Testing the redox coupling between chromium and nitrogen isotopes in modern and ancient redox-stratified depositional systems: the Coorong Lagoon and the Greater McArthur Basin

Abstract

The history of Earth’s atmospheric oxidation following the Great Oxidation Event (GOE) is widely debated and currently poorly constrained. This uncertainty is largely because the use of different geochemical proxies provides a broad range of possible palaeo-redox conditions during the mid-Proterozoic. Such proxies include nitrogen (δ15N) and chromium (δ53Cr) isotopes, which are the focus of this study. These redox-sensitive proxies have recently demonstrated coupled behaviour in both modern seawaters and recent marine sediments, suggesting isotopic fractionation of Cr could result from biologically mediated redox cycling of N. This concept is opposed to Cr isotope fractionation being purely representative of oxidative weathering on continents, thus challenging the reliability of the δ53Cr proxy as a direct tracer for past atmospheric O2 levels. The aim of this study is to test the purported redox coupling of the δ53Cr and δ15N proxies in two redox-stratified depositional systems, specifically investigating (i) modern waters and organic matter from the Coorong Lagoon of South Australia, and (ii) organic-rich shales from the greater McArthur Basin in the Northern Territory (including the Velkerri, Mainoru, Barney Creek and Fraynes Formations). These marine settings display notable redox gradients, allowing insight into the isotopic behaviour of N and Cr through a variety of conditions. Contrary to published data, this study revealed no positive co-variance between δ53Cr and δ15N records. Rather, δ15N changes in both waters and shales are interpreted to largely result from pH-driven volatilisation of NH3, while δ53Cr variations in shales exhibit a systematic temporal increase. This increase likely reflects progressive basin oxygenation, linked to gradually increasing atmospheric O2 during the mid-Proterozoic (i.e. from 1.64 to 1.31 Ga). Thus, the validity of δ53Cr values in marine archives as a palaeo-redox proxy are supported in this instance, with no direct evidence for biologically driven redox cycling of Cr coupled to local N cycling.