Elemental and stable isotope records of organic matter input and its fate in the Pichavaram mangrove–estuarine sediments (Tamil Nadu, India)

In order to evaluate the sources and fate of organic matter (OM) in the Pichavaram mangrove–estuarine ecosystem, Corg, Ntot, C/Natm, and stable isotopes (δ13Corg and δ15Norg) of five 210Pb dated sediment cores were investigated. Core A was collected from the Vellar Estuary, cores B, C, and D from mangrove forests, and core E was from the Coleroon Estuary. In mangrove sediments, Corg, Ntot, and C/Natm concentration varied from 1.2 ± 0.52%, 0.08 ± 0.03%, and 16.6 ± 4.1%, respectively. In the estuaries, Corg, Ntot, and C/Natm varied from 0.37 ± 0.19%, 0.03 ± 0.01%, and 12.3 ± 2.9%, respectively. In mangrove forests the δ13Corg and δ15Norg values varied from − 24.3 ± 1.8‰ and 4.2 ± 0.86‰, whereas in estuaries these values were − 23.2 ± 1.6‰ and 5.1 ± 1.3‰, respectively. The variation represents variable inputs of marine and terrestrial OM in these area, OM degradation, replacement of native vegetation due to increasing salinity, and early diagenetic changes. Some of these changes could be attributed to anthropogenic activities that have been happening in the Pichavaram complex over the last few decades. OM once deposited in the sediments is rapidly decomposed or remineralized in both mangrove and estuarine sites. The rate of Corg burial is more in mangrove forests than estuaries. Early diagenetic changes are modeled in the cores based on C/N stoichiometry. This was most applicable in mangrove sediments (core C) where ~ 56% C was remineralized representing a first order diagenetic model.

Research highlights
► First time five 210Pb dated sediment cores from Indian mangrove were investigated.
► One of few studies from Indian mangroves focusing on δ13Corg and δ15Norg.
► Organic matter (OM) source is shifting from a terrigenous dominated to marine OM.
► The rate of carbon burial is more in mangrove forests than estuaries.
► The carbon burial values indicate that OM is decomposed and rapidly recycled.