d13C and C/N as potential coastal palaeoenvironmental indicators in the Mersey Estuary, UK

The reconstruction of former environments, and ultimately relative sea-level changes, in coastal and estuarine environments normally utilises a range of microfossil proxies, such as diatoms, foraminifera and pollen. As microfossils are sometimes poorly preserved, sparse or absent from coastal sediments, this study assesses the potential of using δ13C and C/N analysis of sedimentary organic matter as an alternative. Contemporary high inter-tidal and sub-tidal surface saltmarsh sediments of the Mersey Estuary, UK, reveal distinctly different δ13C and C/N values, reflecting the contrasting sources of organic matter in relation to ground elevation within the tidal frame. Despite decomposition effects, the relationship between ground elevation, δ13C and C/N, is preserved in the sediment record, with a gradual increase in δ13C and decrease in C/N from supra-tidal, through to inter-tidal and to sub-tidal environments. Holocene variations in δ13C and C/N are compared with stratigraphical palaeoenvironmental changes inferred from diatom and pollen evidence and show a good correspondence. Although δ13C and C/N are not as accurate as microfossil analysis in terms of coastal palaeoenvironmental reconstruction, they may be applied successfully in the field of relative sea-level reconstruction by helping to verify sea-level index points when microfossils are absent.