Abstract
The ecological functions and biogeochemical processes of continental marginal seas are important for the global carbon cycle. In the eastern China marginal seas (ECMS), phytoplankton productivity has increased significantly in recent decades, but the sources and burial processes of marine organic carbon (OC) remain under-studied. We analyzed the contents of marine lipid biomarkers (brassicasterol, dinosterol, and C37 alkenones) in surface suspended particles obtained from seven cruises between 2010 and 2015, and in surface sediments from eight cruises between 2006 and 2012 in the ECMS, to estimate marine OC sources and burial. The correlations between lipid biomarkers and environmental factors were quantified to reveal controlling factors. The study area was divided into four regions according to cluster analysis conducted based on sediment parameters. Our results showed that the concentrations of marine lipid biomarkers in surface water were high near large estuaries such as the Changjiang River Estuary and the Yellow River Estuary, but those in surface sediments were high in mud areas. Nutrient concentration was a key factor controlling phytoplankton biomass in surface water, with high nutrients enhancing the growth of diatoms and dinoflagellates, while haptophytes were more abundant in low-nutrient, high-salinity and cold environments. High marine OC contents calculated from total OC δ13C were mainly associated with fine sediments transported by coastal currents, and finally deposited in mud areas. The proto-burial efficiency of marine OC in the ECMS (7–19%) was markedly higher than the mean value in global marginal seas, with high values being located in the western coast of the ECMS. The proto-burial efficiency of total marine lipid biomarkers (6–24%) was quantified for the first time in our study, with overall values and spatial patterns similar to that of marine OC. A key parameter for marine OC and marine biomarker proto-burial efficiency was sediment grain size. The smaller the sediment grain size was, the better the marine OC and marine biomarker were retained. Our study provides an important basis to elucidate spatial distribution patterns and forcing mechanisms of marine OC in surface water (production process) and surface sediments (burial process), and to estimate carbon budgets in large marginal seas.
Highlights
The ocean is a large carbon reservoir, absorbing about 30% of CO2 released by human activities in the recent global carbon budget, with an average absorption rate of about 2.6 Pg C yr−1 (DeVries, 2014; Gruber et al, 2019), and it plays a key role in the global carbon cycle
Our study revealed the spatial distributions of marine lipid biomarkers in a large area of the eastern China marginal seas (ECMS) by simultaneously analyzing summertime surface suspended particles and surface sediments from 334 stations and 258 stations, respectively
Our results showed that marine organic carbon (OC) generation was higher in estuaries and upwelling areas, which was mainly controlled by nutrient concentrations, while marine OC deposition and burial mainly occurred in mud areas with less dynamic sedimentary environments
Summary
The ocean is a large carbon reservoir, absorbing about 30% of CO2 released by human activities in the recent global carbon budget, with an average absorption rate of about 2.6 Pg C yr−1 (DeVries, 2014; Gruber et al, 2019), and it plays a key role in the global carbon cycle. Marginal seas only occupy about 20% of ocean surface area, about 80–90% of global marine sediment OC is stored there, constituting an important OC reservoir on the earth’s surface (Burdige, 2007; Keil, 2017). There are two main sources of OC in marginal seas, i.e., terrestrial OC transported by river runoff and dust deposition, and marine OC produced by marine primary producers in the euphotic zone (Zonneveld et al, 2010). The role of marine OC in the carbon cycle cannot be ignored This is because human activities have induced serious eutrophication in marginal seas, especially in river estuaries (Anderson et al, 2002). High sedimentation rates in marginal seas contribute to high OC burial efficiency (Burdige, 2007). Previous studies have focused on the sources and burial processes of terrestrial OC (Burdige, 2005; Karlsson et al, 2011; Fang et al, 2015; Smith et al, 2015; Zhao et al, 2021), while these processes of marine OC in marginal seas have not been fully understood
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