The Substantial Yet Unaccounted Carbon Sink: Microbially-Derived Black Carbon in Cold Seep Sediments

Marine sedimentary black carbon (BC) is conventionally attributed to terrestrial biomass and fossil-fuel combustion. However, persistent imbalances in the marine BC budget suggest unaccounted marine sources. This study identifies and quantifies a novel microbially-derived black carbon (mBC) source in deep-sea cold seeps. High-resolution geochemical analysis of Qiongdongnan Basin sediment cores reveals mBC as a significant component, evidenced by extremely depleted stable carbon isotopes in BC (-37.74‰) and total organic carbon (-55.39‰), linking its formation to anaerobic methane oxidation. During active seepage (2011-2020), mBC constitutes 31-43% of sedimentary BC, with a modern accumulation rate of 1.58-3.36 g m^-2 yr^-1. Scaling this rate to global cold seep areas, we estimate that cold seeps contribute 0.52-0.79 Tg mBC yr^-1 as a previously unrecognized sink, accounting for 4.1-5.9% of the total marine BC burial flux. Multiproxy records show a three-stage seep evolution over the past century, governed by shallow fluid dynamics. Isolating the mBC signal also clarifies combustion-derived BC, validating post-2010 regional emission controls. These findings redefine marine BC source apportionment and reveal an important, long-overlooked carbon sequestration pathway, necessitating revisions to marine carbon cycle models to include mBC as a critical sink and a paleo-methane proxy.