Species-dependent chromium isotope fractionation across the Eastern Tropical North Pacific Oxygen Minimum Zone

The stable chromium (Cr) isotope system is an emerging paleoredox proxy. Interpretation of sedimentary and seawater δ⁵³Cr (⁵³Cr/⁵²Cr relative to SRM 979) hinges on our understanding of the isotopic fractionations during Cr sequestration from seawater to sediments. Seawater δ⁵³Cr values reported thus far are for total dissolved Cr. This study reports the δ⁵³Cr of both dissolved Cr (III) and dissolved Cr (VI) across the oxygen minimum zone (OMZ) in the Eastern Tropical North Pacific (ETNP) off Manzanillo, Mexico. Dissolved Cr (III) accounts for about 50% total dissolved Cr in this region. There are no correlations between O₂ and Cr concentrations, or O₂ concentrations and δ⁵³Cr throughout the water column. The δ⁵³Cr of Cr (III) is lower than that of Cr (VI) by a relatively constant offset, and is also uncorrelated with dissolved O₂ concentration. These observations provide part of the foundation for using δ⁵³Cr of the authigenic Cr in organic-rich shales to reconstruct seawater δ⁵³Cr. A survey of literature Cr concentration and δ⁵³Cr data suggests that the Cr isotope fractionation factor in the global ocean is likely more complex than previously thought. More future species-dependent Cr isotope studies could shed new light on the biogeochemical processes controlling the oceanic Cr isotope fractionation.