Abstract
Abstract. Exchanges between sediment pore waters and the overlying water column play a significant role in the chemical budgets of many important chemical constituents. Direct quantification of such benthic fluxes requires explicit knowledge of the sediment properties and biogeochemistry. Alternatively, changes in water-column properties near the sediment-water interface can be exploited to gain insight into the sediment biogeochemistry and benthic fluxes. Here, we apply a 1-D diffusive mixing model to near-bottom water-column profiles of 224Ra activity in order to yield vertical eddy diffusivities (KZ), based upon which we assess the diffusive exchange of dissolved inorganic carbon (DIC), nutrients and oxygen (O2), across the sediment-water interface in a coastal inlet, Bedford Basin, Nova Scotia, Canada. Numerical model results are consistent with the assumptions regarding a constant, single benthic source of 224Ra, the lack of mixing by advective processes, and a predominantly benthic source and sink of DIC and O2, respectively, with minimal water-column respiration in the deep waters of Bedford Basin. Near-bottom observations of DIC, O2 and nutrients provide flux ratios similar to Redfield values, suggesting that benthic respiration of primarily marine organic matter is the dominant driver. Furthermore, a relative deficit of nitrate in the observed flux ratios indicates that denitrification also plays a role in the oxidation of organic matter, although its occurrence was not strong enough to allow us to detect the corresponding AT fluxes out of the sediment. Finally, comparison with other carbon sources reveal the observed benthic DIC release as a significant contributor to the Bedford Basin carbon system.
Highlights
A fraction of the particulate organic matter (POM), generated photosynthetically in the euphotic zone, settles to the sediment, where microbes utilize a variety of electron acceptors to respire this organic material, producing dissolved inorganic carbon (DIC) and nutrients
The application of the 1-D diffusive model, illustrated in Fig. 5b, yields KZ values ranging from 1.0–3.3 cm2 s−1, with uncertainties ranging from 25–30 %, with the exception of the 26 October profile, which yields a value of 25.7 cm2 s−1 with uncertainties near 100 % (Table 1)
This study demonstrates the utility of water-column 224Ra measurements in assessing vertical eddy diffusive coefficients using the simple 1-D diffusion model
Summary
A fraction of the particulate organic matter (POM), generated photosynthetically in the euphotic zone, settles to the sediment, where microbes utilize a variety of electron acceptors to respire this organic material, producing dissolved inorganic carbon (DIC) and nutrients. Sediment pore waters become highly concentrated in many chemical constituents relative to the overlying water column (Moore et al, 2011; Charette et al, 2007), so that small volumes of released fluid can have disproportionately large affects on biogeochemical cycles and budgets (Berelson et al, 1987). Pore water fluxes in the shallow Wadden Sea have been shown to largely control the budgets of numerous important chemical constituents, including alkalinity, silica, manganese and uranium (Moore et al, 2011). Many studies have focused on quantifying the release of pore waters in coastal environments (Colbert and Hammond, 2008; Jahnke and Jahnke, 2000; Hancock et al, 2006; Simmons Jr., 1992), and modelling the biogeochemistry of chemical fluxes across the sediment-water interface (Fennel et al, 2009)
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