Award: OCE-1924050

Chromium stable isotope ratios are being used by geologists to infer atmospheric and oceanic oxygenation conditions in the geological record. The data has been interpreted initially without much data showing how chromium isotopes behave in the modern ocean. Our objective is to provide modern process-interpretable chromium isotope data that can illuminate the processes that generate the geological signal. This data can also shed some light on chemical oxidation-reduction processes in and near regions in the ocean with very little oxygen (Oxygen-Deficient Zones, ODZs). Our major goals are (1) to obtain data on the two environmentally-relevant oxidation states of chromium: reduced Cr(III) and oxidized Cr(VI) concentrations and their isotopic compositions. We focus on ODZs, most intensely on the Eastern Tropical North Pacific (EPNP) but with significant numbers of samples from the Eastern Tropical South Pacific and Arabian Sea. We compare these data to other redox-relevant species such as nitrite, iodine, iron, and nitrogen stable isotopes in order to understand the processes that create stable isotope variations in oxidized and reduced chromium. We have analyzed samples obtained on two U.S. research cruises in the Eastern Tropical North Pacific (ETNP) for total dissolved chromium, chromium(III), and chromium(VI) concentrations and their d53Cr, a measure of differences in the isotope ratios compared to that of a standard [ d53Cr =(53/52Crsample/53/52Crstandard – 1)*1000 ]. These analyses have been reported in a publication and a publicly-available PhD thesis, as well as several conference abstracts and presentations. We have also analyzed these properties from samples sent to us from two other regions, the Eastern Tropical South Pacific (ETSP) and Arabian Sea. We aim to document the reduction of chromium(VI) to chromium(III) in oxygen-deficient zones of the global ocean and the isotopic fractionation that occurs during this process. Although this reduction is thermodynamically favorable in low-oxygen waters, this reduction may require either microbial action or reactions with specific reductants to proceed at a significant rate. We aim to deduce the isotope fractionation factor for the chromium(VI) to chromium(III) reduction and the fractionation between dissolved chromium(III) and chromium(III) scavenged by sinking particles, and the effect of those combined processes on the isotopic composition of total dissolved chromium. We found that the isotope fractionation factor for ODZ chromium(VI) to chromium(III) reduction in a nearshore station from the ETNP is -1.5‰, similar to the -1.3‰ value in offshore stations. The fractionation factor (ε) derived from offshore ETNP station P2 and ETSP offshore station calculated Chromium(VI) data is -1.24 ± 0.04‰, which is within the error of that from ETNP P2 data alone (-1.29 ± 0.04‰). In the ETNP, the weighted average of the scavenged δ53Cr(III) is +0.5 ± 0.4‰ (SD, n=55) relative to the dissolved δ53Cr(III). An anoxic incubation experiment with 0.2 µm filtered suboxic ODZ water shows that 5 nM Fe(II) does not account for the small chromium(VI) reduction observed, as can also be predicted from the aquatic chemistry rate law for Fe(II) reduction of chromium(VI). This reduction in a filtered sample implies that some other reductant (perhaps nitrite) is responsible. Anoxic bottom waters on the ETNP continental slope see the greatest Chromium scavenging with heaviest δ53Cr (+1.85‰). Our estimates of the scavenged chromium isotopic composition are within error of the anoxic and euxinic marine sedimentary δ53Cr. This implies that the vertical transport of chromium to the seafloor and subsequent diagenesis may not generate significant isotopic fractionation for chromium. In the core of the Arabian Sea ODZ (160 ~ 200m) at isopycnal surfaces, GEOSECS station 417 is only slightly heavier in δ53Cr by 0.12 to 0.25‰ than station 419, with a [total chromium] deficit of 0.16 to 0.21 nmol/kg. Our estimates of the scavenged δ53Cr are consistent with the authigenic δ53Cr of marine sediments in different redox conditions. These results suggest that there may be negligible isotopic fractionation of chromium during diagenesis. A strong correlation between sedimentary δ53Cr and δ15N (Gueguen et al., 2016) indicates that sediments may preserve δ53Cr signals from the thermocline as is the case for δ15N (Thunell et al., 2004). Therefore, instead of recording deepwater δ53Cr, authigenic δ53Cr may be more likely to preserve the redox information of the overlying waters. Last Modified: 06/12/2023 Submitted by: Edward A Boyle