Abstract
Abstract. Chromium (Cr) and its isotopes hold great promise as a tracer of past oxygenation and marine biological activity due to the contrasted chemical properties of its two main oxidation states, Cr(III) and Cr(VI), and the associated isotope fractionation during redox transformations. However, to date the marine Cr cycle remains poorly constrained due to insufficient knowledge about sources and sinks and the influence of biological activity on redox reactions. We therefore implemented the two oxidation states of Cr in the Bern3D Earth system model of intermediate complexity in order to gain an improved understanding on the mechanisms that modulate the spatial distribution of Cr in the ocean. Due to the computational efficiency of the Bern3D model we are able to explore and constrain the range of a wide array of parameters. Our model simulates vertical, meridional, and inter-basin Cr concentration gradients in good agreement with observations. We find a mean ocean residence time of Cr between 5 and 8 kyr and a benthic flux, emanating from sediment surfaces, of 0.1–0.2 nmol cm−2 yr−1, both in the range of previous estimates. We further explore the origin of regional model–data mismatches through a number of sensitivity experiments. These indicate that the benthic Cr flux may be substantially lower in the Arctic than elsewhere. In addition, we find that a refined representation of oxygen minimum zones and their potential to reduce Cr yield Cr(III) concentrations and Cr removal rates in these regions in much improved agreement with observational data. Yet, further research is required to better understand the processes that govern these critical regions for Cr cycling.
Highlights
The chromium (Cr) cycle at the Earth’s surface has received continuous attention throughout the past decades to gain better understanding on the mobility of hexavalent Cr (Cr(VI)), a carcinogenic pollutant in the environment (e.g., Wang et al, 1997)
Research was further motivated by technical advances in mass spectrometry that allow for robustly measuring the stable isotopic composition of Cr (e.g., Bonnand et al, 2013; Moss and Boyle, 2019; Wei et al, 2020), the fractionation of which is linked to redox transformations between its two main oxidation states Cr(III) and Cr(VI) (Ellis et al, 2002; Joe-Wong et al, 2019; Wanner and Sonnenthal, 2013; Zink et al, 2010)
The mean ocean residence time of Cr is strongly influenced by the magnitude of the newly introduced benthic flux representing the largest source in most runs of the 500-member tuning ensemble
Summary
The chromium (Cr) cycle at the Earth’s surface has received continuous attention throughout the past decades to gain better understanding on the mobility of hexavalent Cr (Cr(VI)), a carcinogenic pollutant in the environment (e.g., Wang et al, 1997). As redox cycling is known to control oceanic Cr distributions, isotopic fractionation during Cr(VI) reduction and Cr(III) removal have been proposed to account for the principal mechanisms that lead to the well-defined inverse logarithmic relationship between the total dissolved Cr concentration and its stable isotopic ratio (e.g., Janssen et al, 2020, 2021; Moos et al, 2020; Nasemann et al, 2020; Scheiderich et al, 2015). The tightly constrained relationship between the dissolved Cr concentration and the stable isotopic ratio could allow for future model– data intercomparisons using marine sedimentary isotope data to reflect past changes in the marine Cr cycle
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