ANALYSIS OF ANTHROPOGENIC INFLUENCES ON ORGANIC MATTER DEPOSITION IN PLEASANT BAY AND NAUSET MARSH BACK BARRIER SYSTEMS, CAPE COD

Sediment cores taken from the Pleasant Bay and Nauset Marsh back barrier systems in 2014 were shown to be sensitive to anthropogenic activities (Love et al, 2015). Significant increases in volume magnetic susceptibility, correlative across three coring sites, were attributed to the burning of fossil fuels during the Industrial Revolution (ca. 1880 CE). In this study, we utilize stable isotope values and elemental concentrations of carbon, nitrogen, and sulfur to assess ecological and geochemical responses to anthropogenic activity. Age constraints are afforded by the previously identified anthropogenic zone (1880 CE) as well as pending radiocarbon dates. Sediment samples were analyzed for isotope values and element concentrations using an elemental analyzer/ isotope ratio mass spectrometer (EA/IRMS). Mass accumulation rates of organic carbon were calculated for 1880 CE to present, yielding average rates of 6.2 ± 2.0 mg/cm²yr in Meetinghouse Pond (MHP), 11.9 ± 2.8 mg/cm²yr in Frostfish Cove (FFC), and 10.4 ± 1.6 mg/cm²yr in Orleans Town Cove (OTC). Gradual rises of δ¹³C in the anthropogenic zone are identified in all three core sites, reflecting a rise in phytoplankton production likely due to cultural eutrophication. C/N ratios calculated from elemental analysis indicate marine phytoplankton is the main source of organic carbon in both barrier systems. Slightly higher C/N values are seen in MHP, likely due to the coring location’s proximity to land and the resultant influx of terrigenous material. Percent organic carbon increases significantly in OTC (T-test; p=0.002) above the anthropogenic horizon, likely due to increased productivity and the preservation potential of the anoxic basin. Percent organic carbon decreases significantly in MHP (T-test; p=0. 005) at the anthropogenic horizon, while dry bulk density significantly increases (T-test; p<0.001). The MHP observations demonstrate that the flux or inorganic terrigenous matter has increased during this period. Further radiometric dating will provide constrained basal ages and allow for sedimentation and mass accumulation rates for pre-anthropogenic sections. Analyses of carbon signatures has demonstrated anthropogenic ecological impacts, further refining the designation of an anthropogenic horizon in these systems.