Title:

Reconstructing Paleo-Sulfate Methane Transition Zones (SMTZs) in the Japan Trench

In subduction zone trenches, the movement of pore fluids, as well as slope failure, are likely to occur during earthquake ruptures and leave evidence in the sedimentary record. In the Japan Trench, slope failures are well correlated to historic subduction zone earthquakes. During recent IODP Expedition 386, 40 m long piston cores were recovered from 15 sites located in basins along the Japan Trench. These cores provide a new and potentially longer term record of paleoseismic activity. With these cores, we aim to reconstruct the positions of paleo-SMTZs throughout the sediment layers by analyzing total sulfur (TS), δ34STS, and total organic carbon (TOC). These measurements will help us understand the response of paleo-SMTZs to sedimentation and fluid advection induced by past subduction zone earthquakes. Initial shipboard measurements of TS and TOC suggest that organoclastic sulfate reduction (OSR) and anaerobic oxidation of methane (AOM) are the primary processes influencing the position of the SMTZ through time. We present high-resolution TS records for 15 sites, located in the northern, middle, and southern parts of the Japan Trench respectively. In some of these records, consistently low TS suggests OSR dominance and little to no overprinting by the SMTZ. In other records, elevated sulfur corresponds with the modern SMTZ and suggests paleo-SMTZ positions. Downcore patterns of TOC and TS are well correlated, suggesting OSR is the primary control on TS content. However, the presence of methane in these cores, and elevated sulfur relative to TOC, suggest AOM also influences TS content. Pending measurements of δ34STS in these same records will reveal the relative influence of AOM and OSR and confirm past SMTZ positions. We will integrate our new findings with the known sedimentary deposits associated with earthquake activity and the pore fluid geochemistry of these cores to identify potential factors driving changes in the SMTZ over time. These results will ultimately reveal spatial patterns that can be correlated with past megathrust ruptures in the Japan Trench.

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