Widespread dispersal and aging of organic carbon in shallow marginal seas

The occurrence of pre-aged organic carbon (OC) in continental margin surface sediments is a commonly observed phenomenon, yet the nature, sources, and causes of this aged OC remain largely undetermined for many continental shelf settings. Here we present the results of an extensive survey of the abundance and radiocarbon content of OC in surface sediments from the northern Chinese marginal seas. Pre-aged OC is associated with both coarser (>63 mu m) and finer (<63 mu m) sedimentary components; measurements on specific grain-size fractions reveal that it is especially prevalent within the 20-63 mu m fraction of inner shelf sediments. We suggest that organic matter associated with this sortable silt fraction is subject to protracted entrainment in resuspension-deposition loops during which it ages, is modified, and is laterally dispersed, most likely via entrainment within benthic nepheloid layers. This finding highlights the complex dynamics and predepositional history of organic matter accumulating in continental shelf sediments, with implications for our understanding of carbon cycling on continental shelves, development of regional carbon budgets, and interpretation of sedimentary records.