Abstract
BackgroundSeasonal changes in pore water and sediment redox geochemistry have been observed in many near-surface sediments. Such changes have the potential to strongly influence trace metal distribution and thus create seasonal fluctuations in metal mobility and bioavailability.ResultsSeasonal trends in pore water and sediment geochemistry are assessed in the upper 50 cm of littoral kettle lake sediments. Pore waters are always redox stratified, with the least compressed redox stratification observed during fall and the most compressed redox stratification observed during summer. A 2-step sequential sediment extraction yields much more Fe in the first step, targeted at amorphous Fe(III) (hydr)oxides (AEF), then in the second step, which targets Fe(II) monosulfides. Fe extracted in the second step is relatively invariant with depth or season. In contrast, AEF decreases with sediment depth, and is seasonally variable, in agreement with changes in redox stratification inferred from pore water profiles. A 5-step Tessier extraction scheme was used to assess metal association with operationally-defined exchangeable, carbonate, iron and manganese oxide (FMO), organic/sulfide and microwave-digestible residual fractions in cores collected during winter and spring. Distribution of metals in these two seasons is similar. Co, As, Cd, and U concentrations approach detection limits. Fe, Cu and Pb are mostly associated with the organics/sulfides fraction. Cr and Zn are mostly associated with FMO. Mn is primarily associated with carbonates, and Co is nearly equally distributed between the FMO and organics/sulfide fractions.ConclusionThis study clearly demonstrates that near-surface lake sediment pore water redox stratification and associated solid phase geochemistry vary significantly with season. This has important ramifications for seasonal changes in the bioavailability and mobility of trace elements. Without rate measurements, it is not possible to quantify the contribution of various processes to natural organic matter degradation. However, the pore water and solid phase data suggest that iron reduction and sulfate reduction are the dominant pathways in the upper 50 cm of these sediments.
Highlights
Seasonal changes in pore water and sediment redox geochemistry have been observed in many near-surface sediments
Peepers were emplaced in areas separated by not more than ~1.5 m, but were positioned in an attempt to capture maximum heterogeneity
This study demonstrates that organic-rich, littoral lake sediments undergo significant seasonal variations in pore water and sediment redox stratification
Summary
Seasonal changes in pore water and sediment redox geochemistry have been observed in many near-surface sediments. Organic-rich freshwater marsh and lake sediments and pore waters show distinct vertical patterns of redox stratification [1,2,3,4,5,6,7,8]. This vertical zonation results, in part, from oxidation of organic matter being coupled to a succession of increasingly less energetically-favorable terminal electron acceptors, e.g. O2, NO3-, Mn(IV), Fe(III), and SO4-2 [9]. In addition to transport via diffusion of dissolved solutes, redox zonation is influenced by transport via advection and due to macrophyte activity, both via bioirrigation and bioturbation, referring to transport of dissolved solutes and particles, respectively
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