Differentiating the Causes of Aged Organic Carbon in Marine Sediments

Aged organic carbon (OC) is widely observed in surficial sediments deposited on continental margins, resulting from several factors including old OC input and sedimentological processes. While the supply of old OC to marine sediments has been examined and quantified in a range of continent margin settings, there have been few attempts to constrain the extent to which hydrodynamic processes, specifically lateral transport, contribute to the old age of sedimentary OC. Here, we propose a mathematical index, b(AgedOC), to differentiate the causes of sedimentary aged OC, and apply this approach to two typical sediment transport pathways that follow the supply and dispersal of terrestrial OC in the marine environment, and further extrapolate it to submarine canyon, Arctic and deep-sea environments. The index, representing a modification of a widely used quantitative method, provides a novel approach to distinguish the contribution of lateral transport to the pre-depositional aging of OC in the oceanic region. Plain Language Summary Organic carbon (OC) occurring in marine surface continental margin sediments varies in radiocarbon ages, ranging from modern to ancient. While the supply of old terrestrial OC represents a major source of aged OC in marine sediments, sediment redistribution processes in the ocean can also induce old OC ages, although the latter currently remains poorly constrained. We introduce a mathematical method to visualize and distinguish between the causes of aged sedimentary OC that can be applied in regional settings. By applying this method to a suite of surficial sediments from two along-shelf transects that follow the transport and dispersal of terrestrial OC, we demonstrate the importance of hydrodynamic processes on the sedimentary OC ages. Then we further apply this method to other continental margin settings. Delineating between causes leading to old ages of OC in marine sediments bears upon our understanding of the oceanic carbon cycle.