PREVALENCE OF NITRATE IN CAVE SEDIMENTS OF THE UPPER MISSISSIPPI VALLEY

Nitrate is not easily retained by earth materials such as clays. Nitrate is leached from surface soils by percolating water. Thus, sediments rich in nitrate, discovered in mechanical crevice caves along the Minnesota side of Lake Pepin several years ago, seemed to be anomalous. Further sampling, however, reveals that these natural occurrences of elevated nitrate (up to 2.5% dry weight), though concentrated along the Mississippi River, occur over a wide area in the Upper Mississippi Valley (UMV). Of 62 sites sampled in MN, WI, and IL, 98% had elevated (non-zero) accumulations of nitrate. This was an unexpected result, because the area lies well outside the classic saltpeter-forming cave area in the southeastern United States, as defined by Hill (1981).

The predominant soluble cations in our samples are Ca and Mg followed by Na and K. Samples taken from higher, more exposed rock shelters along the bluffs show Mg/Ca molar ratios greater than one, suggesting that evaporation has precipitated calcite and increased the Mg/Ca ratios above the ratio produced from dolomite solution. The predominant anions are nitrate, sulfate, and Cl. The Cl/Br ratios in minority of samples with enough Br to be accurately measured are in the range of atmospheric precipitation to sea water (200-300), i.e. with little evidence of the high Cl/Br ratios in artificial fertilizer. One site, Miles Cave in Hastings, MN, has very low Cl/Br ratios of about 50. Miles Cave lies under a grain elevator and Br containing fumigants may have contaminated the cave.

The cave nitrate in the UMV caves are likely due to wild animal wastes, pending further investigation. The majority of the UMV nitrate accumulations appear to be boreal guano caves, analogous to the guano caves of the southwestern US. The UMV accumulations do not appear to have formed by a groundwater seepage mechanism as described by Hill (1981).

These occurrences of nitrate in UMV crevices are enough to show that the second-hand report that the French fur-trader Le Sueur found saltpeter (more likely, a saltpeter precursor, such as calcium nitrate) in caves along the shores of Lake Pepin in 1700, for making gunpowder in the wilderness, is credible. That report is the earliest reported occurrence of cave saltpeter in North America, predating documentation of French saltpeter manufacture from Missouri caves in 1720.