Organic Matter Sources and Black Carbon Fractions Shape Mercury Partitioning in Intertidal Sediments of the Yangtze Estuary

Abstract

Estuarine intertidal sediments are important sinks for riverine mercury (Hg), yet the effects of organic matter sources and BC fractions on Hg partitioning are still poorly understood. We analyzed sediments from reed-covered and unvegetated flats in the Yangtze Estuary for total Hg (THg), operationally defined Hg fractions, total organic carbon (TOC), total nitrogen (TN), n-alkanes, and BC fractions (char and soot). THg ranged from 60.3 to 228.5 ng g−1, averaging 114.6 ± 38.9 ng g−1. THg was significantly higher in reed-covered sediments and in surface layers, and was positively correlated with TOC and TN (p < 0.01, n = 20), indicating that vegetation-driven organic matter accumulation promoted Hg enrichment. Hg partitioning differed between habitats: reed-covered sediments showed co-dominance of oxidizable Hg and residual Hg, whereas unvegetated flats were dominated by residual Hg (62.0 ± 10.8%), suggesting stronger Hg stabilization in the absence of fresh plant inputs. n-Alkane patterns indicated that vegetation type controlled sediment organic matter composition more strongly than site or depth. While no significant correlations were observed between BC fractions and Hg fractions in individual habitats, soot exhibited significant positive correlations with labile Hg (acid-soluble and reducible Hg) in a depth-averaged analysis of merged surface and subsurface layers (p < 0.05, n = 10), a preliminary signal constrained by the small number of sites, which suggests that soot may facilitate the sedimentary retention of reactive Hg. These results highlight source-specific controls on Hg partitioning beyond bulk TOC.

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