Chromium isotope behaviour in natural waters

Abstract

The isotopes of chromium (Cr) fractionate during terrestrial oxidation reactions that require the presence of oxygen. The main source of Cr to the oceans is via rivers, and thus Cr isotopic signatures (expressed as δ53Cr) preserved in authigenic sediments are increasingly being used to reconstruct the oxygen levels of ancient environments. However, Cr can undergo various reactions in natural waters that may fractionate Cr isotopes. The contribution of these reactions to authigenic sediment δ53Cr values is not well understood, and this limits the interpretation of δ53Cr values measured in ancient archives. This thesis describes the development of a method to accurately and precisely measure Cr isotopic signatures in natural waters with extremely low Cr concentrations, and evaluates the behaviour of Cr isotopes in water samples from a range of environmental settings.

Seawater samples from the Atlantic Ocean Oxygen Minimum Zone (OMZ) were analysed to evaluate whether Cr isotopic fractionation was enhanced under depleted oxygen conditions. Results indicate that the Atlantic OMZ is not sufficiently depleted in oxygen to reduce and remove Cr from seawater, although δ53Cr values (1.08 – 1.72‰) were variable due to adsorption of Cr(III) to particles on the shelf, and hydrological mixing. Black Sea seawater samples that had dissolved oxygen concentrations close to zero were enriched in 52Cr, resulting in a δ53Cr that was 0.38‰ lower than the overlying fully oxygenated waters.

Samples were also taken from the Celtic Sea to investigate the seasonal variation in the Cr isotopic composition of seawater from a biologically productive, oxic shelf sea. Chromium concentrations and δ53Cr values were affected by organic matter cycling, and, as in the Atlantic OMZ region, interactions of Cr with particles on the shelf and hydrological mixing were locally important.
Finally, two transects of the Beaulieu River (UK), a relatively pristine river-estuary system, were sampled to assess the behaviour of Cr during estuarine mixing. Both redox species of Cr (Cr(III) and Cr(VI)) and δ53Cr behaved conservatively during mixing between river water and seawater. The Beaulieu River had relatively low δ53Cr (as low as -0.59‰) compared to most other rivers analysed to date, which suggests that the δ53Cr of Cr supplied to the oceans from rivers may be more variable than previously thought.

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ORCID for Rachael James: orcid.org/0000-0001-7402-2315

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