Site C0002 Research Articles

Thermal properties and thermal structure in the deep-water coalbed basin off the Shimokita Peninsula, Japan

Heat transport properties are significant parameters that affect geothermal processes in deep sedimentary basins. We measured the thermal properties of deep sediment core samples to a depth of approximately 2500 m below sea floor (mbsf), whereas most previous studies have only presented data <800 mbsf for marine sediments, taken from the basin off Shimokita, Japan during Integrated Ocean Drilling Program (IODP) Expedition 337. Thermal conductivity and thermal diffusivity increased with depth, and heat capacity decreased with depth, although the data were highly scattered at greater depths. The increase in thermal conductivity is explained primarily by the porosity reduction of sediment resulting from consolidation during sedimentation. The high variations in thermal conductivity at the similar depth are reflected by various lithological rocks formed at the same core section. Coal exhibits the lowest thermal conductivity of 0.4 Wm−1 K−1, and calcite-cemented sandstone/siltstone exhibits the highest conductivity of approximately 3 Wm−1 K−1. The other thermal properties are also influenced by the porosity and lithological contrast. The correlation between thermal conductivity and porosity is explained by the mixed-law model of the geometric mean.The one-dimensional temperature-depth profile at site C0020 estimated from measured thermal conductivity reveals a regression of the thermal gradient with depth, and the surface heat flow of 30 mWm−2, which is consistent with the heat flow data near this site, was evaluated. The obtained results suggest that the linear geothermal gradient estimated from the average thermal conductivity value at shallow sediment materials could overestimate the temperature at greater depths and underestimate the heat flow.

IODP掘削地点C0002の海底下1000~3000 mから採取された、南海トラフ付加体泥質堆積物試料の摩擦特性

IODP掘削地点C0002の海底下1000~3000 mから採取された、南海トラフ付加体泥質堆積物試料の摩擦特性

Variability in coal facies as reflected by organic petrological and geochemical data in Cenozoic coal beds offshore Shimokita (Japan) - IODP Exp. 337

This study reports on Cenozoic coal seams recovered at Integrated Ocean Drilling Program (IODP) Site C0020 during Expedition 337. IODP Site C0020 is located in a forearc basin formed by the subduction of the Pacific plate off Shimokita Peninsula (Japan). Hole C0020A penetrated 14 coal layers between 1825 and 2466mbsf. Eleven of them were investigated within the frame of this paper. Investigated seams show a slight maturity increase with depth from lignite to sub-bituminous coal. In order to detect temporal changes in maceral and molecular composition and to relate them to changes in vegetation and depositional environment, macro- and micropetrographic data, bulk geochemical parameters, biomarker analysis, stable isotope geochemistry, and vitrinite reflectance measurements were performed. Results were also compared with palynological data obtained from 9 coal samples.Elevated sulfur contents and high ash yields occur in the upper seams (cores 14R to 18R) whereas low sulfur contents and varying ash yields were determined for the lower coal seams (24R to 30R). The maceral composition and biomarker ratios of the uppermost seams argue for coal formation in a paralic environment and brackish, alkaline water conditions. In contrast, lignite samples from the lower part of unit III point to a limnic-fluviatile deposition. Conifers contributed significantly to peat formation in the uppermost seams (from cores 15R to 22R) and in the lowermost lignite seam. In all other samples, angiosperms are considered as the major peat-forming plants. The pollen and spore floras indicate rich angiosperm vegetation, however significant contributions from Pinaceae and Taxodiaceae are evident for all coals. Sporophytes have no dominant influence on the coal flora. Microbial activity in the peat is suggested for instance by higher concentrations of hop-17(21)-ene with increasing contents of hopanes or by low δ13C values of hop-17(21)-ene. The revealed changes in the environment during coal deposition highlight the importance of combined organic petrography, organic geochemistry and palynology to reconstruct palaeoenvironmental conditions.

Crustal fluid and ash alteration impacts on the biosphere of Shikoku Basin sediments, Nankai Trough, Japan.

We present data from sediment cores collected from IODP Site C0012 in the Shikoku Basin. Our site lies at the Nankai Trough, just prior to subduction of the 19 Ma Philippine Sea plate. Our data indicate that the sedimentary package is undergoing multiple routes of electron transport and that these differing pathways for oxidant supply generate a complex array of metabolic routes and microbial communities involved in carbon cycling. Numerical simulations matched to pore water data document that Ca(2+) and Cl(1-) are largely supplied via diffusion from a high-salinity (44.5 psu) basement fluid, which supports the presence of halophile Archean communities within the deep sedimentary package that are not observed in shallow sediments. Sulfate supply from basement supports anaerobic oxidation of methane (AOM) at a rate of ~0.2 pmol cm(-3) day(-1) at ~400 mbsf. We also note the disappearance of δ-Proteobacteria at 434 mbsf, coincident with the maximum in methane concentration, and their reappearance at 463 mbsf, coinciding with the observed deeper increase in sulfate concentration toward the basement. We did not, however, find ANME representatives in any of the samples analyzed (from 340 to 463 mbsf). The lack of ANME may be due to an overshadowing effect from the more dominant archaeal phylotypes or may indicate involvement of unknown groups of archaea in AOM (i.e., unclassified Euryarchaeota). In addition to the supply of sulfate from a basement aquifer, the deep biosphere at this site is also influenced by an elevated supply of reactive iron (up to 143 μmol g(-1)) and manganese (up to 20 μmol g(-1)). The effect of these metal oxides on the sulfur cycle is inferred from an accompanying sulfur isotope fractionation much smaller than expected from traditional sulfate-reducing pathways. The detection of the manganese- and iron-reducer γ-Proteobacteria Alteromonas at 367 mbsf is consistent with these geochemical inferences.

Early to Middle Pleistocene paleoceanographic history of southern Japan based on radiolarian data from IODP Exp. 314/315 Sites C0001 and C0002

During the mid-Pleistocene Transition (MPT), spanning from 0.8 to 1.2 Ma, the prevailing 41 kyr glacial cycles were replaced by longer cycles with a period of ca. 100 kyr. This event was also accompanied by important climate changes, such as the expansion of the Northern Hemisphere Ice Sheet. The impact of the MPT on the Kuroshio Current off southern Japan remains unknown, however, in the absence of a long core covering the period of the MPT. In this area, preservation of calcareous microfossils is poor, but polycystine radiolarians are abundant and well preserved. Therefore, they are an excellent alternative proxy for reconstructing past climate change. Indeed, because numerous studies have examined the geographic distribution of modern radiolarian species, the relationships between several radiolarian assemblage groups (e.g., equatorial to sub-arctic assemblage groups) and climatic conditions are well understood. We examined the impact of the MPT on the Kuroshio Current off the Kii Peninsula, southern Japan, by investigating radiolarian assemblages in deep-sea cores collected during Integrated Ocean Drilling Program Expedition 314/315. We studied the radiolarian assemblages in 293 samples from cores obtained at Site C0001 Holes E and F and Site C0002 Hole D. The age models for these cores, which are based on biostratigraphy and chemostratigraphy, showed that sediments from the period between Marine Isotope Stage (MIS) 15 and MIS 8, which includes the Mid-Brunhes Event, are missing from these cores. However, continuous data are available between MIS 63 (1.82 Ma) and MIS 15 (0.5 Ma). Therefore, using the ecological properties of 11 key radiolarian species, we reconstructed regional paleoceanographic changes between 1.82 and 0.5 Ma. Radiolarian fauna revealed that the relative abundance of Kuroshio and the Central Pacific water taxa (e.g., Didymocyrtis tetrathalamus and the Polysolenia spinosa/lappacea group, respectively) increased drastically during the MPT, possibly indicating a northward shift of the Kuroshio at that time. The relative abundance of Rhizosphaera mediana, an Oyashio/Kuroshio mixed-water species, also increased during this interval. Thus, a southward shift of the Oyashio Current may have occurred synchronously with a northward shift of the Kuroshio during the MPT.

Early sedimentation and deformation in the Kumano forearc basin linked with Nankai accretionary prism evolution, southwest Japan

AbstractEarly forearc basin sedimentation and subsidence appear to be linked with convergent margin evolution, yet they do not follow predictable spatiotemporal patterns. In the Kumano basin of southwest Japan, 3‐D seismic mapping of major unconformities, combined with biostratigraphic age constraints from two Integrated Ocean Drilling Program (IODP) drill sites illustrates the development of several tectonostratigraphic packages during early evolution and initial creation of accommodation space in the forearc basin. Between ∼3.8 Ma and ∼2.06 Ma, a series of slope basins developed between thrust‐anticlines oriented along a plate convergence‐perpendicular axis. A thin and sheet‐like interval, likely slope‐cover, unconformably overlies the slope basin sediments. Deposition of this interval at IODP Site C0009 lasted from ∼2.06 Ma until between ∼1.24 Ma and ∼1.34 Ma and was partly coeval with the early upper Kumano basin sedimentary packages. Landward, postdating the slope‐cover sediments, a third sedimentary package was deposited before ∼0.9 Ma. This section correlates with an interval rich in terrigenous material (e.g., woody debris), indicating a possible distinct source or sediment routing system. Our work supports the idea that early forearc basin sediments may be deposited in an actively deforming outer wedge environment and that the outer‐to‐inner wedge transition of the lower forearc basin is likely a multistage process. The unsteady spatiotemporal nature of inner‐outer wedge coupling may lead to erratic stratigraphic patterns, such as have been observed in forearc basins worldwide.

Sr, Nd, and Pb isotope compositions of hemipelagic sediment in the Shikoku Basin: Implications for sediment transport by the Kuroshio and Philippine Sea plate motion in the late Cenozoic

The provenance of hemipelagic sediments in the Shikoku Basin during late Cenozoic time was studied through the temporal variations in the Sr–Nd–Pb isotopic compositions of detrital sediments from Integrated Ocean Drilling Program Site C0011 from the late Miocene (7 Ma) to the present. Detrital sediments at Site C0011 are interpreted as a mixture of sediments originating from the southwest Japan arc and lands around the East China Sea. Sediments from the East China Sea were transported by the Kuroshio, while Japanese sediments were transported by turbidity currents, bottom currents, and ocean surface currents. The isotopic evidence suggests that the main source of hemipelagic sediments gradually changed from the East China Sea to Japan from 4.4 to 2.9 Ma, in accordance with the northward movement of Site C0011 with the Philippine Sea plate in this period. A contemporaneous increase in grain size also supports this interpretation. The beginning period of these changes, 4.4 Ma, conforms closely to the postulated advent or acceleration of trench-normal subduction of the Shikoku Basin lithosphere.

Title Physical properties of Nankai Accretionary prism sediments at Site C0002, IODP Expedition 348

Title Physical properties of Nankai Accretionary prism sediments at Site C0002, IODP Expedition 348

Stress-state and micro-scale deformations in the hangingwall of the splay fault: Preliminary results from riser drilling at the Site C0002, IODP Expedition 348

Stress-state and micro-scale deformations in the hangingwall of the splay fault: Preliminary results from riser drilling at the Site C0002, IODP Expedition 348

Pore size controls on the base of the methane hydrate stability zone in the Kumano Basin, offshore Japan

AbstractThe base of the methane hydrate stability zone (MHSZ) in the Kumano Basin, offshore Japan, is marked by a bottom‐simulating reflection (BSR) on seismic data. At Integrated Ocean Drilling Program Site C0002, which penetrates this BSR, the in situ temperature profile combined with bulk seawater methane equilibrium conditions suggest that the base of the MHSZ is 428 m below seafloor (bsf), which is 28 m deeper than the observed BSR (400 m bsf). We found that submicron pore sizes determined by mercury injection capillary pressure are sufficiently small to cause 64% of the observed uplift of the base of the MHSZ by the Gibbs‐Thomson effect. This is the most thorough characterization of pore sizes within the MHSZ performed to date and illustrates the extent to which pore size can influence MHSZ thickness. Our results demonstrate the importance of considering lithology and pore structure when assessing methane hydrate stability conditions in marine sediments.

Re-evaluation of temperature at the updip limit of locked portion of Nankai megasplay inferred from IODP Site C0002 temperature observatory

In 2010, the first long-term borehole monitoring system was deployed at approximately 900 m below the sea floor (mbsf) and was assumed to be situated above the updip limit of the seismogenic zone in the Nankai Trough off Kumano (Site C0002). Four temperature records show that the effect of drilling diminished in less than 2 years. Based on in situ temperatures and thermal conductivities measured on core samples, the temperature measurements and heat flow at 900 mbsf are estimated to be 37.9°C and 56 ± 1 mW/m2, respectively. This heat flow value is in excellent agreement with that from the shallow borehole temperature corrected for rapid sedimentation in the Kumano Basin. We use these values in the present study to extrapolate the temperature below 900 mbsf for a megasplay fault at approximately 5,200 mbsf and a plate boundary fault at approximately 7,000 mbsf. To extrapolate the temperature downward, we use logging-while-drilling (LWD) bit resistivity data as a proxy for porosity and estimate thermal conductivity from this porosity using a geometrical mean model. The one-dimensional (1-D) thermal conduction model used for the extrapolation includes radioactive heat and frictional heat production at the plate boundary fault. The estimated temperature at the megasplay ranges from 132°C to 149°C, depending on the assumed thermal conductivity and radioactive heat production values. These values are significantly higher, by up to 40°C, than some of previous two-dimensional (2-D) numerical model predictions that can account for the high heat flow seaward of the deformation front, including a hydrothermal circulation within the subducted igneous oceanic crust. However, our results are in good agreement with those of the 2-D model, which does not include the advection cooling effect. The results imply that 2-D geometrical effects as well as the influence of the advective cooling may be critical and should be evaluated more quantitatively. Revision of 2-D simulation by introducing our new boundary conditions (37.9°C of in situ temperature at 900 mbsf and approximately 56 mW/m2 heat flow) will be essential. Ultimately, in situ temperature measurements at the megasplay fault are required to understand seismogenesis in the Nankai subduction zone.

Pressure dependence of fluid transport properties of shallow fault systems in the Nankai subduction zone

We measured fluid transport properties at an effective pressure of 40 MPa in core samples of sediments and fault rocks collected by the Integrated Ocean Drilling Program (IODP) NanTroSEIZE drilling project Expedition 316 from the megasplay fault system (site C0004) and the frontal thrust (site C0007) in the Nankai subduction zone. Permeability decreased with effective pressure as a power law function. Permeability values in the fault zones were 8 × 10−18 m2 at site C0004 and 9 × 10−18 m2 at site C0007. Stratigraphic variation in transport properties suggests that the megasplay fault zone may act as a barrier to fluid flow, but the frontal thrust fault zone might not. Depth variation in permeability at site C0007 is probably controlled by the mechanical compaction of sediment. Hydraulic diffusivity at shallow depths was approximately 1 × 10−6 m2 s−1 in both fault zones, which is small enough to lead to pore pressure generation that can cause dynamic fault weakening. However, absence of a very low permeable zone, which may have formed in the Japan Trench subduction zone, might prevent facilitation of huge shallow slips during Nankai subduction zone earthquakes. Porosity tests under dry conditions might have overestimated the porosity.

Consolidation state of incoming sediments to the Nankai Trough subduction zone: Implications for sediment deformation and properties

AbstractThe hydromechanical properties of accreted and underthrust sediments are key parameters controlling the mechanics of earthquakes and the development of fluid pressure in subduction zones. We conducted consolidation tests on sediments from the Philippine Sea Plate (PSP) in the Nankai Trough to understand the consolidation state and hydraulic properties of the incoming sediment section before its incorporation into the subduction zone. We used mudstone and sandstone cores sampled from the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment at two reference sites (Site C0011 located on a basement low; and Site C0012 located on a basement high). Our experimental results indicate that most of the mudstone samples are normally consolidated or overconsolidated, with overconsolidation ratios (OCR) ranging from 0.89 to 2.52 at Site C0011 and 0.86 to 3.85 at Site C0012. Higher OCR values at Site C0012, at least at shallow depths, are likely caused by erosional unloading. This implies that Site C0011 may serve as a better geotechnical reference site. We also find that mudstones accreted along the frontal thrust are severely overconsolidated relative to coeval mudstones at Site C0011, which likely reflects enhanced consolidation due to increased horizontal tectonic stress. Sandstones in the incoming section on the PSP exhibit 2–3 orders of magnitude higher in situ permeability than the mudstones, and the siliciclastic sandstone we tested maintains a high permeability at stresses up to at least 70 MPa, suggesting that the sandstones may act as important pathways for drainage or pore pressure translation from depths of several kilometers.

Hydrocarbon seepage and its sources at mud volcanoes of the Kumano forearc basin, Nankai Trough subduction zone

AbstractTwelve submarine mud volcanoes (MV) in the Kumano forearc basin within the Nankai Trough subduction zone were investigated for hydrocarbon origins and fluid dynamics. Gas hydrates diagnostic for methane concentrations exceeding solubilities were recovered from MVs 2, 4, 5, and 10. Molecular ratios (C1/C2 &lt; 250) and stable carbon isotopic compositions (δ13C‐CH4 &gt;−40‰ V‐PDB) indicate that hydrate‐bound hydrocarbons (HCs) at MVs 2, 4, and 10 are derived from thermal cracking of organic matter. Considering thermal gradients at the nearby IODP Sites C0009 and C0002, the likely formation depth of such HCs ranges between 2300 and 4300 m below seafloor (mbsf). With respect to basin sediment thickness and the minimum distance to the top of the plate boundary thrust we propose that the majority of HCs fueling the MVs is derived from sediments of the Cretaceous to Tertiary Shimanto belt below Pliocene/Pleistocene to recent basin sediments. Considering their sizes and appearances hydrates are suggested to be relicts of higher MV activity in the past, although the sporadic presence of vesicomyid clams at MV 2 showed that fluid migration is sufficient to nourish chemosynthesis‐based organisms in places. Distributions of dissolved methane at MVs 3, 4, 5, and 8 pointed at fluid supply through one or few MV conduits and effective methane oxidation in the immediate subsurface. The aged nature of the hydrates suggests that the major portion of methane immediately below the top of the methane‐containing sediment interval is fueled by current hydrate dissolution rather than active migration from greater depth.

Gas hydrate saturation at Site C0002, IODP Expeditions 314 and 315, in the Kumano Basin, Nankai trough

AbstractThe degree of gas hydrate saturation at Integrated Ocean Drilling Program (IODP) Site C0002 in the Kumano Basin, Nankai Trough, was estimated from logging‐while‐drilling logs and core samples obtained during IODP Expeditions 314 and 315. Sediment porosity data necessary for the calculation of saturation were obtained from both core samples and density logs. Two forms of the Archie equation (‘quick‐look’ and ‘standard’) were used to calculate gas hydrate saturation from two types of electrical resistivity log data (ring resistivity and bit resistivity), and a three‐phase Biot‐type equation was used to calculate gas hydrate saturation from P‐wave velocity log data. The gas hydrate saturation baseline calculated from both resistivity logs ranges from 0% to 35%, and that calculated from the P‐wave velocity log ranges from 0% to 30%. High levels of gas hydrate saturation (&gt;60%) are present as spikes in the ring resistivity log and correspond to the presence of gas hydrate concentrations within sandy layers. At several depths, saturation values obtained from P‐wave velocity data are lower than those obtained from bit resistivity data; this discrepancy is related to the presence of free gas at these depths. Previous research has suggested that gas from deep levels in the Kumano Basin has migrated up‐dip towards the southern and seaward edge of the basin near Site C0002. The high saturation values and presence of free gas at site C0002 suggest that a large gas flux is flowing to the southern and seaward edge of the basin from a deeper and/or more landward part of the Kumano Basin, with the southern edge of the Kumano Basin (the location of site C0002) being the main area of fluid accumulation.

Nuclear magnetic resonance characterization of shallow marine sediments from the Nankai Trough, Integrated Ocean Drilling Program Expedition 333

AbstractWe measured nuclear magnetic resonance (NMR) relaxation times on samples from Integrated Ocean Drilling Program Expedition 333 Sites C0011, C0012, and C0018. We compared our results to permeability, grain size, and specific surface measurements, pore size distributions from mercury injection capillary pressure, and mineralogy from X‐ray fluorescence. We found that permeability could be predicted from NMR measurements by including grain size and specific surface to quantify pore networks and that grain size is the most important factor in relating NMR response to permeability. Samples within zones of anomalously high porosity from Sites C0011 and C0012 were found to have different NMR‐permeability relationships than samples from outside these zones, suggesting that the porosity anomaly is related to a fundamental difference in pore structure. We additionally estimated the size of paramagnetic sites that cause proton relaxation and found that in most of our samples, paramagnetic material is present mainly as discrete, clay‐sized grains. This distribution of paramagnetic material may cause pronounced heterogeneity in NMR properties at the pore scale that is not accounted for in most NMR interpretation techniques. Our results provide important insight into the microstructure of marine sediments in the Nankai Trough.

Thermal and transport properties of the sedimentary rocks at Site C0020, IODP Expedition 337 in Sanriku-oki basin off Shimokita Peninsula

Thermal and transport properties of the sedimentary rocks at Site C0020, IODP Expedition 337 in Sanriku-oki basin off Shimokita Peninsula

Radiolarian biostratigraphic scheme and stable oxygen isotope stratigraphy in southern Japan (IODP Expedition 315 Site C0001)

Radiolarian biostratigraphic scheme and stable oxygen isotope stratigraphy in southern Japan (IODP Expedition 315 Site C0001)

Mid-Quaternary decoupling of sediment routing in the Nankai Forearc revealed by provenance analysis of turbiditic sands

Coring during Integrated Ocean Drilling Program Expeditions 315, 316, and 333 recovered turbiditic sands from the forearc Kumano Basin (Site C0002), a Quaternary slope basin (Site C0018), and uplifted trench wedge (Site C0006) along the Kumano Transect of the Nankai Trough accretionary wedge offshore of southwest Japan. The compositions of the submarine turbiditic sands here are investigated in terms of bulk and heavy mineral modal compositions to identify their provenance and dispersal mechanisms, as they may reflect changes in regional tectonics during the past ca. 1.5 Myrs. The results show a marked change in the detrital signature and heavy mineral composition in the forearc and slope basin facies around 1 Ma. This sudden change is interpreted to reflect a major change in the sand provenance, rather than heavy mineral dissolution and/or diagenetic effects, in response to changing tectonics and sedimentation patterns. In the trench-slope basin, the sands older than 1 Ma were probably eroded from the exposed Cretaceous–Tertiary accretionary complex of the Shimanto Belt and transported via the former course of the Tenryu submarine canyon system, which today enters the Nankai Trough northeast of the study area. In contrast, the high abundance of volcanic lithics and volcanic heavy mineral suites of the sands younger than 1 Ma points to a strong volcanic component of sediment derived from the Izu-Honshu collision zones and probably funnelled to this site through the Suruga Canyon. However, sands in the forearc basin show persistent presence of blue sodic amphiboles across the 1 Ma boundary, indicating continuous flux of sediments from the Kumano/Kinokawa River. This implies that the sands in the older turbidites were transported by transverse flow down the slope. The slope basin facies then switched to reflect longitudinal flow around 1 Ma, when the turbiditic sand tapped a volcanic provenance in the Izu-Honshu collision zone, while the sediments transported transversely became confined in the Kumano Basin. Therefore, the change in the depositional systems around 1 Ma is a manifestation of the decoupling of the sediment routing pattern from transverse to long-distance axial flow in response to forearc high uplift along the megasplay fault.

Long-distance fluid and heat transport in the oceanic crust entering the Nankai subduction zone, NanTroSEIZE transect

I examine the potential causes of anomalous seafloor heat flux on the oceanic plate in the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) transect offshore southern Japan. The most prominent anomaly is a ∼50 mWm−2 change in heat flux between Integrated Ocean Drilling Program Sites C0011 and C0012 over a distance of <10 km. I develop thermal models to investigate the effects of hydrothermal circulation in the basaltic basement of the oceanic crust and variations in heat input from the mantle. The <10 km wide transition in surface heat flux reflects a process in the shallow subsurface; variations in heat input from ⩾5 km depth would generate a >30 km wide transition at the seafloor. The observed surface heat flux pattern is indicative of hydrothermal circulation in the basement aquifer and advection of heat from the subducted crust into the aquifer on the incoming plate. For a 600 m thick aquifer, the permeability is likely ⩾7×10−11 m2, and hydrothermal circulation transports at least 300 times more heat than conduction alone. The heat flux from the subduction zone seaward to the incoming plate is consistent with hydrothermal circulation in the subducting crust persisting to ∼100 km landward of the deformation front. Vigorous fluid circulation in the basaltic basement is consistent with both the seafloor thermal anomalies and geochemical anomalies near the sediment–basement interface.