Abstract Submarine landslides occur globally and have the potential to damage seafloor infrastructure and trigger tsunamis. Recently, diatomaceous weak layers have been hypothesized to play a role in triggering submarine landslides on passive continental margins by generating overpressure. Here, we mixed two types of clay, an illite‐rich glaciomarine Boston Blue Clay (BBC) and a smectite‐rich marine clay from Eugene Island (EI) Block 330 offshore Louisiana, each with marine or lacustrine diatoms at mass ratios of 100:00, 90:10, and 80:20 to investigate the consolidated‐undrained shear strength of diatomaceous mudstones. These samples underwent uniaxial consolidation to 950 kPa before undrained shearing. Failure modes vary from bulging failure (BBC mixtures) to distinct failure planes (EI mixtures). Under peak conditions, BBC shows higher shear strength (202 vs. 176 kPa), effective friction angle (32° vs. 24°), and excess pore pressure (500 vs. 360 kPa) than EI, respectively. Adding 20 wt.% marine diatoms increased the peak undrained shear strength (BBC: +47 kPa, EI: +44 kPa) and effective friction angle (BBC: +9°, EI: +10°) similarly in both lithologies. However, excess pore pressure increased more rapidly in EI mixtures (+250 kPa) than in BBC mixtures (+5 kPa). Therefore, EI mixtures are more susceptible to shear failure with increased diatom content than BBC mixtures because of lower shear strengths and friction angles and faster pore‐pressure buildup. When combined with prior hydromechanical data, the findings indicate that diatom contents exceeding 20 wt.% and/or higher sedimentation rates are needed to produce overpressure sufficient to destabilize a passive continental slope.

Abstract Near‐seafloor marine sediments on active margins have been shown to be stronger than those on passive margins. However, the reasons for this are not fully understood. To investigate the role of intrinsic properties, we performed resedimentation and oedometer tests and measured undrained shear strength, grain size, mineralogy, and plasticity on sediments from three active margins (Nankai, Cascadia, Surveyor Fan) and three passive margins (Amazon Fan, Carolina, and New Jersey) and compared them to their in situ field measurements. We find that all shear strengths on resedimented samples fall within the expected range for normal consolidation. However, Nankai and Cascadia exhibit anomalously high undrained shear strength and low porosity in situ, which cannot be reproduced with 1‐D consolidation experiments. These results suggest that other factors occurring on active margins contribute to strengthening near‐seafloor sediments such as the repeated exposure to earthquakes and lateral shear strain induced by non‐uniaxial stress paths.

Microfossils can have a large impact on the hydromechanical properties of marine sediments. Here, we study how these properties change in sediment mixtures containing varying concentrations of diatoms during experimental loading. We mixed an illite‐rich glaciomarine clay known as Boston Blue Clay (BBC) and a smectite‐rich marine clay known as Eugene Island Clay (EI) with marine and lacustrine diatoms in mass ratios of 100:00, 90:10, and 80:20. These mixtures were uniaxially compressed to 100 kPa in resedimentation tests and further loaded to 2 MPa in constant rate of strain consolidation experiments. We found that adding diatoms results in an increase in void ratio, compressibility, and vertical permeability at a given vertical effective stress for both sediments. These changes are due to an increased intraskeletal and interskeletal porosity caused by the porous nature of diatoms and their ability to form stress bridges. With increasing vertical effective stress, sediments lose their permeability at a slower rate when containing diatoms. These changes are most evident in BBC mixtures. When comparing both sediment types, void ratio and permeability decrease faster during burial for the EI mixtures than the BBC mixtures. These results provide new insights into the hydromechanical behavior of microfossil‐rich marine sediments and contribute to our understanding of their potential for overpressure generation and the development of a weak layer.

Calcium carbonate precipitation in ageing ocean crust sequesters carbon dioxide dissolved in seawater through seafloor weathering reactions, influencing atmospheric CO 2 concentrations on million-year timescales. However, this crustal carbon sink, and the extent it balances CO 2 degassing during crustal formation at mid-ocean ridges, remain poorly quantified due to limited sampling of the vast ridge flanks where CO 2 uptake continues for millions of years. Here we quantify the carbon sink hosted within talus breccias that accumulated through mass wasting 61 million years ago during rift faulting at the slow spreading Mid-Atlantic Ridge, cored during International Ocean Discovery Program South Atlantic Transect Expedition 390. After 40 million years of carbonate cementation, these breccias contain ~7.5 wt% seawater-derived CO 2 , 2 to 40 times more than previously cored upper crust. Our estimates of talus breccia abundance based on fault geometries indicate that talus formed at slow-spreading ridges can accommodate a CO 2 sink equivalent to a large proportion of the CO 2 released during accretion of the underlying crust. The proportion of plate divergence accommodated by faulting, and hence talus abundance, increases nonlinearly with decreasing spreading rate. Consequently, past variations in spreading rate may have impacted the balance between ocean crust CO 2 release and uptake in Earth’s carbon cycle.

Definitive evidence for the microbially induced smectite-to-illite (S-I) reaction has previously been shown using culture experiments with pure clay minerals, whereas recognition in nature remains difficult. Here, we investigated the microbially induced S-I reaction in natural sediments during laboratory compression and applied new and previously used techniques that can positively identify the products of this reaction. We performed resedimentation experiments without (control experiment) and with the Fe reducing bacteria Shewanella oneidensis MR-1 (microbially amended experiment) added to natural sediments collected from the Ursa and Brazos-Trinity regions in the Gulf of Mexico during Integrated Ocean Drilling Program Expedition 308. Following these experiments, subsamples were collected and analyzed with bulk and clay fraction X-ray diffraction, micro X-ray fluorescence, electron microprobe elemental mapping, and energy dispersive spectroscopy (EDS) spot analyses at the µm-scale. We found in the microbially amended experiments of both sediment samples that (1) clay fraction XRD scans and quantitative analyses revealed layer collapse, permanent K fixation, and decreased expandability in mixed layered illite-smectite indicating progressive illite formation, (2) electron microprobe mapping and EDS spot analyses both showed a decrease in Si, increases in Al/Si and K, and no change in Al, and (3) layer charge calculated using EDS spot data increased relative to the control experiments. The control and amended experiments of both sediments displayed little to no change in bulk elemental compositions. Our results indicate that the microbially induced S-I reaction occurred to a relatively small, yet systematic, degree in the amended experiments of both sediment samples and that electron microprobe elemental mapping, EDS spot analyses at the µm-scale, and clay fraction mineralogy can be used to infer this reaction in natural sediments, whereas bulk elemental compositions may not. This research will help provide a path forward in recognizing the microbially induced S-I reaction in natural settings and assist in understanding elemental cycling during early diagenesis, sediment pore fluid overpressures, and fault zone behavior.

The South Atlantic Transect (SAT) is a multidisciplinary scientific ocean drilling experiment designed to investigate the evolution of the oceanic crust and overlying sediments across the western flank of the Mid-Atlantic Ridge.This project comprises four International Ocean Discovery Program expeditions: fully staffed Expeditions 390 and 393 (April-August 2022) built on engineering preparations during Expeditions 390C and 395E that took place without science parties during the height of the Coronavirus Disease 2019 (COVID-19) pandemic.Through operations along a crustal flow line at ~31°S, the SAT recovered complete sedimentary sections and the upper ~40-340 m of the underlying ocean crust formed at a slow to intermediate spreading rate at the Mid-Atlantic Ridge over the past ~61 My.The sediments along this transect were originally spot cored more than 50 y ago during Deep Sea Drilling Project Leg 3 (December 1968-January 1969) to help verify the theories of seafloor spreading and plate tectonics.The SAT expeditions targeted six primary sites on 7, 15, 31, 49, and 61 Ma ocean crust that fill critical gaps in our sampling of intact in situ ocean crust with regards to crustal age, spreading rate, and sediment thickness.Drilling these sites was required to investigate the history, duration, and intensity of the low-temperature hydrothermal interactions between the aging ocean crust and the evolving South Atlantic Ocean.This knowledge will improve the quantification of past hydrothermal contributions to global biogeochemical cycles and help develop a predictive understanding of the impacts of variable hydrothermal processes and exchanges.Samples from the transect of the previously unexplored sediment-and basalt-hosted deep biosphere beneath the South Atlantic Gyre are essential to refine global biomass estimates and examine microbial ecosystems' responses to variable conditions in a low-energy gyre and aging ocean crust.The transect is located near World Ocean Circulation Experiment Line A10, which provides a baseline for records of carbonate chemistry and deepwater mass properties across the western South Atlantic through key Cenozoic intervals of elevated atmospheric CO 2 and rapid climate change.Reconstruction of the history of the deep western boundary current and deepwater formation in the Atlantic basins will yield crucial data to test hypotheses regarding the role of evolving thermohaline circulation patterns in climate change and the effects of tectonic gateways and climate on ocean acidification.During engineering Expeditions 390C and 395E, a single hole was cored through the sediment cover and into the uppermost rocks of the ocean crust with the advanced piston corer (APC) and extended core barrel (XCB) systems at five of the six primary proposed SAT sites.Reentry systems with casing were then installed either into basement or within 10 m of basement at each of those five sites.Expedition 390 (7 April-7 June 2022) conducted operations at three of the SAT sites, recovering 700 m of core (77%) over 30.3 days of on-site operations.Sediment coring, basement coring, and wireline logging were conducted at two sites on 61 Ma crust (Sites U1556 and U1557), and sediment coring was completed at the 7 Ma Site U1559.Expedition 393 operated at four sites, drilling in 12 holes to complete this initial phase of the SAT.Complete sedimentary sections were collected at Sites U1558, U1583, and U1560 on 49, 31, and 15 Ma crust, respectively, and together with 257.7 m of sediments cored during earlier operations, more than 600 m of sediments was characterized.The uppermost ocean crust was drilled at Sites U1558, U1560, and U1583 with good penetration (~130 to ~204 meters subbasement), but at the youngest ~7 Ma Site U1559, only ~43 m of basement penetration was achieved in this initial attempt.Geophysical wireline logs were aquired at Sites U1583 and U1560.Expeditions 390 and 393 established legacy sites available for future deepening and downhole basement hydrothermal and microbiological experiments at Sites U1557, U1560, and U1559 on 61, 15, and 7 Ma crust, respectively.

Mineralogical composition is a primary control on the mechanical strength of tight mudrock reservoirs and is a critical rock property in the identification of intervals for hydraulic fracturing. The x-ray fluorescence (XRF) spectroscopy and x-ray diffraction (XRD) analyses were conducted on mudrocks from the Permian Wolfcamp formation and Spraberry Formation, Midland Basin, Texas, to study their chemical composition and potential impact on mechanical rock properties. Additionally, we use a combina-tion of thin-section petrography, scanning electron microscopy, and mineral segmentation mapping to document rock texture, especially the amount and distribution of quartz and calcite cements. We find that samples dominated by extrabasinal grain components show isolated clusters of intergranular cementation in grain-supported packing arrangements, whereas samples domi-nated by intrabasinal grain components show pervasive cementa-tion in matrix-supported grain assemblages. We present a novel workflow using correlative relationships between elemental Si, Al, and Ca to classify mudrocks into chemofacies and predict which chemofacies are cement prone. This workflow identifies four XRF-based chemofacies for the Wolfcamp and Spraberry mudrocks: (1) abundant siliciclastic detrital grain components, (2) intergranu-lar calcite cement, (3) abundant quartz and calcite cement, and (4) pervasive microcrystalline quartz cement. Results show that cement-prone facies of the Wolfcamp and Spraberry, particularly chemofacies 2, correlate to high elastic response and represent the strongest core materials. Our workflow can be applied to any mudrock system with available compositional data sets, such as XRF, XRD, or Fourier transform infrared spectroscopy, aiding in the prediction of mechanical mudrock properties and the development of brittle fractures in unconventional reservoirs.

Recent work has demonstrated elevated shear strength in the uppermost 100 meters below seafloor (mbsf) on seismically active margins. This observation is consistent with the seismic strengthening hypothesis that repeated exposure to earthquake shaking progressively dewaters and densifies sediment, which leads to increased shear strength and slope stability. However, the relative contribution of seismic strengthening versus intrinsic properties on shear strength remain largely unknown. Here, we compare sediments from seismically active and passive margins from scientific ocean drilling sites that exhibit significant shear strength differences. Active margin sites are Nankai (Site C0001), Cascadia (Site 1054), and Southern Alaska (Site U1418), and passive margin sites are Amazon Fan (Site 942), North Carolina Slope (Site 1054), and New Jersey (Site 1073). From each site, we sampled 500 g of sediment equally distributed throughout the top 100 mbsf. We combined samples to create a representative bulk sample per continental margin and reconstituted them with saltwater that matched field-measured salinity. We measured particle size (hydrometer), plasticity states (Atterberg limits), mineralogy (powder X-ray diffraction), compression behavior and permeability (1-D resedimentation experiments), and undrained shear strength (fall cone device). All samples are siliciclastic marine mud that classify as silty clay or clayey silt. Despite the apparent similarity in lithology, sand fraction varies from 0.8 wt. % (Amazon) to 10.3 wt. % (N. Carolina) and clay fraction (<2 mm) varies from 37.7 wt. % (N. Carolina) to 56.0 wt. % (Amazon). Void ratios, measured in resedimentation experiments range from 1.6 (porosity = 62%) (Nankai) to 1.0 (porosity = 50%) (S. Alaska) at a vertical effective stress of 100 kPa. Resedimentation experiments are followed by consolidation to 1 MPa (equivalent to 100 meters of burial depth) and undrained shear strength measurements, which are compared with field-measured shear strengths. We find the previously observed strengthening effect observed in the active margin field- strength is no longer present in the lab-strengths. This suggests that the exposure to seismicity in the field is potentially leading to enhanced shear strength during early burial.

Abstract Micro‐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.

The 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.

Early diagenetic precipitation of authigenic carbonate has been a globally significant carbon sink throughout Earth history. In particular, SO42- and Fe3- reduction and CH4 production create conditions in pore fluids that promote carbonate mineral precipitation; however, these conditions may be modified by the presence of acid-base buffers such as clay minerals. We integrated the acid-base properties of clay minerals into a biogeochemical model that predicts the evolution of pore-water pH and carbonate mineral saturation during O-2, Fe3+, and SO42- reduction and CH4 production. Key model inputs were obtained using two natural clay mineral-rich sediments from the Integrated Ocean Drilling Program as well as from literature. We found that clay minerals can enhance carbonate mineral saturation during O-2 and SO42- reduction and moderate saturation during Fe3+ reduction and CH4 production if the pore-fluid pH and clay mineral pK, values are within similar to 2 log units of one another. We therefore suggest that clay minerals could significantly modify the environmental conditions and settings in which early diagenetic carbonate precipitation occurs. In Phanerozoic marine sediments-where O-2 and SO42- have been the main oxidants of marine sedimentary organic carbon-clay minerals have likely inhibited carbonate dissolution and promoted precipitation of authigenic carbonate.

The permeability of shallow marine sediments is an extremely important parameter to constrain, as it affects fluid and nutrient transport near the sediment-water interface, mediates mass exchange between igneous basement and oceans, and plays a role in seismicity along convergent margins. Determining the permeability of these sediments in the laboratory is difficult because existing methods typically require fully saturated, intact samples of large volume (tens of cm(3)), which are usually not collected with high spatial resolution in scientific ocean drilling operations. We demonstrate how mercury injection capillary pressure (MICP) data may be used to predict the permeability of marine muds using a modification of the widely used Swanson method. Our results show that MICP measurements performed on small, irregular, and, most importantly, unpreserved samples can yield important permeability information. This will improve the spatial resolution of permeability data in the shallow marine subsurface and allow analyses to be performed on the significant quantities of existing legacy core.

Four different empirical models used to describe the one-dimensional normal compression behavior of mudrocks have been evaluated by investigating the behavior of 15 different resedimented mudrocks. These mudrocks originate from a wide variety of geologic origins and were tested in the laboratory over an effective stress range of 0.1-100 MPa. The normal compression of silt-rich, low liquid limit mudrocks is best described by assuming a log-linear relationship between vertical effective stress, sigma(v)' and void ratio. For smectite-rich, higher liquid limit mudrocks their behavior is better described by assuming a log-linear relationship between sigma(v)', and porosity. The assumption of a log-log relationship between (1 + void ratio) and sigma(v)' is not the most appropriate for any particular mudrock type, but gives a reasonably good description of compression behavior for all mudrocks, and is therefore ideal for situations in which the general composition of a sediment is unknown. For all mudrocks, the assumption of an exponential relationship between porosity and sigma(v) gives a very poor fit to the experimental data. As mudrocks undergo compression to high effective stresses, high liquid limit mudrocks display a much greater compressibility and a corresponding larger loss in porosity compared to more silt-rich mudrocks. The porosities of all materials tend to converge into a much narrower range above about 10 MPa, where all mudrocks display a similar compression behavior and a constant value of compression index of 0.21 regardless of their composition. At low effective stresses though, their compression behavior is strongly controlled by composition.

Abstract Muds and mudrocks are important barriers to flow and perform this role over their entire depositional history, from mediating sediment‐ocean methane exchange to preventing leakage from hydrocarbon reservoirs, CO2 sequestration targets, or nuclear waste repositories. It is well established that the percolation threshold, which is the fluid saturation at which percolation occurs, is directly related to the pressure sealing capacity of a mud or mudrock. However, the evolution of the percolation threshold during burial and diagenesis is not well understood. Using a data set of natural muds and mudrocks and resedimented laboratory mixtures, we show that the percolation threshold is strongly controlled by porosity, achieving a minimum of 0.10–0.30 at a porosity of 0.40, and increasing as porosity increases or decreases from that value, to as high as 0.60 at deposition and >0.80 at porosity <0.10. This implies a highly disconnected pore network at high and low porosities.

Slow slip on preexisting faults during hydraulic fracturing is a process that significantly influences shale gas production in extremely low permeability “shale” (unconventional) reservoirs. We experimentally examined the impacts of mineralogy, surface roughness, and effective stress on permeability evolution of slowly slipping faults in Eagle Ford shale samples. Our results show that fault permeability decreases with slip at higher effective stress but increases with slip at lower effective stress. The permeabilities of saw cut faults fully recover after cycling effective stress from 2.5 to 17.5 to 2.5 MPa and increase with slip at constant effective stress due to asperity damage and dilation associated with slip. However, the permeabilities of natural faults only partially recover after cycling effective stress returns to 2.5 MPa and decrease with slip due to produced gouge blocking fluid flow pathways. Our results suggest that slowly slipping faults have the potential to enhance reservoir stimulation in extremely low permeability reservoirs.