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Dissertation/Thesis Abstract

Deciphering the Boring Billion: A combined sedimentological and geochemical study of Mid-Proterozoic carbonates
by Bellefroid, Eric John, Ph.D., Yale University, 2018, 245; 10957226
Abstract (Summary)

The late Proterozoic is a key period in Earth's history. Characterised by a diversification of eukaryotes, a rise in atmospheric oxygen, and a series of major climate perturbations, this period marks the transition to a complex and diverse biosphere. However, while the majority of these geochemical and biotic transformations occurred in the Neoproterozoic, the foundation for these developments transpired earlier in the mid-Proterozoic, a period characterised by very stable geochemical cycling (e.g. δ13C) and little biological diversification, earning it the label of the boring billion" (1.8-0.8 Ga). This period, though, remains heavily understudied. In order to better understand the major biological and geochemical changes of the Proterozoic, the overarching objective of this thesis is to characterise basic biogeochemical cycling in the Mesoproterozoic, to provide a baseline for the Neoproterozoic and develop a better mechanistic understanding of the late Proterozoic transition.

In the first chapter of my thesis, I evaluate changes in the redox state of the surface oceans across the mid-Proterozoic. Ocean and atmospheric oxygen levels were significantly lower in the mid-Proterozoic than in the Phanerozoic, and a redox evolution towards more oxic conditions is one of the factors invoked to mark the end of the boring billion. I have undertaken a combined sedimentological and geochemical study of the Muskwa Assemblage, a well-exposed carbonate mid-Proterozoic basin in Northern British Columbia, Canada. I use sedimentologically based facies assignments to constrain paleodepth and combine these results with REE+Y (Rare Earth Elements and Yttrium) to determine the redox conditions of the water column. I find that REE+Y show clear depth dependent trends, with moderately negative Ce anomalies (Ce/Ce*) above fair-weather wave base, but positive Ce/Ce* between fair-weather wave base and storm-wave base. The preservation of both positive and negative Ce/Ce* indicates an active Mn cycle and a redox gradient near fair-weather wave base. Most importantly, this study shows that shallow marine settings were oxic, but the small range of Ce/Ce* values suggests a dominantly anoxic and low-oxygen ocean.

Following up on this initial work in the Muskwa assemblage, in the next chapter I target the Paleoproterozoic Pethei Group located in NW Canada. The Pethei Group is a well-mapped and well-preserved limestone deep basin to shelf assemblage, which provides an ideal location to test depth-dependent redox structure. Sequentially leaching samples to minimise contamination, I measure REE+Y on limestone samples from across the basin and find similar results to the Muskwa Assemblage. To convert these Ce/Ce* measurements to atmospheric pO2 estimates, I incorporate these geochemical results into a Ce oxidation model that allows me to estimate surface ocean Ce/Ce* values for a range of atmospheric pO2 values. In contrast to prior attempts, I use my sedimentological and geochemical results to constrain the depth of the chemocline and then use a general circulation model of the modern ocean (CIMP-5) to estimate the residence time of surface water at different chemocline depths. I then integrate these with Ce oxidation rates for different dissolved O2 values to estimate the range of Ce/Ce* for different atmospheric pO2 values. I estimate atmospheric pO2 values below 0.1% for both the Muskwa assemblage and the Pethei group, indicating that mid-Proterozoic oceans would have been a hostile environment for complex multi-cellular organisms.

In the last chapter, I investigate an apparent increase in paleo-weathering deduced by a rapid rise in carbonate 87Sr/86Sr through the late-Proterozoic. This increase in weathering has been suggested to be a major driver of the late Proterozoic transition in Earth's surface environment, however the reliability of the carbonate 87Sr/ 86Sr record is highly uncertain. Carbonate Sr isotopes are incredibly sensitive to alteration and the current Proterozoic carbonate 87Sr/ 86Sr record is compiled from published data that use a range of new and outdated dissolution methods, and are often unaccompanied by trace element data, making it difficult to assess sample preservation. To tackle this problem, I streamline and provide a proof of concept for an updated sequential leaching method that allows systematic and rapidly constraints on least-altered 87Sr/86Sr values for bulk carbonates. I then sequentially leach a series of mid-Proterozoic samples across the Mesoproterozoic and early Neoproterozoic. Overall, I show that this improved leaching method increases the quality and reliability of 87Sr/86Sr analysis, greatly expanding the utility of carbonate 87Sr/86Sr. In addition, in the basins I analysed, I find 87Sr/86Sr values that are similar to the current curve suggesting that mid-Proterozoic 87Sr/ 86Sr values are low and stable.

Indexing (document details)
Advisor: Planaysky, Noah J.
School: Yale University
School Location: United States -- Connecticut
Source: DAI-B 79/12(E), Dissertation Abstracts International
Subjects: Sedimentary Geology, Geochemistry
Keywords: Carbonate, Cerium Anomaly, Geochemistry, Oxygen, Strontium Isotope, Weathering
Publication Number: 10957226
ISBN: 978-0-438-26967-5
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