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Title: Potential of chromium isotopes as a tracer of past ocean oxygenation
Author: Bonnand, Pierre
Awarding Body: The Open University
Current Institution: Open University
Date of Award: 2011
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Levels of atmospheric oxygen (02 ) hav(' increaseo from <2o/r:. of the present atlnospheric level (PAL) to values similar to the PAL during the N('oproterozoic. This is likely to have resulteo in significant changes in redox conditions in thp ocpans. The aim of this stud.v 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 carbonatps. To this end, a new method for J 53Cr 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 J53Cr values (+0.55-1.55%0) relative to the continental crust (-0.12%0), 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 J53Cr. Carbonate samples of Phanerozoic age also have high J 53Cr values (0.737-1.994%0), consistent with oxidative weathering on the continents at this time. Most of the Neoproterozoic carbonates have significantly lower J53Cr values than the modern and Phanerozoic carbonates. However, oolitic limestones deposited just prior to the first Cryogenian glaciations, have higher J53Cr (+0.571-1.004%0). Together, the J 53Cr and REE data suggest that the operation of the Cr cycle was significantly different in the Neoproterozoic. Changes in J53Cr reflect eitlwr a shift in the relative importance of riverine and h~'drothermal sources, and/or changes in levels of atmospheric O2 . Modelling indicates that shallow waters were either dysoxic or suboxic during the Cryogenian, ano levels of atmospheric O2 must have been less than 30-40% PAL.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available