The nitrate deposits of rock crevices in the Upper Mississippi Valley

Abstract

Prompted by French fur-trader Pierre-Charles Le Sueur's 1700 report of caves containing saltpeter (potassium nitrate) along the Minnesota shore of Lake Pepin, this study located what are thought to be the original caves and revealed a hitherto unsuspected widespread district of cave nitrate deposits in the Upper Mississippi Valley (UMV). Of the 103 caves sampled from nine geologic formations, 67% had elevated nitrate concentrations (>50 ppm NO3) in their sediments relative to surrounding surface soils (<10 ppm NO3). These nitrate concentrations, up to 3.5 percent dry weight of sediment (35,000 ppm), are comparable to the concentrations found at Mammoth and Dixon caves, Kentucky, an historical nineteenth-century saltpeter mining locality, which range from 0.01 to 4 percent. But the Hill (1981) seeping groundwater model, which satisfactorily accounts for the nitrate deposits of the historic saltpeter caves of the southeastern United States, does not apply in the UMV. Through the application of geochemical and isotopic analysis to soil solutions it was concluded that the source of the nitrate was organic matter such as guano, scats, urine, carcasses, and plant materials, and that the nitrate accumulated because the cave roof protected the soluble nitrate ion from leaching and by blocking out sunlight, prevented plants from growing and thus extracting this nutrient. Le Sueur's claim of finding saltpeter (more likely, a saltpeter precursor, such as calcium nitrate) in Minnesota caves, for making gunpowder in the wilderness, is thus credible.