Weathering during the Great Oxidation Event: Fennoscandia, arctic Russia 2.06 Ga ago

Abstract

The establishment of an oxygen-rich atmosphere between 2.4 and 2.0 billion years ago (Ga) is one of the defining events in Earth history; however, the nature of the transition from an anoxic atmosphere is poorly resolved with interpretations relying heavily on empirical evidence that is poorly calibrated and not supported by numerical modeling. In an attempt to characterize the nature, timing, and impact the Great Oxidation Event (GOE) had on Earth's surface, the Fennoscandian Arctic Russia–Drilling Early Earth Project (FAR-DEEP) targeted the Archean-Proterozoic transition exposed in the eastern part of the Fennoscandian Shield. FAR-DEEP recovered more than 400m of the highly oxidized 2.06Ga Kuetsjärvi Volcanic Formation (KVF), which is potentially a manifestation of when atmospheric oxygen levels exceeded the ultimate Proterozoic steady state. Identifying the timing and extent of oxidation within the KVF are the primary objectives of this study.The presence of secondary hematite within fractures and amygdales indicates that the elevated oxidation state of the KVF is not a primary feature of the rocks. Immediately below the erosional surface marking the top of the KVF the degree of iron oxidation scales with the degree of chemical weathering, implying that oxidation of the KVF is the result of interaction with oxygen-charged groundwaters during subaerial exposure. The incorporation of rounded clasts with conspicuous hematite rinds into the overlying Kolosjoki Sedimentary Formation indicates that the oxidation of the KVF occurred prior to its deposition.The oxidized regolith recovered in the FAR-DEEP drill cores is more oxidized for the degree of weathering observed compared to older Precambrian- and Archean-aged paleosols, but not anomalously so compared to younger Proterozoic paleosols. This observation suggests that the Kuetsjärvi regolith was created during the progressive increase of pO2 towards the Proterozoic steady state. Using a previously published mass balance model to infer paleoatmospheric pCO2 and pO2 we are unable to provide any useful constraint on the paleoatmospheric pCO2 and pO2 levels. We estimate that the paleoatmospheric pCO2 and pO2 levels ca. 2.06Ga ago in Fennoscandia were between 0.4 and 6,500 times present atmospheric level (PAL), and 1×10−3 and 12 times PAL, respectively. The wide range in the calculated pCO2 and pO2 levels is the result of the pCO2 model calculations being extremely sensitive to key parameters that are nearly impossible to estimate accurately for ancient paleosols (e.g. the annual precipitation rate, depth to the groundwater table, and the amount of time it took the soil to form).