Thermal conductivity profilein the Nankaiaccretionary prism at IODP NanTroSEIZE Site C0002: estimationsfromhigh-pressure experiments using input site sediments

Depth profiles of sediment thermal conductivity are required for understanding the thermal structure in active seismogenic zones. During the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a scientific drilling project of the International Ocean Discovery Program (IODP), a borehole was penetrated to a depth of 3, 262.5 m below seafloor (mbsf) at Site C0002. Because core samples obtained from below ~1, 100 mbsf in an accretionary prism are limited, a thermal conductivity profile over such depths usually determined by laboratory measurements using core samples is not available. To obtain the thermal conductivity profile at Site C0002, we used core samples collected from sediments that overlay the incoming subducting oceanic basement at Nankai Trough Seismogenic Zone Experiment Site C0012, which can be considered to have the same mineral composition as the accretional prism at Site C0002. The thermal conductivity of the C0012 core samples was measured at high pressure to simulate subduction by reducing the sample porosity. We measured the thermal conductivity of six core samples from 144–518 mbsf at Site C0012 up to a maximum effective pressure of ~50 MPa, corresponding to depths greater than ~4 km below seafloor. We obtained an empirical relation between thermal conductivity λBulk in Wm⁻¹K⁻¹ and fractional porosity ϕ for the Nankai Trough accretionary prism as λBulk = exp(−1.09ϕ + 0.977). Based on porosity data measured using core/cuttings samples and data derived from P wave velocity logs, we estimate two consistent and complete thermal conductivity profiles down to ~3 km below seafloor in the Nankai Trough accretionary prism. These profiles are consistent with the existing thermal conductivity data measured using limited core samples

Geophysical constraints on microbial biomass in subseafloor sediments and coal seams down to 2.5 km off Shimokita Peninsula, Japan

Abstract To understand the ability of microbial life to inhabit a deep subseafloor coalbed sedimentary basin, the correlation between fluid transport properties and the abundance of microbial cells was investigated based on core samples collected down to about 2.5 km below the seafloor during the Integrated Ocean Drilling Program Expedition 337 off the Shimokita Peninsula, Japan. The overall depth profiles for porosity and permeability exhibited a decreasing trend with increasing depth. However, at depths greater than 1.2 km beneath the seafloor, the transport characteristics of the sediments were highly variable, with the permeability ranging from 10−16 to 10−22 m2 and the pore size ranging from < 0.01 to 100 μm. This is mainly attributed to the diversity of the lithology, which exhibits a range of pore sizes and pore geometries. Fracture channels in coal seams had the highest permeability, while shale deposits had the smallest pore size and lowest permeability. A positive correlation between permeability and pore size was confirmed by the Kozeny-Carman equation. Cell abundance at shallower depths was positively correlated with porosity and permeability, and was less strongly correlated with pore size. These findings suggest that one of the factors affecting the decrease in microbial cell abundance with increasing depth was a reduction in nutrient and water supply to indigenous microbial communities as a result of a decrease in porosity and permeability due to sediment compaction. Anomalous regions with relatively high cell concentrations in coal-bearing units could be explained by the higher permeability and larger pore size for these units compared to the surrounding sediments. Nutrient transport through permeable cleats in coal layers might occur upwards toward the upper permeable sandstone layers, which are well suited for sustaining sizable microbial populations. Conversely, impermeable shale and siltstone with small pores (< 0.2 μm, which is smaller than microbial cell size) may act as barriers to water and energy-yielding substrates for deep microbial life. We propose that the pore size and permeability govern the threshold for microbial habitability in the deep subseafloor sedimentary biosphere