The Marine Biogeochemistry of Chromium Isotopes

Simone Moos, Ph.D., 2018
Edward Boyle, Advisor

This thesis presents a novel analytical technique for measuring Cr isotope signatures of seawater samples, which is then used to investigate Cr cycling in oceanic regimes that may promote Cr reduction (e.g. oxygen deficient zones, ODZs). The first full water column profile of Cr isotope ratios in the North Pacific Ocean (at SAFe station) serves as a fully oxic counterpart to stations located within the ODZ of the Eastern Tropical North Pacific. At one station, Cr concentrations are lower and d53Cr values are heavier within the ODZ. This is consistent with Cr reduction resulting in isotopically lighter, particle-reactive Cr(III), which is scavenged and exported from the water column. A strong correlation of d53Cr and d15NNO3- at this station suggests that Cr reduction may be microbially mediated instead of simply a product of thermodynamic equilibrium. Alternatively, Cr may be reduced by Fe(II). Additionally, a strong Cr reduction signal (lower [Cr], heavier d53Cr) is observed in the anoxic bottom waters of the Santa Barbara Basin, where the same reduction mechanisms may apply. An extreme Cr reduction signal in the oxic bottom waters of the Chukchi shelf may result from Cr reduction by Fe(II), which was released from the underlying anoxic sediments.