Abstract
AbstractAlthough the majority of coastal sediments consist of sandy material, in some areas marine ingression caused the submergence of terrestrial carbon‐rich peat soils. This affects the coastal carbon balance, as peat represents a potential carbon source. We performed a column experiment to better understand the coupled flow and biogeochemical processes governing carbon transformations in submerged peat under coastal fresh groundwater (GW) discharge and brackish water intrusion. The columns contained naturally layered sediments with and without peat (organic carbon content in peat 39 ± 14 wt%), alternately supplied with oxygen‐rich brackish water from above and oxygen‐poor, low‐saline GW from below. The low‐saline GW discharge through the peat significantly increased the release and ascent of dissolved organic carbon (DOC) from the peat (δ13CDOC − 26.9‰ to − 27.7‰), which was accompanied by the production of dissolved inorganic carbon (DIC) and emission of carbon dioxide (CO2), implying DOC mineralization. Oxygen respiration, sulfate () reduction, and methane (CH4) formation were differently pronounced in the sediments and were accompanied with higher microbial abundances in peat compared to sand with ‐reducing bacteria clearly dominating methanogens. With decreasing salinity and concentrations, CH4 emission rates increased from 16.5 to 77.3 μmol m−2 d−1 during a 14‐day, low‐saline GW discharge phase. In contrast, oxygenated brackish water intrusion resulted in lower DOC and DIC pore water concentrations and significantly lower CH4 and CO2 emissions. Our study illustrates the strong dependence of carbon cycling in shallow coastal areas with submerged peat deposits on the flow and mixing dynamics within the subterranean estuary.
Highlights
We investigate the mobilization of peatderived dissolved organic carbon (DOC) and biogeochemical processes that control carbon transformations using a novel flow-through column experiment with natural sediments
The experimental cores were exposed to the 50-day flowthrough treatments, whereas the initial cores were kept as a reference until geochemical analysis at the end of the experiment
Total nitrogen was only detectable in the peat layers and followed the depth distribution of Corg (r2 = 0.85, p = 0.007)
Summary
Sediment column experiments are powerful tools to investigate subprocesses and simulate changes of environmental and hydrological conditions. These changes are accelerated by land subsidence of peatland caused by their large-scale drainage for agricultural use. SGD-borne nitrogen fluxes can be of the same order of magnitude as those delivered by rivers (Seitzinger and Harrison 2008; Knee et al 2010; Knee and Paytan 2012). This may be one factor, driving coastal productivity toward phosphorus limitation (Slomp and Van Cappellen 2004). The most abundant process of marine CH4 production is microbial methanogenesis (Cicerone and Oremland 1988), a form of anaerobic respiration using, for example, CO2 instead of O2 as an electron acceptor
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