The Paleoproterozoic Era has been characterized by an inferred rapid rise of atmospheric oxygen (Great Oxidation Event or GOE) at ~2.4-2.2 Ga. Oxygenation of the surface environment of the Earth was most likely to have been heterogeneous; molecular O2 produced by oxygenic photosynthesis first oxygenated the surface ocean after oxidizing dissolved reduced species such as Fe and S, then excess oxygen was liberated into the atmosphere and transported to deep ocean by ocean circulation. Although the timing of GOE has long been debated, such stepwise oxygenation itself appears to have been widely accepted. Geological and geochemical evidence to support such Paleoproterozoic GOE is largely based on characteristics of sedimentary rocks that formed in rather shallower environments, such as shallow marine or continental settings, namely carbonates, shallow-facies shales, and paleosols. Since there is a general lack of least-metamorphosed deep-facies siliceous sedimentary rocks that were deposited right after the inferred GOE, studies that geochemically constrain the degree of oxygenation of the deep ocean are hampered. We drilled to obtain 2.2 Ga least-metamorphosed deep-facies sedimentary rocks from the Birimian Greenstone Belt, southwest Ghana. The 196m-long core at low-grade metamorphism (upto greenschist facies) comprise of basaltic volcanic rocks and fine-grained sedimentary rocks. Here we report results of major element analysis by XRF, trace and rare earth element analysis by ICP-MS, Fe-speciation analysis, S-speciation analysis, Corg and Ccarb analysis, and Corg and Spy isotope analysis for 25 samples selected from 20m-long sedimentary unit of the core (Hayama, 2019). We estimated the origin of Fe in the samples using mass balance calculation with PAAS to suggest that Fe in the lower part of the core was continental and hydrothermal in origin, while that in the upper part was mostly continental. In a plot of DOP (degree of pyritization) vs. FeHR/FeT, 16 samples with positive Ce anomalies fall within the “oxic” domain, and 8 samples without any Ce anomaly fall within the “euxinic” domain. Trace elements such as redox-sensitive Mo, Cu, Ni, and Zn are neither enriched (EF ≤ 1) nor associated with Spy content, but the “euxinic” samples have mild positive correlation with Al2O3 content. The Corg isotope compositions are near –25‰, independent of stratigraphic height. The data set consistently suggests that redox state in the 2.2 Ga deep ocean was essentially oxic, with temporal anoxic/euxinic transitions. Enhanced continental weathering under an oxic atmosphere would have increased nutrient input into the ocean, primary productivity, consumption of dissolved O2, and sporadic emergence of euxinic conditions. Redox state of the deep ocean right after the inferred GOE at around 2.4 Ga would place important constraints on the co-evolution of the atmosphere, ocean, and biosphere.

Reference(s)

Hayama, H 2019, Redox state of the deep ocean 2.2 billion years ago: Constraints from geochemistry of shales, southwest Ghana: MSc Thesis, Toho University, Japan.