Abstract
At Integrated Ocean Drilling Program (IODP) Site C0008 in the Nankai Trough slope sediment, we discovered in situ temperature anomalies at 80 to 160 m below the seafloor, where pore fluid Cl and δ 18O excursions were identified and interpreted as pore fluid refreshing due to hydrate dissociation. The volume fraction of hydrates is estimated to be approximately 3% and approximately 40% maximum at Holes C0008A and C0008C, respectively. In the vicinity of these anomalies, we discovered negative and positive temperature excursions of up to 1 K measured in situ using the Advanced Piston Corer Temperature (APC-T) tool attached to the shoe of a hydraulic piston corer. They are significantly larger than the uncertainties caused during data acquisition and processing. Frictional heat due to penetration increased the temperature by >10 K, exceeding the gas/hydrate stability temperature at that depth. This heat is partly consumed by hydrate dissociation, which disturbs the thermal decay curve after penetration, but 2D numerical modeling revealed that hydrate dissociation does not significantly change the extrapolated equilibrium temperature. So far, we cannot suggest any acceptable explanation for the observed thermal anomalies, although we strongly suspect that it is related to hydrate dissociation.
Highlights
At Integrated Ocean Drilling Program (IODP) Site C0008 in the Nankai Trough slope sediment, we discovered in situ temperature anomalies at 80 to 160 m below the seafloor, where pore fluid Cl and δ18O excursions were identified and interpreted as pore fluid refreshing due to hydrate dissociation
As part of the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiments (NanTroSEIZE; e.g., Tobin and Kinoshita 2006), shallow holes were drilled at Site C0008 on the slope sediment 1 km seaward of Site C0004, penetrating the mega-splay fault (Figure 1), in order to characterize the lithological features and history of the slope sediment near shallow faults (Expedition 316 Scientists, 2009)
Bottom-simulating reflectors (BSRs) that mark the top of the free gas zone beneath the hydrate stability zone are intermittently identified in the slope sediment or
Summary
At Integrated Ocean Drilling Program (IODP) Site C0008 in the Nankai Trough slope sediment, we discovered in situ temperature anomalies at 80 to 160 m below the seafloor, where pore fluid Cl and δ18O excursions were identified and interpreted as pore fluid refreshing due to hydrate dissociation. We explore the possible relationship between thermal excursion and dissociation of gas hydrates upon penetration. These negative Cl excursions likely reflect pore fluid freshening because of the dissociation of gas hydrates when the core samples that include hydrates are recovered (Expedition 316 Scientists, 2009).
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