The stable chromium isotope system (δ5352CrSRM979) is an emerging powerful proxy that can be used to reconstruct the oxygenation history of Earth’s ocean-atmosphere system, which is intimately linked to the evolution of life. However, the systematics of this isotope system in modern aquatic environment is still poorly understood, which hinders accurate interpretation of deep time data.
Here we report δ5352Cr data from the Connecticut River (CR) and its Estuary System (CRES) and several oxygen minimum zones (OMZ) in the Eastern Tropical North Pacific (ETNP). The CR samples’ δ5352Cr values (-0.2 to 1.0‰) have different distribution compared to the CRES samples, suggesting distinctive Cr cycling between CR and its estuary. The CRES δ5352Cr values range from 0.3 to 1.7‰ and are negatively correlated with Cr concentrations, and this correlation is consistent with seawater samples from other parts of the Atlantic and Pacific Ocean, suggesting a uniform sequestering mechanism of Cr(VI) from the global ocean. Across the OMZs in the ETNP, the Cr(III) and Cr(VI) approximately has 1:1 ratios. Cr(III) δ5352Cr range from 1.2 to 1.3‰ whereas Cr(VI) δ5352Cr range from 1.6 to 1.7‰, which is consistent with previous theoretical prediction and experimental results. Interestingly, Cr(III):Cr(VI) ratio and δ5352Cr are not significantly affected by local redox conditions throughout the water column. Furthermore, the difference in δ5352Cr between Cr(III) and Cr(VI) is relatively small, which suggests that Cr species effect is small compared to observed δ5352Cr excursions in Earth’s history, and thus ancient sedimentary δ5352Cr signal can be used to unravel the redox conditions of seawater. In summary, cumulating δ5352Cr suggest that the Cr isotope system is a promising paleoceanographic redox proxy.
