Stable Carbon Isotopes and Bulk-Sediment Geochemistry As Indicators of Relative Sea-Level Change in Tidal Marshes, Mangroves and Isolation Basins: Application and Developments

Bulk organic stable carbon isotope (S13C) and accompanying bulk organic geochemical measurements have been increasingly used as a relative sea-level (RSL) indicator over the last two decades. Their utility as a RSL indicator is premised on the ability of bulk organic S13C and bulk organic geochemistry to distinguish between organic matter (OM) sources in coastal environments, and to identify changes in OM source contributions in sediment sequences in response to RSL change. We evaluate the performance of bulk S13C and bulk organic geochemistry as a RSL indicator in tidal marsh, mangrove and isolation basin environments. The interpretation of isotope measurements from these environments requires knowledge of the processes controlling contemporary OM S13C, and the influence of decomposition on bulk values. We review in detail the controls on the S13C composition of OM in tidal marshes, mangroves and isolation basins, and advocate wherever possible for the collection of contemporary geochemical datasets corrected for the 13C Suess effect to help inform interpretations. From the wide range of case studies considered, an emerging principle is that the degree of isotopic distinctiveness between OM sources is key in determining how the technique can be deployed as a RSL indicator. This can range in use from the provision of qualitative information on changes in marine influence over time, to the identification of sea-level index points at lithostratigraphic contacts, and most powerfully to the recognition of inter-tidal subenvironments with isotopically well-constrained vertical limits.