C3/C4 VARIATIONS IN HIGHER SALT MARSH SEDIMENTS: AN APPLICATION OF COMPOUND SPECIFIC ISOTOPIC ANALYSIS OF LIPID BIOMARKERS TO LATE HOLOCENE PALEOCLIMATIC RESEARCH

Previous sea-level studies reveal that the carbon isotopic composition of high marsh organics from coastal Maine alternately shifted between C3 and C4 dominated communities over the past 5,000 yrs. In modern marshes, salinity is an important control on the spatial distribution of salt-marsh plant communities. The saline low-marsh areas are typically dominated by C4 communities whereas C3 plants are predominant in the high marsh zones. The focus of this work is to determine if regional climatic effects or more localized effects on salinity influence the fluctuations between C3 and C4 communities in Holocene marshes from coastal Maine. Carbon isotopic and organic geochemical data are presented for surface plant samples and core samples collected from a marsh located in Machiasport, Maine. Chain length distributions of n-alkanes have been used to identify plant material in cores and marsh surface samples. The compound specific carbon isotopic composition of marsh organics is used to monitor the balance between C3 and C4 grasses down core. Foraminiferal distributions allow for the identification of the specific marsh zone of the deposits (low vs. high marsh) and provide a control for sea level. A series of radiocarbon dates will provide temporal control. These data sets in combination allow for plant paleoecology to be monitored through time within specific marsh zones at the Machiasport marsh, and future lab work for samples collected from two additional Maine marshes will allow researchers to determine if a regional climatic signal or more localized signal is controlling marsh plant community composition.