Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean

Abstract

While chromium stable isotopes (δ 53 Cr) have received significant attention for their utility as a tracer of oxygen availability in the distant geological past, a mechanistic understanding of modern oceanic controls on Cr and δ 53 Cr is still lacking. Here we present total dissolved δ 53 Cr, concentrations of Cr (III) and total dissolved Cr, and net community productivity (NCP) from the North Pacific. Chromium concentrations show surface depletions in waters with elevated NCP, but not in lower productivity waters. Observed Cr deficits correspond well with calculated Cr export derived from NCP and Cr:C ratios of natural phytoplankton and marine particulates. Chromium (III) concentrations are stable over the diel cycle yet correlate with NCP, with maxima found in highly productive surface waters but not in lower productivity waters, indicating biological control on Cr (III). The relationship between Cr (III) and δ 53 Cr suggests that δ 53 Cr distributions may be controlled by the removal of isotopically light Cr (III) at an isotopic enrichment factor ( 53 Cr) of −1.08‰ ± 0.25 relative to total dissolved δ 53 Cr, in agreement with the global δ 53 Cr-Cr fractionation factor (−0.82‰ ± 0.05). No perturbation to δ 53 Cr, Cr, or Cr (III) is observed in oxygen-depleted waters (~10 μmol/kg), suggesting no strong control by O 2 availability, in agreement with other recent studies. Therefore, we propose that biological productivity is the primary control on Cr and δ 53 Cr in the modern ocean. Consequently, δ 53 Cr records in marine sediments may not faithfully record oxygen availability in the Late Quaternary. Instead, our data demonstrate that δ 53 Cr records may be a useful tracer for biological productivity.

Modeling the marine chromium cycle: New constraints on global-scale processes

“…This also permitted the application of stable Cr isotopes as a proxy for past changes in oceanic and atmospheric oxygenation (e.g., Frei et al, 2009;Planavsky et al, 2014). Recent investigations revealed that biologically mediated redox reactions may play an important role in the marine Cr cycle (Huang et al, 2021;Janssen et al, 2020;Semeniuk et al, 2016), and the isotopic ratio measured in marine sediments may hence be potentially used for the reconstruction of past changes in biological productivity.…”

“…In the modern ocean, total dissolved Cr concentrations of unpolluted waters typically range between 1 and 6 nmol kg -1 (e.g., Jeandel and Minster, 1987;Nasemann et al, 2020) of which a majority (>70 %) is generally found in the soluble Cr(VI) redox state. Reduction of Cr(VI) is either induced in poorly oxygenated waters and/or catalyzed by Fe(II) and organic matter in oxic waters (Janssen et al, 2020;Kieber and Helz, 1992). As such, Cr(III) concentrations can account for the majority of total dissolved Cr in OMZs (Huang et al, 2021;Rue et al, 1997), and may also be elevated in high productivity surface waters (e.g., Janssen et al, 2020).…”

“…Reduction of Cr(VI) is either induced in poorly oxygenated waters and/or catalyzed by Fe(II) and organic matter in oxic waters (Janssen et al, 2020;Kieber and Helz, 1992). As such, Cr(III) concentrations can account for the majority of total dissolved Cr in OMZs (Huang et al, 2021;Rue et al, 1997), and may also be elevated in high productivity surface waters (e.g., Janssen et al, 2020). Complexation with organic ligands may further increase its solubility (e.g., Murray et al, 1983).…”

“…Complexation with organic ligands may further increase its solubility (e.g., Murray et al, 1983). Trivalent chromium is fairly particle reactive and is thus rapidly removed from the dissolved phase via adsorption on sinking particles, with strong affinities for biological particles (Janssen et al, 2020;Semeniuk et al, 2016;Wang et al, 1997). Hence, subsurface pelagic Cr(III) concentrations are substantially lower than Cr(VI) concentrations typically ranging between 0.0 and 0.3 nmol kg -1 (Janssen et al, 2020).…”

“…Trivalent chromium is fairly particle reactive and is thus rapidly removed from the dissolved phase via adsorption on sinking particles, with strong affinities for biological particles (Janssen et al, 2020;Semeniuk et al, 2016;Wang et al, 1997). Hence, subsurface pelagic Cr(III) concentrations are substantially lower than Cr(VI) concentrations typically ranging between 0.0 and 0.3 nmol kg -1 (Janssen et al, 2020). The redox behavior and the different particle affinities of the two oxidation states therefore control the spatial distribution of Cr in the oceans.…”