AN ISOTOPIC INVESTIGATION OF ORGANIC MATTER CYCLING AT SPRAGUE MARSH, PHIPPSBURG, MAINE

Stable carbon isotope analysis of organic matter preserved in salt marsh sediment cores have been used to reconstruct changes in high and low marsh vegetation through time, yet little is known about the degree to which modern plant biomass is incorporated into the sedimentary record in salt marshes. In this study, stable isotopes are used to examine the cycling of modern C₃ and C₄ vegetation into surface sediments at the Sprague River Marsh, Phippsburg.

A series of quadrats (2m x 2m) were set up in an east-west transect (from high to low marsh) across the north end of Sprague Marsh. From each quadrat, the elevation and the percent plant cover by species were estimated and surface sediments were collected for salinity and carbon isotope analysis. A suite of modern plants were also analyzed for isotopic analysis.

The isotopic composition of the surface soil organic matter reflects the proportion of C₃ and C₄ vegetation biomass in all quadrats except those located in the low marsh. The low marsh surface soils are more depleted in ¹³C than predicted by the composition of the overlying C₄ salt marsh vegetation (Spartina alterniflora), suggesting that isotopically depleted allochthonous material makes up a greater proportion of the organic matter in the low marsh than in the high marsh. The results of the modern study indicate that the carbon isotope composition of salt marsh sediment cores reflects large scale changes in the C₃/C₄ vegetation biomass through time. Independent proxies of marine and terrestrial organic matter deposition are needed when isotopically depleted sedimentary horizons are encountered.