Stable chromium isotope compositions of hydrogenetic ferromanganese crusts potentially linked to primary productivity

Abstract

Stable chromium isotope composition (δ53Cr) is a widely used paleo-redox proxy. However, modern seawater δ53Cr values do not correlate with dissolved O2 concentrations, suggesting that non-O2 factors may influence marine Cr cycling. We present new δ53Cr data for two hydrogenetic ferromanganese (FeMn) crusts deposited in the western- and central-north Pacific Ocean since the late Eocene, a period when the ocean remained largely oxic. Simple isotope mass balance and Rayleigh distillation models suggest that trivalent Cr is likely the major Cr species incorporated from seawater into the Fe phases of hydrogenetic FeMn crusts. The δ53Cr values of FeMn crusts decrease from −0.35‰ in the late Eocene to −0.74‰ in the middle Miocene, followed by increasing to modern values ranging from −0.33 to −0.47‰. The temporal trend of δ53Cr follows that of the regional pelagic sedimentation rate, suggesting that primary productivity may have played a role in shaping the δ53Cr signals recorded in the hydrogenetic FeMn crusts. Our data show that non-O2 factors may influence δ53Cr values recorded in Fe-rich sedimentary archives deposited in a largely oxic ocean, thus adding more complexities to the Cr isotope paleoproxy.