BRENNAN O'CONNELL
R E S E A R C H
I am interested in how sedimentary systems, marine oxygen levels, and life (microbes, animals, and plants) have co-evolved through time. To understand possible links between variables I integrate sedimentology, stratigraphy, and geochemistry in my research. I am a strong advocate for having a thorough understanding of the paleoenvironmental setting and diagenesis prior to any geochemical analysis.
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Nearshore marine environments before the evolution of land plants
Sedimentary environments have fundamentally changed through time with the evolution of plants and animals. There is extensive literature on the evolution of river systems through time, however, relatively little is known about pre-land plant nearshore environments. Nearshore environments such as tidal flats, back-barrier settings, and beach barriers are often vegetated and bioturbated in modern settings. I am working to better constrain the sedimentology and sedimentary processes of Precambrian to early Paleozoic nearshore systems. I am interested in how nearshore environments evolve through time and how changes (if any) are linked with the evolution of plants.
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Precambrian marine oxygenation
Neoproterozoic oceans were potentially low in oxygen – with redox chemistry resembling modern pore fluids. My research is focused on environment-specific ocean oxygenation. For example, during the three periods of the Neoproterozoic Era (the Tonian, Cryogenian, and Ediacaran), how does the redox chemistry of reef and backreef settings change through space (shallow-to-deep) and through time?
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Research starts with detailed stratigraphic, sedimentological, and petrographic work to understand diagenesis and the environmental context – terrestrial, intertidal flats, lagoons, shallow subtidal, backreef, reefal, deep marine, etc. It's impossible to test ancient seawater chemistry directly, so geochemical proxies from each environment are used. Well-preserved carbonate marine cements and ooids are great archives of ancient seawater chemistry because they have a low detrital component and precipitate directly from seawater. I use trace metal abundance data such as iron, manganese, REE+Y profiles, and the cerium anomaly as redox proxies. Iron and cerium oxidize at different oxygen levels, and can therefore be used to place redox constraints on Precambrian oceans.
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Work from my MSc at the University of Oregon:
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Gulf of California ~6 Ma
Expansive tidal flats and grassy salt marshes dominated the tide influenced shallow waters of the northern Gulf of California at ~6 Ma. Our work addressed the following questions: how far north did the Gulf of California extend and what depositional environments existed at the Gulf's northernmost arm? This work then informs tectonic reconstructions of the southwestern United States. We find evidence for high tides (i.e., deposited at sea level) in sediment uplifted 100–200 m over the past 6 Ma. This work suggests activity of young crustal and upper-mantle deformation processes that remain incompletely understood.
