Uranium isotope evidence for limited euxinia in mid-Proterozoic oceans

Abstract

Reconstructing Earth's oxygenation history is key to deciphering environmental controls on early biospheric evolution. During the mid-Proterozoic Eon, low (but potentially variable) atmospheric pO2 led to highly heterogeneous marine redox conditions, with most studies indicating a relatively shallow depth of oxygen penetration. The relative proportion of oxic, anoxic and iron-rich (ferruginous), and anoxic and sulfide-rich (euxinic) conditions on the global seafloor is difficult to quantify, however, due to a general reliance on local redox proxies applied to the temporally discontinuous black shale record. It is particularly important to constrain the global prevalence of euxinic bottom waters because sulfide toxicity has been implicated as a primary constraint on evolution in mid-Proterozoic oceans and limits the solubility of bioessential trace metals such as copper, zinc, and molybdenum. Here, we present a suite of new uranium (U) isotope data from marine carbonate rocks that span the entire mid-Proterozoic interval (∼1.8 to 0.8 Ga). U-isotopes in well-preserved carbonate rocks represent a powerful new proxy that can be used to quantitatively constrain the global extent of marine euxinia. The median δ238U value for mid-Proterozoic carbonate samples is −0.43‰, which is lower than the median value for modern post-depositional carbonate sediments from the Bahamas, yet significantly higher than carbonate δ238U values recorded during other times of expanded anoxia in Earth history, such as during the end-Permian mass extinction. When paired with a three-sink isotope mass balance model, our data indicate that no more than 7% of the global seafloor was euxinic during the mid-Proterozoic Eon, although transient pulses of expanded euxinia are recorded. Although these results challenge early expectations of a sulfide-rich mid-Proterozoic ocean, they are consistent with more recent inferences from the black shale trace metal record. Evidence therefore indicates that euxinic bottom waters were relatively limited in their spatial extent in the mid-Proterozoic oceans.