Abstract
The subterranean estuary (STE) has been historically defined in terms of the mixing of saline and fresh water, in an analogy to surface estuaries. However, redox gradients are also a defining characteristic of the STE and influence its role as a source or sink for metals in the environment. Approaching the STE from a redox-focused biogeochemical perspective (e.g., considering the role of microbial respiration and availability of organic matter) provides the ability to quantify drivers of metal transport across spatial and temporal scales. This study measured the groundwater composition of a shallow STE over 2 years and used multiple linear regression to characterize the influence of salinity and redox chemistry on the behavior of redox-sensitive metals (RSMs) including Mo, U, V, and Cr. Molybdenum and uranium were both supplied to the STE by surface water, but differed in their removal mechanisms and seasonal behavior. Molybdenum showed non-conservative removal by reaction with sulfide in all seasons. Sulfide concentrations at this site were consistently higher than required for quantitative reaction with Mo (10 µM sulfide), evidently leading to quantitative removal at the same depth regardless of season. In contrast, U appeared to depend directly on microbial activity for removal, and showed more extensive removal at shallower depths in summer. Both V and Cr were elevated in meteoric groundwater (2.5–297 nM and 2.6–236 nM, respectively), with higher endmember concentrations in summer. Both V and Cr also showed non-conservative addition within the STE relative to conservative mixing among the observed endmembers. The mobility of V and Cr in the STE, and therefore their supply to the coastal ocean, was controlled by the availability of dissolved organic matter and Fe, suggesting V and Cr were potentially complexed in the colloidal fraction. Complexation by different organic matter pools led to seasonal variations in V but greater interannual variability of Cr. These results reveal distinct behaviors of RSMs in response to seasonal biogeochemical processes that drive microbial activity, organic matter composition, and complexation by inorganic species.
Highlights
Submarine groundwater discharge (SGD), advection of water across the seafloor into the coastal ocean, is an important source of nutrients and trace metals to the sea (Spinelli and Fisher 2002; Burnett et al, 2003; Slomp and Van Cappellen 2004; Beck et al, 2009)
Molybdenum was highest in the surface water and shallow subterranean estuary (STE) groundwater, with the 25 nM contour falling between 50 and 150 cm depth, and the 10 nM contour between 100 and 200 cm depth (Figure 1A)
3.1.2 Uranium The mean U concentration for the shallow freshwater endmember (1.2 ± 0.6 nM) tended to be higher than that for the deep freshwater endmember (0.2 ± 0.2 nM), but both were much lower than estuarine surface water concentrations (7.2 ± 0.6 nM) (Table 1)
Summary
Submarine groundwater discharge (SGD), advection of water across the seafloor into the coastal ocean, is an important source of nutrients and trace metals to the sea (Spinelli and Fisher 2002; Burnett et al, 2003; Slomp and Van Cappellen 2004; Beck et al, 2009). The trace metals Mo, U, V, and Cr form soluble oxyanions under oxidizing conditions, but their reduced species are particle reactive (Bruland and Lohan 2006). Due to their redox-active behavior, these RSMs are widely used as marine paleoproxies to examine oceanic oxygen levels, productivity, and circulation over geologic time scales (e.g., Nameroff et al, 2004; Rimmer et al, 2004; Tribovillard et al, 2006; Algeo et al, 2012). The use of elements like Mo and U as paleoindicators is influenced by material budgets controlling their oceanic concentrations and isotopic composition (Klinkhammer and Palmer 1991; Siebert et al, 2003; Archer and Vance 2008)
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