Deciphering the early oxidation of Earth's surface: evidence from Fennoscandia, arctic Russia

Open Access
Author:
Rybacki, Kyle S
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 12, 2015
Committee Members:
  • Lee Kump, Dissertation Advisor
  • Lee Kump, Committee Chair
  • Hiroshi Ohmoto, Committee Member
  • Maureen Feineman, Committee Member
  • Patrick Joseph Drohan, Committee Member
Keywords:
  • Precambrian oxygen
  • paleosol
  • iron isotopes
  • chromium isotopes
  • Great Oxidation Event (GOE)
  • oxidation
  • U-Th-Pb geochronology
Abstract:
The establishment of an oxygen–rich atmosphere between 2.4 and 2.0 billion years (Ga) ago is one of the defining events in Earth history. Unfortunately, the nature of the transition from anoxia is poorly resolved with interpretations relying heavily on numerical modeling. Recently, it has been hypothesized that oxygen levels may have overshot their ultimate Proterozoic steady state during the transition. The Fennoscandia Arctic Russia – Drilling Early Earth Project (FAR–DEEP) recovered over 400 m of the anomalously oxidized 2.06 Ga old Kuetsjärvi Volcanic Formation, which may be the manifestation of the oxygen overshoot in the geologic record. Detailed petrographic and geochemical analyses resulted in the discovery of an oxidized paleosol that fills in a key gap in the Precambrian paleosol record. Oxidation within the Kuetsjärvi paleosol appears to be strongly correlated with the extent of chemical weathering. U-Th-Pb geochronology constrained the timing of oxidation to between eruption and metamorphism. However, the incorporation of rounded, oxidized KVF clasts with conspicuous hematite rinds into the overlying sedimentary formation indicates that oxidation occurred prior to erosion. Non-traditional, redox sensitive heavy stable isotope analyses revealed that iron was largely oxidized in situ within the Kuetsjärvi paleosol, but that chromium was added to the paleosol profile at drilling site 8B. Given the extent of weathering, the Kuetsjärvi paleosol is oxidized compared to older Precambrian aged paleosols, but not anomalously so compared to younger Proterozoic paleosols. Qualitatively, this observation suggests the oxidation of the Kuetsjärvi paleosol occurred during the progressive increase towards, but not necessarily above, the Proterozoic pO2 steady state. Our modeling results, using a previously published mass balance model, broadly corroborate this interpretation, and suggest that paleoatmospheric pCO2 and pO2 levels ca. 2.06 Ga ago in Fennoscandia were between 0.4 and 6,500 times PAL and 1 x 10–3 and 12 times PAL, respectively. The broad range in estimated pCO2 and pO2 values for the Kuetsjärvi paleosol demonstrate how sensitive the current model is to parameters that are difficult to estimate accurately for Precambrian-aged paleosols (e.g., annual precipitation rate, groundwater table depth, and soil age). Thus, our results reveal the need to refine the current model to improve the accuracy of future paleoatmospheric pCO2 and pO2 during early Earth’s history.