CARBON IN SALT MARSH SEDIMENT ACROSS ESTUARINE SALINITY AND TIDAL GRADIENTS, NORTH CAROLINA, USA

The accumulation of organic matter in sediments beneath tidal wetlands can sequester large quantities of carbon, thereby mitigating anthropogenic CO₂ emissions. Mapping and assessing carbon stocks in coastal sediments necessitates information on how and why carbon burial varies over time and across space. We studied carbon content in multiple cores from each of ten estuarine salt marshes near Cape Lookout, North Carolina. The sites studied range from sheltered, low-salinity high marshes to more exposed and/or more saline low marshes. Mean tidal range varies from < 15 cm to > 45 cm from the northeastern to southwestern marshes, respectively, within the 35 km length of our study area. Estuarine salinity responds to seasonal and episodic hydroclimate shifts, but the average spatial pattern (from ~12 to ~34 psu) reflects tidal flushing; i.e., saltier in southwestern sites with larger tides. Most cores are from broad high-marsh platforms vegetated by Juncus roemarianus, but we also sampled Spartina alterniflora low marshes. The studied cores (34 total) range in depth from 50 to >250 cm depending on marsh sediment thickness.

Carbon content is often calculated from organic matter (OM) content, measured by mass loss-on-ignition (LOI). To explore variability of OM-to-Carbon conversion equations within our localized study area, we analyzed 1,443 paired sample splits by LOI and Cavity Ringdown Spectroscopy for carbon concentration. Overall mean LOI was 34.7 ± 18.0% and mean organic carbon abundance was 16.4 ± 9.8% (1 s.d.).

We find the highest organic carbon concentrations (> 35%) in sheltered, unditched, low-salinity, low tidal-range sites dominated by Juncus. These sites in the northeast part of the study area also contain some of the thickest marsh sediments (up to 3m), representing more than 2,500 years of continuous accumulation. Many marshes throughout the study area, both Juncus and Spartina, have much lower average carbon content (~10%) but carbon abundance generally decreases to the southwest, where carbon content in the upper 75cm of cores averages ~4%. Analyzing carbon content by depth shows a general pattern toward lower carbon densities in the upper parts of most cores. The blue carbon stock in the upper 100 cm of the sediment column varies at sites across our study region from as much as 430 MgC/hectare to less than 180 MgC/hectare.