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Project Progress

Paleosols, preserved ancient soils, are significant to the study of past life, climate, and landscapes on Earth and as analogues for the environment in which terrestrial life might develop on other planets. However, interpretation of the ancient soil record presents many challenges, including difficulty constraining ages, overprinting by thermal events, and deformation of soil textures and structures. To gain insight into element mobility under different soil-forming conditions, M.S. student Amanda Reynolds investigated the geochemical and isotopic characteristics of Paleozoic paleosols and lacustrine sediments formed under varying redox environments. She found that reducing conditions within and between “redbed” paleosols and green gleyed paleosols with similar parent materials led to distinct Sr and Nd isotopic compositions. Ph.D. student Sherry Stafford returned to Finland for additional sampling of the ~2.3 Ga Hokkalampi paleosol of eastern Finland. Recent results, including isotopic and LA-ICPMS analysis with Associate Member Macpherson, suggest preservation of Sm-Nd isochrons that represent the time of soil formation, and a decoupling of iron loss and redox sensitive trace elements, including REE. Moreover, initial strontium isotope data indicate that the Rb-Sr system can be used to determine the age of metamorphism that partially overprinted paleosol geochemistry.

Terrestrial carbonate minerals formed in soil profiles (e.g., calcretes) can also record paleoenvironmental conditions. In collaboration with Associate Member Oliver Chadwick and Ph.D. student Charles Whipkey, soil carbonate was examined in three Quaternary environments: (1) New Mexico soil (dust dominated); (2) Hawaii chronosequence (basalt weathering), and (3) South Point, HI (marine influenced). Strontium isotope measurements were combined with micromorphologic studies, major and trace element analyses, and mass balance calculations in order to examine the locations, rates and mechanisms of mineral weathering within arid to semiarid soils and identify shifts in the relative contributions of silicate weathering and exogenous input to the soil cation budget as a function of climate change. These results can be applied to Precambrian soil carbonates to understand early Earth environments.