Date of Award

8-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Linda C. Kah

Committee Members

Christopher M. Fedo, Annette S. Engel, Michael E. Essington

Abstract

The protracted oxygenation of Earth’s surface environments played a critical role in biospheric evolution during the Proterozoic eon. Initial oxygenation began ~2.3 Ga during the Great Oxidation Event, yet Earth’s oceans did not become fully oxygenated until at least the end of the Neoproterozoic—coincident with the first appearance of metazoans in the fossil record. Patterns of environmental change and evolutionary innovation are more complex and less certain, however, in the prolonged period between these two oxygenation thresholds. The late Mesoproterozoic (1.3 to 1.0 Ga) was marked by increasing biospheric oxygen—evidenced by increased carbon isotopic variability and an increase in marine sulfate concentrations—and an increase in diversity among early eukaryotes. Eukaryotic diversification occurred largely in nearshore environments, yet redox proxy investigations of shallow water Mesoproterzoic strata have been limited, leaving us unable to directly examine the coupled evolution of life and environment in the Mesoproterozoic.

In this study, I investigate the geochemical record of epicratonic and pericratonic strata of the 1.1 Ga Atar/El Mreiti Group, Mauritania, which were deposited in an epeiric sea during sea level highstand. In Chapter I, I explore carbon isotopic heterogeneities across the epeiric sea, and use trace element substitution in carbonate to relate isotopic heterogeneity to chemically distinct water masses between onshore and offshore environments. In Chapter II, I explore the shale-based redox proxy record (iron speciation, pyrite sulfur isotopes, and trace metal concentrations) of Atar/El Mreiti Group strata. Results suggest that euxinia—at least within substrate pore fluids—was common across the epeiric sea, and that the chemocline intersected the seafloor deep in the craton interior. In Chapter III, I explore the affect of nearshore euxinia on trace metal delivery to the global ocean. Model simulations suggest that expanded epeiric seas in the late Mesoproterozoic could have effectively crashed the oceanic Mo reservoir, with potentially disastrous consequences for early eukaryotes with high biochemical demands for Mo. Ultimately, this study is the first to directly examine redox conditions in late Mesoproterozoic epeiric seas, and provides rare insight into the chemical workings of the global ocean during a critical interval in Earth’s biogeochemical evolution.

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