Microbial priming controls terrestrial carbon burial in marine sediments

Marine sediments are a globally important sink for organic carbon of both marine and terrestrial origin, yet far less terrestrial carbon is ultimately buried than rivers deliver to the ocean margins. This long-standing "terrestrial carbon conundrum" has resisted explanation, largely because conventional bulk proxies cannot distinguish marine, terrestrial and microbial-degrader carbon pools. Here we use sedimentary ancient DNA from five records spanning the Bering Sea to the Sea of Okhotsk over the past 30,000 years to reconstruct organic carbon resolved simultaneously by taxonomy and habitat. Integrating these taxon-resolved carbon estimates with environmental proxies across the last glacial-interglacial cycle, we find that terrestrial carbon burial is governed not only by supply but by biogeochemical interactions previously documented only in soils. Microbial processing splits into two competing pathways: under priming, marine organic matter fuels the degradation of terrestrial organic matter, whereas under preferential substrate utilization microbes consume marine organic matter and spare terrestrial carbon. Both pathways are triggered only once marine organic-matter availability crosses a critical, temperature-governed threshold. This temperature-sensitive cascade reveals a previously unrecognized vulnerability of the ocean's carbon sink to global warming.