TY - JOUR T1 - Microbial decomposition of marine dissolved organic matter in cool oceanic crust JF - Nature Geoscience Y1 - 2018 A1 - Shah Walter, Sunita R. A1 - Jaekel, Ulrike A1 - Osterholz, Helena A1 - Fisher, Andrew T. A1 - Huber, Julie A. A1 - Pearson, Ann A1 - Dittmar, Thorsten A1 - Girguis, Peter R. KW - carbon KW - community KW - deep-ocean KW - DISTINCT KW - extraction KW - fluid-flow KW - HEAT KW - MID-ATLANTIC RIDGE KW - SEAWATER KW - WESTERN FLANK AB - Marine dissolved organic carbon (DOC) is one of the largest active reservoirs of reduced carbon on Earth. In the deep ocean, DOC has been described as biologically recalcitrant and has a radiocarbon age of 4,000 to 6,000 years, which far exceeds the timescale of ocean overturning. However, abiotic removal mechanisms cannot account for the full magnitude of deep-ocean DOC loss. Deep-ocean water circulates at low temperatures through volcanic crust on ridge flanks, but little is known about the associated biogeochemical processes and carbon cycling. Here we present analyses of DOC in fluids from two borehole observatories installed in crustal rocks west of the Mid-Atlantic Ridge, and show that deep-ocean DOC is removed from these cool circulating fluids. The removal mechanism is isotopically selective and causes a shift in specific features of molecular composition, consistent with microbe-mediated oxidation. We suggest organic molecules with an average radiocarbon age of 3,200 years are bioavailable to crustal microbes, and that this removal mechanism may account for at least 5% of the global loss of DOC in the deep ocean. Cool crustal circulation probably contributes to maintaining the deep ocean as a reservoir of 'aged' and refractory DOC by discharging the surviving organic carbon constituents that are molecularly degraded and depleted in C-14 and C-13 into the deep ocean. VL - 11 UR - http://www.nature.com/articles/s41561-018-0109-5http://www.nature.com/articles/s41561-018-0109-5.pdfhttp://www.nature.com/articles/s41561-018-0109-5http://www.nature.com/articles/s41561-018-0109-5.pdf IS - 5 ER -