Insights on Organic Carbon Inputs in the Delaware Estuary from Compound-Specific δ13C and Δ14C

Land-derived vascular plant particulate organic matter (POM) is delivered to estuaries via rivers, yet its fate is difficult to constrain between land and sea. POM is intricately linked to sediment dynamics, making it susceptible to deposition, resuspension, and trapping. We assessed the spatial and seasonal variation in algal and vascular plant-derived POM for a transect down the Delaware Estuary, using n-alkane and fatty acid biomarkers and their compound specific δ¹³C values as tracers of organic matter provenance. n-Alkane distributions, and their low δ¹³C values reveal a markedly higher proportion of terrestrial inputs in bottom waters relative to the overlying surface waters throughout the estuary, and suggest significant marsh POM contributions that were previously undocumented. Fatty acid δ¹³C distributions indicate similar sources. Bacterial fatty acids from sediments in the estuarine turbidity maximum (ETM) with isotope values of ~-30 ‰ suggest preferential uptake of terrestrial organic matter by sedimentary bacteria. Lines of evidence from both compound classes highlight marsh contributions to the ETM and in the lower estuary. Compound-specific Δ¹⁴C data from ETM sediments indicate both burial and degradation of aged terrestrial POM. Modelling of estuarine circulation suggests input of marsh POM is driven by lateral pumping in the estuary. The input of wetland POM, long recognized as a critical resource for animal ecology, may be geochemically and volumetrically important in the processing and delivery of terrestrial organic carbon to the coastal ocean.