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 ¹⁴C and ¹³C into the deep ocean.

海洋溶解有机碳（DOC）是地球上最大的减少碳活动储层之一。在深海中，DOC被描述为具有生物顽抗性，其放射性碳年龄为4,000至6,000年，远远超过了海洋倾覆的时间范围。但是，非生物去除机制无法解决深海DOC损失的全部数量。深海水在低温下通过山脊两侧的火山壳循环，但对相关的生物地球化学过程和碳循环知之甚少。在这里，我们对来自安装在中大西洋海岭以西地壳岩石中的两个钻孔观测站的流体中的DOC进行分析，并显示深海DOC已从这些冷循环流体中去除。去除机理是同位素选择性的，并导致分子组成的特定特征发生变化，这与微生物介导的氧化一致。我们建议地壳微生物可生物利用平均放射性碳年龄为3200年的有机分子，并且这种清除机制可能至少占深海DOC全球总损失的5％。凉爽的地壳环流可能会释放出分子中降解并消耗掉的残存有机碳成分，从而有助于维持深海作为“老龄化”和耐火DOC的储集层。并且这种清除机制可能至少占全球深海DOC损失的5％。凉爽的地壳环流可能会释放出分子中降解并消耗掉的残存有机碳成分，从而有助于维持深海作为“老化”和耐火DOC的储集层。并且这种清除机制可能至少占全球深海DOC损失的5％。凉爽的地壳环流可能会释放出分子中降解并消耗掉的残存有机碳成分，从而有助于维持深海作为“老化”和耐火DOC的储集层。¹⁴ C和¹³ C进入深海。