM volcanic centers are close to the projected trace of the Nootka fault, which forms the boundary between the subducting Juan de Fuca plate and the near-stagnant Explorer plate. A gap or attenuated zone between the plates may promote upwelling of enriched asthenosphere that undergoes low-degree decompression melting to generate alkalic basalts that are essentially free of slab input yet occur in an arc setting.

Description: The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off-axis (i.e., erupted within the last 620 ky). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe 8.0 , Zr/Y and (Dy/Yb) N decrease from the rift shoulders to the rift axis, SiO 2 , Na/Ti, Lu/Hf increase and Na 2 O and Na 8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off-axis lava flows. Na 8.0 and Fe 8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 ± 4 km to 43 ± 5 km. These melting paths are consistent with adiabatic melting in normal-temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 ± 7 km to 67 ± 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 ± 40 °C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 ± 2.0 cm yr -1 , a value close to the mean spreading rate (2.9 ± 0.2 cm yr -1 ) over the last 620 ky.

Description: Global Positioning System (GPS) and Differential Interferometric Synthetic Aperture Radar (DInSAR) data, collected from July 2007 to July 2008 on Mt. Etna, are analysed to define the dynamics preceding and accompanying the onset of the eruption on 13 May 2008. Short and long-term comparisons have been made on both GPS and radar data, covering similar time windows. Thanks to the availability of three GPS surveys the year before the eruption onset, an increase in the seawards movement of the NE flank of the volcano has been detected in the few months before the dike intrusion. The GPS ground deformation pattern also shows a slight inflation centred on the western side of the volcano in the pre-eruptive long-term comparison (from July 2007 to May 2008). The GPS has been integrated with DInSAR data by the SISTEM approach, to take advantage of the different methodologies and provide high spatial sampling of the 3D ground displacement pattern. We inverted the SISTEM results in order to model the pressure source causing the observed pre-eruptive inflation. The subsequent emplacement of the eruptive dike was imaged by two GPS surveys carried out on a dense network over the uppermost part of the volcano on May 6 and 13, i.e. a few days before and a few hours after the beginning of the eruption. We inverted this comparison to define the position, geometry and kinematics of the dike. The dike intrusion was also imaged by DInSAR data with temporal baselines of 2-3 months, which confirm strong displacements localized on the summit area, rapidly decreasing towards the middle flanks of the volcano, as detected by very short-term GPS data; furthermore, the comparison between DInSAR and GPS data highlighted the presence of a depressurizing source localized beneath the upper south-western area, acting just after the dike intrusion. Finally, the long period (one year) GPS and DInSAR data were integrated by SISTEM in order to finely depict the 3D ground deformation pattern with the highest spatial resolution. The long-period data allowed the complex kinematics of the volcano to be finely imaged and highlighting the interaction between flank dynamics and magma injection.

Description: A regional survey of alkaline springs in Oman and Ligurian ophiolites shows that the alkaline water compositions significantly vary from one ophiolite to the other and within the same ophiolite. The first order correlation between the Na (and K) and Cl concentrations points to fluid compositions only partly due to evaporation. The scatter around the evaporation line implies that Na and Cl may not be conservative during the alteration of the ultramafic rocks. Mg is almost entirely depleted at pH 〉 10.5 as a result of serpentine formation within the ultramafic body and of brucite (and minor hydrotalcite) precipitation at the springs. Ca accumulates in the high pH fluids and is consumed by Ca-carbonate formation at the springs, by mixing with river waters or by the CO2 supply from the atmosphere. Thermodynamic calculations show that brucite saturation is reached at pH values around 10.5 which triggers major changes in the water composition. The waters evolve from a quartz-saturated low pH continental environment to a brucite-dominated high pH serpentinizing system at low temperature. The highest water salinities are found in springs located along the basal thrust plane of the ophiolite. The highest Al concentrations are found in some springs located on the crustal side of the mantle/crust boundary. This poses the question of the hydrologic pathways and of the role of the mineralogical composition of the altered formations.

Description: [1]  This study examines the effect of gas hydrate formation on seismic wave velocities of fine-grained sediments. Synthesis of gas hydrates in fine-grained sediments has proved to be challenging, and how hydrate formation would affect the seismic wave velocities and stiffness of clay-rich sediments has not yet been fully understood. In this study, CO 2 hydrate was synthesized in remolded and partially water-saturated clayey silt sediments that were originally cored from a hydrate occurrence region in the Ulleung Basin, East Sea, offshore Korea. After achieving excess water conditions, compressional wave and shear wave velocities were measured for different hydrate saturations and under different vertical effective stresses. The results reveal that the compressional wave velocity V P and shear wave velocity V S increase and the stress-dependency of V P and V S decreases as the hydrate saturation S H increases from 0% to ~60%. In particular, the V S ‒S H trend lies between the grain-cementing model and the load-bearing model, suggesting that gas hydrate formation in clayey silt sediments causes weak cementation from a hydrate saturation less than ~28%. The weak cementation in fine-grained sediments can be explained by the breakage of hydrate bonds that are cementing grains during sediment compression and/or the innate weakness in bonding between hydrate crystals and fine mineral grains owing to the presence of unfrozen water films on clay mineral surfaces. In addition, it is found that at low S H the cementation effect on V P is masked by the high stiffness of pore-filling phases, but it becomes pronounced at S H greater than 47%.

Description: [1]  Mountain rivers play a key role in the delivery of particulate organic carbon (POC) to large river systems and the ocean. Due to the extent of its drainage area and runoff, the Amazon River is one of Earth's most important biogeochemical systems. However the source of POC eroded from the humid region of the Eastern Andes and the input of fossil POC from sedimentary rocks (POC fossil ) remains poorly constrained. Here we collected suspended sediments from the Kosñipata River during flood events to better characterise Andean POC, measuring the nitrogen to organic carbon ratio (N/C), stable carbon isotopes ( δ 13 C org ) and radiocarbon ( Δ 14 C org ). Δ 14 C org values ranged from -711‰ to -15‰ and significant linear trends between Δ 14 C org, N/C and δ 13 C org suggested that this reflects the mixing of POC fossil with very young organic matter ( Δ 14 C org  ~ 50‰) from the terrestrial biosphere (POC non-fossil ). Using N/C and Δ 14 C org in an end member mixing analysis, we quantify the fraction of POC fossil (to within 0.1) and find that it contributes a constant proportion of the suspended sediment mass (0.37 ± 0.03%) and up to 80% of total POC. In contrast, the relative contribution of POC non-fossil was variable, being most important during the rising limb and peak discharges of flood-events. The new data shed light on published measurements of ‘old’ POC (low Δ 14 C org ) in Andean-fed tributaries of the Amazon River, with their Δ 14 C org and δ 13 C org values consistent with variable addition of POC fossil . The findings suggest a greater persistence of Andean POC in the lowland Amazon than previously recognised.

Description: [1]  The D’ Entrecasteaux Island (DEI) gneiss domes are fault-bounded domes with ~2.5 km of relief exposing ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic gneisses and migmatites exhumed in an Oligocene-Miocene arc-continent collision and subduction zone subject to Late Miocene to Recent continental extension. Multi-channel seismic (MCS) reflection data and well data show the Trobriand basin formed as a forearc basin caused by southward Miocene subduction at the Trobriand trench. Subduction slowed at ~8 Ma as the margin transitioned to an extensional tectonic environment. Since then, the Trobriand basin has subsided 1–2.5 km as a broad sag basin with few normal faults deforming the basin fill. South of the DEI, the Good enough rift basin developed after extension began (~8 Ma) as the hanging-wall of the north-dipping Owen-Stanley normal fault that bounds the basin's southern margin. The lack of upper crustal extension accompanying subsidence in the Trobriand and Good enough basins suggests depth-dependent lithospheric extension since 8 Ma has accompanied uplift of the DEI gneiss domes. Structural reconstructions of seismic profiles show 2.3 to 13.4 km of basin extension in the upper crust, while syn-rift basin subsidence values indicate at least 20.7 to 23.6 km of extension occurred in the entire crust since ~8 Ma. Results indicating thinning is preferentially accommodated in the lower crust surrounding the DEI are used to constrain a schematic model of uplift of the DEI domes involving vertical exhumation of buoyant, post-orogenic lower crust, far-field extension from slab rollback, and an inverted two-layer crustal density structure.

Description: [1]  The relationship between lithospheric evolution of eastern Eurasia and subduction of the Pacific plate has long been debated. However, the timing and implications of subduction on the tectonics of eastern China are not well constrained. Here, we present new zircon U-Pb ages and Hf isotopes, elemental and Sr-Nd-Pb isotopic data on Cretaceous volcanic rocks from the Ningwu basin, eastern China to further address this issue. Our age data reveal rapid eruption of the volcanic rocks within a short duration from 133 to 130 Ma. The rocks, mostly characterized by shoshonitic and high-K calc-alkaline signatures, display light rare earth element and Pb enrichment, Nb, Ta and Ti depletion, highly radiogenic Sr-Pb isotopic ratios and variable ε Hf (t) (+1.8 to −10), suggesting derivation from an enriched lithospheric mantle metasomatized by marine sediments. The early lavas (133.3 ± 1.1 Ma) show stronger subducted-related signatures than the late lavas (130.1 ± 1.0 Ma), which we interpret to reflect consumption of a significant volume of fusible subducted components in the early melting phase. The large ε Hf (t) variation of late lavas suggests greater involvement of asthenospheric melts and lower crust in their petrogenesis. The youngest age (130 Ma) appears to coincide with an inferred change in the direction of Pacific-Eurasia convergence, manifested as a change from extension to transpression in eastern China. The narrow window of eruption may signify a rapid change of the tectonic regime in the Early Cretaceous.

Description: Mid-ocean ridges magmatism is by and large considered to be mostly dry. Nevertheless, numerous works in the last decade have shown that a hydrous component is likely to be involved in ocean ridges magmas genesis and / or evolution. The petrology and geochemistry of peculiar coarse grained gabbros sampled in the upper part of the gabbroic sequence from the Northern Oman ophiolite (Wadi Rajmi) provide information on the origin and fate of hydrous melts in fast spreading oceanic settings. Uncommon crystallization sequences for oceanic settings (clinopyroxene crystallizing before plagioclase), extreme mineral compositions (plagioclase An% up to 99, and clinopyroxene Mg# up to 96), and the presence of magmatic amphibole, imply the presence of a high water activity during crystallization. Various petrological and geochemical constraints point to hydration resulting from the recycling of hydrothermal fluids. This recycling event may have occurred at the top of the axial magma chamber where assimilation of anatectic hydrous melts is recurrent along mid-ocean ridges, or close to segments ends where fresh magma intrudes previously hydrothermally altered crust. In ophiolitic settings, hydration and remelting of hydrothermally altered rocks producing hydrous melts may also occur during the obduction process. Although dry magmatism dominates oceanic magmatism, the dynamic behavior of fast spreading ocean ridge magma chambers has the potential to produce the observed hydrous melts (either in ophiolites or at spreading centers), which are thus part of the general mid-ocean ridges lineage.

Description: The Seismic Array HiKurangi Experiment (SAHKE) investigated the structure of the forearc and subduction plate boundary beneath the southern North Island along a 350 km transect. Tomographic inversion of first-arrival travel times was used to derive a 15-20 km deep P-wave image of the crust. The refracted phases and migrated reflection events image subducting slab geometry and crustal structure. In the west, Australian Plate Moho depth decreases westward across the Taranaki Fault system from 35 to ~28-30 km. In the east, subducted Pacific Plate oceanic crust is recognised to have a positive velocity gradient, but becomes less distinct beneath the Tararua Ranges, where the interface increases in dip at about 15 km depth from 〈5° to 〉15°. This bend in the subducted plate is associated with vertical clusters in seismicity, splay fault branching, and low-velocity high-attenuation material that we interpret to be an underplated subduction sedimentary channel. We infer that a step down in the decollément transfers slip on the plate interface at the top of a subduction channel to the oceanic crust and drives local uplift of the Tararua Ranges. Reflections from the Wairarapa Fault show that it is listric and soles into the top of underplated sediments, which in turn abut the Moho of the over-riding plate at ~32 km depth, near the downdip end of the strongly locked zone. The change in dip of the Hikurangi subduction interface is spatially correlated with the transition from geodetically determined locked to unlocked areas of the plate interface.

Description: [1]  The origin of crustal-scale silicic magmatism remains a matter of debate, and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper crustal regions. A 2D numerical model demonstrates that injection of mantle-derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels. The injection of mafic magma induces over pressure in the reservoir, which increases crustal stresses and triggers development of brittle/plastic shear zones, and can drive significant surface uplift. The emerging topography causes a non-uniform over pressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones. Based on systematic numerical experiments we investigate the influence of crustal strength and injection rate. The initial upper crustal strength controls the degree of crustal faulting and surface uplift and, therefore, whether felsic magma ascent can be initiated or not. The final upper crustal strength influences the depth and final style of felsic intrusion. The injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage. In contrast, the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant. Our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust, but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels.

Description: The Paleocene-Eocene Thermal Maximum (PETM) is marked by a prominent negative carbon isotope excursion (CIE) of 3-5‰ that has a characteristic rapid onset, stable body, and recovery to near pre-CIE isotopic composition. Although the CIE is the major criterion for global correlation of the Paleocene-Eocene boundary, spatial variations in the position and shape of the CIE have not been systematically evaluated. We measured carbon isotope ratios of bulk organic matter (δ 13 C org ) and pedogenic carbonate (δ 13 C carb ) at six PETM sections across a 16 km transect in the SE Bighorn Basin, Wyoming. Bed tracing and high-resolution floral and faunal biostratigraphy allowed correlation of the sections independent of chemostratigraphy. The onset of the CIE in bulk organic matter at all six sections occurs within a single laterally extensive geosol. The magnitude of the CIE varies from 2.1-3.8‰. The absolute and relative stratigraphic thickness of the body of the CIE in bulk organic matter varies significantly across the field area and under-represents the thickness of the PETM body by 30%-80%. The variations cannot be explained by basinal position and instead suggest that δ 13 C org values were influenced by local factors such as reworking of older carbon. The stratigraphic thickness and shape of the CIE have been used to correlate sections, estimate timing of biotic and climatic changes relative to the presumed carbon isotope composition of the atmosphere, and calculate rates of environmental and biotic change. Localized controls on δ 13 C org values place these inferences in question by influencing the apparent shape and duration of the CIE.

Description: Estimates of the relative motion between the Hawaiian and Louisville hotspots have consequences for understanding the role and character of deep Pacific-mantle return flow. The relative motion between these primary hotspots can be inferred by comparing the age records for their seamount trails. We report 40 Ar/ 39 Ar ages for 18 lavas from 10 seamounts along the Hawaiian-Emperor Seamount Chain (HESC), showing that volcanism started in the sharp portion of the Hawaiian-Emperor Bend (HEB) at ≥47.5 Ma and continued for ≥5 Myr. The slope of the along-track distance from the currently active Hawaiian hotspot plotted versus age is constant (57±2 km/Myr) between ~57 and 25 Ma in the central ~1900 km of the seamount chain, including the HEB. This model predicts an age for the oldest Emperor Seamounts that matches published ages, implying that a linear age-distance relationship might extend back to at least 82 Ma. In contrast, Hawaiian age progression was much faster since at least ~15 Ma and possibly as early as ~27 Ma. Linear age-distance relations for the Hawaii-Emperor and Louisville seamount chains predict ~300 km overall hotspot relative motion between 80 and 47.5 Ma, in broad agreement with numerical models of plumes in a convecting mantle, and paleomagnetic data We show that a change in hotspot relative motion may also have occurred between ~55 Ma and ~50 Ma. We interpret this change in hotspot motion as evidence that the HEB reflects a combination of hotspot and plate motion changes driven by the same plate/mantle reorganization.

Description: ABSTRACT Axial Seamount, an active submarine volcano on the Juan de Fuca Ridge at 46°N, 130°W, erupted in January 1998 along 11 km of its upper south rift zone. We use ship-based multibeam sonar, high-resolution (1-m) bathymetry, sidescan sonar imagery, and submersible dive observations to map four separate 1998 lava flows that were fed from eleven eruptive fissures. These new mapping results give an eruption volume of 31 x 10 6 m 3 , 70% of which was in the northern-most flow, 23% in the southern-most flow, and 7% in two smaller flows in between. We introduce the concept of map-scale submarine lava flow morphology (observed at a scale of 100s of meters, as revealed by the high-resolution bathymetry), and an interpretive model in which two map-scale morphologies are produced by high effusion-rate eruptions: “inflated lobate flows” are formed near eruptive vents, and where they drain downslope more than 0.5-1.0 km, they transition to “inflated pillow flows.” These two morphologies are observed on the 1998 lava flows at Axial. A third map-scale flow morphology that was not produced during this eruption, “pillow mounds,” is formed by low effusion-rate eruptions in which pillow lava piles up directly over the eruptive vents. Axial Seamount erupted again in April 2011 and there are remarkable similarities between the 1998 and 2011 eruptions, particularly the locations of eruptive vents and lava flow morphologies. Because the 2011 eruption reused most of the same eruptive fissures, 58% of the area of the 1998 lava flows is now covered by 2011 lava.

Description: A global map of surface heat flow is presented on a 2° by 2° equal area grid. It is based on a global heat flow data set of over 38,000 measurements. The map consists of three components. Firstly, in regions of young ocean crust (〈67.7Ma) the model estimate uses a half-space conduction model based on the age of the oceanic crust, since it is well known that raw data measurements are frequently influenced by significant hydrothermal circulation. Secondly in other regions of data coverage the estimate is based on data measurements. At the map resolution these two categories (young ocean, data covered) cover 65% of Earth's surface. Thirdly, for all other regions the estimate is based on the assumption that there is a correlation between heat-flow and geology. This assumption is assessed and the correlation is found to provide a minor improvement over assuming that heat flow would be represented by the global average. The map is made available digitally.

Description: We present 36 new 40 Ar- 39 Ar incremental heating age determinations from the Caribbean Large Igneous Province (CLIP) providing evidence for extended periods of volcanic activity and suggest a new tectonomagmatic model for the province's timing and construction. These new 40 Ar- 39 Ar ages for the Curaçao Lava Formation (CLF) and Haiti's Dumisseau Formation show evidence for active CLIP volcanism from 94 to 63 Ma. No clear changes in geochemical character are evident over this period. The CLF has trace element signatures (e.g., Zr/Nb = 10-20) and flat rare earth element (REE) trends consistent with plume volcanism. The Dumisseau Formation also has plume-like geochemistry and steeper REE trends similar to ocean island basalts. Volcanism in the Dumisseau Formation appears to have largely ceased by 83 Ma while at Curaçao it continued until 63 Ma. A rapidly surfacing and melting plume head alone does not fit this age distribution. Instead, we propose that the residual Galapagos plume head, following initial ocean plateau construction, was advected eastward by asthenospheric flow induced by subducting oceanic lithosphere. Slab rollback at the Lesser Antilles and Central America subduction zones created an extensional regime within the Caribbean plate. Mixing of plume with upwelling asthenospheric mantle provided a source for intermittent melting and eruption through the original plateau over a ~30 Ma period.

Description: [1]  Plagioclase and spinel lherzolite thermometry and barometry is applied to an extensive geochemical dataset of young (〈10.5 Ma) primitive basaltic lavas from across Oregon's High Lava Plains, California's Modoc Plateau, and the central - southern Cascades volcanic arc to calculate the depths and temperatures of mantle melting. This study focuses on basalts with low pre-eruptive H 2 O-contents that are little fractionated near-primary melts of mantle peridotite (i.e., basalts thought to be products of anhydrous decompression mantle melting). Calculated minimum depths of nominally anhydrous melt extraction are 40–58 km below Oregon's High Lava Plains, 41–51 km below the Modoc Plateau, and 37–60 km below the central and southern Cascades arc. The calculated depths are very close to Moho depths as determined from a number of regional geophysical studies and suggest that the geophysical Moho and lithosphere-asthenosphere boundary in this region are located in very close proximity to one another (within 5–10 km). The basalts originated at 1185–1383 °C and point to a generally warm mantle beneath this area, but not one hot enough to obviously require a plume contribution. Our results, combined with a range of other geologic, geophysical, and geochemical constraints, are consistent with a regional model whereby anhydrous mantle melting over the last 10.5 Ma in a modern convergent margin and back-arc was driven by subduction-induced corner flow in the mantle wedge, and to a lesser extent toroidal flow around the southern edge of the subducting Juan de Fuca and Gorda plates, and crustal extension-related upwelling of the shallow mantle.

Description: [1]  We determine the 3-D melt geometry of partially molten samples of dunite containing 1.6 and 3.6 volume % of basaltic melt that were held in a piston cylinder apparatus at upper mantle conditions for 430 hours. Our approach involves serial sectioning and high-resolution field emission SEM imaging. Resolution is such that melt pockets approaching ~30 nm in size were resolved while covering an area of ~300 by 230 micron. The principal result of this study is to show that thin layers (typically 100 nm or less in thickness) between adjacent grains observed in 2-D images persist with depth, and are therefore wetted two-grain boundaries. Melt geometries most closely resembling triple junction tubules of the isotropic equilibrium model occur at all three grain edges, but are small compared to larger pockets. The wetted grain boundaries at a dihedral angle 〉0° for this system are inferred to be due to slow expulsion of melt from dynamically reorganizing grain boundaries during steady state grain growth. The attenuation peak observed in forced torsional oscillation experiments on similar samples ( [18]) is likely related to the wetted grain boundaries. Grain growth, driven by surface energy reduction, occurs also at the larger grain sizes expected for the mantle. This suggests the presence of wetted grain boundaries and significant velocity reduction and attenuation in partially molten upper mantle, as observed for example in back-arc basins.

Description: [1]  Thellier-type experiments are a method used to estimate the intensity of the ancient geomagnetic field from samples carrying thermoremanent magnetization. The analysis of Thellier-type experimental data is conventionally done by manually interpreting data from each specimen individually. The main limitations of this approach are: (1) manual interpretation is highly subjective and can be biased by misleading concepts (2) the procedure is time consuming, and (3) unless the measurement data are published, the final results cannot be reproduced by readers. These issues compound when trying to combine together paleointensity data from a collection of studies. Here we address these problems by introducing the Thellier GUI : a comprehensive tool for interpreting Thellier type experimental data. The tool presents a graphical user interface, which allows manual interpretation of the data, but also includes two new interpretation tools: 1) Thellier Auto Interpreter : an automatic interpretation procedure based on a given set of experimental requirements, and 2) Consistency Test : a self-test for the consistency of the results assuming groups of samples that should have the same paleointensity values. We apply the new tools to data from two case studies. These demonstrate that interpretation of non-ideal Arai plots is non-unique and different selection criteria can lead to significantly different conclusions. Hence, we recommend adopting the automatic interpretation approach, as it allows a more objective interpretation, which can be easily repeated or revised by others. When the analysis is combined with a Consistency Test the credibility of the interpretations is enhanced. We also make the case that published paleointensity studies should include the measurement data (as supplementary files or as a contributions to the MagIC database) so that results based on a particular data set can be reproduced and assessed by others.

Description: [1]  The mean composition of mid-ocean ridge basalts (MORB) is determined using a global dataset of major elements, trace elements, and isotopes compiled from new and previously published data. A global catalog of 771 ridge segments, including their mean depth, length, and spreading rate, enables calculation of average compositions for each segment. Segment averages permit weighting by segment length and spreading rate and reduce the bias introduced by uneven sampling. A bootstrapping statistical technique provides rigorous error estimates. Based on the characteristics of the data, we suggest a revised nomenclature for MORB. “ all morb ” is the total composition of the crust apart from back-arc basins, N-MORB the most likely basalt composition encountered along the ridge 〉500 km from hot spots, and D-MORB the depleted end-member. all morb and N-MORB are substantially more enriched than early estimates of normal ridge basalts. The mean composition of back-arc spreading centers requires higher extents of melting and greater concentrations of fluid-mobile elements, reflecting the influence of water on back-arc petrogenesis. The average data permit a re-evaluation of several problems of global geochemistry. The K/U ratio reported here (12,340±840) is in accord with previous estimates, much lower than the estimate of Arevalo et al. (2009). The low Sm/Nd and 143 Nd/ 144 Nd ratio of all morb and N-MORB provide constraints on the hypothesis that Earth has a non-chondritic primitive mantle. Either Earth is chondritic in Sm/Nd and the hypothesis is incorrect, or MORB preferentially sample an enriched reservoir, requiring a large depleted reservoir in the deep mantle.

Description: [1]  Long-term changes of geomagnetic field intensity, including possible dependence on lengths of polarity intervals, provide fundamentally important information for understanding the geodynamo. A positive correlation between paleointensity and polarity interval length was previously suggested from an Oligocene (ca 23–34 Ma) relative paleointensity record at Deep Sea Drilling Program Site 522 in the Atlantic Ocean, which is the only continuous paleointensity data set published so far for this age interval. We have conducted a paleomagnetic study of Eocene to Oligocene sediments at three sites in the eastern equatorial Pacific Ocean. Our objectives include revisiting the issue of the paleointensity-polarity length correlation. Magnetic properties of the sediments meet the frequently used criteria for reliable relative paleointensity estimation. Although short-wavelength normalized remanence intensity fluctuations associated with polarity boundaries and possible geomagnetic excursions agree among the three sites, long-term changes are inconsistent. Apparent positive correlation between normalized intensity and polarity length was observed, but the normalized intensity has an obvious anti-correlation with the ratio of anhysteretic remanent magnetization (ARM) to isothermal remanent magnetization (IRM), which is mainly controlled by the relative abundance of biogenic and terrigenous magnetic minerals. Furthermore, the normalized intensity correlates with sedimentation rate. These facts indicate a lithological contamination on the normalized intensity records. The dependence on ARM/IRM and sedimentation rate is also evident at Site 522. It is inferred that variations in sedimentation rate and the relative abundance of biogenic magnetite on depositional remanent magnetization acquisition efficiency may not be well compensated by the normalization. It is therefore premature to conclude that stronger geomagnetic fields were recorded during longer polarity intervals from currently available normalized intensity records.

Description: Concentrations of cosmogenic iodine, 129 I, in the pore fluid of marine sediments often indicate that the pore fluid is much older than the host sediment, even when vertical flow due to sediment compaction is taken into account. Old pore fluid has been used in previous studies to argue for pervasive upward fluid flow and a deep methane source for hydrate deposits. Alternatively, old pore fluid age may reflect more complex flow patterns. We use a two-dimensional numerical transport model to account for the effects of topography and fractures on pore fluid pathlines when sediment permeability is anisotropic. We find that fluid focusing can cause significant lateral migration as well as regions where downward flow reverses direction and returns toward the seafloor. Longer pathlines can produce pore fluid ages much older than that expected with a one-dimensional compaction model. For steady-state models with geometry representative of Blake Ridge (USA), a well-studied hydrate province, we find pore fluid ages beneath regions of topography and within fractured zones that are up to 70 Ma old. Our results suggest that the measurements of 129 I/ 127 I reflect a mixture of new and old pore fluid. However, old pore fluid need not originate at great depths. Methane within pore fluids can travel laterally several kilometers, implying an extensive source region around the deposit. This type of focusing should aid hydrate formation beneath topographic highs. © 2013 American Geophysical Union. All rights reserved

Description: [1]  Deep-sea ultramafic-hosted vent systems have the potential to provide large amounts of metabolic energy to both auto- and heterotrophic microorganisms in their dispersing hydrothermal plumes. Such vent-systems release large quantities of hydrogen and methane to the water column, both of which can be exploited by autotrophic microorganisms. Carbon cycling in these hydrothermal plumes may, therefore, have an important influence on open-ocean biogeochemistry. In this study, we investigated an ultramafic-hosted system on the Mid-Cayman Rise, emitting metal-poor and hydrogen sulfide-, methane-, and hydrogen-rich hydrothermal fluids. Total organic carbon concentrations in the plume ranged between 42.1 and 51.1 μM (background = 43.2 ± 0.7 μM (n = 5)) and near-field plume samples with elevated methane concentrations imply the presence of chemoautotrophic primary production and in particular methanotrophy. In parts of the plume characterized by persistent potential temperature anomalies but lacking elevated methane concentrations, we found elevated organic carbon concentrations of up to 51.1 μM, most likely resulting from the presence of heterotrophic communities, their extracellular products and vent larvae. Elevated carbon concentrations up to 47.4 μM were detected even in far-field plume samples. Within the Von Damm hydrothermal plume, we have used our data to hypothesize a microbial food web in which chemoautotrophy supports a heterotrophic community of microorganisms. Such an active microbial food web would provide a source of labile organic carbon to the deep ocean that should be considered in any future studies evaluating sources and sinks of carbon from hydrothermal venting to the deep ocean.

Description: [1]  The paleoclimate record is information-rich, yet signifiant technical barriers currently exist before this record can be used to answer scientific questions. Here we make the case for a universal format to structure paleoclimate data. A simple example demonstrates the scientific utility of such a self-describing way of organizing coral data and meta-data. This example is generalized to a universal ontology that may form the backbone of an open-source, open-access and crowd-sourced paleoclimate database. The format is designed to enable semantic searches, and is expected to accelerate discovery on topical scientific problems like climate extremes, the characteristics of natural climate variability, and climate sensitivity to various forcings.

Description: [1]  Today it is widely accepted that during its early evolution the Earth experienced a magma ocean that covered most its surface. The separation of metal from silicate was much facilitated in the environment of such a magma ocean. The differentiation mechanism is known as the ‘metal-rain scenario’. Our study will focus on the settling dynamics of these metal droplets. [2]  Because of the low viscosity of molten silicate and a higher rotation period of the Earth at that time the rotation has a potentially strong influence on the dynamics of the magma ocean. We use numerical 3D fluid simulations to analyze the combined effect of strong rotation and convection on the settling of the iron droplets. [3]  We show that the influence of rotation on the settling depends on the latitude. At the poles the influence of rotation is only marginal. At the equator, three different scenarios can be distinguished. First, at low rotation rates, the particles form a dense layer at the bottom. Second, for higher rotation rates the particles stay mostly suspended and layers form in the temperature field. Third, at higher rotation rates the particles form a ribbon-like structure in the middle of the box. [4]  The influence of rotation on the iron droplets may lead to a scenario where part of the iron is kept in the mantle instead of transported to the core. This would have a strong influence on the later states of the differentiation process and the amountof siderophile elements in the mantel.

Description: [1]  [1] The Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, are the highest feature within the East Antarctic highlands and have been investigated seismically for the first time during the 2007/2008 International Polar Year by the Gamburtsev Mountains Seismic Experiment. Using data from a network of 26 broadband seismic stations and body wave tomography, the P and S wave velocity structure of the upper mantle beneath the GSM and adjacent regions has been examined. Tomographic images produced from teleseismic P and S phases reveal several large-scale, small amplitude anomalies (δV p  = 1.0%, δV s  = 2.0%) in the upper 250 km of the mantle. The lateral distributions of these large-scale anomalies are similar in both the P and S wave velocity models and resolution tests show that they are well resolved. Velocity anomalies indicate slower, thinner lithosphere beneath the likely Meso- or Neoproterozoic Polar Subglacial Basin and faster, thicker lithosphere beneath the likely Archean/Paleoproterozoic East Antarctic highlands. Within the region of faster, thicker lithosphere, a lower amplitude (δV p  = 0.5%, δV s  = 1.0%) slow to fast velocity pattern is observed beneath the western flank of the GSM, suggesting a suture between two lithospheric blocks possibly of similar age. These findings point to a Precambrian origin for the high topography of the GSM, corroborating other studies invoking a long-lived highland landscape in central East Antarctica, as opposed to uplift caused by Permian/Cretaceous rifting or Cenozoic magmatism. The longevity of the GSM makes them geologically unusual, however, plausible analogues exist, such as the 550 Ma Petermann Ranges in central Australia. Additional uplift may have occurred by the reactivation of pre-existing faults, for example during the Carboniferous-Permian collision of Gondwana and Laurussia.

Description: [1]  A multidisciplinary study involving micropaleontological and geochemical tools was carried out in borehole PRGL 1-4 (Promess 1), with the aim of reconstructing the impact of climate change and sea level variation between 133 ka and 406 ka in the upper slope of the Gulf of Lions. We used factor analysis to obtain three main benthic assemblages related to eutrophic, mesotrophic and oxygenated environments; planktic foraminifers were grouped as warm-water and cold-turbulent species. These results were compared with records of CaCO 3 and major and trace elements (Al, Ca, K, Sr) as well as the C/N ratio of organic matter. Power and cross-spectral analysis showed a straightforward relationship between precession minima and thermal stratification of the water column as well as the occurrence of eutrophic bottom conditions during lowstand periods and mesotrophic environments at times of highstand. These eutrophic-mesotrophic oscillations, usually driven by global eustatic change, also involved regional variations in CaCO 3 source to this environment. [2]  During periods of precession maxima, enhancement of northwesterly winds increased primary productivity by mixing, enhancing the percentage of cold-turbulent species in the water column and the proportion of oxygenated benthic species on the bottom. During interglacial stages, these events were recorded by lower biogenic carbonate at the expense of higher silicate-related components most likely due to a higher supply from Pyrenees rivers. The record of oxygenated benthic species can be a good proxy to monitor past changes in Winter Intermediate Water dynamics driven by northwesterly winds.

Description: [1]  A new magnetostratigraphyand magnetic susceptibility cyclostratigraphyfrom Middle Triassic basinal sediments at Rio Sacuz, northern Italy, suggests a depositional duration of ~1myr formost of the670 m carbonate Latemar Platform, giving a high resolution estimate for the duration for the Latemar's deposition. The new magnetostratigraphy from Rio Sacuz clarifies the ambiguities in a previous magnetostratigraphic study of the Latemar caused by lightning strikeremagnetizations.Our Rio Sacuzstudy was unaffected by lightning. Using thermal demagnetization, we show a N-R-N-R sequence at the Latemar-correlated locality of Rio Sacuz. With a polarity interval duration of ~0.25-0.5 myr for the Middle Triassic, this gives a depositional duration of ~1-2 myr. Rock magnetic experiments show that the magnetic carrier is magnetite, suggesting a primary, depositional-ageremanence .Measurement of magnetic susceptibility at one meter intervals from Rio Sacuz reveals eight short eccentricity cycles (~95-125 kyr) bundled into slightly more than two 405 kyr long eccentricity cycles indicating a more precise duration for the Rio Sacuz section of 0.8 to 1 myr. The shorter depositional duration required by this study forces a sub-Milankovitch time scaleof 1-2 kyrfor the Latemar platform's meter-scalerhythmic bedding and provides strong evidence for non-orbitally driven climate change in the Middle Triassic.

Description: [1]  Modern shelf bathymetry bordering the Gulf of Alaska exhibits shelf-crossing sea valleys that suggest focused pathways for ice flow during glacial conditions. Using an integrated seismic data set between the present Yakutat and Alsek Sea Valleys, we investigate the glacial stratigraphic record in order to improve our understanding of the regional glacial system during maximum glacial conditions. Our investigations reveal four glacial unconformities, of which, the latter two are overlain by sediment packages 100 s of m thick. We suggest that these unconformities are indicative of ice advance phases during the Little Ice Age (LIA), the Last Glacial Maxima (LGM), and two pre-LGM advances with glacial retreat sequences preserved from the youngest two. The advances were dominated by ice expanse from the Malaspina Glacial system and Alsek River districts rather than the Hubbard Glacial system and only show distinctive morainal bank development near the shelf edge and near the mouth of, or within, modern bays, fjords, or river valleys. This observation strongly supports rapid and continuous retreat from glacial maxima conditions during climatic warming. All, but the inferred LIA sequence, transgress the shelf and exhibit concentrated erosion in overdeepened troughs, analogous to cross-shelf troughs similar to those observed on other high-latitude glaciated shelves. The two Alaskan troughs discussed here may be end-member examples of high-sediment flux systems due to the temperate glacial setting combined with an actively exhuming orogen.

Description: [1]  Decadal trends of volcanic deformation in the Central Andes Volcanic Zone are identified with Interferometric Synthetic Aperture Radar (InSAR) stacks and time series velocity maps covering an area 19–27°S and 66–69°W. We combine over 750 ERS and Envisat interferograms from 2 descending and 3 ascending tracks. These tracks cover 100,000 km 2 and span 1992 to 2011. Our analysis extends observations at Cerro Blanco, Uturuncu, and Lazufre volcanic centers and uncovers two previously undocumented deformation centers; Cerro Overo in Northern Chile, and Putana Volcano in Southwest Bolivia. Cerro Overo exhibits a transition from steady −0.4 cm/yr deflation to 0.5 cm/yr inflation over several years. Putana Volcano underwent a short-lived episode of uplift between 13 September 2009 and 31 January 2010, with a maximum uplift of 4.0 cm. Cerro Blanco continues −1.0 cm/yr deflation since 1995. Uplift at Lazufre began between 1997–2000 and has gradually accelerated to 3.5 cm/yr since 2005. Uturuncu volcano continues 1.0 cm/yr monotonic uplift since 1992, and shows evidence for a broad moat of subsidence surrounding the uplifting region. Four of the nine deformation events in the CVZ are not obviously associated with a particular volcanic edifice. Furthermore, there is significant spatial and temporal variability of these deformation events within a small geographic area.

Description: [1]  Deep-sea ultramafic-hosted vent systems have the potential to provide large amounts of metabolic energy to both autotrophic and heterotrophic microorganisms in their dispersing hydrothermal plumes. Such vent-systems release large quantities of hydrogen and methane to the water column, both of which can be exploited by autotrophic microorganisms. Carbon cycling in these hydrothermal plumes may, therefore, have an important influence on open-ocean biogeochemistry. In this study, we investigated an ultramafic-hosted system on the Mid-Cayman Rise, emitting metal-poor and hydrogen sulfide-, methane-, and hydrogen-rich hydrothermal fluids. Total organic carbon concentrations in the plume ranged between 42.1 and 51.1 μM (background = 43.2 ± 0.7 μM ( n  = 5)) and near-field plume samples with elevated methane concentrations imply the presence of chemoautotrophic primary production and in particular methanotrophy. In parts of the plume characterized by persistent potential temperature anomalies but lacking elevated methane concentrations, we found elevated organic carbon concentrations of up to 51.1 μM, most likely resulting from the presence of heterotrophic communities, their extracellular products and vent larvae. Elevated carbon concentrations up to 47.4 μM were detected even in far-field plume samples. Within the Von Damm hydrothermal plume, we have used our data to hypothesize a microbial food web in which chemoautotrophy supports a heterotrophic community of microorganisms. Such an active microbial food web would provide a source of labile organic carbon to the deep ocean that should be considered in any future studies evaluating sources and sinks of carbon from hydrothermal venting to the deep ocean.

Description: [1]  Three vertical profiles of rare earth element concentrations and Nd isotopic compositions have been measured in the remote southeast Pacific Ocean. The three stations represent contrasting environments: the oligotrophic center of the gyre (station GYR), the “transition zone” east of the South Tropical Front (station EGY), and the Peru-Chile upwelling marked by a pronounced oxygen minimum (station UPX). Rare earth concentrations display nutrient like vertical profiles except at UPX where surface waters are enriched. At this station Nd isotopic compositions are clearly more radiogenic than in the open ocean, suggesting that boundary exchange process is releasing lithogenic rare earth element from the volcanic Andes. Unexpected radiogenic values ( ε Nd reaching –3.7) are also observed at 2000 m at station GYR in the Upper Circumpolar Deep Water that commonly have ε Nd values around –6. Exchange processes related to hydrothermal activity are suspected to produce this increase in ε Nd in the vicinity of the East Pacific Rise. These results provide some guidance for higher resolution studies planned in this region by the international GEOTRACES program.

Description: [1]  Massive corals offer continuous records of climate locked within their skeleton, with the most commonly applied paleo-thermometer being Sr/Ca. Recently, however, problems with Sr/Ca thermometry indicate that the intrinsic variance of single-core Sr/Ca time series differs between cores. Here, we compare the Sr/Ca records and growth parameters of two Porites lutea colonies sampled from the same reef zone, 0.72 km apart, with two gridded SST datasets, ERSST and HadISST, off NE Madagascar. Specifically, we address seasonal and interannual variability as well as trend differences between records over the same 43 year period. The two gridded SST datasets showed strong seasonality and weak positive ENSO anomalies on a slow 43-year warming trend at significantly different rates. Both the coral Sr/Ca records showed the same clear seasonality and similar amplitudes in SST. However, on interannual timescales they displayed diverging 43-year Sr/Ca trends and opposite responses to weak ENSO anomalies. Moreover, their growth response also differed as one coral showed increasing extension/calcification rates and Sr/Ca ratios (cooling) over the 43 years, while the other coral showed decreasing extension/calcification rates and Sr/Ca ratios (warming). Further, during positive ENSO events the calcification rates of the two corals were negatively correlated, while skeletal density anomalies were opposite. Possible explanations to why these corals are so different may be related to the corals growth response to SST changes. The growth response of individual corals to increasing SST seems to be opposite, which in turn are likely related to biological factors. Consequently, coral growth responses explain much of the inter-colony Sr/Ca variability.

Description: [1]  Modeling the diffusion of elements in olivine from volcanic rocks has recently become one of the most useful techniques to determine the timescales of the processes that occur in magma reservoirs before eruptions. However, many potential users are not versed in the numerical methods needed to solve the diffusion equation for timescale determinations. Here we present DIPRA (Diffusion Process Analysis), a user-friendly computer tool that models easily and intuitively the olivine chemical zoning by performing an automatic, visual, and quick fit to the natural profiles. The code is developed under a finite difference scheme and allows simultaneous modeling of diffusion of Fe, Mg, Mn, Ni, and Ca. DIPRA accounts for most variables that affect the diffusivity, including temperature, pressure, oxygen fugacity, major element composition, and anisotropy. Initial and boundary conditions can be done as complex as desired, including changing boundary composition with time. Such versatility allows modeling the large variety of scenarios that are characteristic of volcanic systems. We also have implemented a methodology to estimate objectively the uncertainties of the timescales from the uncertainties of the data and temperature. We expect that our application will increase the number and quality of timescale determinations from crystal zoning studies. It may be also useful as a teaching resource for higher education courses.

Description: [1]  Soil gas Radon ( 222 Rn) and Thoron ( 220 Rn) activities have been continuously measured during the period of the recent volcanic unrest that occurred at El Hierro, Canary Islands, at two different monitoring stations, namely HIE02 and HIE03. Significant increases in soil 222 Rn activity and 222 Rn/ 220 Rn ratio from the soil were observed at both stations prior to the 2011-2012 submarine eruption off the coast of El Hierro, showing the highest increases before the eruption onset and before the occurrence of the strongest seismic event (M = 4.6). Statistical analysis showed that the long-term temporal trend of filtered radon data matched closely that of seismic energy release during the volcanic unrest. The observed increases of 222 Rn are related to the rock fracturing processes (seismic activity) and the magmatic CO 2 outflow increase, as observed in HIE03 station.

Description: [1]  Mobilization of contaminants by CO 2 -charged brines is one concern relating to injection of CO 2 as part of carbon capture and storage projects. This study monitors the mobility of trace metals in an exhumed CO 2 -charged aquifer near the town of Green River, Utah (USA), where CO 2 -charged brines have bleached red sandstones, and concentrated trace metals at the bleaching reaction front. Mass balance calculations on the trace metal enrichments are used to calculate time-integrated fluid fluxes and show that a significant fraction of the metals mobilized by the CO 2 -rich brines are re-deposited locally. A sequential extraction procedure on metal-enriched samples shows that these metals are incorporated into secondary carbonate and oxide phases which have been shown to grow at the CO 2 -promoted bleaching reaction front. We argue that whilst CO 2 -charged brines are capable of mobilizing trace metals, local metal re-deposition implies that the potential for contamination of overlying freshwater aquifers is low.

Description: [1]  The early Cenozoic experienced at least three short but major hyperthermals associated with disruptions of the global carbon cycle. The largest among those, the Paleocene-Eocene thermal maximum, was associated with a negative carbon isotopic excursion of ~ 2.5‰ that appears to be best explained by the thermal dissociation of methane hydrates due to an initial period of warming. The cause of the initial warming has been attributed to a massive injection of carbon (CO 2 and/or CH 4 ) into the atmosphere; however, the source of the carbon is as yet unknown. The emplacement of a large cluster of kimberlite pipes at ~56 Ma in the Lac de Gras region of northern Canada may have provided the carbon that triggered early warming in the form of exsolved magmatic CO 2 . Our calculations indicate that the estimated 900–1100 Pg of carbon required for the initial ~3°C of ocean water warming associated with the Paleocene-Eocene thermal maximum could have been released during the emplacement of a large kimberlite cluster. The coeval ages of two other kimberlite clusters in the Lac de Gras field and two other early Cenozoic hyperthermals indicate that CO 2 degassing during kimberlite emplacement is a plausible source of the CO 2 responsible for these sudden global warming events.

Description: [1]  Lower Aptian carbonates in the Bab Basin at the southern Neo-Tethys margin record significant environmental changes across Oceanic Anoxic Event 1a (OAE1a). A long-lasting negative shift of carbon-isotope ratios (δ 13 C) associated with a distinct decrease in oxygen-isotope ratios (δ 18 O) in orbitolinid-rich carbonates characterizes the onset of OAE1a (Livello Selli), supporting a hypothesis that a long-lasting volcanic CO 2 emission is a main cause of OAE1a, inducing global warming. A bloom of microencrusters ( Lithocodium and Bacinella ) across the proto-Bab Basin occurred synchronously at the beginning of the subsequent positive δ 13 C excursion, responding to the global carbon-cycle perturbations. The carbonates, formed during the OAE1a, show higher strontium-isotope ratios ( 87 Sr/ 86 Sr) compared with those of global seawater; this was likely caused by a local influx of isotopically heavier strontium, along with nutrients, into the proto-Bab Basin. These biotic proliferations were triggered by an increased nutrient supply induced by intensified continental weathering due to the global warming suggested by the increase in δ 18 O values. Spatial variations in the δ 13 C values among sites in the Bab Basin and its surrounding platform are related to local environmental factors, such as the degree of mixing of basin water with ocean water and local removal of 12 C by metabolic activity at the platform-top. The δ 13 C profile of the studied core indicates global removal of organic carbon of OAE1a began during the early stage of the second-order transgression and lasted until the early stage of the highstand after the OAE1a. The Livello Selli corresponds to the early stage of this transgression.

Description: [1]  Quantifying long-term erosion of tropical shields is crucial to constraining the role of lateritic regolith covers as prominent sinks and sources of CO 2 and sediments in the context of long-term Cenozoic climate change. It is also a key to understanding long-term landform evolution processes operating over most of the continental surface and their control onto the sediment routing system. We study the surface evolution of West Africa over three erosion periods (~ 45-24, ~ 24-11 and ~ 11-0 Ma) recorded by relicts of 3 sub-continental scale lateritic paleolandsurfaces whose age is bracketed by 39 Ar/ 40 Ar dating of lateritic K-Mn oxides. Denudation depths and rates compiled from 380 field stations show that despite heterogeneities confined to early-inherited reliefs, the sub-region underwent low and homogeneous denudation (~ 2-20 m Ma -1 ) over most of its surface whatever the considered time interval. This homogeneity is further documented by a worldwide compilation of cratonic denudation rates, over long-term, intermediate and modern Cenozoic time scales (10 0 – 10 7  yr). These results allow defining a steady-state cratonic denudation regime that is weathering-limited i.e., controlled by the thickness of the (lateritic) regolith available for stripping. Steady-state cratonic denudation regimes are enabled by maintained compartmentalization of the base levels between river knick points controlled by relief inheritance. Under such regimes, lowering of base levels and their fossilization are primarily imposed by long-term eustatic sea level fall and climate rather than by epeirogeny. The expression of steady-state cratonic denudation regimes in clastic sedimentary fluxes remains to be investigated.

Description: [1]  We image anisotropic structure of the upper mantle beneath the Northern Apennines based on body-wave data collected during the RETREAT experiment (2003-2006). Joint analysis of anisotropic parameters evaluated from independent data sets – teleseismic P-wave travel times and shear-wave splitting – allows us to identify regions of different fabrics both in the mantle lithosphere and in the sub-lithospheric mantle. We recognize three regions – the Tyrrhenian, Adriatic and Transition – with their own anisotropic characteristics. The slab-parallel flow prevails in the sub-lithospheric mantle beneath the thin Tyrrhenian plate, while nearly slab-perpendicular orientation of the high velocities dominates on the Adriatic side of the region. The asthenospheric-flow pattern excludes a simple corner-flow model to fit the fabric of the upper mantle in the syn-convergent extensional tectonics and suggests the end of the subduction roll-back. We map fossil anisotropy with inclined symmetry axes within two domains of the thick continental Adriatic lithosphere. We estimate the lithosphere thickness of the Tyrrhenian and Adriatic plates at ~50 km and ~80 km, respectively, the latter being subducted down to no more than ~200 km with indications of inherited frozen-in anisotropic fabric. If a potential detachment at the northern end of the Apennine slab exists then it would have to be narrow and in its initial stage. Synthetic tests of the well-known trade-off between isotropic heterogeneity and evaluated anisotropic parameters, along with combining independent data sets, document a sufficient separation of both effects.

Description: [1]  Seasonal melting of the coastal part of the Greenland ice sheet is investigated using GPS vertical displacement data from coastal stations, combined with data on atmospheric and ocean temperatures. Using a high pass filter and cubic spline models, we estimate five variables describing seasonal uplift, a proxy for proximal mass loss, including duration of the melt season and the amount of summer uplift. Our analysis shows both temporal and spatial variations of uplift. Southern coastal Greenland experienced anomalously large uplift in summer 2010, implying significant melting that year. However, the northwest coast did not experience significant change in uplift at that time. Our data suggest that a combination of warm summer air temperature and warm sub-surface ocean water temperature drove the large mass losses in 2010. Using the uplift pattern of 2008 - 2010, and comparing to atmospheric data and ocean water temperature data, we show that warm Irminger water exerted significant influence on coastal melting in southeastern, southern and southwestern Greenland, reaching about 69ºN in 2010. North of this, Irminger water did not exert significant influence, in effect defining the northward limit of the sub-polar gyre for that year. Thus, short term variability in the coastal GPS uplift signal can be used to infer an oceanographic parameter that has a critical influence on Greenland ice sheet health.

Description: [1]  Along the west coast of North America, climate and marine productivity is affected by seasonal to interannual changes in coastal upwelling. Our understanding of upwelling variability in the past is limited by the short duration of instrumental records. Changes in upwelling intensity are expected to further affect the 13 C/ 12 C ( δ 13 C) and radiocarbon (∆ 14 C) content of marine dissolved inorganic carbon (DIC) due to variable mixing of old, upwelled seawater into surface waters. If seasonal variations in the δ 13 C of DIC are recorded in marine bivalve shells, they may provide valuable information about the extent of upwelling in the past. Comparison of modern Mytilus californianus shells from South California with a 5-year time series of coastal seawater ∆ 14 C and δ 13 C allows an assessment of the suitability of Mytilus shell ∆ 14 C and δ 13 C as proxies of upwelling intensity. We show that both absolute values and the seasonal range in ∆ 14 C are preserved in shell ∆ 14 C allowing its use as an indicator of upwelling intensity. Interpretation of shell δ 13 C is more problematic, with the δ 13 C of shell carbonate lower than seawater DIC by variable amounts (ranging from 0.5-1.5 ‰) due to the incorporation of metabolic carbon. The spatial and temporal variability observed in specimens that grew during the severe El Niño event of 1997-1998 demonstrates how a transect of shells spanning the western North American coastline can be used to reconstruct large-scale patterns of seawater ∆ 14 C variability for specific intervals of interest, such as those associated with ENSO-type phenomena.

Description: ABSTRACT [1]  Countless seamounts occur on Earth that can provide important constraints on intraplate volcanism and plate tectonics in the oceans, yet their nature and origin remain poorly known due to difficulties in investigating the deep ocean. We present here new lithostratigraphic, age and geochemical data from Lower/Middle Jurassic and Lower Cretaceous sequences in the Santa Rosa accretionary complex, Costa Rica, which offer a valuable opportunity to study a small-sized seamount from a subducted plate segment of the Pacific basin. The seamount is characterized by very unusual lithostratigraphic sequences with sills of potassic alkaline basalt emplaced within thick beds of radiolarite, basaltic breccia and hyaloclastite. An integration of new geochemical, biochronological and geochronological data with lithostratigraphic observations suggests that the seamount formed ~175 Ma ago on thick oceanic crust away from subduction zones and mid-ocean ridges. This seamount travelled ~65 Ma in the Pacific before accretion. It resembles lithologically and compositionally “petit-spot” volcanoes found off Japan, which form in response to plate flexure near subduction zones. Also, the composition of the sills and lava flows in the accreted seamount closely resembles that of potassic alkaline basalts produced by lithosphere cracking along the Line Islands chain. We hypothesize based on these observations, petrological constraints and formation of the accreted seamount coeval with the early stages of development of the Pacific plate that the seamount formed by extraction of small volumes of melt from the base of the lithosphere in response to propagating fractures at the scale of the Pacific basin.

Description: ABSTRACT [1]  We located continuous seismic tremorwith coherent amplitude wavetrainsin the Chile Ridge subduction region (~46.5°S)in two clusters north and south of the Chonos Archipelago, between the Chile trench and the North Patagonian forearc. Tremor persisted from December 2004 to February 2007 (the entire period of the Chile Ridge Subduction Project temporary seismic deployment), and lasted 〉 17 hours on six occasions. Tremor in the more active southern cluster reached a maximum duration of 48 hours, and we observed no more than 3 continuous days without tremor activity. The cluster locations coincide with the surface projections of subducted transform faultsformed atthe Chile ridge. We also detected simultaneous, co-located low-frequency microearthquakes with well-defined impulsive waves within the tremor signals distributed from the surface to 40 km depth, suggesting tremors and earthquakes are part of the same process. The periodicity of tremor duration is strongly correlated with semi-diurnal, diurnal, and long period tides, M2, N2, K1, O1, P1, and Mm (12.421, 12.000, 23.934, 25.819, 24.066 hrs and 27.555 days, respectively). We found a significant correlation between tremor occurrence and earth tides when tidal stressis calculated forthe slip plane of a right-lateral strike-slip fault withstrikeN95 0 E, which isnear parallelto subducted transform faults (N78 0 E) of the Chile Ridge,indicatingthat the very small stresses resulting from the combination of ocean loading and solid earth tides (~1kPa) are sufficient to facilitate or suppress tremor production; tremors occur when shear stressesare maximum and wane or are low whenshear stresses are minimum.

Description: [1]  The mechanics and dynamics of thin-skinned compressible thrust wedges with prescribed offsets in the backstop, i.e. transfer zones, are investigated using a three-dimensional finite difference numerical model with a visco-brittle/plastic rheology. The main questions addressed are: (i) What is the influence of the initial length of the backstop offset and (ii) what is the effect of the frictional strength of the main décollement on the structural evolution of the brittle wedges along such transfer zones? Results show that the shorter the backstop offset, the earlier these two thrust planes connect, forming a curved frontal thrust along the entire width of the model. Younger, in-sequence thrusts are formed parallel to this curved shape. Long backstop offsets produce strongly curved thrust faults around the indenting corner. Simulations with a weak basal friction evolve towards almost linear frontal thrusts orthogonal to the bulk shortening direction. Increased basal drag in models with a strong décollement favours propagation of the backstop offset into a transfer zone up to the frontal thrust. These simulations revealed that surface tapers of the wedge in front of the backstop promontory are larger than what the critical wedge theory predicts, whereas the tapers on the other side of the transfer zone are smaller than analytical values. This difference is amplified with increasing length of the backstop offset and/or strength of the décollement. Modelled surface elevation schemes reproduce well the topographic patterns of natural orogenic systems such as the topographic low along the Minab-Zendan transform/transfer fault between the Zagros and Makran.

Description: ABSTRACT [1]  We explore possible models for the seismological signature of the oceanic lithosphere-asthenosphere boundary (LAB) using the latest mineral-physics observations. The key features that need to be explained by any viable model include (1) a sharp (〈20 km width) and a large (5-10%) velocity drop, (2) LAB depth at ~70 km in the old oceanic upper mantle and (3) an age-dependent LAB depth in the young oceanic upper mantle. We examine the plausibility of both partial melt and sub-solidus models. Because many of the LAB observations in the old oceanic regions are located in areas where temperature is ~1000-1200 °K, significant partial melting is difficult. We examine a layered model and a melt accumulation model (at the LAB) and show that both models are difficult to reconcile with seismological observations. A sub-solidus model assuming absorption-band physical dispersion is inconsistent with the large velocity drop at the LAB. We explore a new sub-solidus model, originally proposed by Karato [2012], that depends on grain-boundary sliding. In contrast to the previous model where only the absorption-band behavior was assumed, the new model predicts an age-dependent LAB structure including the age-dependent LAB depth and its sharpness. Strategies to test these models are presented.

Description: [1]  Seismic velocity is a function of bulk vibrational properties of the media whereas electrical resistivity is most often a function of transport properties of an interconnected minor phase. In the absence of a minor conducting phase then the two should be inter-relatable, primarily due to their sensitivity to temperature variation. We develop expressions between shear wave velocity and resistivity for varying temperature, composition and water content based on knowledge from two kimberlite fields; Jagersfontein (Kaapvaal Craton) and Gibeon (Rehoboth Terrane). We test the expressions through comparison between a new high-resolution regional seismic model, derived from surface wave inversion of earthquake data from Africa and the surrounding regions, and a new electrical image from magnetotelluric (MT) data recorded in SAMTEX (Southern African Magnetotelluric Experiment). The data-defined robust linear regression between the two is found to be statistically identical to the laboratory-defined expression for 40 wt ppm water in olivine. Cluster analysis defines five clusters that are all geographically spatially distinct and tectonically relate to (i) fast, cold and variably-wet Kaapvaal Craton, (ii) fast and wet central Botswana, (iii) slow, warm and wet Rehoboth Terrane, (iv) moderately fast, cold and very dry southernmost Angola Craton, and (v) slow, warm and somewhat dry Damara Belt. From the linear regression expression and the MT image we obtain predicted seismic velocity at 100 km and compare it with that from seismic observations. The differences between the two demonstrates that the linear relationship between Vs and resistivity is appropriate for over 80% of Southern Africa. Finally, using the regressions for varying water content, we infer water content in olivine across Southern Africa.

Description: [1]  Ultralow velocity zones (ULVZs) are relatively thin regions directly above the core-mantle boundary (CMB) that exhibit marked seismic P - and S -velocity reductions. A viable explanation for the reduction is the presence of melt fractions within ULVZs. Since melt was found to be denser than solid in melting experiments at lowermost mantle pressures, partially molten ULVZs should exhibit negative buoyancy. Using published experimental data, we present a melting model based on Clapeyron slopes for the formation of ULVZs as partially molten regions above the CMB and apply the resulting melting curves and latent heat effects in fully dynamic, regionally constant viscosity convection simulations of the lowermost mantle. We find that the height of the ULVZs depends only moderately on Rayleigh number but strongly decreases with increasing excess density of melt over solid. The models predict excess density of at least 1% to explain observed heights. The combined effect of topography and latent heat of melting reduces the vigor of mantle convection only very slightly, while if combined with a decrease of the ULVZ viscosity, mantle flow velocities are significantly enhanced near the CMB, and overall mantle temperatures are notably increased. ULVZ heights are found to be insensitive to ULVZ viscosity (for the range isoviscous to 1/100 the viscosity of the ambient mantle).

Description: [1]  The evolution of the mantle wedge below the Colorado Plateau during low-angle subduction has been investigated by analysis of fragments from the Navajo Volcanic Field (NVF), most from serpentinized ultramafic microbreccia (SUM) diatremes. In most SUM-hosted olivine, concentrations of Al and V are 〈 1 ppm, and Cr, Ca, and Na concentrations also are unusually low: most temperatures from three olivine thermometers (Al, Cr, and V-based) are in the range 530 °C to 650 °C. The temperatures are consistent with the presence of chlorite in some inclusions, and they support the utility of olivine thermometry for diverse mineral assemblages in cool peridotite. Most pressures calculated for discrete diopside grains in SUM correspond to depths in the range 80 to 120 km. Diopside is relatively high in Li (~3.5 ppm), and two of five diopside grains have high Sr/Nd. SUM diatreme sources are inferred to be serpentine-rich mélange tectonically eroded from the forearc, transported above the Farallon slab, and incorporated into the lithosphere of the Plateau. Garnet peridotite xenoliths from minettes of the NVF record deeper depths in the range 120 to 150 km. These garnet peridotites also may be from forearc mantle emplaced during Farallon subduction. Calculated temperatures preclude the possibility that asthenosphere was in contact with that lithosphere at or near 150 km depth for tens of m.y. before NVF formation. Structures observed in seismic images of mantle to depths of 150 km below the central Colorado Plateau may be related to Farallon subduction, not inherited from Proterozoic lithosphere.

Description: ABSTRACT Large ignimbrite flare-ups provide records of profound crustal modification during batholith formation at depth. The locations of source calderas and volumes and ages of the eruptions must be determined to develop models for the tectonomagmatic processes that occur during these events. Although high-precision isotopic ages of the ignimbrites are critical, less expensive and more rapid techniques, such as paleomagnetism, can extend the temporal information from dated outcrops. Paleomagnetic and rock magnetic data, including characteristic remanent magnetization (ChRM) and anisotropy of magnetic susceptibility (AMS), from the Altiplano-Puna Volcanic Complex of the Central Andes reliably identify calderas and eight associated Mio-Pliocene ignimbrites. ChRM results indicate a larger between-site error for most ignimbrites, in comparison to within-site scatter. Part of this dispersion may be due to tumescence/detumescence associated with the caldera-forming eruptions, but most of the effect is probably due to the recording of paleosecular variation during cooling and vapor-phase crystallization of the thick ignimbrites. AMS data identify the source calderas for four ignimbrites and provide limits on possible post-emplacement rotations of the deposits. AMS data indicate significant topographic control on inferred flow directions, implying that the flows were dense and/or of low mobility.

Description: [1]  The carbon isotope composition (δ 13 C) of bulk organic matter and two palynomorph groups (scolecodonts and chitinozoans) from the Llandovery-Wenlock strata of Gotland (E Sweden) are compared to gain knowledge about carbon cycling in the Silurian (sub)tropical shelf environment. The δ 13 C values of the palynomorphs are mostly lower than the δ 13 C values of the bulk organic matter, and the δ 13 C values of the benthic scolecodonts are lower than those of the planktonic chitinozoans. While the difference between bulk and palynomorph δ 13 C may be in part a function of trophic state, the lower values of the scolecodonts relative to those of chitinozoans, which are assumed to live in the well-mixed water column, might imply an infaunal mode of life for the polychaetes that carried the scolecodonts. Lower δ 13 C for the scolecodonts in the middle of the section may represent variations in primary marine productivity (supported by acritarch abundance data), oxidation of organic matter in the bottom waters, or genera effects. In general, however, trends between the three data sets are parallel, indicating similarities in the low frequency, environmentally forced controls. The δ 13 C data show a decreasing trend from the base of the section, up to a horizon well below the base of the Upper Visby Formation. At this level, and therefore probably several 10 kyr before the δ 13 C increase in the carbonates, the δ 13 C organic values increase by ~1‰. This perhaps is an expression of a changed composition of the bulk organic matter associated with the extinction events prior to the Llandovery-Wenlock boundary.

Description: [1]  The steady state structure of thermal plumes rising from a small heater is studied in high Prandtl number fluids. We show good agreement between laboratory experiments and numerical simulations. We study the effect of the boundaries on the plume development by numerically simulating the plume rise in very large geometries. The thermal structure of the plume axis is similar for all box sizes considered, but the velocity structure changes strongly as box sizes are increased. We show that the effect of the side boundaries becomes unimportant for large aspect ratio, but that the free-slip top boundary has a strong influence on the velocity structure under all conditions. We show that the use of an outflow boundary condition significantly reduces the influence of the top boundary. Under these conditions we recover to good precision the theoretical predictions for plumes rising in an semi-infinite half-space. The strong influence of the boundaries in high Prandtl number fluids is important in the interpretation of laboratory experiments and numerical simulation for the dynamics of the Earth's mantle.

Description: [1]  Inferring the circulation of the mantle around subducting plates from surface measurements of shear wave splitting patterns remains to date elusive. To assist the interpretation of the seismic signal and its relation with the mantle circulation pattern, we present a new methodology to compute the seismic anisotropy directly from the flow in the upper mantle of 3-D numerical models of Earth-like subduction. This computational strategy accounts for the non-steady-state evolution of subduction zones yielding mantle fabrics that are more consistent with the deformation history than previously considered. In the subduction models, a strong mantle fabric develops throughout the upper mantle with a magnitude of the anisotropy that is proportional to the amount of subduction and is independent of the subduction rate. The sub-slab upper mantle is characterized by two domains with different fabrics: at shallow depth, the mantle entrained with the subducting slab develops trench-perpendicular directed anisotropy due to simple shear deformation, while in the deeper mantle, slab rollback induces pure shear deformation causing trench-parallel extension and fast seismic directions. Subducting plate advance favors the development of the fabric in the entrained mantle domain, while slab retreat increases the trench-parallel anisotropy in the deeper upper mantle. In the deeper domain, the strength of the fabric is proportional to the horizontal divergence of the flow and weakens from the slab edges toward the center. As such, strong trench-parallel anisotropy forms below retreating and relatively narrow slabs or at the margins of wider plates. The synthetic SKS splitting patterns calculated in the fore arc are controlled by the magnitude of the anisotropy in the upper domain, with trench-perpendicular fast azimuths in the center of large plates and trench parallel toward the plate edges. Instead, above relatively narrow, retreating slabs (≤600 km and low subduction partitioning ratio (SPR)), azimuths are trench parallel due to the strong anisotropy in the lower sub-slab domain. In all models, the anisotropy in the back arc and on the sides of the subducting plate is, respectively, trench perpendicular and sub-parallel to the return flow at depth. Results from our regional scale models may help to infer the flow and composition of the upper mantle by comparison with the wide range of subduction zones seismic data observed globally.

Description: [1]  The crystal fabric of a lava has been analyzed for the first time by neutron texture diffraction. In this study we quantitatively investigate the crystallographic preferred orientation of feldspars in the Castello d'Ischia (Ischia Island, Italy) trachytic exogenous dome. The crystallographic preferred orientation was measured with the monochromatic neutron texture diffractometer SV7 at the Forschungszentrum Jülich in Germany and a Rietveld refinement was applied to the sum diffraction pattern. The complementary thin section analysis showed that the three-dimensional crystal shape and the corresponding shape preferred orientation are in agreement with the quantitative orientation distributions of the neutron texture data. The (0 k 0) crystallographic planes of the feldspars are roughly parallel to the local flow bands, whereas the other corresponding pole figures show that a pivotal rotation of the anorthoclase and sanidine crystals was active during the emplacement of this lava dome. In combination with scanning electron microscopy investigations, electron probe microanalysis, XRF, and X-ray diffraction, the Rietveld refinement of the neutron diffraction data indicates a slow cooling dynamic on the order of several months during their crystallization under subaerial conditions. Results attained here demonstrate that neutron texture diffraction is a powerful tool that can be applied to lava flows.

Description: [1]  An analytical method is presented for converting thermochronometric ages to surface erosion or, equivalently, exhumation rate. The method incorporates the two most important thermal processes during cooling by erosion: the dependence of closure temperature on cooling rate and the advection of heat by rock motion toward the Earth's surface. Two thermal models are considered: (1) a steady state model, valid for low erosion rates; and (2) an eroding half-space model, which has no steady state, but captures the transient increase of geothermal gradient with erosion. In each case, it is assumed that data consist of one or more thermochronometric ages, present-day surface geothermal gradient, and topographic information including the elevation at which the age was obtained. Analytical solutions are provided to derive the erosion rate from these data either as an explicit expression for the steady case or as a root-finding problem for the transient case. A graphical method for plotting age against erosion rate and geothermal gradient is presented as a method for solving the root finding problem and for tracking analytical errors in observations of age and surface geothermal gradient. The graphical method is also appropriate for comparing data from different elevations or from different thermochronometric systems. Examples are provided using synthetic data or published data from the literature.

Description: [1]  Phase relations in harzburgite have been determined between 14 and 24 GPa and 1473 and 1673 K. At 1673 K, harzburgite transformed to wadsleyite + garnet + clinopyroxene below 19 GPa and decomposed into an assemblage of ringwoodite + garnet + stishovite above 20 GPa. Certain amounts of akimotoite were produced at still higher pressures (22–23 GPa). Finally perovskite and magnesiowüstite were found to coexist with garnet at 24.2 GPa. Compositions of all the phases were analyzed and elemental partitioning coefficients were determined among co-existing phases. Combining our experimental data with available thermoelastic properties of major minerals in the earth's mantle, we modeled the velocity and density signatures of the stagnated oceanic slabs in the mantle transition zone (MTZ) under eastern China, based on kinematic slab thermal structure analysis. We examined two end-member slab models: a conventional straight slab with deformation thickening, and an undulated slab with an oscillating wavelength of 200 km. We found that an undulated (buckled) slab model yields velocity anomalies (about 1–2% for Vp) that are consistent with seismic tomography models, taking into account of low-pass filtering effects in seismic tomography studies. On the other hand, straight slab models yield velocity anomalies that are too high comparing with seismic tomography models. Our models provide important constraints on the thermal structure, mineralogy, composition, density, and velocities of slab materials in the MTZ.

Description: [1]  Geochemical consequences of composite diapirs formed in subduction zones have been studied using a thermomechanical numerical model of an ocean-continent subduction zone. This model includes dehydration of subducted crust, aqueous fluid transport, partial melting, and melt emplacement. Subduction of crustal material to sublithospheric depth results in the formation of a tectonic rock mélange composed of basalt, sediment and hydrated /serpentinized mantle. At asthenospheric depth, this rock mélange may evolve into partially molten diapirs and rise through the mantle prior to emplacement (relamination) at crustal levels. We have investigated the composition and the geochemical evolution of liquids derived from such composite diapirs by analyzing the differing proportions of the crustal end-members in the source, i.e. basalt and sediment. Our results show that the proportions of the components (in the diapiric mélange) are limited to short-range variations within an interval of X b [=volume fraction of basalt/(basalt + sediment)] = 0.4–0.8, yielding melt with a relatively stable granodioritic major element composition. Hence, granodioritic melt is transported by rising composite diapirs to crustal levels, contributing to the growth of the continental crust. In addition to this, we have calculated Sr and Nd isotopic initial ratios of the diapiric mélange as a function of time, based on the fraction of the components in the mélange. Liquids derived from composite diapirs inherit the geochemical characteristics of the composite source and show distinct temporal variations of radiogenic isotopes depending on the changing values of X b . Partial melting of composite diapirs is therefore expected to produce melt with a constant major element composition, but substantial changes in terms of radiogenic isotopes.

Description: [1]  Axial Volcanic Ridges (AVRs) are found on most slow spreading mid-ocean ridges and are thought to be the main locus of volcanism there. In this study we present high-resolution mapping of a typical, well-defined AVR on the Mid-Atlantic Ridge at 45°N. The AVR is characterised by ‘hummocky terrain’, composed typically of pillowed or elongate pillowed flanks with pillowed or lobate lava flow summits, often with small haystacks sitting on their highest points. The AVR is surrounded by several areas of ‘flat seafloor’, composed of lobate and sheet lava flows. The spatial and morphological differences between these areas indicate different eruption processes operating on and off the AVR. Volcanic fissures are found all around and on the AVR, although those with the greatest horizontal displacement are found on the ridge crest and flat seafloor. Clusters of fissures may represent volcanic vents. Extremely detailed comparisons of sediment coverage and examination of contact relations around the AVR suggest that many of the areas of flat seafloor are a similar age or younger than the hummocky terrain of the AVR. Additionally, all the lavas surveyed have similar degrees of sediment cover, suggesting that the AVR was either built or resurfaced in the same 50 ka timeframe as the flat seafloor.

Description: [1]  The global mid-ocean ridge system is peppered with localities where mantle plumes impinge on oceanic spreading centers. Here, we present new, high resolution and high precision data for 40 trace elements in 573 samples of variably plume-influenced mid-ocean ridge basalts from the Mid-Atlantic ridge, the Easter Microplate and Salas y Gomez seamounts, the Galápagos spreading center, and the Gulf of Aden, in addition to previously unpublished major element and isotopic data for these regions. Included in the data set are the unconventional trace elements Mo, Cd, Sn, Sb, W, and Tl, which are not commonly reported by most geochemical studies. We show variations in the ratios Mo/Ce, Cd/Dy, Sn/Sm, Sb/Ce, W/U, and Rb/Tl, which are expected not to fractionate significantly during melting or crystallization, as a function of proximity to plume-related features on these ridges. The Cd/Dy and Sn/Sm ratios show little variation with plume proximity, although higher Cd/Dy may signal increases in the role of garnet in the mantle source beneath some plumes. Globally, the Rb/Tl ratio closely approximates the La/Sm N ratio, and thus provides a sensitive tracer of enriched mantle domains. The W/U ratio is not elevated at plume centers, but we find significant enrichments in W/U, and to a lesser extent the Mo/Ce and Sb/Ce ratios, at mid-ocean ridges proximal to plumes. Such enrichments may provide evidence of far-field entrainment of lower mantle material that has interacted with the core by deeply-rooted, upwelling mantle plumes.

Description: [1]  The Calabrian Arc subduction system is part of the Africa–Eurasia plate boundary, is one of the most seismically active regions in the Mediterranean Sea, and has been struck repeatedly by destructive historical earthquakes. In this study, we investigate the effects of historical earthquakes on abyssal marine sedimentation through the analysis of the turbidite record. We collected gravity cores in tectonically controlled basins where the eastern Mediterranean pelagic sequence is interbedded with resedimented units. Textural, micropaleontological, geochemical, and mineralogical signatures reveal three turbidite events in the last millennium. We dated the turbidite sequences from two different cores using different radiometric methods, whereas the average time interval between successive turbidite beds was estimated from pelagic sediment thickness and sedimentation rates; chronologies were refined through age modeling that provided age ranges (2 σ ) of each turbidite bed. The results suggest that turbidite emplacement was triggered by three historical earthquakes recorded in the area (i.e., the 1908, 1693, and 1169 events); their magnitude, epicentral location, and associated tsunamis support causative faults located in the Ionian Sea. The source for all the turbidites, as inferred from their mineralogy, is the metamorphic basement outcropping in southern Calabria and/or northeastern Sicily. Turbidite composition and cable breaks for the 1908 event have been used to infer likely traveling paths and seismogenic faults in the subduction system. Our findings suggest that Ionian Sea turbidites represent more than 80% of sedimentation and may be seabed archives of paleo-earthquakes capable of reconstructing seismicity back in time, during several earthquake cycles.

Description: [1]  Shock experiments with pressures ranging from 3 to 30 GPa have been conducted on a mixed assemblage of hexagonal and monoclinic pyrrhotite. All samples were studied with respect to their particular shock-induced microstructures and magnetic properties at high and low temperatures. Up to 8 GPa, microstructures in shocked pyrrhotite are characterized by mechanical deformation producing a damage of the crystal structure. At pressures of 20 GPa and upward, amorphization and mechanical twinning are the dominant structural features induced by shock. Within the lower-pressure range coercivity, saturation isothermal remanent magnetization and coercivity of remanence increase with shock pressures, in agreement with more single-domain (SD)-like behavior. Simultaneously, the λ-peak of hexagonal pyrrhotite decreases and the 34 K transition of monoclinic pyrrhotite broadens and is depressed. Magnetic hardening is triggered by grain-size reduction, but also by the formation of SD within discrete multidomain grains. Planar deformation features subdivide such multidomain grains into lath-shaped domains with average sizes lying in the SD range. The planar deformation features disappear at 20 GPa and irregular, nanometer-sized “amorphous domains” occur instead. Pressure release from 30 GPa finally triggers partial melting of pyrrhotite. The sharp interfaces between molten and crystalline pyrrhotite document a rapid change of thermal conditions. Within molten pyrrhotite, quenched iron crystals occur. The presence of native iron strongly influences the magnetic properties, depending on the particular amount in the studied sample and likely affects the magnetic properties of impact lithologies on Earth and extraterrestrial material.

Description: [1]  Several lines of evidence, including remote triggering of earthquakes and modulation of seismic tremor by Earth tides, suggest that faults weaken when subject to shaking and dynamic stresses associated with the passage of seismic waves. However, the origin of such dynamic weakening is poorly understood. Here, we explore the role of acoustic resonance for dynamic fault weakening using laboratory measurements. Experiments were conducted using a split Hopkinson pressure bar assembly, with dynamic stressing via impact loading. Samples were composed of crushed rock particles from mine tailings with a particle size distribution similar to that found in natural fault gouge. We used pulse shaper techniques and carefully evaluated dynamic stresses recorded at the front and rear of the sample to ensure that dynamic force balance was satisfied. Our experiments document acoustic-induced fluidization and dramatic dynamic weakening. Frictional strength and elastic modulus of simulated fault gouge are reduced by a factor of five to tenvia acoustic fluidization. We find a threshold acoustic pressure for fluidization that varies systematically with gouge zone properties. Our observations could help explain dynamic fault weakening and triggering of earthquake fault slip by dynamic stressing.

Description: [1]  Sediments from the equatorial Pacific Ocean, at the Integrated Ocean Drilling Program sites U1334 and U1335, record reliable magnetic polarity stratigraphies back to ~26.5 Ma (late Oligocene) at sedimentation rates usually in the 5–20 m/Myr range. Putative polarity subchrons that do not appear in current polarity timescales occur within Chrons C5ACr, C5ADn, and C5Bn.1r at Site U1335; and within Chrons C6AAr.2r, C6Br, C7Ar, and C8n.1n at Site U1334. Subchron C5Dr.1n (~17.5 Ma) is recorded at both sites, supporting its apparent recording in the South Atlantic Ocean, and has an estimated duration of ~40 kyr. The Oligocene-Miocene calcareous oozes have magnetizations carried by submicron magnetite, as indicated by thermal demagnetization of magnetic remanences, the anhysteretic remanence to susceptibility ratio, and magnetic hysteresis parameters. Transmission electron microscopy of magnetic separates indicates the presence of low-titanium iron oxide (magnetite) grains with size (50–100 nm) and shape similar to modern and fossil bacterial magnetite, supporting other evidence that biogenic submicron magnetite is the principal remanence carrier in these sediments. In the equatorial Pacific Ocean, low organic-carbon burial arrests microbial pore-water sulfate reduction, thereby aiding preservation of bacterial magnetite.

Description: ABSTRACT How reliable are shear wave splitting measurements as a means of determining mantle flow direction? This remains a topic of debate, especially in the context of subduction. The answer hinges on whether our current understanding of mineral physics provides enough to accurately translate between seismic observations and mantle deformation. Here, we present an integrated model to simulate strain-history-dependent texture development and estimate resulting shear wave splitting in subduction environments. We do this for a mantle flow model that, in its geometry, approximates the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single– and a double-sided subduction case. Results are compared to recent splitting measurements of this region by Di Leo et al. [2012a]. The setting lends itself as a case study, because it is fairly young and, therefore, early textures from the slab's descent from the near-surface to the bottom of the mantle transition zone – which we simulate in our models – have not yet been overprinted by subsequent continuous steady-state flow. Secondly, it allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel sub-slab mantle flow. We demonstrate that although a barrier amplifies trench-parallel sub-slab anisotropy due to mantle flow, it is not necessary to produce trench-parallel fast directions per se . In a simple model of A-type olivine lattice-preferred orientation and one-sided subduction, trench-parallel fast directions are produced by a combination of simple shear and extension through compression and pure shear in the sub-slab mantle

Description: ABSTRACT A composite of twenty-eight trachytic lava flows were recovered from the Jeju Geopark Drilling Project (JGDP) in Jeju Geopark, one of the new seven wonders of Nature declared by UNESCO in 2011. Each trachytic lava flow has a tendency to increase in magnetic grain size from the rapidly cooled brecciated margin and vesicle streaked zone downward into the massive crystalline flow interiors. The brecciated margin and vesicle streaked zone of individual trachytic lava flow contains exclusively fine-grained magnetite as inclusions in plagioclase. High-fidelity paleointensity determinations were obtained from twenty-six (out of 224 examined) samples from JGDP cores. Temporal variation of virtual axial dipole moments (VADMs) calculated from the absolute paleointensity estimates follows the trend of sint-800 data for the interval from ~80 ka to ~360 ka. High VADM from flow 21 possibly represents real intensity peak, as previsously recognized high VADM in Japan at ~336 ka, in Trans-Mexican volcanism ~339, and in Hawaii ~340-350 ka. Perhaps such a strong magnetic intensity near ~325-350 ka might be smoothed out in relative paleointensity records.

Description: The ability to map ocean–floor magnetisation is key to infer past plate motions. The advent of geodesy in the Earth Sciences offered an independent snapshot of contemporary plate kinematics, averaged over decades. Early studies suggested plate–motions steadiness through geological time, because contemporary rates were similar to past–3–Myr averages. Recent data, however, show that geodetic and paleo–magnetic motions exhibit differences beyond confidence ranges, which might suggest geodesy samples time–scales shorter than a few centuries. These differences pose important questions on the character of plate kinematic variability through time. Here I derive inferences on the steadiness of geodetically–derived plate–motions by combining geodynamical arguments with kinematic datasets. I exploit datasets to build scenarios for the recent evolution of most of the major plates. I compare the minimum rate at which torque need to vary to generate these scenarios, with the maximum rate at which geological processes can contribute torque. This allows assessing the geodynamical plausibility of each scenario. Results indicate that plate–motion changes recorded since ~3 Ma must have taken at least 1 Myr to occur. Two exceptions are the Pacific and Australian plates, whose motions changed by less than 5%, and therefore required torque variations that might be built over periods as short as 0.1 Myr. It remains unresolved for how long geodetically–derived plate motions kept steady in the geological past. However, results indicate it is indeed plausible that they did over the past 1 to 2 Myr. These inferences call for a shift in the way we regard the figure of geodetic plate–motions.

Description: Most oceanic islands are due to excess volcanism caused by thermal and/or compositional mantle melting anomalies. We call attention here to another class of oceanic islands, due not to volcanism but to vertical motions of blocks of oceanic lithosphere related to transform tectonics. Sunken tectonic islands capped by carbonate platforms have been previously identified along the Vema and Romanche transforms in the equatorial Atlantic. We reprocessed seismic reflection lines, did new facies analyses and 87 Sr/ 86 Sr dating of carbonate samples from the carbonate platforms. A 50-km-long narrow paleo-island flanking the Vema transform, underwent subsidence, erosion and truncation at sealevel; it was then capped by a 500 m thick carbonate platform dated by 87 Sr/ 86 Sr at ˜11-10 Ma. Three former islands on the crest of the Romanche transverse ridge are now at ˜900 m bsl; they show horizontal truncated surfaces of oceanic crust capped by ˜300 m thick carbonate platforms, with 10 to 6 Ma Sr isotopic ages. These sunken islands formed due to vertical tectonics related to transtension/transpression along long-offset slow-slip transforms. Another tectonic sunken island is Atlantis Bank, an uplifted gabbroic block along the Atlantis II transform (SW Indian Ridge) ˜700 m bsl. A modern tectonic island is St Peter and St Paul Rocks, a rising slab of upper mantle located at the St Paul transform (Equatorial Atlantic). “Cold” tectonic islands contrast with “hot” volcanic islands related to mantle thermal and/or compositional anomalies along accretionary boundaries and within oceanic plates, or to supra-subduction mantle melting that gives rise to islands arcs.

Description: The Integrated Ocean Drilling Program (IODP) Expedition 331 investigated the Iheya North hydrothermal field in the Okinawa Trough. Several post-drilling underwater vehicle investigations were conducted over two years to identify post-drilling changes in fluid discharge pattern, mineral deposition, and fluid chemistry. Drilling-induced high-temperature hydrothermal fluid vents were identified at deep holes not only near the naturally occurring NBC hydrothermal fluid vent (Site C0016) but also at the seafloor ~450 m distal to the NBC vent (Site C0014), where no hydrothermal fluid discharge was observed prior to drilling. A chimney structure at Hole C0016A grew rapidly at the NBC mound crest, where only small chimneys had been found before drilling. A drilling-induced diffuse hydrothermal flow region spread at Site C0014, and this area was newly colonized by the galatheid crab. From a fluid chemistry perspective, the post-drilling hydrothermal fluids were enriched in Cl relative to seawater, although this fluid chemistry was not observed during the 12 years prior to drilling. The Cl-enriched fluid reservoir underlying the subseafloor impermeable layers, observed by IODP Exp. 331, is likely source for the Cl-enriched fluids discharging from the post-drilling vents. The drilling-induced physical disturbance of subseafloor hydrogeological structures would release such fluids to the seafloor. In turn, the rapid chimney growth at the NBC mound crest may also be attributed to highly turbulent fluid flow with the enlarged artificial vent of Hole C0016A, which can contribute to the retention of the fluid-seawater mixture for a sufficiently long period to precipitate sulfide/sulfate minerals on the seafloor.

Description: [1] Geochemical time series analysis of lavas from Kīlauea's ongoing Pu'u ‘Ō'ō eruption chronicle mantle and crustal processes during a single, prolonged (1983 to present) magmatic event, which has shown nearly two-fold variation in lava effusion rates. Here we present an update of our ongoing monitoring of the geochemical variations of Pu'u ‘Ō'ō lavas for the entire eruption through 2010. Oxygen isotope measurements on Pu'u ‘Ō'ō lavas show a remarkable range (δ 18 O values of 4.6-5.6 ‰), which are interpreted to reflect moderate levels of oxygen isotope exchange with or crustal contamination by hydrothermally altered Kīlauea lavas, probably in the shallow reservoir under the Pu'u ‘Ō'ō vent. This process has not measurably affected ratios of radiogenic isotope or incompatible trace elements, which are thought to vary due to mantle-derived changes in the composition of the parental magma delivered to the volcano. High-precision Pb and Sr isotopic measurements were performed on lavas erupted at ~6 month intervals since 1983 to provide insights about melting dynamics and the compositional structure of the Hawaiian plume. The new results show systematic variations of Pb and Sr isotope ratios that continued the long-term compositional trend for Kīlauea until ~1990. Afterwards, Pb isotope ratios show two cycles with ~10 year periods, whereas the Sr isotope ratios continued to increase until ~2003 and then shifted towards slightly less radiogenic values. The short-term periodicity of Pb isotope ratios may reflect melt extraction from mantle with a fine-scale pattern of repeating source heterogeneities or strands, which are about 1-3 km in diameter. Over the last 30 years, Pu'u ‘Ō'ō lavas show 15% and 25% of the known isotopic variation for Kīlauea and Mauna Kea, respectively. This observation illustrates that the dominant time scale of mantle-derived compositional variation for Hawaiian lavas is years to decades.

Description: Currently we lack a systematic and remote method for locating and quantifying diffuse sea floor venting using underwater robotic vehicles. Diffuse flow is characterized by both low temperature and low flux rates, which cannot readily be distinguished using current remote visual, acoustic, or vehicle-mounted environmental sensors. The result is a poor understanding of the distribution, contribution, and context of diffuse flow sources. An underwater structured light imaging system, also used for high resolution seafloor bathymetric mapping, has however shown promise in detecting diffuse flows while completing sea floor imaging surveys at a typical altitude of 3\meter. The system creates sequential bathymetric profiles by imaging a laser line projected on the sea floor. In the presence of venting fluids, the laser line exhibits a detectable level of distortion due to variations in the refractive index along the optical path. By characterizing the degree of distortion it is possible to create maps indicating areas of potential venting with sub-meter spatial resolution. Results from three distinct vent fields are presented and discussed. Analysis of these data sets indicates this system is capable of detecting both small point source vents and near bottom diffuse flow.

Description: ABSTRACT In this contribution, we report rock magnetic, petrographic and anisotropy of magnetic susceptibility (AMS) data from the Pinto Peak intrusion, all of which bear on volcanic construction. Rock magnetic data indicate that the dominant magnetic mineral phase is low-Ti titanomagnetite of multi-domain grain size, the composition of which varies spatially across the intrusion. The intrusion is a porphyritic andesite dominated by Ca-rich plagioclase (〉60%) as well as biotite, amphibole, olivine and opaque minerals. Reflected light petrography reveals mostly euhedral-subhedral (titano)magnetite crystals that often form clustered glomerocrysts and stringers of equant crystals, without exhibiting a consistent mineral alignment fabric. Moderate-to-shallow plunging prolate magnetic susceptibility ellipsoids dominate the northern part of the intrusion while steeply-dipping/plunging magnetic susceptibility ellipsoids are generally restricted to the southern part of the intrusion. The vent facies rocks yield moderate-to-steep oblate susceptibility ellipsoids. We interpret the flow pattern in the north to reflect sub-horizontal flow of magma, filling a tabular sheet-like body associated with propagation of the intrusion to the north. We argue that the southern part of the intrusion represents the ascent site of the magma rising to shallow crustal levels along a steep feeder system. The oblate magnetic fabrics in the vent area plausibly represent flattening against the conduit walls as evidenced by a weak planar flow foliation observed in the vent conduit rocks. On reaching shallow crustal levels, the magma deformed and uplifted the roof rocks leading to gravitational instability. As the slide mass released from the roof, an explosive eruption ensued resulting in the emplacement of the Rocks of Paradise tuff and associated effusive lava flows. Following eruption, magma pressure decreased and the magma drained northward forming the northern intrusive phase.

Description: This study presents the theory, applicability and merits of the new THERIAK_D add-on for the open source Theriak/Domino software package. The add-on works as an interface between Theriak and user-generated scripts, providing the opportunity to process phase equilibrium computation parameters in a programming environment (e.g., C or MATLAB ®). THERIAK_D supports a wide range of features such as calculating the solid rock density or testing the stability of mineral phases along any pressure-temperature (P-T) path and P-T grid. To demonstrate applicability, an example is given in which the solid rock density of a 2D-temperature-pressure field is calculated, portraying a simplified subduction zone. Consequently, the add-on effectively combines thermodynamics and geodynamic modeling. The carefully documented examples could be easily adapted for a broad range of applications. THERIAK_D is free, and the program, user manual, and source codes may be downloaded from http://www.min.uni-kiel.de/~ed/theriakd/ .

Description: We compute high-resolution seismic images from scattered wavefield to detect discontinuities beneath the High Lava Plains (HLP), using data recorded at a dense broadband array. Our images of the HLP and surrounding regions reveal (1) a prominent Moho discontinuity with varying depth, with thinnest crust of 35 km beneath the volcanic track, and thickened crust of ~45 km beneath the Owyhee Plateau (OP); (2) distinct intracrustal velocity reversals beneath regions of pre-2.0 Ma volcanism and within the OP; and (3) intermittent negative velocity discontinuities in the uppermost mantle beneath regions of Holocene volcanism and volcanic centers near Steens Mountain and Newberry volcano. These features exhibit remarkable similarity with those seen in the surface wave tomography and Ps receiver functions. We fail to find evidence for a ubiquitous regional lithosphere-asthenosphere boundary (LAB). In concert with petrological constraints on the equilibration depths of primitive basaltic melts, our results suggest that the present-day HLP mantle lithosphere is thin or absent, perhaps a consequence of episodes of extensive mantle inflow, lithospheric extension, and possibly melting induced by rapid slab rollback and trench retreat. It remains possible, however, that strong E-W seismic anisotropy reported across this region may reduce the effective S -wave velocity contrast to render the LAB less detectable. In contrast, the Owyhee Plateau exhibits a clear LAB, consistent with it being a block of older pre-existing lithosphere. Our images demonstrate the complexity of mantle dynamics in the Cascadian back-arc and the close casual link between subduction-related processes and the origin of HLP volcanism.

Description: The transfer of volatiles from the Earth's interior to the atmosphere occurs through degassing of magma, the dynamics of which assert a significant control on volcanic eruptions. The first and most critical step in degassing is the nucleation of gas bubbles, which requires that a sufficient number of volatile molecules cluster together to overcome the free energy associated with the formation of a new interface between nucleus and surrounding melt. This free energy is a function of surface tension, typically assumed to equate to the macroscopically measurable value. Surface tension estimates inferred from bubble nucleation experiments in silicate melts are, however, lower than direct macroscopic measurements, making it difficult to accurately predict magma ascent and decompression rates from measured bubble number densities in pyroclasts. We provide a potential resolution to this problem through an integrated study of bubble nucleation experiments and modeling thereof, based on non-classical nucleation theory. We find that surface tension between critical bubble nuclei and the surrounding melt depends on the degree of supersaturation and is lower than the macroscopically measured value. This is consistent with the view that far from equilibrium the interface between a nucleus and surrounding metastable bulk phase is diffuse instead of sharp. As a consequence, the increase in nucleation rate with supersaturation is significantly larger at high supersaturations than predicted by classical nucleation theory.

Description: Tholeiite of the oldest oceanic crust was drilled during ODP Legs 129 and 185 at Hole 801C in the western Pacific. Fresh appearing submarine basaltic glass (SBG) was recovered from the tholetiites (~167 Ma; Koppers et al. [2003]) which has been shown to be nearly ideal for determining absolute paleointensity. Paleointensities of the younger, off-axis, alkalic basalts (~160 Ma; Koppers et al. [2003]), overlying the tholeiites, had been studied earlier [ Tauxe , 2006]. Here we report results from the older tholeiitic (on-axis) sequence. We subjected a total of 73 specimens from 17 cooling units to absolute paleointensity experiments. Of these, 30 specimens and 6 cooling unit averages met our strictest reliability criteria, yielding an average of 11.9 ± 3.9 μ T. The bulk of evidence suggests a paleolatitude of the site of 14°S (with an uncertainty of 10°). This translates the intensity to a value for the virtual axial dipole moment of 28 ZAm 2 , slightly lower than values determined from the plagio clase crystals in the three cooling units of the younger alkalic basalts over lying the tholeiites. This value is low when compared to the long-term median value of the field of 42 ZAm 2 . Our results and those of the published literature therefore support the contention of a low magnetic field strength in the Jurassic (average of 28 ± 14 ZAm 2 ; N = 138 individual estimates), as initially suggested by Prévot et al. [1990]. Our interpretation of the body of available data argue for low field strengths for the entire Jurassic extending into the early Cretaceous.

Description: 19 whole-round core samples from the Nankai accretionary prism (IODP Expeditions 315, 316 and 333) from a depth range of 28-128 m below sea floor were experimentally deformed in a triaxial cell under consolidated and undrained conditions at confining pressures of 400-1000 kPa, room temperature, axial displacement rates of 0.01-9.0 mm/min, and up to axially compressive strains of ~64%. Despite great similarities in composition and grain size distribution of the silty clay samples, two distinct ‘rheological groups’ are distinguished: The first group shows deviatoric peak stress after only a few percent of compressional strain (〈 10%) and a continuous stress decrease after peak conditions. Simultaneous to this decrease is a pore pressure increase, indicating contractant behavior characteristic of structurally weak material. The second sample group weakens only moderately at a much higher strength level after significantly higher strain (〉 10%), or does not weaken at all. This is characteristic of structurally strong material. The strong samples tend to be overconsolidated and are all from the drillsites at the accretionary prism toe, while the weak and normally consolidated samples come from the immediate hanging wall of a megasplay fault further upslope. Sediments from the incoming plate are also structurally weak. The observed differences in mechanical behavior may hold a key for understanding strain localization and brittle faulting within the uniform silty and clayey sedimentary sequence of the Nankai accretionary prism.

Description: Secondary microseisms are the most energetic waves in the noise spectra between 3 and 10 sec. They are generated by ocean wave interactions and are predominantly Rayleigh waves. We study the associated noise sources in the North Atlantic Ocean by coupling noise polarization analysis and sourcemapping using an ocean wave model that takes into account coastal reflections. From the Rayleigh wave polarization analysis we retrieve the back-azimuth to the noise sources in the time-frequency domain. Noise source modeling enables us to locate the associated generation areas at different times and frequencies. We analyze the distribution of secondary microseism sources in the North Atlantic Ocean using 20 broadband stations located in the Arctic and around the ocean. To model the noise sources we adjust empirically the ocean wave coastal reflection coefficient as a function of frequency. We find that coastal reflections must be taken into account for accurately modeling 7-10 sec noise sources. These reflections can be neglected in the noise modeling for periods shorter than 7 sec. We find a strong variability of backazimuths and source locations as a function of frequency. This variability is largely related to the local bathymetry. One direct cause of the time- and frequency-dependent noise sources is the presence of sea-ice that affects the amplitude and polarization of microseisms at stations in the Arctic only at periods shorter than 4 sec

Description: We examined the physical properties of an exhumed and fossilized subduction zone megasplay fault by analyzing geophysical logging data obtained by the Nobeoka Thrust Drilling Project, which provide a high-resolution transect of properties across the main fault zone. The footwall cataclasite exhibits higher averages of neutron porosity (~7.6%) and lower values of electric resistivity (~232 Ωm) compared to the hanging wall phyllite (~4.8%, ~453 Ωm). This clear contrast between the hanging wall and footwall may account for the difference in maximum burial and structural variation. Despite the contrast observed between the hanging wall and footwall in macroscopic scale, the resistivity and porosity data from both the hanging wall and footwall can be fit with a single curve using Archie's law, suggesting the similarities in microstructures and mineralogy in this low porosity range. Above the main fault core of the Nobeoka Thrust a brittle damage zone in the hanging wall contains pseudotachylyte as evidence of the seismogenic slip and does not follow Archie's law. Damage zones in the hanging wall are also observed in the modern splay fault at shallow depth in the Nankai Trough but with much thicker width, whereas the footwall damage zone is more extensive in the Nobeoka Thrust. Splay faults may exhibit strong deformation in the hanging wall in the early stage, and as fault rocks get buried deeper and as displacement and physical property contrast increase across the fault, the damage effect may eventually be enlarged in the footwall.

Description: A high-resolution stratigraphy is essential toward deciphering climate variability in detail and understanding causality arguments of events in earth history. Because the middle to late Eocene provides a perfect testing ground for carbon cycle models to reconstruct the transition from a hothouse to an icehouse world, an accurate time scale is needed to decode climate-driving mechanisms. Here we present new results from ODP Site 1260 (Leg 207) which covers a unique expanded middle Eocene section (magnetochrons C18r to C20r, late Lutetian to early Bartonian) of the tropical western Atlantic including the chron C19r transient hyperthermal event and the Middle Eocene Climate Optimum (MECO). To establish a detailed cyclostratigraphy we acquired iron intensity records by XRF scanning Site 1260 cores. We revise the shipboard composite section, establish a cyclostratigraphy and use the exceptional eccentricity modulated precession cycles for orbital tuning. The new astrochronology revises the age of magnetic polarity chrons C19n to C20n, validates the position of very long eccentricity minima at 40.2 and 43.0 Ma in the orbital solutions, and extends the Astronomically Tuned Geological Time Scale back to 44 Ma. For the first time the new data provide clear evidence for an orbital pacing of the chron C19r event and a likely involvement of the very long (2.4 myr) eccentricity cycle contributing to the evolution of the MECO.

Description: As compared to the late Pleistocene, Alkenone-based sea surface temperature (SST) in the Benguela region revealed relatively warm and stable SST recorded between ~3.5 and 2.0 Ma, and coincide with a period of increasing biological productivity as revealed by increasing deposition of biogenic opal. We assess how the hydrological patterns recorded in SST proxies are embedded in the geological record by performing a proxy-proxy comparison. We used Laser-Ablation Inductively Coupled Plasma - Mass Spectrometry to measure the Mg/Ca on the planktonic foraminifera species Globigerina bulloides , allowing in-situ measurements of Mg/Ca on individual foraminiferal tests. Mg/Ca-derived temperatures provide much colder temperatures than alkenone-derived SST by up to 10°C. We build a scenario involving contrasting sensitivities of paleothermometers upon the annual cycle, namely alkenones preferentially capturing SST when the surface ocean is warmer than the mean-annual average SST, and G. bulloides capturing SST when upwelling intensifies. Multi-chamber analysis also suggests that G. bulloides migrates below the sea surface while calcifying its last chambers prior to gametogenesis, allowing the extraction of both surface and subsurface temperature from Mg/Ca measured on different chambers. The range of temperatures recorded between our multiple SST proxies is supported by the range of temperatures simulated with a general circulation model when different seasons, different water depth and different orbital configurations occurring during the late Pliocene are considered. A greater seasonal cycle in SST during the Pliocene can account for alkenone and Mg/Ca-derived temperature contrast, pointing to a radically different mode of upwelling activity in the Benguela region compared to today.

Description: [1] The “laser ablation split stream” (LASS) technique is a powerful tool for mineral scale isotope analyses and, in particular, for concurrent age and Hf isotope composition of zircon. Because LASS utilizes two independent mass spectrometers, a large range of masses can be measured during a single ablation, and thus the same sample volume can be analyzed for multiple geochemical systems. This paper describes a simple analytical setup using a laser ablation system coupled to a single-collector (for U-Pb age determination) and multi-collector (for Hf isotope analyses) inductively coupled plasma mass spectrometer (MC-ICPMS). [2] The ability of LASS Hf-age analysis to extract meaningful Hf isotope compositions in isotopically-zoned zircon is demonstrated using zircons from two Proterozoic gneisses from northern Idaho, USA. These samples illustrate the potential problems associated with inadvertently sampling multiple age and Hf components in zircons, as well as the potential of LASS to recover meaningful Hf isotope compositions. We suggest that such inadvertent sampling of differing age and Hf components is a significant cause of excess scatter in Hf isotope analyses, and demonstrate that the LASS approach offers a robust solution to these issues. [3] The veracity of the approach is demonstrated by accurate analyses of 10 reference zircons with well-characterized age and Hf isotopic composition, using laser spot diameters of 30 and 40 microns. In order to expand the database of high precision Lu-Hf isotope analyses of reference zircons, we present 27 new isotope dilution-MC-ICPMS Lu-Hf isotope measurements of 5 U-Pb zircon standards: FC-1, Temora, R33, QGNG, and 91500.

Description: Magma mixing plays a prominent role in the origins of intermediate magmas in subduction zones. However, the conditions and timescales of magma mixing and how these are linked to subsequent eruption are unclear. Mount Tauhara is the largest dacitic volcanic complex in the Taupo Volcanic Zone, New Zealand. Dacites from Tauhara Volcano have a complex petrography (Qtz + Plag + Amph + OPx + CPx + Oxi ± Oli) that can only have been produced by magma mixing and offer an ideal opportunity to investigate the processes and timescales involved in assembling dacite magmas in a continental subduction zone. Here we present whole-rock and mineral-specific major and trace element and isotopic data for the Tauhara dacites in order to identify the magma mixing end-members, constrain the physical conditions of mixing, and estimate the timescales and relationships between magma mixing, ascent and eruption. These data reveal that four separate mixing events between crystal-rich rhyolites (77–80 wt.% SiO 2 ; 40 ppm Sr) and crystal-poor mafic magmas of basaltic (48 wt.% SiO 2 ; 1340 ppm Sr) to andesitic (55–59 wt.% SiO 2 ; 490–580 ppm Sr) composition occurred to produce the Tauhara dacites. Mixing took place in well-stirred magma chambers located at mid-crustal depths (8–13 km) at temperatures from 840 to 900ºC. The timescales of magma mixing obtained from Ti diffusion in quartz appear to be largely dependent on the temperature and viscosity contrast between the end-members as andesite and rhyolite magma mixed on timescales of 2–7 months, whereas basalt and rhyolite magmas mixed on timescales of 1–2 years. The short magma mixing timescales, combined with the physical properties (e.g., viscosity and density) of the mixed dacite magmas, as compared with those of the end-member magmas, facilitated the ascent and eruption of dacite magmas at Tauhara volcano.

Description: Integrated Ocean Drilling Program (IODP) Hole 1256D successfully sampled a complete section of an intact oceanic crustal sheeted-dike complex (SDC) (from 1061 to 1320 meters below seafloor; mbsf) on a 15 Ma-old Cocos Plate. A series of rock magnetic measurements were carried out to understand the magmatic processes that accreted this end-member, superfast-spread (200 mm/yr full rate) oceanic crust. Results indicate that main ferromagnetic minerals are predominantly pseudo-single domain (titano)magnetite crystals, responsible for both anisotropy of magnetic susceptibility (AMS) and magnetic remanence signals. AMS fabrics were reoriented into a geographic reference frame using magnetic remanence data, and corrected for a counterclockwise rotation of the Cocos Plate relative to the East Pacific Rise (EPR) ca. 15 Ma. Corrected AMS fabrics were then compared with the orientations of chilled margins previously obtained from Formation MicroScanner (FMS) images of the SDC at Hole 1256D. For some samples taken from close to dike margins, a dike-normal orientation of the minimum AMS axes (K min ) of prolate AMS ellipsoids mean that the long axis (K max ) can be used to infer magma flow directions. Subvertical K min orientations in the interior of the dikes, however, may have required settling or compaction of the magma shortly after intrusion, thus rearranging the AMS fabric. Despite this orientation of K min axes, orientation of K max axes indicate a rather constant subhorizontal paleo-flow direction, suggesting that magmas most probably traveled to the surface considerable distances from source regions within the EPR system.