ALBERT

Description: Growing evidence suggests that the low atmospheric CO2 concentration of the ice ages resulted from enhanced storage of CO2 in the ocean interior, largely as a result of changes in the Southern Ocean1. Early in the most recent deglaciation, a reduction in North Atlantic overturning circulation seems to have driven CO2 release from the Southern Ocean**2, 3, 4, 5, but the mechanism connecting the North Atlantic and the Southern Ocean remains unclear. Biogenic opal export in the low-latitude ocean relies on silicate from the underlying thermocline, the concentration of which is affected by the circulation of the ocean interior. Here we report a record of biogenic opal export from a coastal upwelling system off the coast of northwest Africa that shows pronounced opal maxima during each glacial termination over the past 550,000 years. These opal peaks are consistent with a strong deglacial reduction in the formation of silicate-poor glacial North Atlantic intermediate water**2 (GNAIW). The loss of GNAIW allowed mixing with underlying silicate-rich deep water to increase the silicate supply to the surface ocean. An increase in westerly-wind-driven upwelling in the Southern Ocean in response to the North Atlantic change has been proposed to drive the deglacial rise in atmospheric CO2 (refs 3, 4). However, such a circulation change would have accelerated the formation of Antarctic intermediate water and sub-Antarctic mode water, which today have as little silicate as North Atlantic Deep Water and would have thus maintained low silicate concentrations in the Atlantic thermocline. The deglacial opal maxima reported here suggest an alternative mechanism for the deglacial CO2 release**5, 6. Just as the reduction in GNAIW led to upward silicate transport, it should also have allowed the downward mixing of warm, low-density surface water to reach into the deep ocean. The resulting decrease in the density of the deep Atlantic relative to the Southern Ocean surface promoted Antarctic overturning, which released CO2 to the atmosphere.

Description: We report relative paleointensity proxy records from four piston cores collected near the Agulhas Ridge and Meteor Rise (South Atlantic). The mean sedimentation rate of the cores varies from 24 cm/kyr to 11 cm/kyr. The two cores with mean sedimentation rates over 20 cm/kyr record positive remanence inclinations at 40-41 ka coeval with the Laschamp Event. Age models are based on oxygen isotope data from three of the cores, augmented by radiocarbon ages from nearby Core RC11-83, and by correlation of paleointensity records for the one core with no oxygen isotope data. The relative paleointensity proxy records are the first from the South Atlantic and from the high to mid-latitude southern hemisphere. Prominent paleointensity lows at ?40 ka and ?65 ka, as well as many other features, can be correlated to paleointensity records of comparable resolution from the northern hemisphere. The records are attributable, in large part, to the global-scale field, and therefore have potential for inter-hemispheric correlation at a resolution difficult to achieve with isotope data alone.

Description: During Ocean Drilling Program (ODP) Leg 178, eight holes were drilled at three sites (1095, 1096, and 1101) on the continental rise along the western Antarctic Peninsula. The rise sediments proved to be good paleomagnetic recorders and provided continuous magnetostratigraphic records at all three sites. Biosiliceous microfossils, particularly diatoms and radiolarians, were present in the upper Miocene through lower Pliocene sections. In the upper Pliocene to Pleistocene sections, biosiliceous microfossils were rare but calcareous nannofossils and foraminifers were present. This paper summarizes the biostratigraphy and magnetostratigraphy of Leg 178 continental rise sites and is the first attempt at direct calibration of Antarctic biostratigraphic events to the geomagnetic polarity timescale in the Pacific sector of the Southern Ocean.

Description: The Wilkes and Aurora basins are large, low-lying sub-glacial basins that may cause areas of weakness in the overlying East Antarctic ice sheet. Previous work based on ice-rafted debris (IRD) provenance analyses found evidence for massive iceberg discharges from these areas during the late Miocene and Pliocene. Here we characterize the sediments shed from the inferred areas of weakness along this margin (94°E to 165°E) by measuring40Ar/39Ar ages of 292 individual detrital hornblende grains from eight marine sediment core locations off East Antarctica and Nd isotopic compositions of the bulk fine fraction from the same sediments. We further expand the toolbox for Antarctic IRD provenance analyses by exploring the application of 40Ar/39Ar ages of detrital biotites; biotite as an IRD tracer eliminates lithological biases imposed by only analyzing hornblendes and allows for characterization of samples with low IRD concentrations. Our data quadruples the number of detrital 40Ar/39Ar ages from this margin of East Antarctica and leads to the following conclusions: (1) Four main sectors between the Ross Sea and Prydz Bay, separated by ice drainage divides, are distinguishable based upon the combination of 40Ar/39Ar ages of detrital hornblende and biotite grains and the e-Nd of the bulk fine fraction; (2) 40Ar/39Ar biotite ages can be used as a robust provenance tracer for this part of East Antarctica; and (3) sediments shed from the coastal areas of the Aurora and Wilkes sub-glacial basins can be clearly distinguished from one another based upon their isotopic fingerprints.

Description: Detailed biostratigraphy in Site 1006 based on planktonic foraminifers and nannofossils shows large-scale sedimentation rate variability in the Florida Strait west of the Great Bahama Bank. A 'floating' cyclostratigraphy based mainly on resistivity logs and magnetic susceptibility data has been fixed to the biostratigraphy in the absence of magnetostratigraphy. The strongest orbital cycle present is the precessional beat, which is present in the borehole logs throughout the record. Counting the cycles resulted in an accurate time scale and thus a sedimentation rate time series. Spectral analysis of the sedimentation rate time series shows that the short-term cycle of eccentricity (~125 k.y.) and the long term cycle of eccentricity (~400 k.y.) are pervasive throughout the Miocene record, together with the long-term ~2-m.y. eccentricity cycle. The Great Bahama Bank produced pulses of shallow carbonate input once every precessional (sea level) cycle during the Miocene and perhaps two pulses per cycle in the early Pliocene. The amount of sediment exported in these pulses appears to be controlled by eccentricity modulation of the precessional amplitude and therefore the amplitude of the sea-level rise. Finally, an increase in sedimentation rate just after the Miocene/Pliocene boundary is attributed to a change in the location and strength of sediment drift currents in the Florida Strait due to reorganization of the currents following the closure of the Panama Isthmus.

Description: Absolute ages of plutonic rocks from mid-ocean ridges provide important constraints on the scale, timing and rates of oceanic crustal accretion, yet few such rocks have been absolutely dated. We present 206Pb/238U SHRIMP zircon ages from two ODP Drill Holes and a surface sample from Atlantis Bank on the Southwest Indian Ridge. We report ten new sample ages from 26-1430 m in ODP Hole 735B, and one from 57 m in ODP Hole 1105A. Including a previously published age, eleven samples from Hole 735B yield 206Pb/238U zircon crystallization ages that are the same, within error, overlap with the estimated magnetic age and are inferred to date the main period of crustal growth, the average age of analyses is 11.99 ± 0.12 Ma. Any differences in the ages of magmatic series and/or tectonic blocks within Hole 735B are unresolvable and eight well-constrained ages vary from 11.86 ± 0.20 Ma to 12.13 ± 0.21 Ma, a range of 0.27 ± 0.29 Ma, consistent with the duration of crustal accretion observed at the Mid-Atlantic Ridge. An age of 11.87 ± 0.23 Ma from Hole 1105A is within error of ages from Hole 735B and permits previous correlations made between zones of oxide-rich gabbros in each hole. Pb/U zircon ages 〉 0.5 Ma younger than the magnetic age are recorded in at least three samples from Atlantis Bank, one from Hole 735B and two collected along a fault scarp to the East. These young ages may date one or more off-axis events previously suggested from thermochronologic data and support the interpretation of a complex geological history following crustal accretion at Atlantis Bank. Together with results from the surface of Atlantis Bank, dating has shown that while the majority of Pb/U SHRIMP zircon ages record the short-lived (〈 0.5 Ma) phase of crustal accretion on-axis, results from several samples precede and post-date this period by 〉 1 Ma suggesting a complex and prolonged magmatic/tectonic history for the crust at Atlantis Bank.

Description: A limiting factor in the accuracy and precision of U/Pb zircon dates is accurate correction for initial disequilibrium in the 238U and 235U decay chains. The longest-lived-and therefore most abundant-intermediate daughter product in the 235U isotopic decay chain is 231Pa (T1/2 = 32.71 ka), and the partitioning behavior of Pa in zircon is not well constrained. Here we report high-precision thermal ionization mass spectrometry (TIMS) U-Pb zircon data from two samples from Ocean Drilling Program (ODP) Hole 735B, which show evidence for incorporation of excess 231Pa during zircon crystallization. The most precise analyses from the two samples have consistent Th-corrected 206Pb/238U dates with weighted means of 11.9325 ± 0.0039 Ma (n = 9) and 11.920 ± 0.011 Ma (n = 4), but distinctly older 207Pb/235U dates that vary from 12.330 ± 0.048 Ma to 12.140 ± 0.044 Ma and 12.03 ± 0.24 to 12.40 ± 0.27 Ma, respectively. If the excess 207Pb is due to variable initial excess 231Pa, calculated initial (231Pa)/(235U) activity ratios for the two samples range from 5.6 ± 1.0 to 9.6 ± 1.1 and 3.5 ± 5.2 to 11.4 ± 5.8. The data from the more precisely dated sample yields estimated DPazircon/DUzircon from 2.2-3.8 and 5.6-9.6, assuming (231Pa)/(235U) of the melt equal to the global average of recently erupted mid-ocean ridge basaltic glasses or secular equilibrium, respectively. High precision ID-TIMS analyses from nine additional samples from Hole 735B and nearby Hole 1105A suggest similar partitioning. The lower range of DPazircon/DUzircon is consistent with ion microprobe measurements of 231Pa in zircons from Holocene and Pleistocene rhyolitic eruptions (Schmitt (2007; doi:10.2138/am.2007.2449) and Schmitt (2011; doi:10.1146/annurev-earth-040610-133330)). The data suggest that 231Pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions, and excess initial 231Pa may be more common in zircons than acknowledged. The degree of initial disequilibrium in the 235U decay chain suggested by the data from this study, and other recent high precision datasets, leads to resolvable discordance in high precision dates of Cenozoic to Mesozoic zircons. Minor discordance in zircons of this age may therefore reflect initial excess 231Pa and does not require either inheritance or Pb loss.

Description: We present new isotopic data for sedimentary planktonic foraminifera, as well as for potential water column and sedimentary sources of neodymium (Nd), which confirm that the isotopic composition of the foraminifera is the same as surface seawater and very different from deep water and sedimentary Nd. The faithfulness with which sedimentary foraminifera record the isotopic signature of surface seawater Nd is difficult to explain given their variable and high Nd/Ca ratios, ratios that are often sedimentary foraminifera, ratios that are often much higher than is plausible for direct incorporation within the calcite structure. We present further data that demonstrate a similarly large range in Nd/Ca ratios in plankton tow foraminifera, a range that may be controlled by redox conditions in the water column. Cleaning experiments reveal, in common with earlier work, that large amounts of Nd are released by cleaning with both hydrazine and diethylene triamine penta-acetic acid, but that the Nd released at each step is of surface origin. While further detailed studies are required to verify the exact location of the surface isotopic signature and the key controls on foraminiferal Nd isotope systematics, these new data place the use of planktonic foraminifera as recorders of surface water Nd isotope ratios, and thus of variations in the past supply of Nd to the oceans from the continents via weathering and erosion, on a reasonably sure footing.

Description: Few astronomically calibrated high-resolution (〈=5 kyr) climate records exist that span the Oligocene?Miocene time interval. Notably, available proxy records show responses varying in amplitude at frequencies related to astronomical forcing, and the main pacemakers of global change on astronomical time-scales remain debated. Here we present newly generated X-ray fluorescence core scanning and benthic foraminiferal stable oxygen and carbon isotope records from Ocean Drilling Program Site 1264 (Walvis Ridge, southeastern Atlantic Ocean). Complemented by data from nearby Site 1265, the Site 1264 benthic stable isotope records span a continuous ~13-Myr interval of the Oligo-Miocene (30.1?17.1 Ma) at high resolution (~3.0 kyr). Spectral analyses in the stratigraphic depth domain indicate that the largest amplitude variability of all proxy records is associated with periods of ~3.4 m and ~0.9 m, which correspond to 405- and ~110-kyr eccentricity, using a magnetobiostratigraphic age model. Maxima in CaCO3 content, d18O and d13C are interpreted to coincide with ~110 kyr eccentricity minima. The strong expression of these cycles in combination with the weakness of the precession- and obliquity-related signals allow construction of an astronomical age model that is solely based on tuning the CaCO3 content to the nominal (La2011_ecc3L) eccentricity solution. Very long-period eccentricity maxima (~2.4-Myr) are marked by recurrent episodes of high-amplitude ~110-kyr d18O cycles at Walvis Ridge, indicating greater sensitivity of the climate/cryosphere system to short eccentricity modulation of climatic precession. In contrast, the responses of the global (high-latitude) climate system, cryosphere, and carbon cycle to the 405-kyr cycle, as expressed in benthic d18O and especially d13C signals, are more pronounced during ~2.4-Myr minima. The relationship between the recurrent episodes of high-amplitude ~110-kyr d18O cycles and the ~1.2-Myr amplitude modulation of obliquity is not consistent through the Oligo-Miocene. Identification of these recurrent episodes at Walvis Ridge, and their pacing by the ~2.4-Myr eccentricity cycle, revises the current understanding of the main climate events of the Oligo-Miocene.

Description: Pelagic sediments from the subtropical South Atlantic Ocean contain geographically extensive Oligocene ooze and chalk layers that consist almost entirely of the calcareous nannofossil Braarudosphaera. Poor recovery and the lack of precise dating of these horizons in previous studies has limited our understanding of the exact number of acmes, their timing and durations, and the causes of their recurrence. Here we present a high-resolution, astronomically tuned stratigraphy of Braarudosphaera oozes (29.5-27.9 Ma) from Ocean Drilling Program Site 1264 in the subtropical southeastern Atlantic Ocean. We identify seven acme events in the Braarudosphaera abundance record. The longest lasting acme event corresponds to a strong minimum in the ~2.4-My eccentricity cycle, and four acme events coincide with ~110-ky and 405-ky eccentricity maxima. We propose that eccentricity-modulated precession forcing of the freshwater budget of the South Atlantic Ocean resulted in the episodic formation of a shallow pycnocline and hyperstratification of the upper water column. We speculate that stratified surface water conditions may have served as a virtual sea floor, which facilitated the widespread Braarudosphaera acmes. This explanation reconciles the contrasting distribution patterns of Braarudosphaera in the modern ocean, limited largely to shallow water coastal settings, compared to their relatively brief and expanded oceanic distribution in the past.

Description: Neodymium isotopes are becoming widely used as a palaeoceanographic tool for reconstructing the source and flow direction of water masses. A new method using planktonic foraminifera which have not been chemically cleaned has proven to be a promising means of avoiding contamination of the deep ocean palaeoceanographic signal by detrital material. However, the exact mechanism by which the Nd isotope signal from bottom waters becomes associated with planktonic foraminifera, the spatial distribution of rare earth element (REE) concentrations within the shell, and the possible mobility of REE ions during changing redox conditions, have not been fully investigated. Here we present REE concentration and Nd isotope data from mixed species of planktonic foraminifera taken from plankton tows, sediment traps and a sediment core from the NW Atlantic. We used multiple geochemical techniques to evaluate how, where and when REEs become associated with planktonic foraminifera as they settle through the water column, reside at the surface and are buried in the sediment. Analyses of foraminifera shells from plankton tows and sediment traps between 200 and 2938 m water depth indicate that only ~20% of their associated Nd is biogenically incorporated into the calcite structure. The remaining 80% is associated with authigenic metal oxides and organic matter, which form in the water column, and remain extraneous to the carbonate structure. Remineralisation of these organic and authigenic phases releases ions back into solution and creates new binding sites, allowing the Nd isotope ratio to undergo partial equilibration with the ambient seawater, as the foraminifera fall through the water column. Analyses of fossil foraminifera shells from sediment cores show that their REE concentrations increase by up to 10-fold at the sediment-water interface, and acquire an isotopic signature of bottom water. Adsorption and complexation of REE3+ ions between the inner layers of calcite contributes significantly to elevated REE concentrations in foraminifera. The most likely source of REE ions at this stage of enrichment is from bottom waters and from the remineralisation of oxide phases which are in chemical equilibrium with the bottom waters. As planktonic foraminifera are buried below the sediment-water interface redox-sensitive ion concentrations are adjusted within the shells depending on the pore-water oxygen concentration. The concentration of ions which are passively redox sensitive, such as REE3+ ions, is also controlled to some extent by this process. We infer that (a) the Nd isotope signature of bottom water is preserved in planktonic foraminifera and (b) that it relies on the limited mobility of particle reactive REE3+ ions, aided in some environments by micron-scale precipitation of MnCO3. This study indicates that there may be sedimentary environments under which the bottom water Nd isotope signature is not preserved by planktonic foraminifera. Tests to validate other core sites must be carried out before downcore records can be used to interpret palaeoceanographic changes.

Description: Synthetic seismograms are constructed from check shot-corrected velocity and density measurements collected during Ocean Drilling Program (ODP) Leg 180 at Sites 1109, 1115, and 1118. The synthetic seismograms facilitate direct correlation of a coincident multichannel seismic (MCS) profile with borehole data collected at the three sites. The MCS data and the synthetic seismograms correlate very well, with most major reflectors successfully reproduced in the synthetics. Our results enable a direct calibration of the MCS data in terms of age, paleoenvironment, and subsidence history. Seismic reflectors are time correlative within stratigraphic resolution but are often observed to result from different lithologies across strike. Our results facilitate the extrapolation of the sedimentation history into an unsampled section of Site 1118 and enable a full correlation between the three sites using all the data collected during ODP Leg 180. This study forms the foundation for regionalizing the site data to the northern margin of the Woodlark Basin, where the transition from continental rifting to seafloor spreading is taking place.

Description: During the late Miocene epoch, about seven million years ago, large areas of the continents experienced drying, enhanced seasonality, and a restructuring of terrestrial plant and animal communities. These changes are seen throughout the subtropics, but have typically been attributed to regional tectonic forcing. Here we present a set of globally distributed sea surface temperature records spanning the past 12 million years based on the alkenone unsaturation method. We find that a sustained late Miocene cooling occurred synchronously in both hemispheres, and culminated with ocean temperatures dipping to near-modern values between about 7 and 5.4 million years ago. The period of maximum cooling coincides with evidence for transient glaciations in the Northern Hemisphere and with a steepening of the pole-to-equator temperature gradient, as well. We thus infer that late Miocene aridity and terrestrial ecosystem changes occurred in a global context of increasing meridional temperature gradients. We conclude that a global forcing mechanism, such as the previously hypothesized decline in atmospheric CO2 levels between eight and six million years ago, is required to explain the late Miocene changes in temperature, climate and ecosystems.

Description: Recognizing and deciphering transient global warming events triggered by massive release of carbon into Earth's ocean-atmosphere climate system in the past are important for understanding climate under elevated pCO2 conditions. Here we present new high-resolution geochemical records including benthic foraminiferal stable isotope data with clear evidence of a short-lived (30 kyr) warming event at 41.52 Ma. The event occurs in the late Lutetian within magnetochron C19r and is characterized by a ~2°C warming of the deep ocean in the southern South Atlantic. The magnitudes of the carbon and oxygen isotope excursions of the Late Lutetian Thermal Maximum are comparable to the H2 event (53.6 Ma) suggesting a similar response of the climate system to carbon cycle perturbations even in an already relatively cooler climate several million years after the Early Eocene Climate Optimum. Coincidence of the event with exceptionally high insolation values in the Northern Hemisphere at 41.52 Ma might indicate that Earth's climate system has a thermal threshold. When this tipping point is crossed, rapid positive feedback mechanisms potentially trigger transient global warming. The orbital configuration in this case could have caused prolonged warm and dry season leading to a massive release of terrestrial carbon into the ocean-atmosphere system initiating environmental change.

Description: Compaction curves for 11 samples from the mixed sediments and calcareous chalk with clay from the Caribbean Sites 999 and 1001 are discussed with reference to compaction curves for calcareous ooze and chalk of the Ontong Java Plateau (Leg 130). The burial history is discussed from preconsolidation data and present burial conditions and suggests a removal of ~400 m of sediment at the hiatus 166 meters below seafloor (mbsf) at Site 1001. This interpretation predicts a previous burial to 〉500 mbsf for depth intervals containing microstylolites, which corresponds to observations at Sites 999 and 807 (Ontong Java Plateau). Thus, data from three sites from two widely separate regions indicate that microstylolites in carbonates form at minimum burial depths deeper than 500 m. No direct link between formation of microstylolites and cementation was found, suggesting that dissolution and precipitation are not necessarily related. Porosity rebound during core retrieval could not be detected for soft sediments, whereas a porosity rebound of ~2% was deduced for deeper, cemented intervals. Comparing the compaction curves, two distinct rates of porosity loss are noted: (1) samples dominated by clay (〉45% insoluble residue) compact at a higher rate than samples dominated by fine-grained carbonate and (2) fine-grained carbonate supported samples (with 〈45% insoluble residue) compact at the same rate irrespective of the content of nonsupporting microfossils or pore-filling clay.

Description: Major and minor (Mn, Sr, Ba, V, Cr, Ni, Co, Zn, Cu, Zr, Y, Sc) elements and mineralogic compositions were determined on bulk sediments collected during Ocean Drilling Program Leg 135. Three classes of sediment samples from holes drilled in the Lau Basin are discriminated by mineralogy and major element data. Samples labeled Class 1 are significantly enriched in biogenic calcite and occur predominantly in the northern part of the basin (Sites 834-835), whereas those of Class 3 are mostly enriched in volcanogenic material and are predominant in the central part of the basin (Sites 836-839). The minor element composition records the effects of the hydrothermal activity on the sediments. In the northern area of the basin (Sites 834-835), sedimentation is characterized by higher accumulation rates of the carbonate and hydrothermal fractions. These sediments are probably reworked predominantly, transported in the water column, and then settled locally. Thus, ponded sediments are probably responsible to this high accumulation rates. Diagenetic processes altered the volcanic material to a grade corresponding to the stability of phillipsite. In the central area of the basin (Sites 836-839), sedimentation is characterized by the action of bottom currents preferentially reworking the carbonate and hydrothermal fractions. Volcanogenic accumulation rates are greater at these sites than in the northern Lau Basin. Alteration of volcanic material is more important deeper in the holes and records authigenesis of clay rich in Fe-Mg, most likely smectite. Locally, clay minerals have apparently incorporated Cr and other ore-forming elements.

Description: Interstitial water samples from Sites 834 through 839, drilled during Ocean Drilling Program Leg 135 in the backarc Lau basin (Southwestern Pacific), have been analyzed for major elements, manganese, copper, strontium, barium, vanadium, and 87Sr/86Sr isotopic composition values. The concentration-depth profiles of the major chemical components show almost straight concentration gradients at all sites, and seem to reflect slight alteration of volcanic material. However, in the lower part of the sedimentary cover, where volcanogenic material is abundant and where diagenetic minerals occur, systematic decreases in calcium, strontium, manganese, copper, and vanadium concentrations are observed. A downwelling flow of bottom seawater, which affected the diagenetic chemical signature of the interstitial water, is probably responsible for the recorded chemical features. This hypothesis is supported by strontium isotope data obtained from interstitial water samples at Site 835. It is also in accordance with data from heat flow and physical properties.

Description: Isotopic compositions of marine sediments and fossils have been investigated from northern basins of the Mediterranean to help constrain local oceanographic and climatic changes adjacent to the uplifting Alps. Stable C and O isotope compositions of benthic and planktonic foraminifera from the Umbria-Marche region (UMC) have an offset characteristic for their habitats and the changes in composition mimic global changes, suggesting that the regional conditions of climate and the carbon cycle were controlled by global changes. The radiogenic isotope composition of these fossil assemblages allows recognition of three distinct periods. In the first period, from 25 to 19 Ma, high epsilon-Nd values and low 87Sr/86Sr of sediments and fossils support intense tectonism and volcanism, related to the opening of the western Mediterranean. In the second period, from 19 to 13 Ma the 87Sr/86Sr ratio of Mediterranean (UMC) deviate from the global ocean, which is compatible with rapid uplift of the hinterland and intense influx of Sr from Mesozoic carbonates of the western Apennines. This local control on the seawater was driven by a humid and warm climate and indicates restricted exchange of water with the global ocean. Generally, the epsilon-Nd values of the fossils are very similar to those of Indian Ocean water, with brief periods of a decrease in the epsilon-Nd values coinciding with volcanic events and maybe sea level variation at 15.2 Ma. In the third period, from 13 to 10 Ma the fossils have 87Sr/86Sr similar to those of Miocene seawater while their epsilon-Nd values change considerably with time. This indicates fluctuating influence of the Atlantic versus the Paratethys and/or locally evolved seawater in the Mediterranean driven by global sea level changes. Other investigated localities near the Alps and from the ODP 900 site are compatible with this oceanographic interpretation. However, in the late early Miocene, enhanced local control, reflecting erosion of old crustal silicate rocks near the Alps, results in higher 87Sr/86Sr.

Description: New trace element analyses are presented for Leg 180 dolerites, basalts from the Papuan Ultramafic Belt (PUB), and basement rocks of Woodlark Island. The Leg 180 dolerites are similar to those from Woodlark Island in being derived from an enriched source but differ from the PUB, which came from a source similar to normal mid-ocean ridge basalts. A reliable 40Ar/39Ar age of 54.0 ± 1.0 Ma has been obtained by step heating of a whole-rock sample from Site 1109, and a similar but less reliable age was obtained for a sample from Site 1118. Plagioclase from Site 1109 did not give a meaningful age. This age is broadly similar to ages from the Dabi volcanics of the nearby Cape Vogel and for the PUB.

Description: We have performed U-Th isotope analyses on pure aragonite samples from the upper sections of Leg 166 cores to assign each aragonite-rich sediment package to the correct sea-level highstand. The uppermost sediment package from each of the four sites investigated (Sites 1003, 1005, 1006, and 1007) yielded a Holocene U-Th age. Sediment packages from deeper in the cores have suffered diagenesis. This diagenesis consists of significant U loss (up to 40%) in the site nearest the platform (Site 1005), slight U gain in sites further from the platform, and continuous loss of pure 234U caused by alpha recoil at all sites. The difference in diagenesis between the sites can be explained by the different fluid-flow histories they have experienced. Site 1005 is sufficiently close to the platform to have probably experienced a change in flow direction whenever the banks have flooded or become exposed. Other sites have probably experienced continuous flow into the sediment. Although diagenesis prevents assignment of accurate ages, it is sufficiently systematic that it can be corrected for and each aragonite-rich package assigned to a unique highstand interval. Site 1005 has sediment packages from highstands associated with marine isotope Stages 1, 5, 7, 9, and 11. Site 1006 is similar, except that the Stage 7 highstand is missing, at least in Hole 1006A. Site 1003 has sediment only from Stage 1 and 11 highstands within the U-Th age range. And Site 1007 has sediment only from the stage 1 highstand. This information will allow the construction of better age models for these sites. No high-aragonite sediments are seen for Stage 3 or Substages 5a and 5c. Unless rather unusual erosion has occurred, this indicates that the banks did not flood during these periods. If true, this would require the sea level for Substages 5a and 5c to have remained at least ~10 m lower than today.

Description: The mechanics of ocean circulation during the Late Cretaceous greenhouse interval remain contested (MacLeod and Hoope, 1992, doi:10.1130/0091-7613(1992)020〈0117:ETIBWB〉2.3.CO;2; Frank and Arthur, 1999, doi:10.1029/1998PA900017; MacLeod and Huber, 2001; Abramovich et al., doi:10.1029/2009PA001843; Isaza-Londono et al., doi:10.1029/2004PA001130; MacLeod et al., 2005, doi:10.1130/G21466.1), with the role of North Atlantic Deep Water in ocean circulation particularly debated: the relative warming of the North Atlantic during the termination of the greenhouse interval has been attributed to heat piracy from North Atlantic Deep Water formation (Isaza-Londono et al., doi:10.1029/2004PA001130; MacLeod et al., 2005, doi:10.1130/G21466.1), but the sources of Cretaceous deep water have been difficult to resolve. Nd isotopes as captured by seafloor sediments and expressed as epsilon-Nd(t) reflect the region in which the water mass was formed. Here we present epsilon-Nd(t) measurements from Cretaceous- to Palaeogene-aged sediments from four cores in the tropical North Atlantic. Before 69 Myr ago, we find extremely low epsilon-Nd(t) values of about -16, consistent with the presence of a warm, saline deep water mass formed in the low latitudes (MacLeod et al., 2008, doi:10.1130/G24999A.1; Jiménez Berrocoso et al., 2010, doi:10.1130/G31195.1). By 62 Myr ago, epsilon-Nd(t) values had risen to -11, similar to values reported from the northern North Atlantic over the past 65 million years, but lower than most contemporaneous values in the South Atlantic (Robinson et al., 2010, doi:10.1130/G31165.1) and Pacific oceans ((MacLeod et al., 2008, doi:10.1130/G24999A.1; Frank et al., 2005, doi:10.1029/2004PA001052 ). We therefore suggest that the epsilon-Nd(t) shift reflects the increasing influence of a northern-sourced water mass at this site, indicating the onset or intensification of deep- or intermediate-water formation in the North Atlantic 69 Myr ago. Our findings support the heat piracy model and imply that circulation patterns during the greenhouse interval were different from those of the subsequent relatively temperate interval.

Description: Neodymium isotopes of fish debris from two sites on Demerara Rise, spanning ~4.5 m.y. of deposition from the early Cenomanian to just before ocean anoxic event 2 (OAE2) (Cenomanian-Turonian transition), suggest a circulation-controlled nutrient trap in intermediate waters of the western tropical North Atlantic that could explain continuous deposition of organic-rich black shales for as many as ~15 m.y. (Cenomanian-early Santonian). Unusually low Nd isotopic data (epsilon-Nd(t) ~-11 to ~-16) on Demerara Rise during the Cenomanian are confirmed, but the shallower site generally exhibits higher and more variable values. A scenario in which southwest-flowing Tethyan and/or North Atlantic waters overrode warm, saline Demerara bottom water explains the isotopic differences between sites and could create a dynamic nutrient trap controlled by circulation patterns in the absence of topographic barriers. Nutrient trapping, in turn, would explain the ~15 m.y. deposition of black shales through positive feedbacks between low oxygen and nutrient-rich bottom waters, efficient phosphate recycling, transport of nutrients to the surface, high productivity, and organic carbon export to the seafloor. This nutrient trap and the correlation seen previously between high Nd and organic carbon isotopic values during OAE2 on Demerara Rise suggest that physical oceanographic changes could be components of OAE2, one of the largest perturbations to the global carbon cycle in the past 150 m.y.

Description: The shoaling and final closure of the Central American Seaway (CAS) resulted in a major change of the global ocean circulation and has been suggested as an essential driver for strengthening of Atlantic Meridional Overturning Circulation (AMOC). The exact timing of CAS closure is key to interpreting its importance. Here we present a reconstruction of deep and intermediate water Nd and Pb isotope compositions obtained from fossil fish teeth and the authigenic coatings of planktonic foraminifera in the eastern equatorial Pacific (Ocean Drilling Program (ODP) Site 1241) and the Caribbean (ODP Sites 998, 999, and 1000) covering the final stages of CAS closure between 5.6 and 2.2 Ma. The data for the Pacific site indicate no significant Atlantic/Caribbean influence over this entire period. The Caribbean sites show a continuous trend to less radiogenic Nd isotope compositions during the Pliocene, consistent with an enhancement of Upper North Atlantic Deep Water (UNADW) inflow and a strengthening of the AMOC. Superimposed onto this long-term trend, shorter-term changes of intermediate Caribbean Nd isotope signatures approached more UNADW-like values during intervals when published reconstructions of seawater salinity suggested complete closure of the CAS. The data imply that significant deep water exchange with the Pacific essentially stopped by 7 Ma and that shallow exchange, which still occurred at least periodically until approximately 2.5 Ma, may have been linked to the strength of the AMOC but did not have any direct effect on the intermediate and deep Caribbean Nd isotope signatures through mixing with Pacific waters.

Description: New surface water records from two high sedimentation rate sites, located in the western subtropical North Atlantic near the axis of the Gulf Stream, provide clear evidence of suborbital climate variations through marine isotope stage (MIS) 5 persisting even into the warm peak of the interglaciation (substage 5e). We found that the amplitude of suborbital climate oscillations did not vary significantly for the whole of MIS 5, implying that ice volume has little or no influence on the amplitude of suborbital climate variability in this region. Although some records suggest that longer suborbital variations (4-10 kyr) during MIS 5 are linked to deepwater changes, none of the existing records is of sufficient resolution to assess if a linkage occurred for oscillations shorter than 4 kyr. However, when examined in conjunction with published data from the Norwegian Sea, new evidence from the subpolar North Atlantic suggests that coupled surface-deepwater oscillations occurred during the penultimate deglaciation. This supports the hypothesis that during glacial and deglacial times, ocean-ice interactions and deepwater variability amplify suborbital climate change at higher latitudes. We suggest that during the penultimate deglaciation the North Atlantic deepwater source varied between Nordic Sea and open North Atlantic locations, in parallel with surface temperature oscillations.

Description: Micropaleontological and geochemical analyses reveal distinct millennial-scale increases in carbonate preservation in the deep Southeast Atlantic (Cape Basin) during strong and prolonged Greenland interstadials that are superimposed on long-term (orbital-scale) changes in carbonate burial. These data suggest carbonate oversaturation of the deep Atlantic and a strengthened Atlantic Meridional Overturning Circulation (AMOC) during the most intense Greenland interstadials. However, proxy evidence from outside the Cape Basin indicate that AMOC changes also occurred during weaker and shorter Greenland interstadials. Here we revisit the link between AMOC dynamics and carbonate saturation in the deep Cape Basin over the last 400 kyr (sediment cores TN057-21, TN057-10 and ODP Site 1089) by reconstructing centennial changes in carbonate preservation using mm-scale X-ray fluorescence (XRF) scanning data. We observe close agreement between variations in XRF Ca/Ti, sedimentary carbonate content and foraminiferal shell fragmentation, reflecting a common control primarily through changing deep-water carbonate saturation. We suggest that the high-frequency (sub-orbital) component of the XRF Ca/Ti records indicates the fast and recurrent redistribution of carbonate ions in the Atlantic basin via the AMOC during both long/strong- and short/weak North Atlantic climate anomalies. In contrast, the low-frequency (orbital) XRF Ca/Ti component is interpreted to reflect slow adjustments through carbonate compensation, and/or changes in the deep-ocean respired carbon content. Our findings emphasize the recurrent influence of rapid AMOC variations on the marine carbonate system during past glacial periods, providing a mechanism for transferring the impacts of North Atlantic climate anomalies to the global carbon cycle via the Southern Ocean.

Description: Sediment samples from both Site 165-999/165-1000 (Atlantic) and Site 202-1241 (Pacific) were chosen at 1Ma intervals over the period 0.3-9.3Ma. Samples were washed and sieved 〈150µm. Splits of the sediment fraction were picked completely to obtain, where possible, at least 30 specimens each of planktic foraminifer species Globigerinoides sacculifer and Globorotalia tumida, on which outline analysis (Fourier) was performed. Sea surface and thermocline temperatures were reconstructed from palaeoenvironmental proxies (UK37' and Tex86H respectively).

Description: Sediments recovered during Ocean Drilling Program (ODP) Leg 138 in the eastern equatorial Pacific Ocean were analyzed for variations in eolian accumulation rate and mean grain-size. Latitudinal and temporal patterns of these parameters showed important changes in the intensity of atmospheric circulation and eolian flux associated with the intertropical convergence zone (ITCZ) and suggested that eolian input parameters could be used to define its paleoposition through time. Modern atmospheric circulation in the equatorial region is weakest in the intertropical convergence zone and increases as the trade winds are approached to the north and south. Thus, the expected spatial pattern of eolian grain size would have the finest material deposited beneath the ITCZ and a coarsening of material in both directions away from this zone. Sediments from ODP Leg 138 show this pattern for much of the Pleistocene and Pliocene but, prior to about 4 Ma, begin to lose the northern coarse component suggesting that the ITCZ was located north of its present position during the late Miocene. Eolian flux records also show a latitudinal pattern of deposition associated with the position of the ITCZ that, similar to eolian grain-size variability, suggests a more northerly position of the ITCZ during the late Miocene. Overall, the regional input of eolian material to the equatorial Pacific has decreased throughout the late Neogene. This reduction in eolian input reflects climatic changes to relatively wetter conditions in the continental eolian source regions beginning during the late Pliocene.

Description: Seismicity patterns offshore Costa Rica (Central America) at the Middle America Trench have led to speculation that large (moment magnitude, Mw ~7.0) earthquakes are associated with subducting topographic highs. In areas of high basement topography, a regionally extensive nannofossil chalk unit is exposed at the seafl oor on the incoming plate, whereas in regions of low basement topography, hemipelagic clay-rich sediment is exposed. Because the entire sediment section is subducted at this margin, lithologic variation in the uppermost subducting sediments may control plate boundary fault behavior. Our laboratory experiments reveal that the chalk is frictionally strong (µ = 0.71-0.88) and characterized by velocity weakening and stick-slip behavior, notably at elevated temperature. In contrast, the hemipelagic sediment is weak (µ = 0.22-0.35) and in many cases velocity strengthening. We suggest that the presence of frictionally unstable carbonates at bathymetric highs may play a key, previously unrecognized, role in governing earthquake nucleation.

Description: The Leg 173 Site 1067 and 1068 amphibolites and metagabbros from the west Iberia margin exhibit variable whole-rock compositions from primitive to more evolved (Mg numbers = 49-71) that are generally incompatible trace and rare earth element enriched (light rare earth element [LREE] = 11-89 x chondrite). The Site 1067 amphibolites are compositionally similar to the basalts reported at Site 899 from this same region, based on trace and rare earth element contents. The Site 1068 amphibolites and metagabbros are similar to the Site 899 diabases but are more LREE enriched. However, the Sites 1067 and 1068 amphibolites and metagabbros are not compositionally similar to the Site 900 metagabbros, which are from the same structural high as the Leg 173 samples. The Leg 173 protoliths may be represented by basalts, diabases, and/or fine-grained gabbros that formed from incompatible trace element-enriched liquids.

Description: The lower slope and toe-of-slope sediments of the western flank of the Great Bahama Bank (Sites 1003 and 1007) are characterized by an intercalation of turbidites and periplatform ooze. In general, turbidites form up to 12% of the total mass of the sedimentary column. Based primarily on data from the Bahamas, it has been postulated that steep-sided carbonate platforms shed most of their sediments into the basin during sea-level highstands when the platforms are flooded. This highstand shedding is assumed to be less pronounced along platforms with a ramp-like depositional profile where sediment production is not restricted to sea-level highstand. Miocene to Pliocene sediments recovered in five drill holes during Leg 166 at the western margin of the Great Bahama Bank reveal that turbidite distribution follows a complex pattern that is dependent on several factors such as sedimentation rates, sea-level changes, and slope morphology. To identify the depositional sequences in the cores, the depths of seismic-sequence boundaries were used. The distribution of turbidites within sedimentary sequences varies strongly. Generally, turbidites are clustered at the upper and/or lower portions of the sequences indicating deposition of carbonate turbidites during both highstand and lowstand of sea level. Analyses of the Miocene turbidites show that (1) during high sea level, 60% of all turbidites were deposited at Site 1003 (309 out of 518 turbidites), while during low sea level, two thirds of all turbidites were deposited at Site 1007 (332 out of 486 turbidites); (2) the average thickness of highstand turbidites is 1.5 times higher than the average thickness of lowstand turbidites; and (3) the turbidites display slight differences in composition and sorting. In general, highstand turbidites are less sorted and contain an abundant amount of shallow-water constituents such as green algae, red algae, shallow-water benthic foraminifers (miliolids), and intraclasts. The lowstand turbidites are better sorted and contain abundant planktonic foraminifers and micrite. To complicate matters, highstand and lowstand turbidites seem to be deposited at different locations on the slope. At the lower slope (Site 1003), more turbidites were deposited during highstands, while at the toe of the slope, turbidites were dominantly deposited during sea-level lowstands. The result is a slope section with laterally discontinuous turbidite lenses within periplatform ooze, which is controlled by the interplay of sea-level changes, sediment production, and platform morphology.

Description: The phase relations of natural volcaniclastic sediments from the west Pacific Ocean were investigated experimentally at conditions of 3-6 GPa and 800-900 °C with 10 wt.% added H2O (in addition to ~ 10 wt.% structurally-bound H2O) to induce hydrous melting. Volcaniclastic sediments are shown to produce a sub-solidus assemblage of garnet, clinopyroxene, biotite, quartz/coesite and the accessory phases rutile ± Fe-Ti oxide ± apatite ± monazite ± zircon. Hydrous melt appears at temperatures exceeding 800-850 °C, irrespective of pressure. The melt-producing reaction consumes clinopyroxene, biotite and quartz/coesite and produces orthopyroxene. These phase relations differ from those of pelagic clays and K-bearing mid ocean ridge basalts (e.g. altered oceanic crust) that contain phengite, rather than biotite, as a sub-solidus phase. Despite their relatively high melt productivity, the wet solidus for volcaniclastic sediments is found to be higher (825-850 °C) than other marine sediments (700-750 °C) at 3 GPa. This trend is reversed at high-pressure conditions (6 GPa) where the biotite melting reaction occurs at lower temperatures (800-850 °C) than the phengite melting reaction (900-1000 °C). Trace element data was obtained from the 3 GPa run products, showing that partial melts are depleted in heavy rare earth elements (REE) and high field strength elements (HFSE), due to the presence of residual garnet and rutile, and are enriched in large ion lithophile elements (LILE), except for Sr and Ba. This is in contrast to previous experimental studies on pelagic sediments at sub-arc depths, where Sr and Ba are among the most enriched trace elements in glasses. This behavior can be partly attributed to the presence of residual apatite, which also host some light REE in our supra-solidus residues. Our new experimental results account for a wide range of trace element and U-series geochemical features of the sedimentary component of the Mariana arc magmas, including imparting a substantial Nb anomaly to melts from an anomaly-free protolith.