Molybdenum record from black shales indicates oscillating atmospheric oxygen levels in the early Paleoproterozoic

Abstract

The early Paleoproterozoic witnessed Earth's first major oxygenation, referred to as the Great Oxidation Event or GOE. The GOE began around 2.45 billion years ago (Ga) and progressed over hundreds of millions of years, as evidenced by multiple redox indicators, before coming to an abrupt end by ca. 2.06 Ga. The details of the GOE and the extent of oxygenation are still not resolved, however, and it is not clear how redox conditions across the GOE compare with those during the middle Proterozoic. In order to investigate the evolution of deep-ocean redox conditions during the GOE, we present Mo concentration and isotope data together with Fe speciation values for three key organic matter-rich shale units of the early Paleoproterozoic age (2.32–2.06 Ga). In addition, we present a new graphical representation of modeling suggesting that the oceanic Mo isotope system is highly sensitive to the balance between anoxic/suboxic and euxinic conditions until deep-ocean oxygenation, similar in scale to modern ocean oxygenation, is reached. Our approach indicates rising, yet oscillating atmospheric oxygen at 2.32 Ga, leading to an abrupt increase in Mo supply to the oceans and large Mo isotope variations under non-steady state conditions. The low seawater δ⁹⁸Mo value based on the ca. 2.32 Ga black shales (0.32 ± 0.58‰) suggests that the oceans were still largely anoxic with locally developed euxinic conditions. Between 2.2 and 2.1 Ga, during the peak of the Lomagundi carbon isotope excursion, we observe higher δ⁹⁸Mo_SW values (1.23 ± 0.36‰) together with lower Mo concentrations in euxinic shales ([Mo] = 6.3 ± 9.0 ppm). We suggest that a decrease in the continental Mo input flux in the later part of the GOE was the main cause of this trend. Lower sulfide availability on the continents after protracted sulfide weathering associated with the early stages of the GOE, and efficient Mo removal in poorly oxygenated oceans under weakly euxinic conditions would both have contributed to the contraction of the Mo oceanic reservoir. By ca. 2.06 Ga, the Mo isotope composition of seawater, as inferred from euxinic black shale intervals, became significantly lighter (0.70 ± 0.21‰), reflecting an increased rate of quantitative Mo removal due to the more widespread development of strongly euxinic conditions. Counterintuitively, seawater Mo concentrations recovered, likely due to an increase in the Mo input, which in turn might reflect enhanced weathering of organic carbon-rich shales deposited during the Lomagundi Event.