Abstract
We explore the dynamics of the subsurface sulfur, iron and carbon cycles in salt marsh sediments from East Anglia, United Kingdom. We report measurements of pore fluid and sediment geochemistry, coupled with results from laboratory sediment incubation experiments, and develop a conceptual model to describe the influence of bioturbation on subsurface redox cycling. In the studied sediments the subsurface environment falls into two broadly defined geochemical patterns – iron-rich sediments or sulfide-rich sediments. Within each sediment type nearly identical pore fluid and solid phase geochemistry (in terms of concentrations of iron, sulfate, sulfide, dissolved inorganic carbon (DIC), and the sulfur and oxygen isotope compositions of sulfate) are observed in sediments that are hundreds of kilometers apart. Strictly iron-rich and strictly sulfide-rich sediments, despite their substantive geochemical differences, are observed within spatial distances of less than five meters. We suggest that this bistable system results from a series of feedback reactions that determine ultimately whether sediments will be sulfide-rich or iron-rich. We suggest that an oxidative cycle in the iron-rich sediment, driven by bioirrigation, allows rapid oxidation of organic matter, and that this irrigation impacts the sediment below the immediate physical depth of bioturbation. This oxidative cycle yields iron-rich sediments with low total organic carbon, dominated by microbial iron reduction and no methane production. In the absence of bioirrigation, sediments in the salt marsh become sulfide-rich with high methane concentrations. Our results suggest that the impact of bioirrigation not only drives recycling of sedimentary material but plays a key role in sedimentary interactions among iron, sulfur and carbon.
Highlights
Salt marshes are highly productive coastal wetlands that are flushed daily or monthly with seawater (Howarth, 1993; Alongi, 1998; Chmura et al, 2003)
During oxidation of organic carbon, bacteria can respire a variety of molecules; the order in which these electron acceptors are used broadly reflects the decrease in free energy yield associated with their reduction coupled to organic carbon oxidation (Froelich et al, 1979)
Our sampling presented here and in recent work demonstrates that sediments in salt marshes across East Anglia in the United Kingdom can be divided into two types: iron-rich and sulfide-rich sediment (Figures 2, 3; Hutchings et al, 2019)
Summary
Salt marshes are highly productive coastal wetlands that are flushed daily or monthly with seawater (Howarth, 1993; Alongi, 1998; Chmura et al, 2003). The depth in sediments where oxygen is depleted, other electron acceptors are utilized by microbial populations to respire and continue to oxidize organic carbon. These alternative electron acceptors are used in order of decreasing energy yield: denitrification, using nitrate (NO3−) as the electron acceptor; manganese and iron reduction, with Mn4+ and Fe3+, in mineral form, being the electron acceptors, respectively; sulfate reduction, using aqueous SO42−, and organic matter fermentation into methane (methanogenesis) (Froelich et al, 1979)
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