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Project Progress

Preserved ancient soils (paleosols) record the interaction of Earth’s lithosphere with its atmosphere and hydrosphere, thus providing one of the few records of ancient atmospheric composition and Precambrian paleoclimate. Capo and her Ph.D. student Sherry Stafford, in collaboration with J. Marmo, are completing their investigation of the Paleoproterozoic Hokkalampi paleosol of eastern Finland, which has the potential to provide constraints on the rise of oxygen in the Earth’s early atmosphere. Their Sm-Nd work demonstrates that the rare Earth elements (REE), important pedogenic and atmospheric tracers, have not been substantially remobilized since pedogenesis at ~2.4 Ga, despite obvious resetting of the Rb-Sr system around 1.8 Ga, and provide the basis for evaluating mobility and “pedogenic fidelity” of selected geochemical tracers. Stafford has begun initial work on an iron isotope investigation of the paleosol to identify possible biologic and/or pedogenic mass fractionation processes.

An important part of these investigations is evaluating modern analogues of ancient geochemical systems, providing a framework for interpreting the geologic record. In ongoing work centered on the formation of soil carbonate, significant variations in rare Earth element abundance and fractionation were observed in pedogenic carbonate from a single profile in the U.S. desert southwest. The preliminary results indicate a complicated interplay between different sources of REE to the profile (atmospheric and weathering of parent material), and mobility of REE within the profile. These processes must be understood in order to interpret REE variations in Precambrian soils. In another modern analogue investigation, iron isotope ratios are being measured in sedimentary pyrite from different depositional environments. Ultimately, the isotopic variations could be correlated with specific biological processes that would then be extended to the Precambrian Earth.