Chemosynthetic Organic Matter Reveals a Protracted Episode of Past Methane Seepage Activities at a Newly Discovered Cold Seep Site (Mannar Basin) Off the East Coast of India

Abstract Here, we present organic geochemical evidence of past methane‐seepage events in a sediment core retrieved from a recently discovered methane cold‐seep site off Mannar Basin, east coast of India. The significant negative C‐isotope excursion of organic matter (δ 13 C TOC ) in the seep core compared to a reference non‐seep core suggests a substantial contribution of methanotroph‐derived organic carbon to the seep sediment organic pool. Based on archeal biomarker data, and the presence of relict chemosymbiont‐bearing Calyptogena shells, we propose a protracted period of episodic methane seepage events spanning across the Marine Isotope Stage‐3 to mid‐Holocene, with a declining seepage intensity toward the Holocene. The episodic depletion in δ 13 C TOC reveals four intermittent high methane flux events. Our study highlights that δ 13 C TOC can be used as an important proxy for reconstructing the dynamic evolution of past methane‐seep activities, particularly when integrated with lipid biomarkers and their isotopic fingerprints.