Chromium stable isotope systematic – implications for the redox evolution of the earth

Lasse Nørbye Døssing

Chromium stable isotope systematic – implications for the redox evolution of the earth

Lasse Nørbye Døssing

Geologi

Abstract

The isotopic composition of chromium (Cr) holds great promise as a paleo-redox proxy.Whereas the reduction of oxidized Cr(VI) to Cr(III) yields a well-defined kineticfractionation, the fractionation imparted during oxidative weathering is only described theoretically. This thesis demonstrates that Cr isotopes fractionate during oxidative weathering of modern soil systems. The result is the retention of light Cr(III) and the release of heavy Cr(VI) to runoff. Deviations in Cr isotope compositions from mantle inventory values are ultimately attributed to oxidative weathering in modern systems and thus indicate the presence of oxidizing redox species.
To track paleo-redox processes deep in the Earth’s history, a number of ancient soil horizons (e.g. the Drakenstein and Nsuze paleosols) formed ~2.2 and ~3.0 billion years ago have been analyzed. These horizons document similar behavior of Cr isotopes as modern soil profiles and indicate that oxidative weathering in the terrestrial environment started well before and after the Great Oxidation Event ~2.3 billion years ago. The signals of oxidative weathering on land are traceable in contemporaneous marine sediments such as Banded Iron Formations. This demonstrates the power of this novel proxy for tracking redox dynamics through Earth’s history.

Originalsprog	Engelsk

Forlag	Department of Geography and Geology, University of Copenhagen
Antal sider	175
Status	Udgivet - 2012

Citationsformater

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title = "Chromium stable isotope systematic – implications for the redox evolution of the earth",

abstract = "The isotopic composition of chromium (Cr) holds great promise as a paleo-redox proxy.Whereas the reduction of oxidized Cr(VI) to Cr(III) yields a well-defined kineticfractionation, the fractionation imparted during oxidative weathering is only described theoretically. This thesis demonstrates that Cr isotopes fractionate during oxidative weathering of modern soil systems. The result is the retention of light Cr(III) and the release of heavy Cr(VI) to runoff. Deviations in Cr isotope compositions from mantle inventory values are ultimately attributed to oxidative weathering in modern systems and thus indicate the presence of oxidizing redox species. To track paleo-redox processes deep in the Earth{\textquoteright}s history, a number of ancient soil horizons (e.g. the Drakenstein and Nsuze paleosols) formed ~2.2 and ~3.0 billion years ago have been analyzed. These horizons document similar behavior of Cr isotopes as modern soil profiles and indicate that oxidative weathering in the terrestrial environment started well before and after the Great Oxidation Event ~2.3 billion years ago. The signals of oxidative weathering on land are traceable in contemporaneous marine sediments such as Banded Iron Formations. This demonstrates the power of this novel proxy for tracking redox dynamics through Earth{\textquoteright}s history. ",

author = "D{\o}ssing, {Lasse N{\o}rbye}",

note = "Ph.d.-grad opn{\aa}et ved mundtligt forsvar 22. juni 2012",

year = "2012",

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AU - Døssing, Lasse Nørbye

N1 - Ph.d.-grad opnået ved mundtligt forsvar 22. juni 2012

PY - 2012

Y1 - 2012

N2 - The isotopic composition of chromium (Cr) holds great promise as a paleo-redox proxy.Whereas the reduction of oxidized Cr(VI) to Cr(III) yields a well-defined kineticfractionation, the fractionation imparted during oxidative weathering is only described theoretically. This thesis demonstrates that Cr isotopes fractionate during oxidative weathering of modern soil systems. The result is the retention of light Cr(III) and the release of heavy Cr(VI) to runoff. Deviations in Cr isotope compositions from mantle inventory values are ultimately attributed to oxidative weathering in modern systems and thus indicate the presence of oxidizing redox species. To track paleo-redox processes deep in the Earth’s history, a number of ancient soil horizons (e.g. the Drakenstein and Nsuze paleosols) formed ~2.2 and ~3.0 billion years ago have been analyzed. These horizons document similar behavior of Cr isotopes as modern soil profiles and indicate that oxidative weathering in the terrestrial environment started well before and after the Great Oxidation Event ~2.3 billion years ago. The signals of oxidative weathering on land are traceable in contemporaneous marine sediments such as Banded Iron Formations. This demonstrates the power of this novel proxy for tracking redox dynamics through Earth’s history.

AB - The isotopic composition of chromium (Cr) holds great promise as a paleo-redox proxy.Whereas the reduction of oxidized Cr(VI) to Cr(III) yields a well-defined kineticfractionation, the fractionation imparted during oxidative weathering is only described theoretically. This thesis demonstrates that Cr isotopes fractionate during oxidative weathering of modern soil systems. The result is the retention of light Cr(III) and the release of heavy Cr(VI) to runoff. Deviations in Cr isotope compositions from mantle inventory values are ultimately attributed to oxidative weathering in modern systems and thus indicate the presence of oxidizing redox species. To track paleo-redox processes deep in the Earth’s history, a number of ancient soil horizons (e.g. the Drakenstein and Nsuze paleosols) formed ~2.2 and ~3.0 billion years ago have been analyzed. These horizons document similar behavior of Cr isotopes as modern soil profiles and indicate that oxidative weathering in the terrestrial environment started well before and after the Great Oxidation Event ~2.3 billion years ago. The signals of oxidative weathering on land are traceable in contemporaneous marine sediments such as Banded Iron Formations. This demonstrates the power of this novel proxy for tracking redox dynamics through Earth’s history.

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