Black Carbon Formation at Cold Seeps and Its Potential Contribution to the Marine Black Carbon Budget

The long-term carbon sequestration capacity of black carbon (BC) is among the factors controlling climate change dynamics. Culture experiments have revealed that anaerobic methanotrophic archaea (ANME) can produce BC, but their impact on the marine BC cycle has not been thoroughly investigated. This study examined the geochemical properties of BC preserved in seep carbonates from the South China Sea and the Gulf of Mexico. δ¹³C_BC values as low as -45.2‰ were observed in seep carbonates, representing the first documented case of ANME-derived BC in natural settings. With the observed range of BC isotope compositions and further calculations with a Bayesian mixing model, microbially produced BC may account for 2-33% of the total BC enclosed in the studied seep carbonates. By integrating data from various empirical studies, the analyses of radiocarbon (Δ¹⁴C) compositions suggests that BC in the deep sea is generally older than the associated organic carbon. Given that the organic carbon found at seeps is typically older than the organic carbon in the surrounding sediment not affected by seepage, BC originating from seeps could serve as a plausible source of aged dissolved BC in the deep sea.