Chromium Isotope Constraints on the Mid-Proterozoic redox: evidence from δ53Cr of carbonates from the greater McArthur Basin, northern Australia

Abstract

The Great Oxygenation Event (GOE) and the Neoproterozoic Oxygenation Event (NOE) are interpreted to have made the most profound and permanent surface redox changes in Earth’s history. Changes in redox conditions between these two oxygenation events (i.e. mid-Proterozoic; 1.8-0.8 Ga) are poorly understood where environmental stability with persistently low atmospheric oxygen is assumed (<0.1% PAL; Present Atmospheric Levels). This period also witnessed the first appearance of primitive eukaryotes, however Eukarya diversification was determined to be effectively stagnant presumably due to sustained low atmospheric oxygen levels (pO2). More recent studies found evidence of relatively high mid-Proterozoic pO2, well in excess of 1% PAL, sufficient to promote diversification. The importance of better understanding the past redox conditions heightens due to the contrasting pO2 estimates that plausibly swayed the Eukarya diversification.

This study presents stable Cr isotope (δ53Cr) values in mid-Proterozoic organic-rich carbonates of the Limbunya and McArthur Groups from the greater McArthur Basin. Analysed values from -0.293‰ to +1.389‰, present the oldest documented positively fractionated mid-Proterozoic δ53Cr values in marine carbonate units ca. 1.64 Gyrs ago, suggestive of a fluctuating, but increasing pO2 at the time of a generally reducing environment and supporting a permissive environment for Eukarya diversification. However, it is likely that its unstable nature probably inhibited wider and earlier Eukarya diversification, should pO2 levels truly be a barrier for evolution.