Terrestrial paleoenvironmental reconstruction, from mountaintops to sea

摘要

This work investigates three aspects of past conditions on land: the nature of a greenhouse-icehouse climate transition, the growth of a mountain range and its rain shadow, and the availability of atmospheric oxygen. The tools applied to these tasks are sedimentology and stable isotope geochemistry, particularly the stable isotopes of hydrogen (deltaD), oxygen (delta 18O), and chromium (delta53Cr), which are useful for tracking (bio)geochemical processes of interest that occur in surface environments.;In Chapter 1, I present a new record of the Eocene-Oligocene transition (EOT), the first terrestrial record from the southern hemisphere to observe this rapid shift from a greenhouse to icehouse climate. The EOT is well described in marine records, but its terrestrial expression is only known from a handful of sites, of which all are in the Northern Hemisphere and many are continental. Using the hydration water of volcanic glass, I reconstructed precipitation deltaD at high resolution across this transition. Using modern regional deltaD-temperature (T) relationships and Eocene-Oligocene deltaD-T relationships from climate modeling, I reconstruct terrestrial air temperature change during this climate transition. The results show that temperature fell by 5°C before experiencing a partial rebound to 2°C below pre-EOT levels, a pattern that also appears in marine records. The timing and size of this climate event is consistent with northern hemisphere terrestrial records, suggesting that (1) the EOT was a globally synchronous climate shift and (2) the cooling was likely driven by falling atmospheric pCO2 levels.;In Chapter 2, I investigate the paleoclimate imprint left by the Patagonian Andes, using the strength of the rain shadow as a way to track the size of the range during the Cenozoic. The traditional view of the Patagonian Andes is that the region had low topography until the Middle-Late Miocene, when topography grew to approximately its modern size. However, this timing uplift is largely based on coincidence with relatively minor tectonic activity and a single equivocal paleoclimate study. Geologic evidence indicates that the majority of shortening and magmatism in the Patagonian Andes occurred during the Late Cretaceous. I use volcanic glasses from three sedimentary sections to reconstruct a composite history of precipitation deltaD from the Paleocene to Miocene. In order to meaningfully interpret the strength of the rain shadow, I estimate the contribution of global climate change on the precipitation deltaD record. This climate change-corrected record is consistent with topography equivalent to modern since the Paleocene, requiring that substantial uplift must have taken place prior to that time. This conclusion is consistent with existing geologic constraints. Given the timing of shortening and magmatism, I suggest that surface uplift of the Patagonian Andes largely occurred during the Late Cretaceous.;Chapter 3 is a paleosol-based record of atmospheric oxygen since 3.0 Ga. Because paleosols form at the atmosphere-lithosphere interface, they are ideal recorders of atmospheric composition. Stable chromium isotopes are largely fractionated by weathering occurring at atmospheric pO 2 above 0.1% of present levels, making them a valuable indicator of redox state. I apply this isotope system to a suite of well-preserved paleosols spanning 3.0 to 0.323 Ga to track changes in atmospheric pO 2 over that time interval. By considering both the delta53 Cr and total Cr loss or gain relative to parent, I detect small-scale redox cycling of Cr in the terrestrial environment, which began at or prior to 2.685 Ga, and large-scale redox cycling, which began between 1.1 and 0.6 Ga. This suggests that transient pulses ("whiffs") of high levels of oxygen prior to the Great Oxidation Event were short in time and/or space, but that pO2 remained relatively low until the Neoproterozoic, an idea consistent with marine delta53Cr records.