The development and application of uranium, molybdenum, and vanadium stable isotope ratios as redox-proxies in samples from modern times and the early earth

Abstract

The metals Molybdenum (Mo), Uranium (U), and Vanadium (V) belong to the group of redox-sensitive elements and may be used to track redox processes in geological samples. They all show characteristic isotope fractionation that may be captured and preserved in marine sediments so that precise and accurate measurements may reveal past environmental changes of the depositional redox conditions in the oceans. Reconstructing past redox conditions must always rely on a solid understanding of causal present-day processes and hence, their investigation is the basis for reliable interpretations. In this thesis, the isotope fractionation of Mo, U, and V is investigated to improve our understanding of present-day processes during their geochemical cycling and to apply this knowledge to gain information on past environments. The behavior of V in seawater highly depends on speciation, pH and Eh and V isotopes are likely a very sensitive proxy for redox changes in the aquatic environment. In order to use V isotopes for this purpose, the V isotope composition of seawater needs to be analyzed. The first part of this thesis deals with a new method development to separate V from a seawater matrix, purifying this fraction and optimizing mass spectrometric measurements. Therefore, different separation procedures were tested, such as a Fe co-precipitation and ion-chromatographic exchange both followed by additional ion-exchange especially optimized for seawater samples. The final ion-exchange method was applied repeatedly to North Sea seawater and yielded a δ51VNist-3165 = 0.62 ‰ ± 0.25 ‰ (n = 17 single analyses from 6 separately processed aliquots) which is in agreement with a recently published V isotope composition of open-ocean seawater within analytical error (Wu et al., 2019). The results show that, compared to other measured environmental compartments, V isotopes exhibit a high isotope signature in seawater. An additional seawater sample from the Southern Ocean resulted in a δ51VNist 3165 = 2.53 ‰ ± 0.21 ‰ (n = 5), which shows a significantly fractionated signature compared to open-ocean and North Sea. These first-order results need further confirmation and ultimately will help to apply V isotopes as an additional tool in paleo-oceanography. The second part of this thesis targets the investigation of Mo and U isotope fractionation in sediments from the Black Sea and the Cariaco Basin, which are the modern analogues for marine anoxic depositional conditions during past times. Both isotope proxies have been used extensively as single proxies and here, their inter-relationships and responses to the same conditions are aimed to be understood. Hence, both signatures on the same samples were analyzed. The results show that Mo scavenging is mainly controlled by water column sulfide chemistry, whereas U immobilization mainly occurs at the sediment-water interface and upper cm of the sediment pile. The feedback between U isotope ratios of the water column and the sediment stems from the seawater δ238U controlling the sediment δ238U. Although both elements show highly different removal pathways, the isotope signatures of sediments exhibit an inverse correlation in the Black Sea and the Cariaco Basin, with the Cariaco Basin trend being generally lower in δ238U and δ98Mo. This correlation indicates a direct relationship between the responses of Mo and U to surrounding conditions and removal efficiencies which are mainly controlled by sulfate reduction rates and resulting dissolved sulfide concentrations. The results are reproduced in a coupled water column and reactive transport model and the different slopes of the correlation in both basins can be explained with varying degrees of basin restriction, sulfate reduction rates, and isotope compositions of the respective water columns. Hence, the coupled application of Mo and U isotopes will also strengthen results in paleo-redox studies. The third part of this thesis discusses the application of the U isotope proxy for the reconstruction of Archean and Early Proterozoic anoxic conditions. Here, U isotope fractionation may help to disentangle the complex evolution of molecular oxygen during the Archean and Proterozoic times. The Archean is characterized by overall anoxic conditions in the atmosphere and hydrosphere, however, the temporal and spatial evolution of free oxygen is highly debated among scientists. Several lines of evidence indicate a change to slightly oxic conditions around 2.46 Ga to 2.33 Ga (Bekker et al., 2004; Luo et al., 2016; Gumsley et al., 2017) and this period has been named the Great Oxidation Event (GOE). The increase in free oxygen ultimately resulted in oxidative weathering of continental crust and caused geochemical cycling of redox-sensitive elements. Authigenic enrichment of U in marine anoxic organic-rich or Fe-rich sediments as well as marine carbonates is monitored relative to Post-Archean Average Shale (PAAS) and indicates an increase in U cycling during the late Archean. Interestingly, this increased authigenic U enrichment is accompanied with considerably fractionated δ238U values, mainly towards lower values compared to continental crust. These low U isotope compositions are best explained by U isotope fractionation as a consequence of the onset of partial U mobilization during oxidative weathering of uraninite, just before the GOE, i.e. roughly at ~2.5 Ga. After the GOE, the observed δ238U values indicate the onset of modern-style weathering of uraninite and other U-bearing minerals with quantitative U mobilization and limited U isotope fractionation in a continuously anoxic deep ocean.