Isotopic Evidence for Microbial Black Carbon Formation in Methane‐Rich Sediments of the South China Sea

Abstract

Black carbon (BC), a refractory component of organic carbon, exhibits poorly constrained environmental behavior. Recent culture experiments suggested anaerobic methanotrophic archaea (ANME) produce BC via anaerobic oxidation of methane, a pathway supported by studies of seep carbonates — mineral deposits restricted to settings characterized by high methane flux. However, it remains unconfirmed in sediments with low methane flux, settings that are far more widespread in the global ocean. Here, we examine BC distribution and δ ¹³ C _BC values in two seepage‐influenced sediment cores (PC‐01, PC‐02) and a reference core (C‐S05) from the Haima seeps in the South China Sea. Core PC‐01, influenced by higher methane flux, shows significantly ¹³ C‐depleted BC (−28.9‰ to −25.4‰) at the sulfate‐methane transition zone compared to global non‐seep sediments (−26.4‰ to −10.9‰). Furthermore, not all seepage‐affected sediments exhibit this signal, as demonstrated by core PC‐02. This discrepancy is likely governed by site‐specific variations in methane flux. Consequently, when methane flux is low, the limited production of ANME‐derived BC is insufficient to measurably shift the isotopic signature of the bulk BC pool. A Bayesian mixing model estimates that ANME‐derived BC accounts for ∼6.3% of the total BC at core PC‐01. Although quantitatively minor, ANME‐derived BC may represent a previously overlooked source of potentially aged BC in marine environments, consistent with microbial BC production in methane‐rich sediments.