Natural variations of the 51V/ 50V isotope composition: A new redox tracer?

In the proposed study, stable Vanadium isotope ratios of marine and riverine environments will be analyzed systematically for the first time to explore the potential of V isotope signatures as tracers for marine redox and water chemistry variations.

Investigation of stable V isotope ratios is still in its infancy, because numerous analytical challenges were overcome only a few years ago. Theoretical investigations have concluded that low-temperature redox transitions from VIII to VV can result in V isotope fractionation of up to ~6 per mil. Even for V adsorption onto Fe oxides, V isotope fractionation of up to ~1.5 per mil is possible. A V purification and analyses procedure has been successfully set up and tested for different sample matrices by the applicant at the Leibniz University Hannover.

Preliminary V isotope analyses included early Cambrian black shales, a sediment sample taken from the euxinic regime of the Black Sea, and a water sample from the North Sea. Preliminary results based on these samples indicate that significant V isotope fractionation can occur due to redox transitions, in accordance to previous theoretical predictions.

In this study, mechanisms of stable V isotope fractionation during V-reduction will be investigated by analyses of sediments from sub-oxic to euxinic environments, as well as water samples from restricted basins (Black Sea, Baltic Sea). The overall goal is to develop a new paleo-redox tracer that may be used similarly to and combined with other isotopic tracers (e.g. Fe, Mo, U), in order to reconstruct the redox evolution of the oceans, and atmosphere from the Archean to the Phanerozoic. However, V isotopes can only be successfully applied as a paleo-redox proxy if the oceanic V isotope cycle is fully understood.

Accordingly, V isotope compositions of all important oceanic sinks (hydrothermal Fe (oxyhydr)oxide precipitates, carbonates, Fe-Mn deposits, reduced sediments), as well as rivers (the dominant oceanic V source), will be investigated. To elucidate V isotope fractionation during non-redox processes, e.g. V adsorption and co-precipitation with Fe oxides, simple laboratory experiments are included. The outcome of this study will unravel (i) how the 51V/50V isotope signature depends on redox conditions, (ii) the V signature of the major oceans (Atlantic, Pacific, Indian Ocean) is uniform, and (ii) how V mobilization and precipitation may affect the marine V isotope composition. In an ideal case, non-redox processes result in only minor V isotope fractionation, and the oceanic V mass balance is dominated by the extend of reducing sediments. Potential recorders of paleo-seawater may be Fe-Mn deposits or carbonates. According to the multiple valences of V (III-IV-V), or in combination with other redox proxies (Fe, Mo, U), V isotopes may finally provide a more detailed view on the redox evolution of past oceans and distinguish between anoxic and euxinic environments or even provide a distinct bio-signature.