Description: The vast majority of freshly produced oceanic dissolved organic carbon (DOC) is derived from marine phytoplankton, then rapidly recycled by heterotrophic microbes. A small fraction of this DOC survives long enough to be routed to the interior ocean, which houses the largest and oldest DOC reservoir. DOC reactivity depends upon its intrinsic chemical composition and extrinsic environmental conditions. Therefore, recalcitrance is an emergent property of DOC that is analytically difficult to constrain. New isotopic techniques that track the flow of carbon through individual organic molecules show promise in unveiling specific biosynthetic or degradation pathways that control the metabolic turnover of DOC and its accumulation in the deep ocean. However, a multivariate approach is required to constrain current carbon fluxes so that we may better predict how the cycling of oceanic DOC will be altered with continued climate change. Ocean warming, acidification, and oxygen depletion may upset the balance between the primary production and heterotrophic reworking of DOC, thus modifying the amount and/or composition of recalcitrant DOC. Climate change and anthropogenic activities may enhance mobilization of terrestrial DOC and/or stimulate DOC production in coastal waters, but it is unclear how this would affect the flux of DOC to the open ocean. Here, we assess current knowledge on the oceanic DOC cycle and identify research gaps that must be addressed to successfully implement its use in global scale carbon models.

Description: Highlights • Continental margin-scale spatial variability in C values among grain size fractions is presented. • Two different hydrodynamic modes influencing in 14C heterogeneity are identified. • A new index (H14 index) is defined to describe overall 14C heterogeneity within marine surface sedimentary OC. Abstract The deposition and long-term burial of sedimentary organic matter (OM) on continental margins comprises a fundamental component of the global carbon cycle. A key unknown in interpretation of carbon isotope records of sedimentary OM is the extent to which OM accumulating in continental shelf and slope sediments is influenced by dispersal and redistribution processes. Here, we present results from an extensive survey of organic carbon (OC) characteristics of grain size fractions (ranging from 〈20 to 250 μm) retrieved from Chinese marginal sea surface sediments in order to assess the extent to which the abundance and isotope composition of OM in shallow shelf seas is influenced by hydrodynamic processes. Our findings show that contrasting relationships exist between 14C contents of OC and grain size in surface sediments associated with two different hydrodynamic modes, suggesting that transport pathways and mechanisms imparted by the different hydrodynamic conditions exert a strong influence on 14C contents of OM in continental shelf sediments. In deeper regions and erosional areas, we infer that bedload transport exerts the strongest influence on (decreases) OC 14C contents of the coarser fraction, while resuspension processes induce OC 14C depletion of intermediate grain size fractions in shallow inner-shelf settings. We use the inter-fraction spread in 14C values, defined here as 14H , to argue that the hydrodynamic processes amplify overall 14C heterogeneity within corresponding bulk sediment samples. The magnitude and footprint of this heterogeneity carries implications for our understanding of carbon cycling in shallow marginal seas.