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

Abstract

ABSTRACTSources of organic matter (OM) in lower Narmada and Tapi river-estuaries were examined using organic carbon to nitrogen ratio (Corg/N), stable carbon isotope ratio (δ13Corg) and lignin phenol biomarkers. The signature of lower Corg (av. 0.50%) and higher δ13Corg (av. –20.3‰) in Narmada and, higher Corg (av. 0.85%) and lower δ13Corg (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 Corg, lower Ad/Alv = 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 Corg/N and δ13Corg 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.