CARBON ISOTOPIC EVIDENCE OF ANAEROBIC MICROBIAL PROCESSES INVOLVED IN THE FORMATION OF SPHAEROSIDERITES IN PALEOSOLS OF THE CRETACEOUS DAKOTA FORMATION
M. E. Ellis, G. A. Ludvigson, M. E. Raymo, T. S. White, L. D. Young, L. A. Gonzalez, and B. J. Witzke
Sphaerosiderites (SPH) are abundant in buried gleysols of the Albian-Cenomanian Dakota Formation of western Iowa and eastern Nebraska, and are thought to preserve proxy records of the oxygen isotopic composition of shallow groundwaters (Ludvigson et. al., 1998, Geology 26:1039-1042). Different SPH horizons are interpreted to have formed in either purely meteoric phreatic fluids or mixed meteoric-marine fluids, on the basis of carbon-oxygen isotope trends and the absence/presence of pyrite inclusions. In purely freshwater systems, all DIC involved in the formation of SPH is believed to have been generated solely from the decay of plant material (mean = -24.2 ; S.D. = 1.0 ; N = 12), whereas in mixed fluid systems the involvement of marine DIC is considered likely. SPH horizons thought to have formed in meteoric systems have d13C values that range between -44 to -3 (mean = -17.9 ; S.D. = 10.5 ; N = 123), attributable to microbial enrichment and depletion of DIC relative to to source material. SPH with high d13C values are believed to have precipitated from sites of methanogenesis, whereas those with low d13C values are thought to have formed near methanotrophic environments. d13C values of SPH from brackish environments range from -44 to -1 (mean = -16.4 ; S.D. = 9.2 ; N = 81), with higher d13C values not directly attributable to microbial processes, as DIC may be derived from marine bicarbonate. Accordingly, the distribution of SPH d13C values from mixed fluids is skewed toward higher values, whereas SPH d13C values precipitated from meteoric fluid systems have a bimodal distribution, with a mode in the -36 to -29 range that we interpret as indicating that methanotrophy is a volumetrically more important process in this environment. These results indicate a need for understanding the role of reduced iron minerals in redox processes in modern wetland soils.