Potential of Chromium Isotopes as a Tracer of Past Ocean Oxygenation

Bonnand, Pierre (2011). Potential of Chromium Isotopes as a Tracer of Past Ocean Oxygenation. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000ee1f

Abstract

Levels of atmospheric oxygen (O2) have increased from <2% of the present atmospheric level (PAL) to values similar to the PAL during the Neoproterozoic. This is likely to have resulted in significant changes in redox conditions in the oceans. The aim of this study is to provide new constraints on redox conditions in shallow seawater during the Neoproterozoic by analysis of the rare earth element (REE) concentration and chromium (Cr) isotopic composition of marine carbonates. To this end, a new method for δ53Cr analysis in low concentration samples has been developed. In addition, the Cr isotopic composition of modern and Phanerozoic carbonates, and seawater, have been determined to provide new constraints of the environmental behaviour of Cr isotopes.

Modern seawater is characterised by heavy δ53Cr values (+0.55-1.55%) relative to the continental crust (-0.12%), which indicates that Cr isotopes are fractionated during oxidative weathering. Importantly, the Cr isotopic composition of modern carbonates, precipitated in shallow water, is within the range of seawater. This suggests that fractionation of Cr isotopes during carbonate precipitation is minimal, and carbonates should provide a record of seawater δ53Cr. Carbonate samples of Phanerozoic age also have high δ53Cr values (0.737-1.994%), consistent with oxidative weathering on the continents at this time.

Most of the Neoproterozoic carbonates have significantly lower δ53Cr values than the modern and Phanerozoic carbonates. However, oolitic limestones deposited just prior to the first Cryogenian glaciations, have higher δ53Cr (+0.571-1.004%). Together, the δ53Cr and REE data suggest that the operation of the Cr cycle was significantly different in the Neoproterozoic. Changes in δ53Cr reflect either a shift in the relative importance of riverine and hydrothermal sources, and/or changes in levels of atmospheric O2 . Modelling indicates that shallow waters were either dysoxic or suboxic during the Cryogenian, and levels of atmospheric O2 must have been less than 30-40% PAL.

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