Carbon isotopic and biochemical fingerprints of sedimentary organic matter in lower Narmada and Tapi rivers, north-west coast of India

Sources of organic matter (OM) in lower Narmada and Tapi river-estuaries were examined using organic carbon to nitrogen ratio (C_org/N), stable carbon isotope ratio (δ¹³C_org) and lignin phenol biomarkers. The signature of lower C_org (av. 0.50%) and higher δ¹³C_org (av. –20.3‰) in Narmada and, higher C_org (av. 0.85%) and lower δ¹³C_org (av. –22.8‰) in Tapi highlight the difference in OM characteristics of both systems, nevertheless they flow adjacent to each other. The OM in Tapi was predominated by fresh plant tissues, as indicated by higher Λ8 = 1.2–3.2 mg/100 mg C_org, lower Ad/Al_v = 0.22–0.46 and lower LPVI = 17–23. The plant OM present in the sediments of Narmada was previously degraded and mixed with soil. The OM contribution from algae was restricted to estuarine stations in both the rivers, which was ∼50% in Narmada and ∼35% in Tapi. The contribution of OM from fresh vascular plant was higher (20% to 63%) in Tapi than Narmada (2% to 35%). This study illustrated the benefits of using lignin phenols along with C_org/N and δ¹³C_org to identify the potential OM sources in two large river–estuary systems of India, which highlighted the complex interaction of natural and anthropogenic processes acted on the terrestrial OM compositions.