Terrestrial Paleoclimate of the Cenozoic: Insights from and Developments of the Soil Carbonate Clumped Isotope Thermometer

Abstract

Land temperature at the surface of the Earth is a first-order parameter used to describe climate, but reliable and widespread measurements of this basic parameter through geologic time has eluded geochemists and geologists for decades. The carbonate clumped isotope geothermometer, which uses the bond-ordering of the carbon and oxygen isotopes in a carbonate mineral, is a relatively new tool that can be used to measure surface temperature on geologic timescales. Chapter 1 introduces carbonate clumped isotope geochemistry. In the subsequent four research chapters, this thesis first refines fundamental methodologies, then improves our understanding of the proxy-system by examining modern soil carbonates, and finally applies the thermometer to reconstruct terrestrial paleoclimate in the early Paleogene. In Chapter 2, I empirically create a clumped isotope-temperature relationship that can be used to estimate the growth temperature of natural carbonates. I rule out the two primary hypotheses that were proposed to explain discrepancies between existing ∆47 -temperature calibrations: 1) synthesis methods caused kinetic isotope effects in calibration samples, and/or 2) the temperature of the acid which was used to digest the calcite for analysis created an analytical artifact. I precipitated >56 synthetic calcite samples at known temperatures, replicating the methodologies used by previously published discrepant calibrations. I analyzed the samples by digesting the calcite at both 90 °C and 25 °C. My results showed that the temperature-∆47 relationship does not vary with precipitation method or acid digestion temperature. Instead, I suggest that the previously observed variations in empirical calibrations were largely due to poor sample replication and small number statistics. I also show the importance of using appropriate constants in the calculation of ∆47 to correct for the¬ mass interference between 13C and 17O during mass spectrometry (as described in companion paper, Schauer, Kelson et al., 2016). Through this research, I produced a new and robust empirical calibration that can be used to calculate growth temperatures from natural calcite materials. Chapter 3 investigates the seasonal bias in soil carbonates by re-examining in aggregate the published ∆47 -temperature data from >200 Holocene soil carbonate samples. In this synthesis, I re-calculate and update ∆47¬ values to reflect modern standards in methodology. The updated data confirm the general assumption that most soil carbonates have a ∆47 temperature that is warm-season biased. However, importantly, I show that modern soil carbonates have ∆47-temperatures that differ from mean annual air temperatures by -5 to 24 °C. The variation in the magnitude of seasonal bias can be partially explained by differences in the annual timing of rain/snow, the texture of the host soil matrix, and vegetative cover. This variation in seasonal bias has profound implications for using soil carbonate ∆47 in paleoclimate applications and underscores the need for a process-based understanding of soil carbonate formation. Chapter 4 builds an understanding of soil carbonate formation that is based in soil physics and chemistry by numerically predicting the timing of calcite growth in a 1D soil profile. Using a software package called HYDRUS-1D (Šimůnek et al., 2009), I calculate soil water content, soil temperatures, soil CO2 productions and concentrations, and carbonate chemistry. I show that the timing of large rain events controls the timing of carbonate accumulation in a soil profile. During storms, soil respiration increases, which increases soil CO2 and dissolves soil carbonate. The subsequent re-precipitation of soil carbonate as soil CO2 decreases post-storm makes up the majority of the total preserved soil carbonate. This result undermines the existing assumption that soil carbonates form slowly during evaporation during dry periods. I show that soil texture and the timing and character of rainfall will control the seasonal bias recorded by soil carbonates. This process-based understanding of soil carbonate formation will enable more nuanced and accurate interpretations of paleo-temperatures and meteoric waters. Chapter 5 uses the clumped isotope composition of fossilized soil carbonates collected in the Tornillo Basin of Big Bend National Park, Texas (30°N) to investigate the greenhouse climate of the Paleocene and early Eocene. I first use a combination of textural evidence and thermal modeling to identify that the isotopic signatures of the soil carbonates collected reflect primary environmental signals. The clumped isotope temperature record derived from these carbonates show an increase from 25 ± 4 to 32 ± 2 °C from the Paleocene to the Eocene, respectively, and a corresponding increase in calculated 18O of soil waters. These temperatures are lower than the temperatures predicted by global circulation models given the high pCO2 conditions and the subtropical location; they are also similar to modern summer temperatures. These results may suggest habitable, near-coast environments persisted even during the peak-CO2 conditions of the early Cenozoic. In conclusion, this thesis expands the ability to reconstruct terrestrial paleoclimates with carbonate clumped isotope thermometry and highlights future challenges. Applications of clumped isotope thermometry are no longer limited by analytical precision in most settings. Instead, robust paleoclimate reconstructions hinge upon accurate characterizations of the seasonal bias of the proxy and diagenetic alteration. A more complete understanding of the evolution of Cenozoic climate on land awaits further analyses from stratigraphic sections that are spatially diverse and temporally overlapping.