Cadmium and Chromium Isotopes as Ancient Seawater Proxies

Cadmium (Cd) and chromium (Cr) isotopes are among an emerging group of palaeoenvironmental proxies which have been referred to as non-traditional or heavy metal stable isotopes. In the application of these novel isotope systems to the marine sedimentary rock record, the ultimate goal is to reconstruct, on a detailed time scale, past global scale processes such as climate change – its manifestations in the biotic realm, and its drivers in the atmosphere-hydrosphere interface. Through detailed understanding of these past processes, we may better construct current models of modern climate change, which is of ever-increasing importance in our rapidly warming earth.

Cadmium, in its behaviour as a marine micronutrient, is a proposed palaeoproxy of surface water primary productivity, as its use in biogenic processes is accompanied by a characteristic fractionation in the Cd isotope system (δ114Cd). However, the pathways by which Cd and δ114Cd are preserved in the rock record are numerous and subject to ongoing study, wherein processes such as remineralization and adsorption may distort the original surface seawater δ114Cd signal.

Linked to bioproductivity is redox – whether it be in the oxidative surface weathering of continentally-derived nutrient sources to the ocean, in the oxygenation states of surface waters, or in the oxic, sub-oxic, or even anoxic states of the underlying water column and sediment-water interface through which elements pass as they are incorporated into sedimentary archives. Chromium, a redox sensitive metal, preserves changes in redox conditions in its isotopic signature (δ53Cr), and therefore becomes an ideal complement to the application of the δ114Cd palaeoproxy system. In simplified modern marine setting, we would expect to see positively fractionated δ114Cd, indicative of enhanced surface water bioproductivity, coupled with positively fractionated δ53Cr, characteristic of oxygenated surface seawater. Yet, the δ53Cr signal too may become intricately nuanced in its journey to preservation in the rock record, through similar complexities such as fluctuating water column redox, remineralization, and adsorption.

This thesis evaluates a combined Cd-Cr approach in three common marine sedimentary archives – iron formations, black shales, and carbonates. From these archive types, lithologies have been specifically chosen for their depositional and temporal proximity in relation to global-scale climatic events, events which should be reflected in the Cd-Cr system. This thesis presents data from: (i) (Pre)-GOE Paleoproterozoic Cauê Iron Formation of the São Francisco Craton, (ii) Steptoean Positive Isotope Excursion (SPICE)-recording black shales of the Alum Shale Formation, and (iii) enriched carbonate dominated sequences of the Belqa Group, deposited near (though not concurrent with) the Cretaceous-Paleogene (K/Pg) mass extinction event.

Through chromatographic procedures developed to separate Cd and Cr from the same sample aliquot, this project uses high-precision Thermal Ionization Mass Spectrometry (TIMS) to measure the relative isotope abundances of Cd and Cr in 73 samples. This isotope data is interpreted in combination with select major, trace, and rare earth element data measured from these same samples.

Results for the Cauê Iron Formation indicate primary controls of adsorption and co- precipitation on negatively fractionated δ114Cd values, and oxidative fluid-processes and ferrous iron (Fe(II)) mediated reduction resulting in negatively fractionated δ53Cr, which obscures primary seawater signals in the Cd-Cr system. Cd and Cr results from the Alum Shale Formation highlight not only influence of euxinic depositional environments on δ114Cd, but also how these environments manifest in negatively fractionated δ53Cr. Finally, Cd-Cr data from the K/Pg Belqa Group reflect the processes by which enhanced oxygenated surface water productivity recorded by δ114Cd and δ53Cr result in conditions of extreme metal enrichment.

In two of the three studies (those of the Alum Shale Formation and Belqa Group), this thesis successfully characterizes ancient seawater through δ114Cd and δ53Cr measured from the rock record. In the Cauê Iron Formation study this is not accomplished, however, δ114Cd and δ53Cr become unexpected and effective tools for tracing the paragenetic evolution of high-grade Feore. In all studies presented in this thesis, the importance of a thorough evaluation of the numerous fractionation pathways of δ114Cd and δ53Cr is highlighted for future applications of the Cd-Cr palaeoproxy system.