Hadal trenches (>6,000 m water depth) have been revealed as hotspots of organic carbon burial and microbial respiration in the ultra-deep ocean environment. However, an understanding of the anaerobic metabolic pathways and rates, as well as carbon-silicon cycling in the hadal trench sediments remains very fragmentary because of the shallow nature of traditional coring penetration. Using materials collected during IODP Expedition 386 in the Japan Trench and a reaction-transport model, we provide a regional quantitative assessment of organic carbon turnover by sulfate reduction, anaerobic oxidation of methane, methanogenesis, in addition to silicate weathering and authigenic carbonate and clay formation. We show that rapid burial of relatively labile organic carbon resulting from subduction earthquakes triggers organic carbon and methane turnover at rates comparable to those in continental margin sediments, thereby stimulating active silicate weathering and authigenic carbonate formation. Despite vigorous organic carbon turnover, the vast majority of organic carbon has been buried, implying the important role of tectonic associated events in translocating and preserving organic carbon in the deepest part of the ocean. These results demonstrate, for the first time, active coupled carbon-silicon cycling in the hadal trench sediments, which has implications for the subduction zone carbon budget and ocean deep biosphere.

Hadal trenches (>6,000 m water depth) have been revealed as hotspots of organic carbon burial and microbial respiration in the ultra-deep ocean environment. However, an understanding of the anaerobic metabolic pathways and rates, as well as carbon-silicon cycling in the hadal trench sediments remains very fragmentary because of the shallow nature of traditional coring penetration. Using materials collected during IODP Expedition 386 in the Japan Trench and a reaction-transport model, we provide a regional quantitative assessment of organic carbon turnover by sulfate reduction, anaerobic oxidation of methane, methanogenesis, in addition to silicate weathering and authigenic carbonate and clay formation. We show that rapid burial of relatively labile organic carbon resulting from subduction earthquakes triggers organic carbon and methane turnover at rates comparable to those in continental margin sediments, thereby stimulating active silicate weathering and authigenic carbonate formation. Despite vigorous organic carbon turnover, the vast majority of organic carbon has been buried, implying the important role of tectonic associated events in translocating and preserving organic carbon in the deepest part of the ocean. These results demonstrate, for the first time, active coupled carbon-silicon cycling in the hadal trench sediments, which has implications for the subduction zone carbon budget and ocean deep biosphere.

Hadal trench;,silicate weathering,organic carbon degradation