Organic matter diagenesis in hadal setting: Insights from the pore-water geochemistry of the Mariana Trench sediments

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The diagenetic processes of organic matter (OM) in hadal sedimentary environments (>6000 m water depth) are much less studied than those in other accessible environments. In this study, we quantify OM oxidation processes using pore-water geochemical measurements, fluxes, and diagenetic rate calculations in sediments along a depth transect from abyssal to hadal sites (5500–10257 m) in the Mariana Trench. The total benthic O2 consumption, and depth-integrated rates of nitrification and denitrification are positively correlated with water depth, indicating that OM diagenesis is enhanced in the deep sites of the hadal zone. The negative linear correlation between water depth and oxygen/nitrate penetration depths further supports this conclusion. In the abyssal sites, aerobic respiration dominates OM degradation, and anaerobic processes are negligible. In contrast, denitrification plays an important role in anaerobic OM degradation, and accounts for approximately 5% of the depth-integrated total OM mineralization at the deepest hadal site. Moreover, our results suggest that direct coupling between nitrification and denitrification is significant, and may define the turnover of N in hadal sediments. Although they are not without uncertainties, our results shed new light on the importance of denitrification in sedimentary diagenesis in the deepest part of the Earth's ocean, and could have important implications for understanding the current state of biogeochemical cycles in the hadal zone.