CARBON IN COASTAL WETLAND SEDIMENT SEQUENCES: DECADAL TO MILLENNIAL PATTERNS IN NORTH CAROLINA SALT MARSHES

Organic carbon accumulation in coastal wetlands sequesters atmospheric CO₂, potentially for thousands of years. Many studies of net carbon burial rates consider relatively short time (and depth) spans, whereas buried carbon stocks per unit area are usually reported as aggregates based on total marsh-sediment thickness at a site. While necessary for quantifying carbon budgets, both approaches can obscure large variations in carbon content with depth within marsh deposits. Much recent research has addressed the threat to tidal wetlands posed by the ongoing acceleration of global sea-level rise. However, numerous other environmental factors affect marsh vegetation communities and the sediment properties of estuarine wetlands, including its buried carbon content.

We measured bulk density, organic carbon content, and stable carbon isotopic composition of sediment samples at 2-cm depth intervals in cores from estuarine Spartina alterniflora and Juncus roemarianus marshes in eastern Carteret County, North Carolina. The thickness of organic matter-rich salt marsh deposits at the eight sites we cored ranged from 1.8 to 3.2 m. Most cores contained more than 2 m of marsh sediment containing >10% carbon by weight. Age-depth models based on radiocarbon dates indicate these cores span the Common Era; i.e., 2m below the marsh surface corresponds to about 0 CE (~2,000 years ago). At most core sites, maximum organic carbon concentrations (30-40% depending on site) occurred between 500 and 1100 CE, prior to, and overlapping with the Medieval Climate Anomaly. Subsequently, between 1250 and 1450 CE (i.e., Little Ice Age onset), carbon concentrations abruptly dropped, to between 5 and 20%, and remained low until ~1600 CE. These abrupt fluctuations in down-core carbon content do not coincide with major changes in sea-level rise rates. Therefore we hypothesize they result from interacting effects of temperature, hydrology (e.g., drought), and/or shifts in estuarine salinity and tidal range. The latter could have been driven by the collapse of Outer Banks barrier islands as reported by others, which increased inlet connectivity between our estuarine study area and the ocean. Barrier island collapse could accompany sea-level rise and other climate impacts, and thus may alter the function of coastal marshes as carbon sinks.