TRACE METAL CHEMISTRY AND MINERALOGY OF SEDIMENTS HOSTED IN CAVES OF THE SPRINGFIELD (MO) PLATEAU: A LINK TO SUBTERRANEAN BIODIVERSITY?

We used X-ray fluorescence (XRF) and X-ray diffraction (XRD) to compare the heavy metal chemistry and mineralogy of sediments accumulating in caves in rural and urban areas to the chemistry and mineralogy of sediments from a control cave in a relatively undisturbed watershed in the Springfield (MO) Plateau. Sediment from the control cave exhibited the smallest peak sizes for Zn and Mn and a moderately-sized Pb peak. Sediment from the rural caves exhibited larger peaks of Zn and Mn and a smaller Pb peak. Sediment from the urban cave had the largest Zn, Mn and Pb peaks. X-ray diffraction indicated that all samples contained calcite and quartz, which is unsurprising since the caves are hosted in the Burlington Limestone, which is noted for abundant chert nodules. However, minor mineralogy varied widely, and included traces of clay minerals, metal oxides, metal hydroxides, garnet, and complex aluminum-sulfates. Additionally the carbon content of some samples was high enough to trigger an elemental carbon peak that was not consistent with graphite or diamond. These data indicate that trace amounts of base and heavy metals present in sediment are not hosted in sulfides or other primary phases. Instead, these metals likely occur as cations adsorbed onto the surfaces of clay minerals or as metal hydroxides such as wulfingite, an amphoteric zinc hydroxide mineral that was indicated by XRD analysis. Metals hosted in these phases may be liberated by dilute acid solutions. This finding has important implications for the health of troglobitic species, and may explain the apparent relationship between species presence and low metal concentrations in cave sediments outlined in an earlier pilot study. The presence of metals as hydroxides or adsorbed phases in sediments could prove harmful to troglobites that spend a significant portion of their life cycle in sediment (e.g. isopods), especially those species that digest sediment. The acidic digestive tract may increase metal concentrations in primary consumers, potentially reducing fecundity. In addition, secondary consumers may suffer from bioaccumulation as they prey upon species with higher tissue metal concentrations.