Refining Pedogenic Carbonate Proxies for Improved Reconstructions of Past Terrestrial Environmental Conditions.

Abstract

Continental paleoclimate records provide a means to assess regional climate variability through time and assess how the evolution of the terrestrial biosphere has driven and responded to environmental change. Fossil soils (paleosols) are a particularly useful paleoclimate archive, because they are widely distributed throughout the geologic record. Carbonate clumped isotope paleothermometry is an exciting new proxy for paleosols, as it has the potential to assess temperature seasonality. Yet the processes underlying soil carbonate formation and clumped isotope temperature resetting must be further understood before this proxy can be effectively applied. My dissertation centers on improving understanding of the processes controlling soil carbonate formation and critically evaluating the potential resetting of clumped isotope carbonate data from terrestrial deposits. In Chapter 2, I use modern samples to explore seasonal biases associated with the clumped isotope composition of soil carbonate. The results demonstrate that soil carbonate can form at or below mean annual temperatures. The cold nature of these results is explained by the annual timing of soil water depletion, which is driven by patterns of seasonal precipitation and evapotranspiration. In Chapter 3, modern soil environmental data are compiled to examine how soil temperatures relate to surface air temperatures and to quantify systematic biases that will affect paleosol proxies. Seasonal fluctuations in soil moisture are used to predict the seasonal timing of pedogenic carbonate formation. Soil temperature data indicate that pedogenic carbonate is more likely to record warm season bias relative to mean annual air temperature. In Chapter 4, I use clumped isotope and organic biomarker analyses on the 1.1 Ga Nonesuch Formation to explore how easily the clumped isotope thermometer can be reset on geologic samples and to evaluate the performance of new solid-state reordering models. Using a solid-state reordering model, I illustrate that the synsedimentary and early-diagenetic calcite were partially reset to elevated temperatures. Taken together, these results illustrate factors that must be considered when producing environmental reconstructions from pedogenic carbonate and other terrestrial archives. These findings provide guidance on how to extract accurate paleoclimate information from paleosol carbonate and highlight the need for a process-based understanding of pedogenic carbonate formation.