Estimating the abundance of iron sulfide minerals in Minnesota freshwater sediments in relation to pore water and sediment chemistry and wild rice growth

Abstract

Empirical observations of growth of wild rice (Zizania sp.) in wetlands, lakes, and rivers appear to indicate that higher sulfate concentrations lead to a growth deficiency in wild rice. Many growth locations of wild rice in Northern Minnesota freshwater systems have been affected by sulfate leakage, and are within the vicinity of the Mesabi and Cuyuna Iron Ranges (historic and current iron mining) and the Duluth Complex (proposed copper and nickel mining). The dissolved sulfate is reduced to sulfide by microbes in the sediment, and may be sequestered as iron sulfide minerals (e.g., greigite, Fe3S4, and pyrite, FeS2). These minerals are likely to be concentrated where iron is common in local terrestrial geology; pore water pH, as well as organic matter and water content of bottom sediments, are probably also significant. Samples were collected from over 100 research sites spread widely across Minnesota, including water bodies that are naturally high and low in sulfate and iron, as well as water bodies that display recent sulfate increases due to local mine drainage. Petrographic microscopy was used to identify and qualitatively estimate pyrite abundance, while scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) was employed to determine sediment mineralogy at high magnification. Environmental magnetic analyses were conducted on samples selected based on qualitative abundance of pyrite. Preliminary results reveal that samples associated with mining drainage have elevated magnetic grain concentrations with a larger proportion of high-coercivity minerals (e.g., hematite, Fe2O3, and goethite, FeO(OH)) than those from non-mining sites. These minerals are common products of acid mine drainage, and are formed as Fe-bearing silicate minerals are hydrolyzed during acid leaching. The grain size distributions of magnetic minerals were similar for mining and non-mining sites, although lake samples with higher sulfate concentrations appear to correlate with coarser magnetic minerals. Results from this investigation and previously determined chemical analyses data were statistically compared through simple regression analysis, correlation matrix analysis, and Pearson correlation coefficient calculations.