Source-age dynamics of estuarine particulate organic matter using fatty acid δ13C and Δ14C composition

Abstract

This study used a multiproxy approach to elucidate the source and age composition of estuarine particulate organic matter (POM) using bulk stable isotopes (δ13CPOC), fatty acid (FA) biomarkers, and compound specific isotopic analyses in surface waters along the Delaware River and Bay (Delaware Estuary, hereafter). δ13C values of FA (δ13CFA) ranged more widely (−30.9‰ to −21.8‰) than δ13CPOC (−27.5‰ to −23.5‰), providing greater insight about POM sources along the estuary. δ13C values of C16:0 phospholipid FA (primarily, aquatic sources) increased along the salinity gradient (−29.8‰ to −23.4‰), while δ13CFA values of long-chain neutral fatty acid (terrestrial sources) decreased (−28.6‰ to −30.9‰). δ13CFA values for C18's FA indicated the importance of marsh-derived organic matter within Delaware Estuary. Compound specific radiocarbon analysis showed the heterogeneous age structure of FA associated with POM (FAPOM). 14C ages of FA ranged from modern (postbomb) to 1790 BP; aged FA (120 BP to 1700 BP) derived primarily from the watershed, whereas modern FA were produced within Delaware Estuary. 14C ages of short-chain FA (aquatic sources) reflected differences in the age of dissolved inorganic carbon along the estuary and had older 14C ages at the river end-member. 14C ages of FA from terrigenous sources were older than water and sediment residence times indicating this source derived from the watershed. This study is the first to document the complex age distribution of FAPOM along the estuarine salinity gradient and shows that inorganic carbon sources, watershed inputs and autochthonous production contribute to variation in the ages of POM.