The research presented here is an effort to characterize changes in marine oxygen availability across a portion of the later Cambrian noted for unique evolutionary dynamics and which includes a significant global oceanographic event known as the SPICE event (Steptoean Positive Carbon Isotope Excursion). Previous studies have revealed the SPICE caused large changes to the global cycles of carbon, sulfur, uranium, molybdenum and the overall trace metal content of seawater. Furthermore, the initiation of these changes appears to have been temporally coupled with marine extinctions across several paleocontinents raising the possibility of a common causal linkage between all these features. In particular, expanding marine anoxia has been invoked as the most parsimonious explanation for these co-occurring features. The research presented here tests this hypothesis directly across a range of spatial scales using the iron speciation paleoredox proxy to characterize redox conditions within individual basins and to facilitate comparison of conditions between basins. In addition to these analyses, we apply a new proxy, thallium stable isotopes to this interval to assess potential global changes in deoxygenation across the event. These iron speciation analyses showed shallow environments deoxygenated coincident with the initiation of the SPICE and extinction horizons, and these conditions were dominantly ferruginous. Importantly, this work also shows deeper-water environments were deoxygenated prior to and remained so across the event and these environments were also largely. Last we looked at changes in thallium isotopes during this same interval to see if this deoxygenation would be recorded as a positive shift across the interval if expanded anoxia were to impact the areal extent of manganese-oxide sedimentation and burial. We found it did record these changes, but with a different expression than during other more recent events explored using the isotope system. We attribute these differences to the unique chemical structure of the oceans during the Cambrian, which as documented herein were widely oxygen-deficient in their deeper depths. Given this recognition we suggest that thallium isotope studies in deep time should account for this redox structure of ancient oceans likely common under the less-oxygenated atmospheres of the ancient Earth. / Doctor of Philosophy / The research presented here is a story about oxygen in the oceans during an ancient portion of Earth history within the Cambrian Period (around 500 million years ago), soon after animal life first appears in the geologic record. The emerging biosphere of this time seems to have been particularly prone to extinctions, leading to the idea that environmental conditions, such as oxygen availability at the seafloor created difficult circumstances for animals in these ancient seas. This work seeks to quantify the levels of marine oxygenation at this time, however this remains a fundamental challenge because they cannot be directly measured from the rocks we study. Therefore, we rely on how the presence or absence of oxygen changed the chemistry of these rocks at the time they were sediments deposited on the seafloor. Here we use the behavior of two different elements, iron (Fe) and thallium (Tl), to understand changes in oxygen in the oceans around a large, globally-recorded extinction event called the SPICE event. Studying how much iron is concentrated in certain minerals in the rocks formed during this event allowed us to track how changes in oxygen may relate to these notable extinctions. We found that shallow coastal areas changed from oxygenated to deoxygenated at the same time of the extinctions, suggesting a direct role for this environmental shift in the biological crisis. Furthermore, we compared other locations from around the world using more new iron measurements in conjunction with previously published ones compiled by a collaborative geochemistry database project. This work revealed the deeper oceans were deoxygenated prior to and across the SPICE event and that the decline in oxygen in shallower environments was where most environmental change occurred during this time. Last we looked at changes in thallium isotopes during this same interval to see if this deoxygenation changed its global cycle. We found it did record global changes, but they were expressed differently than during other more recent events that have been studied. We attribute these differences to the unique chemical structure of the oceans during the Cambrian, which were widely oxygen-deficient in their deeper depths.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/109821 |
| Date | 06 May 2022 |
| Creators | Leroy, Matthew Alexander |
| Contributors | Geosciences, Gill, Benjamin C., Romans, Brian W., Eriksson, Kenneth A., Xiao, Shuhai |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet, application/pdf, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet, application/pdf |
| Rights | Creative Commons Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
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