Abstract
Coastal sediments play a fundamental role in processing anthropogenic trace metal inputs. Previous studies have shown that terrestrial organic matter (OM) is a significant vector for trace metal transport across the land-to-sea continuum, but little is known about the fate of land-derived metal-OM complexes in coastal sediments. Here, we use a comprehensive set of sediment pore water and solid-phase analyses to investigate how variations in terrestrial OM delivery since the 1950s have influenced trace metal accumulation and diagenesis in a human-impacted boreal estuary in the northern Baltic Sea. A key feature of our dataset is a strong correlation between terrestrial OM deposition and accumulation of metal-OM complexes in the sediments. Based on this strong coupling, we infer that the riverine input of terrestrial metal-OM complexes from the hinterland, followed by flocculation-induced settling in the estuary, effectively modulates sedimentary trace metal sequestration. While part of the trace metal pool associated with these complexes is efficiently recycled in the surface sediments during diagenesis, a substantial fraction is permanently buried as refractory metal-OM complexes or through incorporation into insoluble sulfides, thereby escaping further biological processing. These findings suggest that terrestrial OM input could play a more pivotal role in trace metal processing in coastal environments than hitherto acknowledged.
Highlights
Excess human-induced trace metal(loid) loading to aquatic environments may entail serious risks to coastal ecosystems and human health due to their toxicity, persistence and effective bioaccumulation in food chains (Rainbow, 2007; Rainbow and Luoma, 2011; Yi et al, 2011; Amato et al, 2015; Richir and Gobert, 2016)
General diagenetic zonation At all stations, the profiles of major pore water constituents (SO42−, ΣS2−, NH4+, HPO42−, Fe2+ and Mn2+) display a broadly similar pattern (Fig. 2) that conforms to the classical diagenetic zonation of coastal sediments, reflecting sequential consumption of electron acceptors linked to microbial organic matter (OM) oxidation (e.g. Froelich et al, 1979)
In contrast to Cd, we suggest that the gradual downward increase in As, Co, Cu, and Ni in F5 within the sulfatemethane transition zone (SMTZ) reflects their progressive incorporation into pyrite, which is consistent with previous studies on trace metal sequestration in sulfide-bearing sediments (Huerta-Diaz and Morse, 1992; Scholz and Neumann, 2007; Morgan et al, 2012)
Summary
Excess human-induced trace metal(loid) ( referred to as trace metals) loading to aquatic environments may entail serious risks to coastal ecosystems and human health due to their toxicity, persistence and effective bioaccumulation in food chains (Rainbow, 2007; Rainbow and Luoma, 2011; Yi et al, 2011; Amato et al, 2015; Richir and Gobert, 2016). These riverborne constituents become exposed to multitudinous sorption/desorption, flocculation/deflocculation and aggregation/breakup processes, which often lead to non-conservative behavior of trace metals across estuarine gradients (Sholkovitz, 1978; Boyle et al, 1977; Millward, 1995; Dassenakis et al, 1997; Premier et al, 2019). In these processes, the fate and environmental effects of associated trace metals are largely dependent upon their speciation, which directly influences metal reactivity, bioavailability, and toxicity (Passos et al, 2010; Canuto et al, 2013). Trace metal species sourced from anthropogenic activities are more mobile than the lithogenic background pool of metals (Salomons and Förstner, 1980; Yuan et al, 2004; Heltai et al, 2005; Passos et al, 2010) and more readily enter aquatic food chains
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