Centers of organic carbon burial and oxidation at the land-ocean interface

Continental margin systems collectively receive and store vast amounts of organic carbon (OC) derived from primary productivity both on land and in the ocean, thereby playing a central role in the global carbon cycle. The land-ocean interface is however extremely heterogeneous in terms of terrigenous input, marine primary productivity, sediment transport processes and depositional conditions (e.g. such as bottom water oxygen level). Continental margins are also highly dynamic, with processes occurring over a broad range of spatial and temporal scales. The rates of OC burial and oxidation are consequently variable over both space and time, hindering our ability to derive a global picture of OC cycling at the land-ocean interface. Here, we review the processes controlling the fate of organic matter in continental margin sediments, with a special emphasis on “hot spots” and “hot moments” of OC burial and oxidation. We present a compilation of compositional data from a set of illustrative settings, including fjords, small mountainous river margins, large deltaic systems and upwelling areas. Bulk OC stable isotope and radiocarbon compositions reveal the diversity and complexity characteristic of OC buried in marginal seas. This primarily relates to differences in marine and terrestrial inputs, the composition of the terrestrial component (e.g. vascular plant OC, soil, and petrogenic OC inputs), and processes modulating the fate of OC within the marine environment (e.g. priming). This widely contrasting behavior of OC among these systems illustrates that the reactivity of OC is a product of its chemical composition and regional conditions. Interpreted in the context of bulk compositional data as well as that obtained on specific molecular markers (e.g. lignin-derived phenols), the possibility exists to tease apart complex mixtures of terrestrial and marine inputs, and to shed light on the role of the myriad of depositional and post-depositional processes. Finally, we discuss a set of hot topics that warrant further investigation - such as the role of photochemistry, fungi, halogenation and reactive iron in dictating the fate of OC in the (changing) coastal ocean.