GEOCHEMISTRY OF BIOGENIC PYRITE AND FERROMANGANESE COATINGS FROM A SMALL WATERSHED - A BACTERIAL CONNECTION

Present-day groundwater in an alluvial aquifer in Holocene floodplain deposits in east-central Alabama contains 0.1-4 mg/L Fe, 0.1-0.7 mg/L Mn, similar to 1-10 mu g/L each of Co, Ni, As, Zn, La, and Ce, and 40- 175 mu g/L Ba. There is a distinct correspondence between trace elemen ts present in groundwater and chose concentrated on ferromanganese coa tings on present-day stream alluvium in the study area. This indicates that the reduction and dissolution of such coatings in the alluvial a quifer, probably mediated by Fe- and Mn-reducing bacteria, has been a major control on groundwater chemistry. Authigenic euhedral pyrite cry stals up to 1.5 cm in diameter replace lignitic macro wood fragments n ear the base of the alluvial aquifer, and sulfur isotope data (delta(3 4)S values from +3 to -40%(CDT)) indicate that pyrite precipitated as a consequence of bacterial sulfate reduction in and adjacent to the ir regularly distributed wood fragments. The authigenic pyrite contains s everal hundred parts per million of As, Co, and Ni, indicating that th ese trace elements were coprecipitated in pyrite during bacterial sulf ate reduction. Results suggest a strong geomicrobiological control on trace element cycling in the study area.