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Project Progress

(1) Precambrian paleosols as indicators of paleoatmosphere composition
Steep Rock Group, Superior Province, Canada (Capo, Stewart, Stafford, Macpherson, Ohmoto): Paleosols (preserved ancient soils) can preserve a record of past atmospheric chemistry, though the evidence is often complicated by later diagenetic and metamorphic events. Our current efforts focus on characterization of both weathering-induced changes in bulk chemistry and distribution of trace elements among pedogenic and metamorphic phases in Precambrian paleosols. Preliminary results from a >2.7 Ga paleosol developed on tonalite shows geochemical signatures consistent with either formation in a reducing environment or formation under oxidizing conditions followed by hydrothermal alteration. Initial laser ablation-ICP-MS experiments show distinctive rare earth element patterns from coexisting carbonate, Fe-rich matrix materials, and silicate minerals.
(2) Paleoenvironmental determination: Precambrian and modern carbonates
Eastern Transvaal district, South Africa (Stewart, Capo, Watanabe, Ohmoto): A 2.6 Ga carbonate-rich sequence at the Schagen locality contains evidence of photoautotrophic organisms that may have lived in a terrestrial playa/paleosol environment. Strontium isotope results from sequential leaching experiments are consistent with multiple stages of carbonate formation in a shallow freshwater depositional environment, with variable interaction between surface-derived waters and ultramafic host rock. Our data suggest that the C isotope signatures measured from this paleosol section represent primary signatures.
Paleoproterozoic marine carbonates (Stewart, Capo, Bau): Early Proterozoic marine carbonates can potentially provide important constraints on ancient oceanic element transport and oxidation state. We have begun a Sr-Nd isotopic study of Paleoproterozoic sequences from Australia and South Africa. Initial results suggest that original marine 87Sr/86Sr ratios are preserved in several of the samples, with values as low as 0.7041 in 2.5-2.6 Ga Hammersly carbonates, and 0.7051 in 2.4 Ga Moodrai carbonates. The Nd isotope work in progress will be one of the first to examine 143Nd/144Nd of Precambrian carbonate sediments.
Nama Group, Namibia (Stewart, Capo, Ono, Ohmoto): Limestones from the Nama group span the important period in Earth’s history when shelled organisms first appeared. We show that microdrilled and whole rock samples of ~0.6 Ga sedimentary rocks from the Zebra River Member of the Zaris Formation (ZR Series) fall within the range of Vendian-Cambrian marine 87Sr/86Sr values, while data from the shelly fossil containing Kuibis Subgroup at Hauchabfontain (HF Series) are suggestive of a freshwater incursion. We are currently attempting to distinguish between isotopic effects of deposition in lacustrine systems and overprinting of marine limestones by meteoric waters, either of which may affect ancient carbonates.
Modern terrestrial carbonates (Stewart, Capo, Chadwick, Pretti, Minervini, Reynolds): We are conducting isotopic studies of modern lacustrine analogues including Pleistocene-Holocene lake sediments from Searles and Owens Lake, California, and soil carbonate in continental and tropical environments (Hawaii and New Mexico). Our goal is to identify means of distinguishing between marine, lacustrine, pedogenic, and diagenetically altered carbonate, with the hope of better characterizing ancient carbonate depositional environments. Initial work has focused on relating modern-day cation fluxes to the strontium isotopic evolution of lacustrine and pedogenic systems.