HR: 08:30h
AN: B21C-03 INVITED     [Abstracts]
TI: Black Shales as Sources of Trace Metals to the Weathering Environment
AU: * Cruse, A M
EM: anna.cruse@okstate.edu
AF: Oklahoma State University, 105 NRC, Stillwater, OK 74078-3031 United States
AB: Black shales are known to be enriched in a host of trace metals (Fe, Zn, Pb, U, V, Cr, Ni, Mo), although the causal mechanisms remain highly debated. Pennsylvanian black shales of midcontinent North America are no exception, with many beds enriched in transition metals by as much as two to three orders of magnitude relative to the Post-Archean Shale Standard (PAAS). High-resolution sampling of the Hushpuckney Shale Member in several cores indicates that bulk concentrations of a broad suite of transition metals are highly enriched within the black shale facies compared to the over- and underlying gray shales. However, the level of enrichment varies substantially in cores from different locations (Iowa, Missouri and Oklahoma). For example, Fe/Al and Pb/Al ratios in the Oklahoma core are as much as six and thirty times higher, respectively, than those calculated for PAAS, whereas in Iowa, Fe/Al ratios exhibit only minor enrichment relative to PAAS. However, the concentrations of other metals, such as Zn, U, V and Mo in the Iowa core, are several hundred times higher than PAAS. Such enrichments of bulk metal concentrations in black shales relative to organic-lean gray shales have previously been attributed to paleoceanographic conditions (specifically, the redox condition of bottom or pore waters). However, based on geographic differences of metal enrichments in the same strata, it is clear that other factors will attenuate or enhance the primary redox effects. The rate of siliciclastic input, organic matter input and potential secondary fluid sources, such as hydrothermal waters, will affect metal concentrations in specific cores. These processes also affect the phase in which metals are sequestered. For example, in Pennsylvanian black shales, Fe occurs predominantly as a sulfide, while Mo and Zn appear to be controlled by the organic matter reservoir. Rocks do not weather; minerals do. Therefore, in order to fully evaluate black shales as a source of potentially toxic metals to the environment, studies of metal concentrations in these rocks need to begin to focus on the specific phase in which the metal is found, rather than relying on bulk values. Bulk metal concentrations, in conjunction with other geochemical parameters, provide an initial framework within which the specific nature of metal sequestration-and therefore, their potential for weathering-can be evaluated.
DE: 0404 Anoxic and hypoxic environments (4802, 4834)
DE: 0461 Metals
DE: 1051 Sedimentary geochemistry
DE: 1055 Organic and biogenic geochemistry
DE: 1065 Major and trace element geochemistry
SC: Biogeosciences [B]
MN: Fall Meeting 2005