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Abstract:

Devils Lake is a 200-km2 terminal lake that contains sodium sulfate type water. Dissolved-solids concentrations range from about 3,500 mg/L to 10,000 mg/L depending on location. To investigate geochemical processes in the bottom sediments of Devils Lake, sediment cores were collected at two sites in the western half of the lake during a period of bottom-water oxygen depletion. The upper 10 cm of the sediments consist of about 60 weight percent silicates (quartz, feldspar, and clays), 35 weight percent carbonates, and 5 weight percent organic material. At depths between 1 and 3 cm in the sediments, bacterial sulfate reduction and associated degradation of organic material cause minima in sulfate concentrations and δ13C values of dissolved inorganic carbon and maxima in alkalinity, ammonia, phosphate, and sulfide concentrations and δ14S values of dissolved sulfate. Downward increases of sodium, magnesium, potassium, and calcium concentrations result from upward diffusion of ions from saline pore water and dissolving sulfate minerals below 30-cm depth in the sediments.

High-magnesium calcite (8 mole percent MgCO3) is the most abundant carbonate at the sediment surface. With increasing depth, abundances of high-magnesium carbonate decrease and abundances of low-magnesium calcite, aragonite, and dolomite increase. Carbon-isotope compositions of bulk carbonates range from δ13C = –0.7 to +0.5‰. These values are close to equilibrium with dissolved inorganic carbon in lake water (δ13C = –2.0‰), but far from equilibrium with dissolved inorganic carbon in pore water (δ13C = –16.3 to –10‰). Disequilibrium between pore water and carbonates suggests that the carbonates did not recrystallize substantially in the presence of pore water. Therefore, the change of carbonate-mineral proportions with depth in the sediments is due mainly to temporal changes in the proportions of endogenic, detrital, and biologic carbonates that were deposited on the lake bottom rather than postdepositional carbonate diagenesis.

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