Widespread dispersal and aging of organic carbon in shallow marginal seas

Abstract

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.

Deconvolving the Fate of Carbon in Coastal Sediments

“…We show that the controls on sedimentation signature are highly spatially variable, implying that development of accurate carbon budgets and interpretation of sedimentary records for such marginal sea systems requires further sub-division into discrete zones. Utilization of the two criteria adopted here (TOC and 14 C) results in the deconvolution of the complex mosaic of organic matter properties observed by Bao et al (2016) into five distinct "impact zones". These zones each have geochemical characteristics with different factors contributing to the distinctive geochemical signatures including carbon and sediment sources, as well as reworking and redistribution processes.…”

“…The self-clustering approach adopted here harnesses information held not only in TOC content (Seiter et al, 2004) but also the 14 C signature of the TOC in order to resolve sources and processes contributing to the spatially heterogeneous mosaic of carbon signals that characterize marginal sea sedimentary environments. The latter provides key constraints on carbon sources and the effect of sediment reworking on organic matter composition (Bao et al, 2016;Tao et al, 2015). Combining both geochemical characteristics, we delineate the CMS system into distinct "carbon impact zones" that take into account both carbon provenance and sedimentation processes and afford a greater degree of spatial resolution than hitherto available (Honjo et al, 2008;Longhurst & Glen Harrison, 1989;Seiter et al, 2004), providing a powerful new framework for constraining regional carbon budgets.…”

“…In addition to diverse, and time-evolving inputs, complex processes operating within the CMS impart strong gradients in carbon processing and dynamics that hinder extrapolations and derivation of system-wide properties and fluxes. This complexity in the CMS clearly manifests itself in a spatially highly resolved 14 C dataset of surface sediment OC (Bao et al, 2016).…”

“…Increasing anthropogenic disturbances have strongly impacted the movement of carbon from land to ocean on a global scale, leading to altered terrestrial sediment fluxes (Bauer et al, 2013;Ciais et al, 2013;Keil, 2017;Regnier et al, 2013;Syvitski et al, 2003). Marginal seas are complex and dynamic areas exhibiting a high degree of spatial and temporal heterogeneity (Bao et al, 2016;Cai, 2011). Globally, delta and shelf environments represent hotspots of carbon sequestration, accounting for >85% of OC buried in modern marine sediments (Hedges & Keil, 1995).…”

“…Unfortunately, the lack of data points in this region precludes rigorous statistical analysis and assessment of correlations. Moreover, sharp differences within this dataset preclude quantitative analyses, likely owing to the distinctive characteristics of OC associated with sediments rich in petrogenic OC supplied from Taiwan (Bao et al, 2016;Kao et al, 2014). The impact of these inputs are superimposed on those due to hydrodynamic sorting, Changjiang inflow, marine productivity, conspiring to limit the success of clustering analysis in this region.…”