Integrated Ocean Drilling Program Research Articles

Eocene‐Oligocene Intensification of the Deep Western Boundary Current in the North Atlantic Ocean

AbstractThe role played by ocean circulation in major transitions in Earth's climate is debated. Here, we investigate the physical evolution of the Deep Western Boundary Current (DWBC) in the western North Atlantic Ocean through the late Eocene‐to‐mid Oligocene (35−26 Ma) using terrigenous grain size and geochemistry records of marine sediment cores. Our records cover the most pivotal transition in Cenozoic climate history, the Eocene‐Oligocene Transition (EOT; ∼33.7 Ma), when Earth first became sufficiently cool to sustain large ice sheets on Antarctica. To assess changes in deep‐water circulation in the northwest Atlantic across the EOT, we assembled sortable silt (10–63 μm) grain‐size and Nd, Hf, and Pb radiogenic isotope records at two Integrated Ocean Drilling Program (IODP) drill sites on the Newfoundland ridges (Sites U1406 and U1411). These records reveal an overall gradual increase in sortable silt abundance (SS%) at both sites with no change in sediment provenance. We interpret a steady, long‐term invigoration of the DWBC, likely driven by deepening of the Greenland‐Scotland Ridge and resultant enhanced inflow of waters sourced from deep‐water production sites in the Nordic Seas to the North Atlantic Ocean. Our results do not support abrupt and widespread invigoration of bottom current activity in the North Atlantic synchronous with accelerated cooling and Antarctic ice growth at the EOT. Instead, our records suggest that the DWBC started to intensify before this pivotal event in Cenozoic climate history (at ∼35 Ma) and then further strengthened gradually across the EOT (∼34 Ma) and through the early‐to‐mid Oligocene (∼34‒26 Ma).

Lithium isotopic composition of low-temperature altered oceanic crust and its implications for Li cycling

To investigate the Li fluxes and its isotopic fractionation during low-temperature seafloor alteration, we present the Li isotope compositions of fifty fresh and altered basalts drilled from the Integrated Ocean Drilling Program (IODP) 329 Expedition Sites U1365 and U1368, which show different low-temperature (<150 °C) alteration styles and intensities (i.e., total volume of secondary minerals and degrees of elemental enrichment) with meager sedimentation rates (0.71 and 1.1 m/Myr, respectively). The fresh basalts (δ7Li vary from +2.7 to +6.4) show Li isotope compositions consistent with the published N-MORB (+3.5 ± 1.0‰), whereas the altered basalts exhibit considerably higher δ7Li values (+1.7‰ to +11.6‰). Furthermore, their δ7Li values show positive correlations with alkali elements and loss on ignition and follow a mixing trend between a MORB-like endmember and a Li-rich seawater-derived endmember (+9.2 ± 0.2‰), which indicates the effects of low-temperature seafloor alteration on Li isotopic compositions. Based on previously reconstructed paleo-seawater Li isotope compositions, the Li isotopic fractionation scale between seawater and altered basalts is estimated (+12.6‰ and + 18.0‰ for Sites U1365 and U1368, respectively), which further supports previous conclusions that the scale of Li isotope fractionation changes over time. A mass calculation suggests the upper limits of the Li-uptake fluxes during low-temperature seafloor alteration are 14.9 ± 2.2 × 109 mol/year and 3.6 ± 1.3 × 109 mol/year for Sites U1365 and U1368, respectively. In addition, our estimation shows the Li-uptake flux during low-temperature seafloor alteration in the Cretaceous is comparable to the total Li-input flux, whereas, in the Neogene, the Li-uptake flux during low-temperature seafloor alteration is much lower than that of the Li-input flux, which suggests a large diagenetic Li sink may be necessary to maintain the seawater Li budget balance if we assume the seawater Li concentration is roughly constant.

Multimethod dating of ice-rafted dropstones reveals hidden localized glacial erosion in Wilkes Subglacial Basin, Antarctica

Abstract The Antarctic ice sheet blankets &amp;gt;99% of the continent and limits our ability to study how subglacial geology and topography have evolved through time. Ice-rafted dropstones derived from the Antarctic subglacial continental interior at different times during the late Cenozoic provide valuable thermal history proxies to understand this geologic history. We applied multiple thermochronometers covering a range of closure temperatures (60–800 °C) to 10 dropstones collected during Integrated Ocean Drilling Program (IODP) Expedition 318 in order to explore the subglacial geology and thermal and exhumation history of the Wilkes Subglacial Basin. The Wilkes Subglacial Basin is a key target for study because ice-sheet models show it was an area of ice-sheet retreat that significantly contributed to sea-level rise during past warm periods. Depositional ages of dropstones range from early Oligocene to late Pleistocene and have zircon U-Pb or 40Ar/39Ar ages indicating sources from the Mertz shear zone, Adélie craton, Ferrar large igneous province, and Millen schist belt. Dropstones from the Mertz shear zone and Adélie craton experienced three cooling periods (1700–1500 Ma; 500–280 Ma; 34–0 Ma) and two periods of extremely slow cooling rates (1500–500 Ma; 280–34 Ma). Low-temperature thermochronometers from seven of the dropstones record cooling during the Paleozoic, potentially recording the Ross or Pan-African orogenies, and during the Mesozoic, potentially recording late Paleozoic to Mesozoic rifting. These dropstones then resided within ~500 m of the surface since the late Paleozoic and early Mesozoic. In contrast, two dropstones deposited during the mid-Pliocene, one from the Mertz shear zone and one from Adélie craton, show evidence for localized post-Eocene glacial erosion of ≥2 km.

The LILY Database: Linking Lithology to IODP Physical, Chemical, and Magnetic Properties Data

AbstractDuring each expedition of the International Ocean Discovery Program and its precursor, the Integrated Ocean Drilling Program (jointly referred to as IODP), vast arrays of data are collected from drill cores. These data, which are accessible from the IODP LIMS (Laboratory Information Management System) database, include physical, chemical, and magnetic properties collected semi‐continuously along cores using automated track systems, as well as a variety of analyses conducted on discrete subsamples taken from the cores. In addition, the lithology of all cores is described based on visual characteristics of the surface of split cores, visual examination of smear slides and thin sections, and compositional or mineralogical information derived from geochemical analyses. We extract basic lithologic information from this complex array of descriptive information and then tie that information to all other measurements. This new database is referred to as LIMS with Lithology (LILY). LILY currently contains over 34 million data from 89 km of core recovered on 42 expeditions conducted 2009–2019. Some uses of LILY include identifying the abundance of different lithologies, finding data from core intervals with a specific lithology, assessing the efficacy of coring systems in different lithologies, or characterizing and analyzing physical, chemical, and magnetic properties based on lithology. We illustrate the use of LILY by computing the grain density by lithology from over 24,000 moisture and density measurements and then use those grain densities, along with the large IODP bulk density dataset, to compute a new high‐resolution porosity dataset with over 3.7 million new porosity estimates.

Last Deglacial Environmental Change in the Tropical South Pacific From Tahiti Corals

AbstractOn glacial‐interglacial time scales, changes in the Earth's orbital configuration control climate seasonality and mean conditions. Tropical coral skeletons can be sampled at a sufficient resolution to reconstruct past seasonality. Here, last deglacial Porites skeletons from Integrated Ocean Drilling Program Expedition 310 to Tahiti are investigated and, supported by a modern calibration, monthly resolved time series in geochemical proxies (Sr/Ca, δ18O, δ13C) are constructed. For most of the deglaciation, Sr/Ca seasonality was similar to modern (0.139 ± 0.010 mmol mol−1; 2.8 ± 0.2°C) reflecting the small change in insolation seasonality. However, during the Younger Dryas, high values in Sr/Ca seasonality (0.171 ± 0.017 mmol mol−1; 3.4 ± 0.3°C) suggest a reduced mixed layer depth and enhanced influence of the South Pacific Subtropical Gyre due to South Pacific Convergence Zone (SPCZ) inactivity. Furthermore, high amplitudes in Younger Dryas skeletal δ18O (0.40 ± 0.22 ‰) and δ13C (0.86 ± 0.22 ‰) seasonality compared to modern (δ18O = 0.29 ± 0.08 ‰; δ13C = 0.27 ± 0.08 ‰) point to elevated winter‐summer discrepancies in rainfall and runoff. Mean coral Sr/Ca variability suggests an influence of Northern Hemisphere climate events, such as the Younger Dryas cooling (+0.134 ± 0.012 mmol mol−1;−2.6 ± 0.2°C), or the Bølling–Allerød warming (+0.032 ± 0.040 mmol mol−1; −0.6 ± 0.4°C). Deglacial mean coral Δδ18O (δ18Oseawater contribution to skeletal δ18O), corrected for the ice volume effect, was elevated pointing to more saline, thus dryer conditions, likely due to a northward migration of the SPCZ. Seasonal cycles in coral δ13C were likely caused by variations in linear extension rates that were reduced during the last deglaciation (1.00 ± 0.6 cm year−1) compared to today (1.6 ± 0.3 cm year−1).

Nonlinear increase in seawater 87Sr ∕ 86Sr in the Oligocene to early Miocene and implications for climate-sensitive weathering

Abstract. The 87Sr / 86Sr of marine carbonates provides a key constraint on the balance of continental weathering and hydrothermal Sr fluxes to the ocean, and the mid-Oligocene to mid-Miocene period features the most rapid rates of increase in the 87Sr / 86Sr of the Cenozoic. Because previous records of the 87Sr / 86Sr increase with time were based on biostratigraphically defined age models in diverse locations, it was difficult to unambiguously distinguish million-year-scale variations in the rate of 87Sr / 86Sr change from variations in sedimentation rate. In this study, we produce the first 87Sr / 86Sr results from an Oligocene to early Miocene site with a precise age-model-derived orbital tuning of high-resolution benthic δ18O at Equatorial Pacific Ocean Drilling Program (ODP) Site 1218. Our new dataset resolves transient decreases in 87Sr / 86Sr, as well as periods of relative stasis. These changes can be directly compared with the high-resolution benthic δ18O at the same site. We find that slowing of the rate of 87Sr / 86Sr increase coincides with the onset of Antarctic ice expansion at the beginning of the mid-Oligocene glacial interval, and a rapid steeping in the 87Sr / 86Sr increase coincides with the benthic δ18O evidence for rapid ice retreat. This pattern may reflect either northward shifts in the Intertropical Convergence Zone precipitation to areas of nonradiogenic bedrock and/or lowered weathering fluxes from highly radiogenic glacial flours on Antarctica. We additionally generate the first 87Sr / 86Sr data from ODP Site 1168 on the Tasman Rise and Integrated Ocean Drilling Program (IODP) Site 1406 of the Newfoundland Margin during the Oligocene to early Miocene to improve the precision of age correlation of these Northern Hemisphere and Southern Hemisphere midlatitude sites and to better estimate the duration of early Miocene hiatus and condensed sedimentation.

Effect of Size Fractions of Foraminifera on Reconstructing the Paleoceanography of Cretaceous Oceanic Anoxic Event 2

Abstract The globally recognized Cretaceous Oceanic Anoxic Event 2 was recorded at Integrated Ocean Drilling Program Site U1407 in the North Atlantic in sediments that were deposited under varying oxygen concentrations, conditions often associated with decreased foraminiferal test size. A &amp;gt;63-µm size fraction is typically used for collecting foraminifera that live in low-oxygen and deep-sea settings, particularly for paleoceanographic reconstructions where key environmental indicator species may be under-represented or absent in coarser sizes. Using 31 sediment samples from the site, we test the best size fraction to utilize when exploring biotic responses to an oceanic anoxic event. Counts of benthic and biserial (heterohelicid) planktic foraminifera from the standard &amp;gt;63-µm and finer 38–63-µm sieve sizes are compared to determine if critical information is lost if the smaller size fraction is not used. Results show similar trends in relative abundances for both size fractions. There are higher abundances in 38–63-µm residues, but species identification and counts are overly time-consuming for obtaining the general trends. We recommend the use of &amp;gt;63-µm sieves for foraminifera collected from low-oxygen sediments but encourage examination of the 38–63-µm fraction to verify intervals recognized as barren are truly absent of foraminifera.

Enhanced ocean deoxygenation in the Bering Sea during MIS 11c

Accelerated Arctic warming has raised concerns about future environmental conditions in the Bering Sea, one of the world's most productive marine ecosystems. Marine Isotope Stage (MIS) 11 (424–374 ka), a period with orbital parameters similar to those of the current interglacial (Holocene), is thought to be a suitable analog to predict future marine environments. Here, we reconstruct paleoredox changes in the Bering Sea over the last 800 kyr using high-resolution U/Th ratios from four sites, which were sampled by the Integrated Ocean Drilling Program (IODP) Expedition 323. During the early stages of MIS 11 (MIS11c), four IODP Expedition 323 sites (U1339, U1340, U1343, and U1345) with different water depths (1100–1900 m) had highly elevated U/Th ratios, indicating a substantial loss of dissolved oxygen (O2) in the intermediate waters of the Bering Sea. High U/Th ratios were closely associated with extremely high %biogenic opal in the Bering Sea and an exceptionally warm Arctic climate. We speculate that during this period, the highest amplitude deglacial warming led to the onset of a super interglacial (MIS 11c) in the Arctic regions, via Arctic amplification. The exceptionally warm Arctic climate resulted in particularly high export productivity in the Bering Sea via physiological effects, a reduction in sea ice, and increased river discharge. Due to high productivity, increased organic matter degradation caused severe hypoxia in the intermediate waters of the Bering Sea. Our study shows that severe hypoxia may also appear in the Bering Sea if Arctic warming accelerates in the future.

Surface water mass dynamics at IODP Site U1313 through Principal Component Analysis: Evidence from coccolith assemblages in the ∼25–7 kyr interval

Coccolith assemblages from Integrated Ocean Drilling Program Site U1313 (North Atlantic; 41°0′N, 32°57.4′W) are analysed to assess temporal changes in surface water conditions from the Last Glacial Maximum (LGM) to the Middle Holocene (∼25–7 kyr). We perform Principal Component Analysis (PCA), involving both calcareous nannofossil assemblage data and other paleoenvironmental proxies. We reconstruct sea-surface dynamics using calcareous nannofossil assemblage variations and comparison with other previously published proxy data from Site U1313 including C37:4 alkenone, dolomite/calcite and quartz/calcite ratios, Uk’37 SST, as well as the sedimentation rate.We use variations in paleoproductivity proxies, cold and warm water group abundance, and the PCA results to infer North Atlantic Current (NAC) and North Atlantic Transitional Waters (NATW) migration through the studied interval. These proxies are also used to highlight variations in the Subtropical Gyre dynamics. Warming episodes are recorded during the LGM and Heinrich Stadial (HS) 1 as a result of northward expansion of the Subtropical Gyre. High coccolithophore productivity episodes characterize HS 1 and 2, and are concomitant with increasing terrigenous input. Both processes are considered a consequence of iceberg influence, which transport nutrients and ice-rafted debris to the area. A southward shift of the Subpolar Front and NAC favour the expansion of cold water from 25.3 ka to the end of the Younger Dryas, whereas the early Holocene is characterized by NATW influence resulting from northward migration of the NAC and Azores Current.

Lowering of the kinetic barrier of clay-mineral reactions along a seismogenic fault: Example from the Nankai subduction zone, Japan

An analysis of clay minerals in core samples around the megasplay fault of the Nankai Trough, Japan, recovered during the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment reveals a local progression of the smectite-to-illite conversion (S–I) reaction in the slip zone, possibly associated with local thermogenesis caused by seismic slip on the fault. Our semiquantitative X-ray diffraction analysis demonstrates that the illite content in mixed-layer illite/smectite is ∼15% higher in the gouge than in the host sediments. The thermal history of the fault, constrained in a previous study, requires a reduction of the apparent activation energy by 20–30% compared with the literature value to explain the observed illitization. The Shirako Fault in the Miura–Boso accretionary complex, which is thought to have been activated in a tectonic setting similar to that of the megasplay fault of the Nankai Trough, also shows a 20–30% reduction in the apparent activation energy. These results suggest that the kinetic barrier of the smectite–illite reaction is lowered as a result of mechanochemical processes in seismogenic fault zones.

Revisiting the South China Sea MORBs: Mg isotope and whole-rock geochemical constraints

Mid-Ocean Ridge Basalts (MORBs) from the South China Sea (SCS) have attracted great attention from geologists in the past decade, but their petrogenesis still remains controversial. Here, we show that the Mg isotopes of MORBs from Integrated Ocean Drilling Program (IODP) Sites U1431, U1433 and U1434 are heterogeneous and present the mechanisms causing such heterogeneity. The MORBs from Sites U1431, U1433 and U1434 have δ26Mg values ranging from −0.35‰ to −0.21‰, from −0.34‰ to −0.13‰, and from −0.30‰ to −0.20‰, respectively. Together with all the published data, we found the MORBs at Site U1431 with high MgO contents (> 9 wt%) usually have δ26Mg values that are generally lower than the normal mantle (−0.25‰ ± 0.04‰), whereas some of the Site U1433 MORBs are higher than the normal mantle. Shallow-level geological processes like seawater alteration have negligible effects on the Mg isotope fractionation of our samples. Elemental ratio indicative of the degree of partial melting (like Sm/Nd) from the same site also shows no relationship with the δ26Mg variations. Olivine is proposed to have light Mg isotopes. Considering the high MgO (> 9 wt%) contents of low-δ26Mg samples and the petrographic evidence, we propose that their slightly light Mg isotopic compositions of U1431 MORBs may result from the enrichment of olivine, rather than reflecting the mantle source. Moreover, low Ce/Pb ratios and most variable δ26Mg values of some U1433 MORBs suggest that lower continental crustal materials have contributed to their source. As all the previously published MORBs are evolved, we use the Revpet to estimate the mantle melting conditions of SCS MORBs, including the Site U1500 samples recovered from the northern margin of SCS. Compared with the global MORBs data, there is no thermal anomaly in SCS MORBs. Together with our new Mg isotope data, therefore, we propose that the role of the Hainan mantle plume in the opening and spreading of the SCS may be insignificant.

Barium isotope compositions of altered oceanic crust from the IODP Site 1256 at the East Pacific rise

This study reports barium (Ba) isotope compositions of intact altered oceanic crust (AOC) recovered from Integrated Ocean Drilling Program (IODP) Site 1256, East Pacific Rise (EPR), to investigate the behavior of Ba isotopes during both low-temperature seawater alteration and high-temperature hydrothermal alteration processes in the oceanic crust. The Ba isotope compositions of the volcanic section and sheeted dike complex have large variations and are mostly heavier than those of the fresh EPR MORB, with δ138/134Ba ranging from −0.01‰ to +0.39‰, which may be attributed to the modification of Ba isotopes by seawater and hydrothermal fluid during low-temperature and high-temperature alteration. The samples from the plutonic complex display much lighter Ba isotope compositions than the fresh EPR MORB, having δ138/134Ba values of −0.22‰ to −0.06‰, which may reflect the influence of late magmatic fluids. Because of the large Ba isotope variation in the AOC and the obvious difference in Ba isotope compositions between the AOC and the upper mantle, the recycling of the AOC could result in Ba isotope heterogeneity in the mantle and further indicate the application of Ba isotopes for tracing crustal material recycling in the Earth's mantle.

Well‐log integration and seismic‐to‐well tie off George V Land (Antarctica)

AbstractCenozoic sediments from the continental rise off the George V Land consist of silty/clayey facies variably rich in diatom ooze; these sediments hold a record of the glacial history of the Wilkes Subglacial Basin and are important for estimating the contribution of the East Antarctic Ice Sheet to global sea level changes during past transition to warmer climates. This is fundamental to predict future scenarios related to the global warming. The petrophysical properties of Antarctic marine sediments are influenced by the ice sheet dynamics and may affect the amplitude of seismic reflections. Through a seismic‐to‐well tie procedure, we investigate the origin of high amplitude reflections from Miocene‐early Pliocene deposits identified in two seismic lines crossing at the Integrated Ocean Drilling Program Expedition 318 drill site U1359. Downhole and core log measurements are preconditioned and merged to obtain complete velocity and density records from the sea floor to the bottom of the deepest of the four wells drilled at this site. We generate a synthetic trace by convolving the reflectivity series with the seismic wavelet obtained from the sea‐floor reflection and match the synthetic trace to the seismic data with a time variant cross‐correlation procedure. This procedure established a robust time‐depth relationship, not achievable from the available small‐offset seismic data. To our knowledge, this is the first seismic‐to‐well tying in the George V Land area. Based on results from synthetic data, the anomalous high amplitude seismic package can be linked to changes in density of sediments. Such changes are interpreted as representative for the alternation of diatom‐rich (warm climate) and silty‐clay layers with ice‐rafted debris (cold climate) inside the deposits. We suggest that the analysis of the characteristics and the distribution of similar seismic anomaly around Antarctica can give insight into the modality of past Antarctic ice sheet dynamics.

British–Irish Ice Sheet and polar front history of the Goban Spur, offshore southwest Ireland over the last 250 000 years

Deep Sea Drilling Program (DSDP) Site 548 was cored in 1984 at a water depth of 1256 m on the Goban Spur, offshore southwest Ireland. Coring retrieved a ~100‐m‐thick Pleistocene contourite sequence. This study uses planktonic foraminiferal assemblage and benthic foraminiferal oxygen isotope analyses to establish an age model for the upper 40 m of this core. This site's multidisciplinary analyses of planktonic foraminiferal assemblages, lithic grains, facies and calcium carbonate concentration reveal a 250 000‐year record of the North Atlantic polar front variability and British–Irish Ice Sheet (BIIS) history. The sequence is characterized by alternations of ice rafted debris (IRD) laden pelagic mud facies with calcium carbonate‐rich silty sand contourite facies that track glacial/interglacial cycles. The polar front migrated southward across the area several times during glacial maxima and stadial periods, while warmer Mediterranean Outflow Water (MOW) flowed northward across the region during interglacial and interstadial periods depositing contourites. Lithic analyses reveal a complex history of IRD deposition associated with iceberg calving from the Laurentide Ice Sheet and northwest European ice sheets, mainly the BIIS. Comparison between the Goban Spur (DSDP Site 548) and the Celtic Margin (MD03‐2692) and central North Atlantic Integrated Ocean Drilling Program (IODP) Site U1308 suggests differences between the ‘non‐Laurentide Ice Sheet’ Heinrich Events (HE) 6 and 3 at the Goban Spur, with IRD from the BIIS being prominent during HE 6 and IRD from other European ice sheets north of the BIIS likely being more dominant during HE 3. The nature of lithics in IRD‐rich horizons during Terminations 3, 3A, 2 and 1 suggests significant iceberg calving episodes preceding BIIS retreat during the onset of interstadial intervals.

Palynological implications for the paleoclimate and paleoceanographic reconstruction of the East Sea since the early Pleistocene at IODP site U1430

Palynomorphs, including pollen and dinoflagellate cysts from marine sediments, can provide a history of paleoclimate and paleoceanography. We present new palynological profiles based on a core from the Integrated Ocean Drilling Program Site U1430, which was drilled in the Ulleung Plateau East Sea. This core covers the past 2 Ma and spans the Mid-Pleistocene Transition (MPT, 1.2–0.8 Ma) and Mid-Brunhes Event (MBE, ∼0.43 Ma). Terrestrial and marine palynomorph records indicate orbital-scale variations in paleoclimate and paleoceanography, reflecting changes in the interconnections between vegetation, climate, and the ocean. Before the MPT, the high pollen taxa ratio and abundant temperate pollen suggest a relatively warm temperature associated with astronomical forcing-induced insolation changes. At ca. 1.3 Ma, boreal coniferous pollen increased, suggesting intensification of the East Asian winter monsoon (EAWM). The long-term trend of boreal coniferous pollen concentration was similar to that of benthic δ18O. This orbital scale boreal conifer pollen assemblage variation indicates Quaternary EAWM evolution controlled by changes in global ice volume and climatic cooling. The marine palynological data reflect the ocean circulation history of the study area, where sea level fluctuations and tectonic activity played important roles around the marginal sea. The abundance of warm-water dinoflagellate species after ca. 1.9 Ma, including Tuberculodinium vancampoae, indicates Tsushima Warm Current intrusion related to the expansion of the Korea (Tsushima) Strait. Nematosphaeropsis labyrinthus and other cold-water species increased markedly during the MPT, suggesting a low sea surface temperature and weakened vertical mixing of the East Sea due to the cooled climate and sea ice during the MPT. The marked sea level fluctuation associated with the glacial–interglacial cycling after the MBE resulted in an assemblage composition shift from cold to warm water species. Tsuga and Betula thrived during mild winters, primarily influenced by winter insolation. These genera were particularly prosperous in the early Pleistocene, coinciding with a strong intrusion of the Tsushima Warm Current (TWC). Our findings should be considered in developing a comprehensive understanding of the paleoclimate and paleoceanographic history of northern East Asia across the major climate transitions of the Pleistocene.

Eocene to Oligocene cooling and ice growth based on the geochemistry of interglacial mudstones from the East Antarctic continental shelf

AbstractThe Eocene-Oligocene Transition at c. 34 million years ago (Ma) marked the global change from greenhouse to icehouse and the establishment of the East Antarctic Ice Sheet (EAIS). How the ice-sheet behaviour changed during interglacials across this climate transition is poorly understood. We analysed major, trace and rare earth elemental data of late Eocene interglacial mudstone from Prydz Bay at Ocean Drilling Program Site 1166 and early Oligocene interglacial mudstone from Integrated Ocean Drilling Program Site U1360 on the Wilkes Land continental shelf. Both sites have comparable glaciomarine depositional settings. Lithofacies and provenance at Site 1166 in Prydz Bay are indicative of a late Eocene glacial retreat in the Lambert Graben. Palaeoclimate proxies, including the Chemical Index of Alteration, mean annual temperature and mean annual precipitation, show a dominant warm and humid palaeoclimate for the late Eocene interglacial. In contrast, at Site U1360, in the early Oligocene, the provenance and interglacial weathering regime remained relatively stable with conditions of physical weathering. These results confirm that the EAIS substantially retreated periodically during late Eocene interglacials and that subglacial basins probably remained partially glaciated during interglacials in the earliest Oligocene.

Formation of lower fast-spread oceanic crust: a structural and geochemical study of troctolites in the Hess Deep Rift (East Pacific Rise)

Troctolites were recovered during Integrated Ocean Drilling Program Expedition 345 at the Hess Deep Rift, next to fast-spreading East Pacific Rise. These troctolites are divided into three groups based on textural differences: coarse-grained (1–10 mm in length) troctolite, fine-grained (~ 2 mm in length) troctolite, and skeletal olivine-bearing troctolite. All troctolites exhibit a magmatic fabric. The major-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites are intermediate between those of Hess Deep gabbros and harzburgites. The trace-element compositions of olivine, plagioclase, and clinopyroxene in the troctolites overlap with those of troctolites from slow-spread crust, but they record no petrographic evidence indicating assimilation of mantle peridotite. Thermodynamic calculation for mineral chemistry showed that fractional crystallization of melt is the dominant process responsible for the formation of the troctolites. The fine-grained troctolite was crystallized with high crystallization rate resulting from hot melt injection into colder wall gabbro. In contrast, interactions between the unsolidified troctolite containing interstitial melt and newly injected melt resulted in the formation of the skeletal olivine-bearing troctolite. While our results demonstrate that the troctolites exhibit multiple melt injections and partial dissolution of a troctolite precursor, fractional crystallization is the dominant process for the creation of the lower crust in the Hess Deep Rift.

Peak-ring magnetism: Rock and mineral magnetic properties of the Chicxulub impact crater

The Chicxulub impact event at ca. 66 Ma left in its wake the only complex crater on Earth with a preserved peak ring, characterized by a well-developed magnetic anomaly low. To date, little is known about its magnetic properties. The joint Integrated Ocean Drilling Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drill core M0077A revealed that the peak ring consists of uplifted and strongly deformed granitoid basement rocks overlain by a 130-m-thick impact melt and suevite layer. Pre- and postimpact hydrothermal systems affected this basement with maximum temperatures up to 450 °C. We used microscopy, mineral chemistry, temperature-dependent magnetic susceptibility, and hysteresis properties to characterize the magnetic mineralogy of pre-, syn-, and postimpact rocks. Compared to its amount of pure, stoichiometric shocked magnetite, the granitoid basement shows low magnetic susceptibility, which is in line with earlier experimental studies indicating that shock reduces magnetic susceptibility. Cation-substituted magnetite with varying compositions in the melt rocks carries a higher induced and remanent magnetization compared to the basement. In the granitoid basement, magnetite was partially oxidized to hematite by a pre-impact hydrothermal event, but at lithological contacts with high-temperature impact melt rock, this hematite was locally retransformed back to magnetite. Elsewhere in the granitoid basement, the temperature reached in the hydrothermal system was too low for hematite retransformation. It was also too low to anneal all the lattice defects in the shocked magnetite, which likely occurs above 540 °C. The presence of shocked magnetite in the granitoid basement well explains the magnetic anomaly low due to its unusually low induced magnetization.

Micropaleontological and Isotopic Perspective of Surface Water Mass Variability in the NE Atlantic from MIS 6 to 5e (188 to 115 ka)

The penultimate glaciation (marine isotope stage (MIS) 6) is considered regionally extreme compared to the last glacial maximum, in which the European ice sheets had a vast areal extent. In contrast to the last deglaciation (19–7 ka), the penultimate deglaciation (140–130 ka) hosts one of the most rapid oceanographic changes of the late Pleistocene. In this study, we reconstructed changes in the near-surface and thermocline in the central to northeast Atlantic by analyzing sediments from two Integrated Ocean Drilling Program Expedition 306 sites. Sites U1313 (41°00.6′ N, 32°57.4′ W) and U1314 (56°21.9′ N, 27°53.3′ W) were drilled on the eastern flank of the mid-Atlantic ridge and Gardar Drift of the eastern subpolar North Atlantic, respectively. We analyzed planktonic foraminiferal assemblages, ice-rafted debris (IRD), and oxygen isotopes in two planktonic foraminifers, Globigerina bulloides, and Globorotalia inflata, from MIS 6 to 5e (185–115 ka). Warmer and colder sea-surface conditions were marked by a change in the relative abundance of polar, subpolar, and transitional planktonic foraminifers. Oxygen isotopes in G. bulloides and G. inflata suggest that the thermocline deepened at the subtropical Site U1313 during MIS 6. The lack of Globorotalia inflata prevented us from profiling the mixed layer and thermocline at the subpolar Site U1314. In contrast to MIS 6, the mixed layer and thermocline were re-stratified during the last interglacial. The lack of major IRD events at both sites suggests the stability of the Laurentide ice sheet during MIS 6 compared to the subsequent glaciation. The presence of Heinrich event 11 indicates the discharge of freshwater that freshened the sea surface, resulting in mixing between the mixed layer and thermocline. Our results were placed into a broader context using published data that shed light on the sensitivity of freshwater discharge to the North Atlantic and the following changes with a transition from a penultimate glacial to an interglacial period in surface circulation.

Genome characterization of two novel deep-sea sediment fungi, Penicillium pacificagyrus sp. nov. and Penicillium pacificasedimenti sp. nov., from South Pacific Gyre subseafloor sediments, highlights survivability

BackgroundMarine deep subsurface sediments were once thought to be devoid of eukaryotic life, but advances in molecular technology have unlocked the presence and activity of well-known closely related terrestrial and marine fungi. Commonly detected fungi in deep marine sediment environments includes Penicillium, Aspergillus, Cladosporium, Fusarium, and Schizophyllum, which could have important implications in carbon and nitrogen cycling in this isolated environment. In order to determine the diversity and unknown metabolic capabilities of fungi in deep-sea sediments, their genomes need to be fully analyzed. In this study, two Penicillium species were isolated from South Pacific Gyre sediment enrichments during Integrated Ocean Drilling Program Expedition 329. The inner gyre has very limited productivity, organic carbon, and nutrients.ResultsHere, we present high-quality genomes of two proposed novel Penicillium species using Illumina HiSeq and PacBio sequencing technologies. Single-copy homologues within the genomes were compared to other closely related genomes using OrthoMCL and maximum-likelihood estimation, which showed that these genomes were novel species within the genus Penicillium. We propose to name isolate SPG-F1 as Penicillium pacificasedimenti sp. nov. and SPG-F15 as Penicillium pacificagyrus sp. nov. The resulting genome sizes were 32.6 Mbp and 36.4 Mbp, respectively, and both genomes were greater than 98% complete as determined by the presence of complete single-copy orthologs. The transposable elements for each genome were 4.87% for P. pacificasedimenti and 10.68% for P. pacificagyrus. A total of 12,271 genes were predicted in the P. pacificasedimenti genome and 12,568 genes in P. pacificagyrus. Both isolates contained genes known to be involved in the degradation of recalcitrant carbon, amino acids, and lignin-derived carbon.ConclusionsOur results provide the first constructed genomes of novel Penicillium isolates from deep marine sediments, which will be useful for future studies of marine subsurface fungal diversity and function. Furthermore, these genomes shed light on the potential impact fungi in marine sediments and the subseafloor could have on global carbon and nitrogen biogeochemical cycles and how they may be persisting in the most energy-limited sedimentary biosphere.