Lipid biomarkers and organic matter carbon isotopes in estuarine sediments as proxies for evaluating seawater intrusion

Coastal wetlands and estuarine sediments are characteristics of tide-dominated environments and retain a record of seawater intrusion and possibly sea-level changes. A variety of methods including the C/N ratio and delta C-13 of bulk organic matter in the sediment have been employed in estuarine studies, but they are generally non-specific indicators. Here we report using lipid biomarkers to evaluate the extent of seawater intrusion based on an estuary in eastern China, Xinyanggang. Along the river from the estuary mouth upstream to the freshwater head, the vegetation shifts quickly from salt-tolerant Spartina (C-4) to the less tolerant reed Phragmites (C-3), both of which have distinguished delta C-13 values and biomarker distribution. The delta C-13 values of particulate organic matter (POM) and surface sediment decreased from the estuary mouth upstream, indicating the reduced contributions from Spartina and marine phytoplankton to the POM and surface sediment and increased inputs from Phragmites. The C-32/C-30 alkanol and cholesterol/sitosterol ratio decreased in the surface sediments, faithfully recording the variations in the contributions from Spartina and Phragmites. The combination of biomarker distribution and organic matter delta C-13 in the sediments can be used as indicators for sea water intrusion into the estuary/river. (C) 2015 Elsevier Ltd. All rights reserved.