C3/C4 variations in salt-marsh sediments: An application of compound specific isotopic analysis of lipid biomarkers to late Holocene paleoenvironmental research

In salt-marshes, bulk carbon isotope values reflect organic matter contributions not only from salt-marsh plants, but also from algal and bacterial inputs, as well as allochthonous terrestrial organic matter. In the present study, compound specific isotope analysis was applied to a core and modern plant samples collected from a Machiasport, Maine salt-marsh. We sampled 10 plant species common to the marsh including Spartina alterniflora, Spartina patens, Juncus gerardi, Solidago sempervirens, Salicornia europa, Potentilla anserina, Atriplex patula, Plantago maritima, Suaeda maritima and Limonium nashii. Isolation of two n-alkanes (C27 and C29) present in marsh plant samples allowed for the reconstruction of past plant communities present in the cored deposits. For the C27 and C29 homologues there is a 3-10‰ separation in carbon isotopic composition between C3 and C4 species. Carbon isotopic values down core show a transition from a C4 dominated plant community to a C3 dominated community, attributable to the migration of lower marsh zones onto brackish/freshwater marsh zones during a late Holocene relative sea-level rise. The two homologues used have the potential to provide paleoenvironmental information in future core work, where the sea-level variable will be controlled.