GEOCHEMICAL ANALYSIS OF CAP-CARBONATES AND CAP-LIKE CARBONATES OF THE DEATH VALLEY REGION

The snowball Earth hypothesis of Hoffman et al. (1998) proposes dramatic post-glacial chemical weathering as large concentrations of carbon were removed from the atmosphere. This weathering would result in a large input of terrigenous material into the oceans. We sampled cap carbonates (Noonday Dolomite) and cap-like carbonates (Beck Spring Dolomite) of the Death Valley region in order to explore elemental changes in post-snowball Earth oceans. An Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used to analyze a suite of 25 trace elements. The elements U, V, Ni, Co, Cr, and Fe show a systematic decrease in concentration from moderate values at the base of the sections to low values in the middle of the sections, and rapidly increasing concentrations at the top of the sections, suggesting systematic changes in oxidation state throughout the sections. Manganese, which accumulates in relatively oxic sediments, shows a different, yet compatible, trend due to its geochemical behavior. Manganese concentration profiles are high near the bottom of the section, and gradually decrease from the middle to the top of the section. To further support our ICP-OES data, Raman spectroscopy was used. Uranium inclusions were found within different parts of the Noonday formation. Reduced U (as uraninite, UO2) was found in the highest sections (where U concentrations exceeded 85 mg/kg), while uranophane, a uranyl silicate indicative of more oxic conditions, was identified in the middle of the section (which is from the region that is low in total U).