Cryptic Carbon Cycle at Methane‐Seeps of the South China Sea: Evidence From Porewaters

Abstract Anaerobic oxidation of methane (AOM) governs methane consumption at seeps, yet δ 13 C‐CH 4 values in the sulfate‐methane transitions are commonly lower than predicted from preferential 12 C utilization by methanotrophic archaea, suggesting the influence of a different methane source. To address this isotope discrepancy, we used high‐resolution ion concentrations and carbon isotope data (CH 4 and DIC) of porewaters from the Haima seep, South China Sea, to reconstruct the carbon cycle using a reaction‐transport model. Model simulations demonstrate that in situ methanogenesis cannot explain the extreme 13 C depletion. Instead, the shift towards 13 C‐depleted CH 4 and DIC is primarily driven by isotopic equilibration during AOM, modulated by the carbon isotope fractionation factor and reverse AOM reaction flux under stable conditions. This mechanistic framework reconciles the paradoxical 13 C‐depletion in CH 4 by emphasizing the role of carbon isotope equilibration during AOM, offering an effective method for tracing the cryptic methane cycle in seep environments.