Integrated Ocean Drilling Program Research Articles

Occurrence characteristics of authigenic pyrite in the deep-sea environment and its paleoceanographic implications based on core sediments from IODP Expedition 342 Site U1406

Although authigenic pyrite (FeS2) in marine sediment is an important proxy for oxygen conditions and microbial activities at its formation site, very little is known about its morphological characteristics in the deep-sea (> 3500 m) environment and their paleoceanographic implications. Here, we report the occurrence characteristics of authigenic pyrite including texture, size distribution, and relative content in North Atlantic deep-sea sediments from Integrated Ocean Drilling Program (IODP) Expedition 342 Site U1406. The results of electron microscopy show octahedral pyrite microcrystals of 1.5–4.0 μm, the spherical texture of greigite (Fe3S4), and large pyrite framboids (average, 25 μm) relative to those of the continental margin. These features may demonstrate an unchanged supersaturation level of the pore water for octahedral pyrite after its initial formation, replacement of the precursor greigite by pyrite, and an expanded pyrite framboid formation zone with a low sulfate reduction rate in deep-sea sediments, respectively. Although the size distribution of pyrite framboids measured in this study falls into the range of pyrites formed in the sulfate–methane transition zone, based on onboard geochemical data and previous research, these pyrites most likely formed through organoclastic sulfate reduction during diagenesis. We assessed the implications of these findings for paleoceanographic changes, as the formation of diagenetic pyrite is controlled by the supply rate and reactivity of organic matter, which are closely related to ocean productivity and circulation. A comparison between the profiles of relative pyrite content and benthic foraminiferal oxygen isotope (δ18Obf) values indicated that these factors were anti-phased during the late Oligocene. Based on our results, we conclude that the formation of diagenetic pyrite was favored during interglacial periods due to increases in the amount and reactivity of organic matter driven by more energetic formation of proto-North Atlantic Deep Water. Our findings provide new insights into the formation of authigenic pyrite in deep-sea environments and its potential as a paleoceanographic proxy.

Twenty years of MSPs: Technologies and Perspectives

Mission-Specific Platforms (MSPs) have been important members of the scientific ocean drilling family for two decades, operating alongside their impressive siblings JOIDES Resolution and D/V Chikyu. Over this time, 10 MSP expeditions were implemented in 7 different oceans and seas. These MSPs capitalised on alternative platforms and methods, and enabled the scientific community to access new geographical areas and new geological targets that could not be drilled by the JOIDES Resolution or Chikyu. Offshore heave-compensated wireline coring, onshore-mining-style wireline coring, remote seafloor drilling and giant piston coring have all featured on MSP expeditions. Equally diverse were the environments in which these technologies were deployed, from the ice-covered Arctic Ocean to the tropical seas of the Great Barrier Reef, and from the shallow waters of the Yucatan shelf to the ultra-deep hadal depths of the Japan Trench. In this paper we reflect on the diverse technologies of MSPs, how they increased scientific ocean drilling capability in the Integrated Ocean Drilling Program (2003−2013) and International Ocean Discovery Program (2013–2024), and the scientific achievements that they enabled.

Distinct seasonal changes and precession forcing of surface and subsurface temperatures in the mid-latitudinal North Atlantic during the onset of the Late Pliocene

Abstract. The Late Pliocene marks the intensification of Northern Hemisphere glaciation (iNHG), offering a unique opportunity to study climate evolution and ice-sheet-related feedback mechanisms. In this study, we present high-resolution Mg / Ca-based sea surface temperatures (SSTs) and subsurface temperatures (SubTs) derived from the foraminiferal species Globigerinoides ruber and Globorotalia hirsuta, respectively, at the Integrated Ocean Drilling Program (IODP) Expedition 306 Site U1313 in the mid-latitudinal North Atlantic during the early Late Pliocene, 3.65–3.37 million years ago (Ma). We find distinct differences between our new G. ruber Mg / Ca-based SST record and previously published alkenone-based SST records from the same location. These discrepancies in both absolute values and variations highlight distinctly different seasonal influences on the proxies. The G. ruber Mg / Ca-based SST data were primarily influenced by local summer insolation, showing a dominant precession cycle. Conversely, the variations in alkenone-based SST, dominated by the obliquity and lacking the precession cycle, are found to be more indicative of cold-season changes, despite previous interpretations of these records as reflecting annual mean temperatures. A simultaneous decline in Mg / Ca-based SST and SubT records from 3.65 to 3.5 Ma suggests a diminished poleward oceanic heat transport, implying a weakening of the North Atlantic Current (NAC). A comparison with Early Pleistocene G. ruber Mg / Ca-based SST records shows a shift in the dominant climatic cycle from precession to obliquity, alongside a marked increase in amplitude, indicating an enhanced influence of obliquity cycles correlated with the expansion of Northern Hemisphere ice sheets.

Low-high latitude forcing on the East Asian winter monsoon evolution since the last glacial maximum

Abstract The East Asian winter monsoon (EAWM) has a profound effect on the winter climate in East Asia. The modern EAWM variability is tightly linked to the high-latitude Northern Hemisphere climate change through the Siberian High and can also be regulated by the low-latitude El Niño-Southern Oscillation through oceanic or atmospheric teleconnections. However, the Quaternary EAWM evolution has long been only attributed to the high-latitude climate change, resulting in the uncertainty in interpreting the out-of-phased EAWM variation recording in the East Asian continent and marginal seas. Here we presented a sediment record at Integrated Ocean Drilling Program Site U1427 in the southern Japan Sea to reconstruct the EAWM evolution since the last glacial maximum. By combining our record with previous reconstructions and simulations, we found the synchronous relationship between winter monsoon in northern and southern regions of East Asia from ∼24 to 8 ka, but anti-correlated relationship since ∼8 ka. We proposed the winter insolation and Atlantic meridional overturning circulation were the main drivers from last glacial to early Holocene, and then ENSO became a dominant factor in controlling the regional heterogeneity of EAWM evolution in the middle and late Holocene. This research explains much of the controversy in the Quaternary EAWM records and highlights the low-high latitude interaction in East Asian winter climate change.

Oceanography of the Eastern Equatorial Pacific Ocean Across the Oligocene‐Miocene Transition

AbstractThe functioning of the Pacific Ocean—the world's largest ocean—during a warmer‐than‐present paleoclimate state remains underexplored. We present planktonic and benthic foraminiferal stable oxygen (δ18O) and carbon (δ13C) isotope records from Integrated Ocean Drilling Program (IODP) Site U1334 that span the Oligocene‐Miocene Transition (OMT) interval, from 24.15 to 21.95 million years ago (Ma). We reconstruct (sub‐)surface and deep‐water conditions and provide better constraints on the physical and chemical oceanography of the eastern equatorial Pacific Ocean (EEP). Positive trends in planktonic and benthic foraminiferal δ18O values, mark a largely uniform imprint of increased land‐ice volume/global cooling on surface‐ and deep‐waters. We document a delayed planktonic foraminiferal δ18O increase across the OMT as well as an increase in the amplitude variability of planktonic foraminiferal δ18O values on eccentricity timescales during the early Miocene. We interpret this as an enhanced glacioeustatic sea‐level control on Atlantic‐Pacific salinity exchange through the Central American Seaway (CAS) or as the onset of more variable surface currents and oceanic fronts in the EEP. Positive trends in planktonic and benthic foraminiferal δ13C values characterize the whole‐ocean depletion in 12C linked to organic carbon burial during the Oligocene‐Miocene carbon maximum (CM‐OM). However, this depletion is more pronounced in the planktonic foraminiferal δ13C record, especially during ∼400 Kyr eccentricity minima, reflecting an increase in nutrient upwelling and the efficacy of the biological carbon pump (BCP) when global temperatures decreased across the OMT and during the early Miocene. Our study highlights the dynamic behavior of the EEP in a warmer‐than‐present unipolar icehouse state.

An exceptional record of soft-sediment deformation within Pliocene deposits of Faro Drift (SW Iberia margin) - IODP Expedition 339 Sites U1386 and U1387

The occurrence of soft-sediment deformation structures (SSDS) have long been recognized in several types of sedimentary environments and deposits. However, their presence in contourite drift deposits is still unreported in the literature. In this work, we present the first detailed description of SSDS found within the Pliocene sedimentary record of the Faro Drift, recovered during the Integrated Ocean Drilling Program (IODP) Expedition 339. The Faro Drift is the largest contourite drift of the Contourite Drift Depositional System developed in the Gulf of Cadiz since the Late Miocene by the circulation of the Mediterranean Outflow Water. The SSDS were identified in archive-halves of core sections located between ∼458 and ∼ 510 m below seafloor (mbsf) (hole U1386C), and between ∼599 and ∼ 670 mbsf (hole U1387C). Their identification and characterization was made by visual core description, structural geometrical analysis in core-scan high-resolution images, and scanning electron microcopy (SEM) analysis in selected intervals. The SSDS were classified based on the exhibited geometry, structural configuration and respective kinematics. The main deformation process and potential trigger were inferred from the geometrical and kinematics analysis. We identified five categories of SSDS: i) microfaults (normal and thrust faults), ii) slump sheet (formed by several types of folds, such as eye-folds, fish-hook folds, spiral folds), iii) convolute bedding, iv) folds within debrite mudclasts', and v) sigmoidal-like structures. Although the first three are well known types of SSDS, the folds within debrite mudclasts' and sigmoid-like structures have been scarcely recognized and described at core-scale. The inferred deformation processes responsible for the formation of these SSDS were i) brittle deformation by hydrofracturing and compaction faulting (microfaults), ii) hydroplastic (ductile) deformation (slump folds, folds within debrite mudclasts'), iii) liquefaction (convolute bedding), iv) shearing by flow movement (sigmoid-like structures). The most probable triggering agents seem to have been overloading, downslope movement of slump sheet and debris flow, and shearing by currents.

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.

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).

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.