Ring Indices: The Potential Archaeal Lipid Indicator For Methane Seepage Intensity at Cold Seeps

Abstract Methane removal efficiency at cold seeps typically depends on fluid flow dynamics, necessitating robust proxies to quantify seepage intensity and further provide their implications for the carbon cycle. We analyzed lipid biomarkers of Haima cold seeps sediments (South China Sea (SCS)) to develop improved GDGT‐based seepage intensity indices. Anaerobic oxidation of methane (AOM), mediated by anaerobic methane‐oxidizing archaea (ANME) and sulfate‐reducing bacteria (SRB) is evidenced by the low sn2 ‐hydroxyarchaeol/archaeol ratios (<1.1), abundant glycerol dialkyl glycerol tetraethers, and Δδ 13 C isoprenoids‐methane values of −63‰ to −7‰ (average −35‰). Combined δ 13 C values (isoprenoids to −107‰; SRB lipids −43‰ to −20‰) and unresolved complex mixtures, we demonstrated that thermogenic methane with minor microbial contributions and oil degradation serve as the major carbon sources. We developed the ring indices (Ring Index (RI) and modified RI‐OH’) and methane index (MI) as the methane seepage tracers. The ring indices indicate robust inverse correlations with MI in seep carbonates and sediments ( R 2 > 0.62). Ring Index values consistently remain below 2.5 when MI exceeds 0.4, while RI‐OH’ demonstrates a marked increase at MI > 0.7. Both indices demonstrate rapid increases (ΔRI = 0.5–2; ΔRI‐OH' = 0.01–0.3) as MI approaches 1 (ΔMI = 0.02), showing 0.5–100× sensitivity enhancements from ANME‐derived isoGDGTs 1–3 and OH‐GDGTs 1–2 accumulation. Furthermore, our analysis of AOM‐derived aragonite proportion in cold‐seep carbonates reveals the correlation with RI and MI ( R 2 = 0.21), confirming the utility of RI for qualitative methane flux assessment. This dual‐proxy approach integrates ring indices with MI through microbial membrane adaptation, establishing comprehensive methane seepage dynamics tracers.