Early Paleozoic Ocean Redox Dynamics: Perspectives from Uranium Isotopes of Sedimentary Rocks
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The trajectory of global ocean oxygenation could have greatly influenced the metazoan evolutions because O2 could provide valuable energy to support biological activities. Remarkable metazoan diversifications occurred during the Late Neoproterozoic (i.e., Ediacaran; 635–539 Ma) and Early Paleozoic (i.e., Cambrian, Ordovician, and Silurian; ca. 538–419 Ma), such as the appearance of Ediacaran Biota and the “Cambrian Explosion”. However, an increasing number of studies suggest that a near-modern level of Earth’s surface oxygenation was not established during the Late Neoproterozoic (ca. 680 Ma), but rather during the Devonian (ca. 419–359 Ma). Therefore, it is of great importance to understand the co-evolution of global ocean redox conditions and metazoan diversifications during the Early Paleozoic (ca. 538–419 Ma). The uranium isotope compositions (δ238U) from sedimentary rocks (e.g., organic-rich mudrocks, carbonates) have been used as a global ocean redox proxy and provided insights on ocean redox dynamics. Understanding the local bottom water redox conditions is crucial to interpret δ238U values, as different δ238U offsets occur under various redox settings. Relatively larger δ238U offsets are observed in sediments from modern euxinic basins compared with the other redox settings, suggesting seawater δ238U values are sensitive to the extent of global euxinic seafloor area. Uranium isotope mass balance modelling could be further used to quantitatively estimate the areal extent of euxinic seafloor in the oceans. In this thesis, U isotopes from sedimentary rocks are used to investigate ocean redox conditions, with a focus during the Ordovician Period (ca. 487–443 Ma) when there is rapid evolutionary change. The coupled use of molybdenum and uranium isotope compositions from euxinic organic-rich mudrocks are investigated to better reconstruct ancient ocean redox conditions. Local depositional conditions of each formation were firstly examined by sedimentary Fe speciation, covariations between Mo and TOC, and between Mo and U enrichment factors. The Mo and U isotope compositions from individual formations were observed to exhibit negative, positive, and no correlations, suggesting different controlling mechanisms (e.g., bottom water H2S concentrations, basin restrictions, global ocean redox conditions). This study provides a general framework of using coupled Mo-U isotopes from the same euxinic organic-rich mudrocks to disentangle the effects of local depositional environment and global ocean redox states. Specifically for the Ordovician, a positive correlation of Mo-U isotope data from the late Katian Fjäcka Shale suggests an episodic ocean oxygenation event prior to the Hirnantian. The Late Ordovician mass extinction event (LOME; ca. 445–443 Ma) wiped out 85% of species. However, metazoan biodiversity started to decline during the Katian (ca. 453–445 Ma; prior to the LOME) and coeval global ocean redox conditions are not well understood. The Katian organic-rich sedimentary rocks in southern Ontario, namely the Collingwood Member (upper Lindsay Formation) and succeeding Rouge River Member (lower Blue Mountain Formation), were deposited during the Taconic Orogeny. Samples of both units were collected from several drillcores that cover southern Ontario. Paleosalinity (strontium/barium and sulfur/total organic carbon) and paleoredox (redox sensitive trace metals, Fe speciation, and Corg : P ratios) proxies were used to constrain the local depositional environment of both units. In addition, the δ238U of both units were used to deduce coeval ocean redox conditions. Lower estimated seawater δ238U during deposition of the Collingwood Member suggests an expansion of global ocean euxinia, whereas higher seawater δ238U during deposition of the Rouge River Member represents a contraction of ocean euxinia. A three-sink U isotope mass balance model suggests a global ocean euxinic seafloor area of 0.5–31.6% and 0.2–2.0% during deposition of the Collingwood Member and Rouge River Member, respectively. Combined with other studies, fluctuating ocean redox conditions occurred during a decline of biodiversity prior to the LOME. The base Stairsian mass extinction event (BSME; ca. 482 Ma), accompanied with a positive carbon isotope excursion (CIE), is one of the best studied mass extinction events in the Tremadocian, Early Ordovician (ca. 487–471 Ma). New trace metal concentrations and δ238U of carbonates from three sections (along a proximal-to-distal transect: Ibex area, Shingle Pass, Meiklejohn Peak, respectively) in the Great Basin (western USA) were analyzed to quantitatively constrain the role of global ocean euxinia on the mass extinction event. Carbonate δ238U data show different trends among the three sections. The proximal Ibex section shows a negative δ238U excursion during the CIE, whereas the distal Shingle Pass section only has one sample with unusually low δ238U and the Meiklejohn Peak section does not have any samples with unusually low δ238U. The lowest δ238U values from each of the Ibex and Shingle Pass sections are associated with the highest Mn/Sr ratios in those sections, suggesting diagenetic overprints. Carbonate δ238U data from the other two distal sections likely record the open ocean δ238U signals and limited variations in these sections suggest no significant change in global ocean euxinia during the BSME. A three-sink U isotope mass balance model suggests 0.2–15.8% global euxinic seafloor area during the studied interval. Although there was no expansion of euxinia, there is evidence of expanded ocean suboxia-anoxia based on concurrent positive carbon and sulfur isotope excursions during the BSME. Limited changes in global ocean euxinia are further proposed during the post-SPICE Cambrian and Early Ordovician because other carbon isotope perturbations during this time are smaller than that associated with the BSME. Combined with previous studies, fluctuating ocean redox conditions were possibly the key character during the Early Paleozoic (ca. 538–419 Ma), though limited non-traditional metal isotope data are available for the Early-Middle Ordovician (ca. 487–458 Ma) and Silurian (ca. 443–419 Ma). The notable “Cambrian Explosion” has been suggested to coincide with pulses of ocean oxygenation, however, several recent studies proposed that this metazoan radiation could be facilitated by overall dynamic Cambrian ocean redox conditions. Nonetheless, more studies are needed to better understand the Early Paleozoic ocean redox conditions and the co-evolution of metazoans. For example, there is a great potential to study marine redox conditions during the “Great Ordovician Biodiversification Event” as metal isotope data during this event have not been reported.
Cite this version of the work
Xinze Lu (2022). Early Paleozoic Ocean Redox Dynamics: Perspectives from Uranium Isotopes of Sedimentary Rocks. UWSpace. http://hdl.handle.net/10012/18925