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Project Progress

Evolution of Atmospheric O2, Climate, and the Terrestrial Biosphere (dm)

We are continuing work on paleosols (ancient soil horizons) in order to determine atmospheric conditions and evidence for possible terrestrial biota at the time of soil formation. We are focusing on the major and trace element characteristics of the ~3 Ga Steep Rock paleosol, Ontario, Canada. We have found striking trace element and Nd isotope patterns through a combination of whole rock and laser ablation inductively coupled plasma mass spectrometry (ICP-MS) analysis, including a possible preserved Sm-Nd isochron recording soil formation age. We have also initiated work on the ~2.4 Ga Hokkalampi paleosol, Finland. Results show possible preservation of Sm-Nd soil formation ages from at least two profiles in the Hokkalampi sequence, with a range of redox conditions from oxidized, Fe-rich areas to reduced, Fe-depleted areas. We are also comparing oxidized and reduced paleosols from the Permo-Pennsylvanian Monongahela Group. This work also shows rare earth element (REE) fractionation during pedogenesis, independent of redox conditions. The paleosol isotopic results provide the groundwork for development of an in situ geochronology instrument for planetary surfaces. To understand the similarities and differences between modern and ancient soils, we are working to study the isotopic and geochemical evolution of Hawaiian basalts.

Additional work centers on ancient and modern carbonate as recorders of secular variations in marine chemistry, depositional environment, and paleoclimate indicator. We continue to apply Nd isotopes to Paleoproterozoic marine carbonate sequences from Australia and South Africa. Our work to date suggests that most REE in the carbonate is derived from hydrothermal rather than continental sources, consistent with banded iron formation studies. Our studies of Pleistocene carbonate from Owens Lake, California are designed to understand the water-carbonate speciation of REE, and to relate chemical variations to climate shifts. This provides the baseline for studies of ancient carbonates.