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 µm) and finer (<63 µm) sedimentary components, with measurements on specific grain size fractions revealing that it is especially prevalent Page 2 of 17 within the 20-63 µm 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 pre-depositional 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.

Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon (14C) ages on two groups of molecular tracers of plant-derived carbon—leaf-wax lipids and lignin phenols—from a globally distributed suite of rivers. We find significant negative relationships between the 14C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil 14C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change–induced perturbations of soil OC turnover and stocks.

Compound‐specific radiocarbon analysis was performed on different grain‐size fractions of surficial sediments to examine and compare lateral transport times (LTTs) of organic carbon. 14C aging of long‐chain leaf wax fatty acids along two dispersal pathways of fluvially derived material on adjacent continental margins implies LTTs over distances of ~30 to 500 km that range from hundreds to thousands of years. The magnitude of aging differs among grain size fractions. Our finding suggests that LTTs vary both temporally and spatially as a function of the specific properties of different continental shelf settings. Observations suggest that 14C aging is widespread during lateral transport over continental shelves, with hydrodynamic particle sorting inducing age variations among organic components residing in different grain sizes. Consideration of these phenomena is of importance for understanding carbon cycle processes and interpretation on sedimentary records on continental margins.

Surface sediments from the Changjiang Estuary and adjacent shelf were analyzed using a variety of bulk and molecular techniques, including grain size composition, sediment surface area (SSA), elemental composition (C, N), stable carbon isotopic composition (δ 13 C), n-alkanes, lignin phenols, and glycerol dialkyl glycerol tetraether lipids to obtain a more comprehensive understanding of the sources and fate of sedimentary organic carbon (SOC) in this dynamic region. Bulk N/C ratios of 0.09 to 0.15, δ 13 C of À24.4‰ to À21.1‰, branched/isoprenoid tetraether index of 0 to 0.74, n-alkane content of 0.02 to 0.37 mg g À1 organic carbon (OC), and lignin content (Λ 8 ) of 0.10 to 1.46 mg/100 mg OC and other related molecular indices in these samples indicate a mixed source of marine, soil, and terrestrial plant-derived OC in the study area. A three-end-member mixing model using principal component analysis (PCA) factors as source markers and based on Monte Carlo (MC) simulation was constructed to estimate the relative contributions of OC from different sources. Compared with traditional mixing models, commonly based on a few variables, this newly developed PCA-MC model supported bulk and biomarker data and yielded a higher-resolution OC inputs to different subregions of this system. In particular, the results showed that the average contributions of marine, soil, and terrestrial OC in the study area were 35.3%, 47.0%, and 17.6%, and the highest contribution from each OC source was mainly observed in the shelf, inner estuary, and coastal region, respectively.