TY - BOOK

T1 - Chromium isotope uptake in carbonates

T2 - An experimental and application-based evaluation of the potential use of chromium isotopes as a paleo-climatic tracer in the marine environment

AU - Rodler, Alexandra

N1 - Ph.d.-grad opnået ved mundtligt forsvar 11. marts 2016

PY - 2015/12

Y1 - 2015/12

N2 - Chromium (Cr) is a redox sensitive element potentially capable of tracing fine-scale fluctuations of the oxygenation of Earth’s early surface environments and seawater. The Cr isotope composition of carbonates could perhaps be used as paleo-redox proxy to elucidate changes in the geological past related to the rise of oxygen and the evolution of the biosphere. However, before the Cr isotopesystem can be applied to faithfully delineate paleo-environmental changes, careful assessment of the signal robustness and a thorough understanding of the Cr cycle in Earth system processes is necessary. Processes that potentially fractionate Cr isotopes, perhaps during deposition, burial and alteration need to be constrained.Previous studies have shown that Cr isotopes are fractionated during oxidative weathering on land, where heavy Cr isotopes are preferentially removed with Cr(VI) while residual soils retain an isotopically light Cr signature. Cr(VI) enriched in heavy Cr isotopes is then transported via river waters to the oceans and sequestered into marine sediments. Marine chemical sediments such asbanded iron formations and modern marine carbonates have proven useful in recording the Cr isotope composition of contemporaneous seawater. Marine carbonates are ubiquitous throughout Earth’s rock record rendering them a particularly interesting archive for constraining past changes in ocean chemistry. This thesis includes an investigation of the fractionation behavior of Cr isotopesduring coprecipitation with calcium carbonate in order to test the reliability of the Cr carbonate compositions. Several experimental approaches have been employed to elucidate the fractionation behavior of Cr isotopes when Cr(VI) is incorporated into calcium carbonate phases. These results indicate that at lower Cr concentrations typical for seawater, marginal to no Cr isotope fractionationcan be expected. This represents a first step towards enabling a reliable application of Cr isotopes recorded in ancient carbonates and towards constraining the environmental information they can provide. The Cr isotope proxy was also applied to natural carbonates from several contemporaneoussections, along with other commonly used paleo-proxies, to decipher redox changes during build-up and retreat of one of the major late Neoproterozoic glaciations. These carbonate δ53Cr signatures were sensitive to changes in continental weathering balanced between detrital contamination andoxidative weathering on land and were capable of tracing fluctuating redox conditions. However, a possible diagenetic alteration of the Cr signal as well as Cr contribution from detrital contamination need to be taken into consideration when using ancient carbonates.The redox changes of past surface environments can be explored using the Cr isotope composition of ancient marine carbonates, where a marginal offset compared to contemporaneous seawater δ53Cr is expected and the degree of contamination and later diagenetic alteration can be evaluated. Improved understanding of processes that affect the Cr isotope composition duringdeposition and burial of the archive they are recorded in is expected to improve our understanding of redox processes of Earth’s surface environments in the geological past.

AB - Chromium (Cr) is a redox sensitive element potentially capable of tracing fine-scale fluctuations of the oxygenation of Earth’s early surface environments and seawater. The Cr isotope composition of carbonates could perhaps be used as paleo-redox proxy to elucidate changes in the geological past related to the rise of oxygen and the evolution of the biosphere. However, before the Cr isotopesystem can be applied to faithfully delineate paleo-environmental changes, careful assessment of the signal robustness and a thorough understanding of the Cr cycle in Earth system processes is necessary. Processes that potentially fractionate Cr isotopes, perhaps during deposition, burial and alteration need to be constrained.Previous studies have shown that Cr isotopes are fractionated during oxidative weathering on land, where heavy Cr isotopes are preferentially removed with Cr(VI) while residual soils retain an isotopically light Cr signature. Cr(VI) enriched in heavy Cr isotopes is then transported via river waters to the oceans and sequestered into marine sediments. Marine chemical sediments such asbanded iron formations and modern marine carbonates have proven useful in recording the Cr isotope composition of contemporaneous seawater. Marine carbonates are ubiquitous throughout Earth’s rock record rendering them a particularly interesting archive for constraining past changes in ocean chemistry. This thesis includes an investigation of the fractionation behavior of Cr isotopesduring coprecipitation with calcium carbonate in order to test the reliability of the Cr carbonate compositions. Several experimental approaches have been employed to elucidate the fractionation behavior of Cr isotopes when Cr(VI) is incorporated into calcium carbonate phases. These results indicate that at lower Cr concentrations typical for seawater, marginal to no Cr isotope fractionationcan be expected. This represents a first step towards enabling a reliable application of Cr isotopes recorded in ancient carbonates and towards constraining the environmental information they can provide. The Cr isotope proxy was also applied to natural carbonates from several contemporaneoussections, along with other commonly used paleo-proxies, to decipher redox changes during build-up and retreat of one of the major late Neoproterozoic glaciations. These carbonate δ53Cr signatures were sensitive to changes in continental weathering balanced between detrital contamination andoxidative weathering on land and were capable of tracing fluctuating redox conditions. However, a possible diagenetic alteration of the Cr signal as well as Cr contribution from detrital contamination need to be taken into consideration when using ancient carbonates.The redox changes of past surface environments can be explored using the Cr isotope composition of ancient marine carbonates, where a marginal offset compared to contemporaneous seawater δ53Cr is expected and the degree of contamination and later diagenetic alteration can be evaluated. Improved understanding of processes that affect the Cr isotope composition duringdeposition and burial of the archive they are recorded in is expected to improve our understanding of redox processes of Earth’s surface environments in the geological past.

UR - https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122252978805763

M3 - Ph.D. thesis

BT - Chromium isotope uptake in carbonates

PB - Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen

ER -