Integrated Ocean Drilling Program Expedition Research Articles

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).

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.

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.

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.

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.

Plio‐Pleistocene Ocean Circulation Changes in the Gulf of Alaska and Its Impacts on the Carbon and Nitrogen Cycles and the Cordilleran Ice Sheet Development

AbstractThe modern Gulf of Alaska (GOA) is a Cordilleran Ice Sheet (CIS) region, estimated to be important for nutrient cycling and CO2 exchange. Little is known of the GOA evolution over the Pliocene and Pleistocene as well as its impact on the CIS development, when other evidence for changing North Pacific circulation has emerged. We analyzed Integrated Ocean Drilling Program Expedition 341 Site U1417 sediments, which extend through the Plio‐Pleistocene transition (4–1.7 Ma), focusing on productivity‐related biomarkers (alkenones, brassicasterol), siliceous microfossils and bulk carbon and nitrogen stable isotopes. Our results show two dominant water column regimes: one characterized by high silica and low organic matter (OM) preservation, containing microorganism remains from a mix of habitats (4–3.7 Ma) and a second characterized by low biogenic silica and increased OM preservation of microorganisms from dominantly open ocean habitats (3.33–3.32 Ma and 2.8–1.66 Ma). An increase of phytoplankton diversity (3.7–3.35 Ma, 3.19–2.82 Ma) characterizes the two transitions of water column conditions, from oxygenated to reductive, that we attribute to a change from ocean mixing to strong stratified conditions with some occasional mixing. The biogeochemical changes in the GOA follow 400 and 100 kyr eccentricity cycles which are also reflected in changes in the CIS. We conclude that the CIS expansion created high nutrient low chlorophyll conditions in the GOA during the Mid Piacenzian Warm Period and the early Pleistocene. In turn, positive feedbacks increased marine productivity export, atmospheric CO2 drawdown and further CIS expansion.

Hydromechanical Effects of Micro‐Organisms on Fine‐Grained Sediments During Early Burial

AbstractMicro‐organisms are known to change fluid flow and permeability processes in subsurface environments, but this has only been demonstrated for coarse‐grained sediments and fractures. For fine‐grained sediments (mudstones), little is known about the effects of micro‐organisms on hydromechanical properties. Here, we investigated the influence of micro‐organisms on the porosity, permeability, and compressibility of fine‐grained sediments. We performed resedimentation experiments with and without micro‐organisms added to two reconstituted, fine‐grained sediment samples. These sediments were collected from the Ursa and Brazos‐Trinity Basins in the Gulf of Mexico during Integrated Ocean Drilling Program Expedition 308. Micro‐organisms caused a systematic, yet small increase in compression index for both sediments. Changes to permeability caused by micro‐organisms, while relatively minor, were greater for the Ursa sediment than the Brazos‐Trinity sediment. Additionally, the effect of micro‐organisms on permeability is greater at higher porosities and lower vertical effective stresses. Differences in permeability behavior between the two sediments are likely due to differences in sediment properties and nutrients for microbial growth. We therefore suggest that the effectiveness of micro‐organisms at altering fluid flow in fine‐grained sediments is dependent on burial depth (porosity as a function of vertical effective stress) and the grain size, pore and pore throat size, and specific surface area of a sediment. Characterizing the effects of micro‐organisms on the hydromechanical properties of fine‐grained sediments can further our understanding of the controls on pore pressure near the sediment–water interface in marine environments and aid in bioclogging practices around contaminated sites in terrestrial environments.

Implications for megathrust slip behavior and pore pressure at the shallow northern Cascadia subduction zone from laboratory friction experiments

Geological records of past megathrust earthquakes on the Cascadia subduction zone give a valid reason to assume that another event will occur in the future. Both the magnitude and tsunamigenic potential of such an earthquake depend on the extent of seismic rupture and slip on the shallow megathrust, which are still under debate for Cascadia. The absence of shallow slow slip events at northern Cascadia has recently been interpreted to result from a megathrust that is locked and potentially seismogenic all the way to the trench. A crucial factor controlling both the nucleation and propagation of an earthquake is the frictional slip behavior of the material in the fault system, of which little is currently known for this setting. Here, we report on laboratory friction experiments on all major lithologies of Juan de Fuca plate sediments being input to the northern Cascadia subduction zone. The material was obtained from cores recovered during Integrated Ocean Drilling Program Expedition 301 at Site U1301 in the depth range of 65 to 260 meters below the seafloor. From tests on both intact and powdered samples under in-situ effective normal stress and room temperature, we observe a significant decrease in friction coefficient from 0.76 to 0.19 with increasing depth. A frictionally weak hemipelagic illite-rich unit located directly above the basement is expected to host the décollement upon subduction. This unit reveals predominantly velocity-strengthening frictional behavior at slip velocities of ∼10−9 to 10−4 m/s, and exhibits no slow slip events in the laboratory, indicating a frictionally stable material. Coulomb wedge modeling based on our friction measurements and interpretation of décollement stratigraphic location suggests significant overpressure on the décollement near the trench. Our data offer a possible explanation for the lack of shallow slow slip activity on the northern Cascadia plate-boundary fault, and raise the possibility that the megathrust is not locked near the trench.

Source-to-sink pathways of clay minerals in the cadiz contourite system over the last 25 kyrs: The segregational role of mediterranean outflow water

Despite major advances in our understanding of the interactions between bottom currents and sedimentary deposits over the last forty years, few studies have focused on the nature of fine particles in contourite depositional systems (CDS). XRD analyses of marine sediments can be used to improve our understanding of fine-grained sediment sources and settling processes. This work presents a detailed sedimentological study of sediment cores collected over the middle slope of the Gulf of Cadiz as part of Integrated Ocean Drilling Program Expedition 339 and the 2001 CADISAR cruise. We performed high-resolution clay mineral analyses to reconstruct the pathways of fine-grained particles from their sources to their deposition along the contourite depositional system of the Gulf of Cadiz (source-to-sink approach). The clay mineral associations reflect the major contribution of the Guadalquivir River and North African rivers/dusts to fine particles settling over the middle slope. Our results suggest that size segregation deposition processes along the path of the Mediterranean Outflow Water (MOW) are responsible for the distinct clay mineral associations between sites located under the upper MOW and the lower MOW. We observed changes of sedimentation rates over the contourite depositional system throughout the last 25 kyrs. We propose that these changes are due to temporal variations in the vertical distribution of the upper and the lower MOW whose concentrations in suspended particulate matter are drastically different. Sea-level and large scale atmospheric changes (e.g., ITCZ migration) over this period induced major variations in the distance of river mouths to the Gulf of Cadiz CDS, and in the amount of Northwest African dust delivered to this depositional system, respectively. Climate changes therefore modified fine particle sources and pathways, which considerably influenced clay minerals settling in the middle slope of the Gulf of Cadiz since the Last Glacial Maximum.

Redefining the Subsurface Biosphere: Characterization of Fungi Isolated From Energy-Limited Marine Deep Subsurface Sediment.

The characterization of metabolically active fungal isolates within the deep marine subsurface will alter current ecosystem models and living biomass estimates that are limited to bacterial and archaeal populations. Although marine fungi have been studied for over fifty years, a detailed description of fungal populations within the deep subsurface is lacking. Fungi possess metabolic pathways capable of utilizing previously considered non-bioavailable energy reserves. Therefore, metabolically active fungi would occupy a unique niche within subsurface ecosystems, with the potential to provide an organic carbon source for heterotrophic prokaryotic populations from the transformation of non-bioavailable energy into substrates, as well as from the fungal necromass itself. These organic carbon sources are not currently being considered in subsurface energy budgets. Sediments from South Pacific Gyre subsurface, one of the most energy-limited environments on Earth, were collected during the Integrated Ocean Drilling Program Expedition 329. Anoxic and oxic sediment slurry enrichments using fresh sediment were used to isolate multiple fungal strains in media types that varied in organic carbon substrates and concentration. Metabolically active and dormant fungal populations were also determined from nucleic acids extracted from in situ cryopreserved South Pacific Gyre sediments. For further characterization of physical growth parameters, two isolates were chosen based on their representation of the whole South Pacific Gyre fungal community. Results from this study show that fungi have adapted to be metabolically active and key community members in South Pacific Gyre sediments and potentially within global biogeochemical cycles.

The Impact of Grain Size on the Hydromechanical Behavior of Mudstones

AbstractThe consolidation behavior of mudstone is controlled by the clay fraction and affects a wide range of deformation and fluid transport processes in accretionary wedge systems. I analyze the compression and permeability behavior of six sediment mixtures based on uniaxial resedimentation and constant rate of strain consolidation data. The sediment mixtures are composed of varying proportions of hemipelagic mudstone and silt‐size silica resulting in clay fractions ranging from 56% to 32% by mass. The hemipelagic mudstone is from Site C0011 drilled seaward of the Nankai Trough, offshore Japan, during Integrated Ocean Drilling Program Expedition 322. I show that porosity and compression index consistently decrease and permeability consistently increases with decreasing clay fraction over vertical effective stresses ranging from 0 to 21 MPa. Backscattered electron microscope images reveal that matrix porosity declines and large, jagged pore throats are being preserved in compaction shadows between silt grains as the clay fraction decreases. I compare the behavior of reconstituted samples with that of intact core and field measurements and interpret that increasing creep with depth and different mudstone fabrics explain differences in their compression curves. Finally, I provide empirical compression and permeability models that describe the evolution of porosity (void ratio) and permeability with vertical effective stress and as a function of grain size. Characterizing the in situ hydromechanical properties of subduction inputs is critical in order to relate input sediments to those at frontal thrust regions and predict pore fluid pressures and compaction‐driven fluid sources within the outer part of the accretionary prism.

Mineral Geochemistry of Basaltic Rocks from IODP Expeditions 334 and 344: Implications for Magmatic Processes of Cocos Ridge Segment Being Subducted Beneath the Middle America Trench

The Cocos Ridge, which is subducted beneath the Central American Volcanic Arc, has a complex tectonic evolution history due to plume-ridge interaction between the Galápagos plume and the Cocos—Nazca spreading center. This study presents major and trace element analyses of plagioclase and clinopyroxenes hosted by Cocos Ridge basaltic rocks that were drilled in three holes (U1381A, U1381C and U1414A) of Sites U1381 and U1414 on the Cocos Ridge close to the Middle America Trench during the Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344. The results show that (1) plagioclases are mainly bytownite and labradorite with subordinate andesine, which are enriched in light rare earth elements (LREE) and some large-ion lithophile elements (LILE) and exhibit marked positive Eu anomalies; and (2) that clinopyroxenes are augites, which are depleted in highly incompatible elements such as LREE and LILE, have nearly flat heavy rare earth elements patterns (HREE) and lack Eu anomalies in chondrite-normalized rare earth element (REE) diagrams. During the ascent to the surface, the primary magmas experienced fractional crystallization of plagioclase, clinopyroxene, Ti-Fe oxides and possibly olivine (complete replacement of olivine by secondary minerals). The crystallization temperatures of plagioclase phenocrysts and microlites are 1050 to 1269 °C, and 866 to 1038 °C, respectively, and the pressures of plagioclase phenocrysts are 0.3–0.7 GPa. The crystallization temperatures of clinopyroxene phenocrysts/micro-phenocrysts is 1174–1268 °C, similar to those of plagioclase phenocrysts, suggesting some of clinopyroxene and plagioclase phenocrysts cotectic crystallized during early stage of magmatic evolution. In addition, the equilibrium pressures of clinopyroxene phenocrysts/micro phenocrysts are 0.02–0.97 GPa, implying that the clinopyroxene started to crystallize within the mantle, and magma evolution has undergone an early crystallization stage with clinopyroxene and no plagioclase.

Holocene Spatiotemporal Redox Variations in the Southern Baltic Sea

Low oxygen conditions in the modern Baltic Sea are exacerbated by human activities; however, anoxic conditions also prevailed naturally over the Holocene. Few studies have characterized the specific paleoredox conditions (manganous, ferruginous, euxinic) and their frequency in southern Baltic sub-basins during these ancient events. Here, we apply a suite of isotope systems (Fe, Mo, S) and associated elemental proxies (e.g., Fe speciation, Mn) to specifically define water column redox regimes through the Baltic Holocene in a sill-proximal to sill-distal transect (Lille Belt, Bornholm Basin, Landsort Deep) using samples collected during the Integrated Ocean Drilling Program Expedition 347. At the sill-proximal Lille Belt, there is evidence for anoxic manganous/ferruginous conditions for most of the cored interval following the transition from the Ancylus Lake to Littorina Sea but with no clear excursion to more reducing or euxinic conditions associated with the Holocene Thermal Maximum (HTM) or Medieval Climate Anomaly (MCA) events. At the sill-distal southern sub-basin, Bornholm Basin, a combination of Fe speciation, pore water Fe, and solid phase Mo concentration and isotope data point to manganous/ferruginous conditions during the Ancylus Lake-to-Littorina Sea transition and HTM but with only brief excursions to intermittently or weakly euxinic conditions during this interval. At the western Baltic Proper sub-basin, Landsort Deep, new Fe and S isotope data bolster previous Mo isotope records and Fe speciation evidence for two distinct anoxic periods but also suggest that sulfide accumulation beyond transient levels was largely restricted to the sediment-water interface. Ultimately, the combined data from all three locations indicate that Fe enrichments typically indicative of euxinia may be best explained by Fe deposition as oxides following events likely analogous to the periodic incursions of oxygenated North Sea waters observed today, with subsequent pyrite formation in sulfidic pore waters. Additionally, the Mo isotope data from multiple Baltic Sea southern basins argue against restricted and widespread euxinic conditions, as has been demonstrated in the Baltic Proper and Bothnian Sea during the HTM or MCA. Instead, similar to today, each past Baltic anoxic event is characterized by redox conditions that become progressively more reducing with increasing distance from the sill.

The role of pre-subduction sediment diagenesis in a shallow tsunami-generated slip, Sunda Trench, south of Sumatra

AbstractThe progressive development of microfabrics in sediments was investigated in an active subduction zone using cores recovered during the Integrated Ocean Drilling Program Expedition 362. A sequence of Paleocene–Holocene rock was recorded at Site U1480, which is located in the Sunda Trench, south of Sumatra. Hemipelagic clayey sediments from the drilled succession were recovered from a depth of 1327.23 m. Cardhouse fabrics were observed initially, consisting of clay flakes with edge-to-edge (E–E) or edge-to-face (E–F) contacts. These initial microfabrics developed into compacted bookhouse fabrics (which are random and consolidated), and consist of clay flakes with E–F or face-to-face (F–F) contacts as the domain. These microfabrics developed due to individual particle sliding. Bookhouse fabric developed into a shape-preferred orientation of clay minerals with F–F contacts as a shaly fabric due to domain sliding. Finally, clay flakes with F–F contacts deformed into an undulated shape due to further increased overburden pressure. These pervasive fabrics comprising clay flakes stacked with F–F contacts resisted shear deformation due to their strong cohesion. This strong cohesion in the undulated shaly fabric might be an important factor in the locking of shallow tsunami-generating slip in subduction zones.

Quantification of gas hydrate saturation and morphology based on a generalized effective medium model

Numerous models have been developed for prediction of gas hydrate saturation based on the microstructural relationship between gas hydrates and sediment grains. However, quantification of hydrate saturation and morphology from elastic properties has been hindered by failing to account for complex hydrate distributions. Here, we develop a generalized effective medium model by applying the modified Hashin-Shtrikman bounds to a newly developed cementation theory. This model is validated by experimental data for synthetic methane and tetrahydrofuran hydrates. Good comparison of model predictions with experimental measurements not only reveals its ability to merge the results of contact cementation theory and effective medium theory, but also indicates its feasibility for characterizing complex morphologies. Moreover, the results of inverting acoustic measurements quantitatively confirm that for synthetic samples in “excess-gas” condition gas hydrates mainly occur as a hybrid-cementing morphology with a low percentage of pore-filling morphology, whereas for pressure-core hydrate-bearing sediments in natural environments they exist as matrix-supporting and pore-filling morphologies with a very low percentage of hybrid-cementing morphology. The hydrate saturations estimated from sonic and density logs in several regions including northern Cascadia margin (Integrated Ocean Drilling Program Expedition 311, Hole U1326D and Hole U1327E), Alaska North Slope (Mount Elbert test well) and Mackenzie Delta (Mallik 5L-38), are comparable to the referenced hydrate saturations derived from core data and resistivity, and/or nuclear magnetic resonance log data, confirming validity and applicability of our model. Furthermore, our results indicate that ~8% hybrid-cementing, ~33% matrix-supporting and ~59% pore-filling hydrates may coexist in the fine-grained and clay-rich marine sediments on the northern Cascadia margin, whereas ~10% hybrid-cementing, ~54% matrix-supporting and ~36% pore-filling hydrates may coexist in the coarse-grained and sand-dominated terrestrial sediments of the Alaska North Slope and Mackenzie Delta.

Fault weakening caused by smectite swelling

The large slip along the shallow subduction interface during the 2011 Tohoku-Oki earthquake (Mw9.0) caused a huge tsunami that struck the northeast coast of Honshu, Japan. The Integrated Ocean Drilling Program Expedition 343 JFAST program revealed that the fault zone is composed primarily of smectite. Our swelling experiments using the fault material demonstrated that the swelling pressure systematically increases with a decrease in sample porosity. Based on in situ porosity estimations in the IODP borehole, the swelling pressure of the fault is as high as 8 MPa, which is comparable to the effective normal stress at the drill site (~ 7 MPa). This also suggests that the modified effective confining pressure of the fault is quite low or potentially zero, meaning that fault strength is governed mainly by cohesion rather than frictional strength. The fault may therefore be intrinsically weak, which could enhance the coseismic displacement toward the trench when earthquake slip propagates from depth.

The Late Mesozoic‐Cenozoic Arctic Ocean Climate and Sea Ice History: A Challenge for Past and Future Scientific Ocean Drilling

AbstractOver the past 3–4 decades, coincident with global warming and atmospheric CO2 increase, Arctic sea ice has significantly decreased in its extent as well as in thickness. When extrapolating this alarming trend, the central Arctic Ocean might become ice‐free during summers within about the next 2–5 decades. Paleoclimate records allow us to better understand the processes controlling modern climate change and distinguish between natural and anthropogenic forcing. In this context, detailed studies of the earlier Earth history characterized by a much warmer global climate with elevated atmospheric CO2 concentrations are important. The main focus of this review paper is the long‐term late Mesozoic‐Cenozoic Arctic Ocean climate history from Greenhouse to Icehouse conditions, with special emphasis on Arctic sea ice history. Starting with some information on the Cretaceous Arctic Ocean climate, this paper will concentrate on selected results from Integrated Ocean Drilling Program (IODP) Expedition 302 (Arctic Ocean Coring Expedition (ACEX)), the first scientific drilling in the permanently ice‐covered Arctic Ocean, dealing with the Cenozoic climate history. While these results from ACEX were unprecedented, key questions related to the Cenozoic Arctic climate history remain unanswered, largely due to the major mid‐Cenozoic hiatus (if existing) and partly to the poor recovery of the ACEX record. Following‐up ACEX and its cutting‐edge science, a second scientific drilling on Lomonosov Ridge with a focus on the reconstruction of the continuous and complete Cenozoic Arctic Ocean climate history has currently been proposed and scheduled as IODP Expedition 377 for 2021.

Meteoric 10Be Reveals a Young, Active Accretionary Prism and Structurally Complex Décollement in the Vicinity of the 2011 Tohoku Earthquake Rupture

AbstractWe present new meteoric 10Be concentration data from marine sediment cores recovered during Integrated Ocean Drilling Program Expedition 343 (JFAST) that help constrain the age, origin, and internal structure of the frontal prism at the Japan trench in the vicinity of the 2011 Tohoku‐oki M9 earthquake rupture. 10Be sediment ages from the lower portion of the frontal prism range from ~0–10 Ma, with >60% of analyzed samples above the décollement yielding young ages <2 Ma. Repetition and inversion of high‐over‐low 10Be‐concentration sediments indicate the presence of stratigraphic inversions that correspond to faulting and imbrication of late Miocene and Quaternary sediments. The density of faults inferred indicates that the frontal prism has a fault spacing on the order of 10 s of meters, and the identification of faults in the underthrust section suggests that the plate boundary décollement may be a zone with multiple slip surfaces. Comparison of 10Be concentrations in the frontal prism with those of the incoming and forearc slope sediments indicates that the majority of the prism is sourced from accretion of Pacific Plate sediments, rather than from reworked frontal prism or slope sediments. These data suggest that over at least the past ~1–2 Ma, the décollement preferentially has localized at or near the base of the incoming sediment section, with relatively efficient sediment accretion occurring even in the presence of subducted horst‐and‐graben topography.

The Latest Spreading Periods of the South China Sea: New Constraints From Macrostructure Analysis of IODP Expedition 349 Cores and Geophysical Data

AbstractMacrostructures preserved in deformed rocks are essential for the understanding of their evolution, especially when the deformation is weak and hard to discriminate in regional scale or purely through geophysical data. In order to resolve the inconsistency between NS trending fracture zones and NE oriented spreading fabrics of the South China Sea during the latest spreading stage, we analyzed macrostructures identifiable from the basalt and consolidated sediment samples of the Integrated Ocean Drilling Program (IODP) Sites U1431 and U1433. These two sites are close to the East and Southwest relict spreading ridges and provide critical information on the latest spreading stages. The structures in the basalt of both sites suggest two dominant orientations of NS and NE. At U1431, sediments show mainly WNW trending slickensides, different from that of basalt. At U1433, no structures were found in postspreading sediment. Thus, NE and NS trending structures in basalt are most possibly formed by seafloor spreading. Crosscutting relationship suggests that NE trending structures formed first, followed by NS and finally WNW trending structures. These observations are consistent with geophysical features. Magnetic anomalies and ocean bottom seismometer velocity suggest that the latest relict ridge of the East Subbasin coincides with the EW trending seamount chain. Located between the relict ridges of East and Southwest Subbasins, NS trending Zhongnan‐Liyue Fracture Zone had acted as the latest transform fault. Based on the above evidences, we proposed that the South China Sea may have experienced a short period of NS oriented spreading after earlier SE spreading. These results resolve the previous inconsistencies.