Variability in radiocarbon ages of biochemical compound classes of high molecular weight dissolved organic matter in estuaries

High molecular weight dissolved organic matter (HMW-DOM, >1000 Da) represents a major fraction (>30%) of dissolved organic carbon (DOC) in the ocean and thus plays an important role in the global biogeochemical cycling of carbon and many other elements. Its organic sources and formation mechanisms, however, are still not well understood especially in estuarine and coastal regions where multiple natural and anthropogenic sources contribute to total HMW-DOM. In this paper we report our measurements of natural radiocarbon (14C) abundances and stable carbon isotope (13C) compositions of the major biochemical compound classes: amino acids, carbohydrates and lipids separated from eight HMW-DOM samples collected from five US estuaries as part of our on-going study of sources, distribution and transport of chromophoric dissolved organic matter (CDOM) in estuarine and coastal waters. Distinct differences in both 14C and 13C values were found among the bulk HMW-DOM samples as well as the individual compound classes. Radiocarbon ages of the major compound classes varied by as much as 27,000 years in a single sample. The calculated average radiocarbon ages of the compound fractions of HMW-DOM indicate that the total lipid fraction is very “old”, while the acid-insoluble fraction is slightly younger. Total amino acid and carbohydrate fractions, however, have relatively modern apparent 14C ages. The significant variability in 14C ages among the compound classes indicates not only multiple organic carbon sources but also different formation and turnover pathways controlling the cycling of different biochemical components of HMW-DOM in estuarine and coastal waters.