Understanding the Fate of Organic Matter in a Tidal Salt Flat Environment: A Case Study from Western Rann of Kachchh, India

The stable carbon isotopic composition of sedimentary organic matter has been studied from a ~1m deep trench located in the tidal salt flat environment in the western part of the Great Rann of Kachchh. The lithology constitutes mostly of sand with an intermediate clay deposit layer. In this location, the organic matter content in the sediments can be sourced from both marine and terrestrial environments. Moreover, in the subsurface, various stages of microbial degradation of the organic matter can also affect the bulk stable organic carbon isotopes in the sediments. Therefore, the δ13C of organic matter has been studied in two forms: one is the bulk sedimentary organic matter (δ13CSOM) and the other after treating the sediment in a wet chemical process that removes the labile and easily/partially degraded organic carbon from the sediments through oxidation, known as the oxidation resistant organic carbon (δ13COROC). Due to mixing from various organic sources the δ13CSOM shows no systematic variation along the depth of the trench and shows a mean value of -20.3‰. This value possibly indicates a mixed isotopic signal from terrestrial, marine as well as microbially degraded organic carbon source. On the other hand, the δ13COROC values minimizes the randomization in the temporal profile as is shown by the bulk δ13CSOM. It shows the signature of the most oxidation resistant organic carbon present in the sediments and removes the labile carbon. From the limited dataset of δ13COROC, we infer that a mixture of C3 and C4 might be the dominant source of organic carbon in this area with δ13COROC value going as low as -24.0‰ in the middle part of the trench. The δ13COROC shows a mean value of -22.4‰ which is overall depleted by ~2‰ from the bulk δ13CSOM. This depletion indicates that a 13C enriched component is getting removed from the system during the oxidation. This enriched component can be either labile marine organic matter or C4 organic matter or a mixture of both sources.