Global high rates of anaerobic methane oxidation at marine cold seeps recorded by isotope bonding in seep carbonates

Microbial anaerobic oxidation of methane (AOM) regulates the flux of methane (CH ₄ ) released from anoxic marine sediments into the ocean. However, variations in AOM rates, especially at dynamic environments such as cold seeps, are difficult to constrain, hindering robust projections of marine CH ₄ flux in a changing climate. Here, we quantify AOM rates at global marine cold seeps based on isotope bonding in methane-derived authigenic carbonates. We show through model simulations that ¹³ C- ¹⁸ O bond abundance in these carbonates quantitatively records AOM rates at marine cold seeps and validate this proxy using compiled datasets of seep carbonate isotope compositions and independent AOM rate measurements. Accordingly, we estimate a mean AOM rate of 6.3 3.9 10.6 mol·m ⁻² ·yr ⁻¹ and a consumption of 2.1 1.3 3.5 Tg·yr ⁻¹ CH ₄ at modern global cold seeps, but potentially 1.7 0.9 3.1 times as high an AOM rate and CH ₄ sink over the last ~100 kyr. Our results highlight the important role of cold seeps in the global marine CH ₄ cycle despite their relatively minuscule footprint.