ALBERT

Description: Changes in the Earth's magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere. Paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes (polarity reversals), but dating uncertainties have hindered progress in this understanding. Here we report a paleomagnetic record from multiple lava flows on Tahiti that bracket the Matuyama-Brunhes (M-B) polarity reversal ∼771 thousand years ago. Our high-precision 40 Ar/ 39 Ar ages constrain several rapid and short-lived changes in field orientation up to 33,000 years prior to the M-B reversal. These changes are similar to ones identified in other less well-dated lava flows in Maui, Chile and La Palma that occurred during an extended period of reduced field strength recorded in sediments. We use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates. This prolonged 33,000-year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial-to-millennial scale polarity changes in the presence of a strongly weakened dipole field.

Description: We studied the effect of 973 K heating in argon atmosphere on the magnetic and structural properties of a magnetite-bearing ore, which was previously exposed to laboratory shock waves between 5 and 30 GPa. For this purpose magnetic properties were studied using temperature-dependent magnetic susceptibility, magnetic hysteresis and low-temperature saturation isothermal remanent magnetization. Structural properties of magnetite were analyzed using X-ray diffraction, high-resolution electron microscopy and synchrotron-assisted X-ray absorption spectroscopy. The shock-induced changes include magnetic domain size reduction due to brittle and ductile deformation features and an increase in Verwey transition temperature due to lattice distortion. After heating, the crystal lattice is relaxed and apparent crystallite size is increased suggesting a recovery of lattice defects documented by a mosaic recrystallization texture in the 20 GPa heated sample. The structural changes correlate with a modification in magnetic domain state recorded by temperature-dependent magnetic susceptibility, hysteresis properties and low-temperature saturation isothermal remanent magnetization. These alterations in both, magnetic and structural properties of magnetite can be used to assess impact-related magnetic anomalies in impact structures with a high temperature overprint.

Description: δ 18 O and δD of fluid inclusions in carbonates provide insights into temperatures and fluid chemical compositions prevailing during the carbonate precipitation, however various analytical restrictions limit a wider application of this proxy. This paper presents a new fluid inclusions isotopic analytical line coupled to an online cavity ring-down spectrometer that increased the analytical productivity up to ten carbonate samples per working day. This efficiency allowed for the first time to assess the reliability a large set of water samples with size ranging from 0.1 to 1 µL. Good reproducibility (± 0.5 ‰ for δ 18 O and ± 2 ‰ δD; 1σ) is obtained for water quantity superior or equal to 0.3 μL and no evidence of memory effect is found. The line is further tested using two types of natural carbonates: (1) modern speleothems samples from caves for which δ 18 O and δD values of drip water were measured and (2) diagenetic carbonates for which the δ 18 O of the parent water were independently back-calculated from carbonate clumped isotope Δ 47 measurements. Speleothem fluid inclusion values despite falling close to the Global Meteoritic Water Line are not always representative of the isotopic composition of the parent drip water. Results on diagenetic cements show that the δ 18 O water values measured in fluid inclusions agree, within 1%, with the δ 18 O water independently derived from Δ 47 measurements. Overall, this study confirms the reliability and accuracy of the developed analytical line for carbonate fluid inclusion analyses with a good reproducibility obtained for water quantity above 0.3 μL.

Description: The permeability of the oceanic crust exerts a primary influence on the vigor of hydrothermal circulation at mid-ocean ridges, but it is a difficult to measure parameter that varies with time, space, and geological setting. Here we develop an analytical model for the poroelastic response of hydrothermal exit-fluid velocities and temperatures to ocean tidal loading in a two-layered medium to constrain the discharge zone permeability of each layer. The top layer, corresponding to extrusive lithologies (e.g., seismic layer-2A) overlies a lower-permeability layer, corresponding to intrusive lithologies (e.g., layer-2B). We apply the model to three basalt-hosted hydrothermal fields (i.e., Lucky Strike, Main Endeavour, and 9°46'N L-vent) for which the seismic stratigraphy is well-established, and for which robust exit-fluid temperature data are available. We find that the poroelastic response to tidal loading is primarily controlled by layer 2A permeability, which is about 3 orders of magnitude higher for the Lucky Strike site (∼10 −10 m 2 ) than the 9°46'N L-vent site (∼ 10 −13 m 2 ). By contrast, layer 2B permeability does not exert a strong control on the poroelastic response to tidal loading, yet strongly modulates the heat output of hydrothermal discharge zones. Taking these constraints into account, we estimate a plausible range of layer 2B permeability between ∼10 −15 m 2 and an upper-bound value of ∼10 −14 (9°46'N L-vent) to ∼10 −12 m 2 (Lucky Strike). These permeability structures reconcile the short-term response and long-term thermal output of hydrothermal sites, and provide new insights into the links between permeability and tectono-magmatic processes along the global mid-ocean ridge.

Description: Calcifying organisms face increasing stress from the changing carbonate chemistry of an acidifying ocean, particularly bivalve larvae that live in upwelling regions of the world, such as the coastal and estuarine waters of Oregon (USA). Arguably the first and most significant developmental hurdle faced by larval oysters is formation of their initial prodissoconch I (PDI) shell, upon which further ontological development depends. We measured the minor metal compositions (Sr/Ca, Mg/Ca) of this aragonitic PDI shell and of post-PDI larval Crassostrea gigas shell, as well as the water they were reared in, over ∼20 days for a May and an August cohort in 2011, during which time there was no period of carbonate under-saturation. After testing various methods, we successfully isolated the shell from organic tissue using a 5% active chlorine bleach solution. Elemental compositions (Sr, Mg, C, N) of the shells post treatment showed that shell Sr/Ca ranged from 1.55 to 1.82 mmol/mol; Mg/Ca from 0.60 to 1.11 mmol/mol, similar to the few comparable published data for larval oyster aragonite compositions. We compare these data in light of possible biomineralization mechanisms: an amorphous calcium carbonate (ACC) path, an intercellular path, and a direct-from-seawater path to shell formation via biologically induced inorganic precipitation of aragonite. The last option provides a mechanistic explanation for: 1) the accelerated precipitation rates of biogenic calcification in the absence of a calcifying fluid; 2) consistently elevated precipitation rates at varying ambient-water saturation states; and 3) the high Ca-selectivity of the early-larval calcification despite rapid precipitation rates.

Description: We investigated the (paleo)latitudinal dependence of angular dispersion sets of virtual geomagnetic poles (VGPs) for the Permian-Carboniferous Reversed Superchron (PCRS: 262-318 Ma). In order to analyze the paleosecular variation during this period we prepared different paleomagnetic datasets from scientific databases and recent literature, based on selection criteria which provided high degree of refinement. Model G of McFadden et al. (1988) was fitted to the VGP dispersion data, providing the shape parameters a and b , which were further compared with similar results for the Cretaceous Normal Superchron (CNS) and other periods of different geomagnetic reversal rates throughout the last 3 Ga. Our results indicate high similarity between the angular VGP dispersion from the PCRS and CNS (a low VGP dispersion at low paleolatitudes, and strong paleolatitudinal dependence), in contrasting difference for periods of higher reversal rates (the last 5 Ma). Despite the geodynamic differences related to both Phanerozoic superchrons, such evidence could point out similar heat flux conditions in the CMB, which may favor compatible stability conditions throughout these magnetozones. Notably, two additional observations which arose from the shape parameters relations for both Phanerozoic superchrons and Precambrian datasets demand further investigation: (i) a pattern of increase for the b / a ratio throughout the 0.5-1.6 Ga interval was observed, which could be partially explained by the Maya Superchron, recently proposed by Gallet et al. (2012); (ii) the b × a relationship for both Phanerozoic superchrons and the 2.45-2.82 Ga interval pointed to the presence of one or more long-term magnetozones throughout the Paleoproterozoic.

Description: Foraminiferal taphocoenose characteristics, notably allochthonous tests in surface sediment samples from the Yangtze River Estuary and adjacent areas in the East China Sea, were examined during (July, 2016) and after typhoon Nepartak (February, 2017) crossed this region. A comparison of transport indices from five sample stations reveals that this typhoon significantly strengthened sediment transport, although the transport in predominate direction (TWC north branch) occurs all year. Moreover, findings from a comprehensive impact of currents, topography, test size and shape, as well as other abiotic factors, show that transport occurs in the order (from stronger to weaker): south-north path ≫ southeast-northwest path 〉 nearshore-northeast path. Along the south-north path, the TWC north branch transports larger tests of taxa Ammonia compressiuscula and Cavarotalia annectens as bed load along the gentle seabed in a rolling or saltating process. Results from the southeast-northwest path show smaller and bulging tests of the dominant species Epistominella naraensis and Bolivina robusta may be transported as suspension load along the steep submarine valley by the TWC northwest branch. On the nearshore-northeast path, only part of allochthonous tests Ammonia beccarii vars. was transported, probably due to distance travelled and weakening of the typhoon. Furthermore, we also infer the northeastern area as a sink zone that accumulates grains of allochthonous tests predominantly from the southern source zone, and a selection of tests from littoral species originating from the nearshore source zone. It is important to identify and independently count allochthonous test samples if a large number of these tests are recorded. Findings from this study provide a reference to identify and investigate typhoon modern effects in areas with complex currents and frequent strong physical dynamic events, as well as their geological records.

Description: Recent sophisticated global data compilations and magnetic surveys have been used to investigate the nature of magnetization in the lower crust and upper mantle. Two approaches to constraining magnetizations are developed, providing minimum (0.01 SI) and maximum (0.04 SI) susceptibility estimates, given some assumed thickness (15+ km here). These values are higher than are found in many continental rocks. Are there rocks deeper in the crust or upper mantle that are more magnetic than expected, or are the model assumptions incomplete? What is the magnetic behavior of deep crustal and upper mantle rocks, when slightly cooler than the Curie or Néel temperatures of their magnetic minerals, after being exhumed from locations of high-grade metamorphism at greater depth? Different sets of equilibrium metamorphic minerals can be considered that would form under different conditions. Results on 1501 samples from the Western Gneiss Region (WGR) Norway, mainly from mafic and ultramafic bodies subducted to depths of 60-200 km and temperatures of 750 up to 950°C at the very highest pressures, show that rocks did not fully equilibrate to the dominant metamorphic-facies conditions. There is a large variation in petrophysical properties, oxide minerals, and mineral assemblages in WGR samples, though they cannot explain the broad high-amplitude (deep-seated) anomalies measured in this region. The presence of magnetite, and exsolved titanohematite and hemo-ilmenite in samples, shows those magnetic phases are preserved even at eclogite-facies conditions, in part because complete eclogite-facies equilibrium was rarely achieved.

Description: Reconstructions of the carbonate compensation depth (CCD) in the past have been used to inform hypotheses about the nature of weathering, tectonics, climate change, and the major ion content of the world's oceans over the Cenozoic. These reconstructions are sensitive to uncertainties in the input data, in particular, the paleodepth estimates of sediment cores. Here we propose that a significant, previously unconsidered contributor to uncertainties in paleodepth estimates is from dynamic topography produced by radial stresses exerted on the Earth's surface by the convecting mantle; these stresses can warp the ocean floor by hundreds of meters over broad regions and also vary significantly over millions of years. We present new reconstructions of the equatorial Pacific and Indian Ocean CCDs over the last 30 Myr and 23 Myr, respectively, which demonstrate an overall deepening trend since the Miocene, and illustrate the possible effect of long-term changes in dynamic topography on these reconstructions.

Description: Submarine groundwater discharge (SGD) has been recognized as an important pathway for nutrients into estuaries, coasts and the adjacent seas. In this study, 222 Rn was used to estimate the SGD-associated nutrient fluxes into an aquaculture area in a typical tropical bay (Maowei Sea, China). The SGD into the Maowei Sea during June of 2016 was estimated to be 0.36±0.33 m d −1 and was associated with SGD-derived dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicon (DSi) fluxes (mol d −1 ) of (4.5±5.5)×10 6 , (5.3±9.1)×10 4 and (9.4±9.3)×10 6 , respectively. The SGD-derived nutrients (i.e., DIN, DIP and DSi) were more than 1.9, 0.9 and 3.6 times the amounts in the local river input and served as dominant sources in the nutrient budgets in the Maowei Sea. Moreover, the N/P ratios in the SGD around the Maowei Sea were high (mean: 64), and these ratios likely exceeded the environmental self-purification capacity, thereby enhancing the biomass and changing the phytoplankton community structure. Therefore, SGD processes with derived nutrients may affect the biogeochemical cycles and marine ecological environment in the Maowei Sea. Furthermore, the N/P ratios (∼67) in oysters are very close to those in the SGD in the Maowei Sea; this coincidence suggests that the high N/P ratios in the SGD are likely to be one of the most important sources that support oyster aquaculture, which might weaken the burden of water eutrophication in the Maowei Sea.