Abstract
The burial of organic carbon (OC) in the river-dominated margin plays an important role in global carbon cycle, but its accumulation mechanism is not well understood. Here, we examined the concentration and distribution of water-extractable organic matter (WEOM) and base-extractable organic matter (BEOM) in surface sediments from the lower Yangtze River, estuary, and the East China Sea. Chemical characteristics of the WEOM and BEOM were described by multiple ultraviolet-visible and fluorescence spectral indicators. Concentrations of both WEOM and BEOM showed significant correlations with sediment grain size, suggesting that mineral surface area is a key factor for OC loadings on sediments. Three components (C1, C2, and C3) extracted from fluorescence excitation emission matrices-parallel factor analysis were assigned as terrigenous humic-like substance, mixed terrigenous/aquatic humic-like substance, and microbial protein-like substance, respectively. From the lower Yangtze River to the East China Sea, the C1%, specific UV absorbance at 254 nm (SUVA254), and humification index (HIX) of the WEOM decreased, while the C3%, fluorescence index (FI), and biological index (BIX) of the WEOM increased. This suggested the loss of terrigenous OC and addition of microbial OC in the WEOM. While for BEOM, the overall increase of C1% and HIX and the decrease of C3% and FI suggested selective removal of microbial OC and preferential preservation of terrigenous OC. Our study demonstrates complex behaviors of sediment organic matter (OM) during the land-to-sea transport that is largely controlled by the binding strength of OM–sediment association, and that the formation of BEOM is an important pathway for accumulation of terrigenous OM in the river-dominated margin.
Highlights
Each year, rivers discharge approximately 0.4 × 1015 g of particulate organic carbon and dissolved organic carbon (DOC) to the sea (Hedges et al, 1997), representing a key conduit linking carbon pools of continent to ocean
Given that the lower Yangtze River is close to Shanghai, a megacity with over 23 million inhabitants (Figure 1), the anthropogenic input could be another source to the sedimentary OC that was reflected by the enrichment of protein-like, biolabile substance in the dissolved organic matter (DOM) of water columns (Guo et al, 2014)
The biological index (BIX), fluorescence index (FI), specific UV absorbance at 254 nm (SUVA254), C1%, and C3% correlated with water salinity and sediment grain size for the water-extractable organic matter (WEOM) and base-extractable organic matter (BEOM), respectively
Summary
Rivers discharge approximately 0.4 × 1015 g of particulate organic carbon and dissolved organic carbon (DOC) to the sea (Hedges et al, 1997), representing a key conduit linking carbon pools of continent to ocean. During the transport from land to sea, dissolved organic matter (DOM) and particulate organic matter (POM) were partly degraded and transformed from each other (Bianchi et al, 2002; He et al, 2016). The physical and chemical protection by sediment matrix is an important mechanism for the stabilization of organic carbon (OC) in environments (Mayer, 1994; Kleber et al, 2007; Schmidt et al, 2011; Lalonde et al, 2012). The investigation of concentration, composition, and transformation of WEOM and BEOM could shed light on the degradation and preservation of OM in aquatic environments
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