PALEOREDOX GEOCHEMISTRY OF THE MIDDLE CAMBRIAN WHEELER SHALE (UTAH, USA): IMPLICATIONS FOR EARLY CAMBRIAN ENVIRONMENTS

Reconstructing paleoredox conditions is significant for paleoecological studies, as nearly all metazoans rely upon oxygen to live. Trace fossil models are a very effective proxy for oxygen concentrations due to their ease of use and applicability in the field. Actualistic trace fossil models are unreliable, however, when nonactualistic environmental conditions prevail in the depositional environment. This is the case during the Ediacaran-Cambrian transition when Proterozoic-style firm unlithified substrates, often with seafloor microbial mats, persisted in many shallow subtidal environments because of the prolonged development of typical Paleozoic levels of bioturbation. In such situations, geochemistry may be particularly useful for understanding paleoredox conditions. The solubility of specific trace metals in marine bottom waters are sensitive to changes in redox conditions, and thus their concentrations in sediments fluctuate with respect to changes in dissolved oxygen. By analyzing the relationship between redox sensitive metals, a paleoredox curve may be developed. To demonstrate the utility of this geochemical analysis, 0.31m of the upper part of the middle Cambrian Wheeler Shale of Utah was analyzed on a mm-scale both for bioturbation levels (ii) and paleoredox conditions. Measurements of enrichment factors for four metals—Ni, V, Zn, and Cr—as well as paleoredox indices for V/Cr and Ni/Co were taken using a Bruker S4 Pioneer XRF. Preliminary results indicate relatively low (ii~1) levels of bioturbation underlying an oxic water column, as determined through the above geochemical proxies. This supports the hypothesis that the Wheeler Shale was dominated by firm Proterozoic-style substrates and contrasts with previous actualistic trace fossil model based paleoredox studies of this unit. This further suggests that, in the presence of nonactualistic environmental conditions, trace fossil models based on modern conditions may not be the most reliable method for reconstructing paleoredox conditions.