At the precipice of the Great Oxygen Crash: Redox-sensitive metal geochemistry in the Paleoproterozoic Onega Basin

• Author / Creator

  Mand, Kaarel

• The middle Paleoproterozoic era (~2200–1800 million years ago) was host to some of the most significant perturbations in Earth’s elemental cycles, including the largest ever excursion of carbon isotope ratios in sedimentary rocks, known as the Lomagundi-Jatuli Event. This event has been linked to a contemporaneous “overshoot” in atmospheric oxygen concentrations to near-modern values, as recorded by geochemical redox proxies in rocks from this time. After ~2060 million years ago the Lomagundi-Jatuli isotope excursion waned and, closely thereafter, the O2 overshoot receded. However, there are several competing explanations as to the mechanisms underpinning these global events, and their effect on the evolution of life remains tentative.

  One of the most complete successions to record the Lomagundi-Jatuli Event and its termination is the Onega Basin in NW-Russia. Geochemical data from this succession features prominently in global redox studies but have produced contradictory interpretations due to uncertainty regarding local basinal configuration and redox conditions. In this thesis, I use redox-sensitive metal concentration and isotope ratio analyses of Onega Basin sedimentary rocks to provide constraints on the local depositional setting, assess proxy evidence for global redox conditions, and explore their implications for the mechanisms governing global carbon and oxygen cycle shifts, and the emergence of eukaryotic life forms.
  Mudstone iron isotope analyses in the upper Zaonega Formation, middle part of the Onega Basin sedimentary succession, are consistent with the operation of an ancient benthic Fe shuttle, implying restricted hydrographic conditions and redox stratification. Though this implies basinal modulation of trace metal cycling, high metal concentrations and isotope signatures of occasional oxic iron drawdown attest to replenishment of the basin with oxidized water masses, allowing the sediments to record signals of global marine chemistry.

  Molybdenum, uranium, and rhenium concentrations in the upper Zaonega Formation, as well as uranium isotope ratios, are the highest reported from organic-rich mudstones older than the Neoproterozoic. To explain such values, the contemporaneous oceans must have been highly oxygenated. Given that the Zaonega Formation post-dates the end of the Lomagundi-Jatuli Event by several million years, it means that the carbon isotope excursion and the O2 overshoot were to some extent decoupled. This finding disagrees with the most-cited explanation for these events, which invokes a pulse of elevated organic carbon burial, and urges consideration of alternative mechanisms.

  Finally, fractionated chromium isotope ratios are found throughout a >2400 m thick section of the Onega Basin sedimentary rocks. Since chromium isotope fractionations require appreciable levels of atmospheric O2 to form, these findings mean that stable, high-pO2 conditions must have been the norm throughout the entire period during which this succession accumulated. This stands in contrast to the following mid-Proterozoic time period, which is characterized by low and unstable levels of pO2. The acquisition of the mitochondria in eukaryotic organisms has commonly been linked to atmospheric oxygenation, but despite the long period of stable oxygenation discovered here, diverse eukaryote fossils only become common in the mid-Proterozoic. Therefore, intrinsic biological factors must have controlled the timing of mitochondrial acquisition, instead of background redox levels.

• Subjects / Keywords

  • geochemistry
  • paleoproterozoic
  • trace metals

• Graduation date

  Spring 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-8tde-s562

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.