EARLY PALEOZOIC MARINE ENVIRONMENTS: LINKING BIOGEOCHEMISTRY, ECOPHYSIOLOGY, AND THE FOSSIL RECORD'

The fossil and geochemical records of the early Paleozoic Era document profound ecological, evolutionary and environmental changes in Earth’s oceans. Temporal correlations between sedimentary geochemical perturbations and the extinctions and radiations observed in the fossil record suggest that changing marine environments may have been responsible for many key evolutionary or ecological changes. However, establishing mechanistic relationships between marine environments and animal ecosystems in Earth's history is complicated by the imperfect nature of the fossil and geochemical archives that record them. In this thesis I develop numerical and statistical modeling approaches to connect records and concepts in paleobiology, sedimentary geochemistry, marine ecology, and oceanography that otherwise speak obliquely to each other. I then apply these models, in combination with new geological data where applicable, to investigate longstanding problems in early Paleozoic Earth history.

In Chapter 1, I investigate the relationship between global ocean deoxygenation, the Late Ordovician mass extinction and the delayed early Silurian recovery of marine animals.  I generate new iron speciation, metal concentration, δ⁹⁸Mo and δ²³⁸U geochemical data for early Silurian black shales from the Murzuq Basin, Libya. I further develop a stochastic δ⁹⁸Mo and δ²³⁸U mass balance model to investigate the influence of global marine euxinia (anoxic, sulfidic conditions) on sedimentary records of these trace metal stable isotopes. I evaluate the new geochemical data in combination with published carbonate δ²³⁸U data to provide uncertainty-bound constraints on the intensity of Hirnantian-Rhuddanian euxinia. The model results add new support to correlations between global deoxygenation and the second pulse of the Late Ordovician mass extinction. Furthermore, using this model-data approach I extend the duration of Late Ordovician-early Silurian widespread anoxia to at least 3 million years, providing new environmental context for the delayed post-extinction recovery of marine animals in the early Silurian.

In Chapter 2, I test the hypothesis that a step-wise oxygenation of Earth’s atmosphere and oceans to near-modern levels occurred in the late Neoproterozoic. This hypothesized Neoproterozoic oxygenation event broadly correlates with the Cambrian explosion, leading to inferences that oxygen may have driven the geologically rapid appearance of major marine animal groups. Here, I use statistical models to generate new reconstructions of trace metal (Mo and U) proxies commonly used to reconstruct Neoproterozoic oxygenation that are deconvolved from other processes known to impact their preservation in sedimentary rocks. I apply similar approaches to generate a new record of sedimentary organic carbon through the Neoproterozoic. I integrate these statistical reconstructions with a combined biogeochemical modeling approach, linking 3D oceanographic simulations under different atmospheric oxygen and marine productivity levels to models of the reconstructed proxies. I establish that there were at least two major increases in atmospheric oxygen and marine productivity through the Neoproterozoic and Paleozoic. My reconstructions indicate that there was no late Neoproterozoic oxygenation of the global ocean. However, shallow shelf environments would have experienced an increase in dissolved oxygen and primary production, increasing environmental oxygen and food supply around the same time as the Cambrian explosion.

In Chapter 3, I propose and test a new physiological mechanism for the decrease in background extinction rates of marine animals through the Phanerozoic Eon. Despite being observed 40 years ago in the first quantitative paleobiological analyses, no consensus has been reached for the observed decrease in marine extinction rates over the Phanerozoic or the especially high magnitude extinctions in the early Paleozoic. I use the Metabolic Index to demonstrate that atmospheric oxygenation can act as a key control on marine animal extinction rates. The minimum required environmental oxygen supply for marine ectotherms increases exponentially with environmental temperature. This observation from modern marine animal physiology indicates that limited atmospheric oxygenation in the early Paleozoic may have driven the high observed rates of extinction through the same time interval by reducing the thermal safety margins of marine animals. I provide quantitative support for this new hypothesis using a combined ensemble Earth system and ecophysiological modeling experiment. I thus establish that atmospheric oxygen is a more powerful control on the magnitude of warming-driven marine extinctions than other Earth system boundary conditions such as baseline climate state or continental configuration.

For the zoom information please contact Richard Stockey (rstockey@stanford.edu)