Degradation and Aging of Terrestrial Organic Carbon within Estuaries: Biogeochemical and Environmental Implications

Estuaries are action zones for organic carbon (OC) degradation and aging. These processes influence the nature of terrestrial OC (OC terr ) export and the magnitude of OC terr burial in marginal seas, with important environmental implications such as CO 2 release and hypoxia. In this study, we determined the contents and carbon isotopic compositions ( 13 C and 14 C) of bulk OC and fatty acids (FAs) as well as the sedimentological characteristics of suspended particulate matter (SPM) samples collected from two sites over four seasons and of surface sediment samples from three sites in the Pearl River estuary (PRE) to evaluate processes controlling OC terr degradation and aging along an estuarine gradient. We found that the abundance-weighted average C 24–32 FA 14 C ages increased by an average of ∼1170 years for SPM and by an average of ∼3440 years in PR/PRE sediments, along the ∼60 km PRE transect. These increases in the FA age coincided with an 86% decrease in the corresponding mineral surface area-normalized FA loading along the sediment transport pathway, implying that selective degradation of labile and younger OC resulted in apparent OC aging. These measurements reveal an important shift in the nature of OC, with implications for biogeochemical cycling within estuaries and for regional environmental changes.