Large freshwater lakes, despite their socioeconomic importance, are insufficiently characterized in terms of their geochemical cycling. In systems such as Lake Superior, contributions of several important processes, including those affecting biological productivity, remain poorly quantified. To understand the geochemical controls on sediment diagenesis, we investigated sediments in well-oxygenated temperate Lake Superior and tropical meromictic Lake Malawi. We characterized solid-sediment and porewater geochemistry, calculated diagenetic rates and fluxes, and investigated temporal and geographic variability for the cycles of carbon, nitrogen, phosphorus, iron, and sulfur. Revised nutrient budgets (for N and P) were constructed for both sediment and water column, suggesting a significant contribution of sediments to the geochemical cycling in both lakes. Sedimentation rate and the depth of oxygen penetration (OPD) were found to strongly affect the dynamics of carbon and nutrients. In Lake Superior, the deep (>4 cm) oxygenation of sediments in low-sedimentation areas regulates the remineralization rates of carbon and phosphorus, controls denitrification rates, and creates an unusual sulfur cycle driven by the oxidation of organic sulfur to sulfate. It also makes these deeply oxygenated sediments qualitatively distinct from sediments in nearshore high-sedimentation areas, necessitating their separate consideration in geochemical budgets. Comparisons against data from marine environments suggest that sediment processes in large lakes (both temperate and tropical) can be described by the same quantitative relationships as in marine sediments, facilitating the transfer of knowledge.
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University of Minnesota (Minneapolis, Minnesota)
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