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

Stable chromium isotope composition (& delta;53Cr) is a widely used paleo-redox proxy. However, modern seawater & delta;53Cr values do not correlate with dissolved O2 concentrations, suggesting that non-O2 factors may influence marine Cr cycling. We present new & delta;53Cr data for two hydrogenetic ferromanganese (Fe-Mn) 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 Fe-Mn crusts. The & delta;53Cr values of Fe-Mn crusts decrease from -0.35%o in the late Eocene to -0.74%o in the middle Miocene, followed by increasing to modern values ranging from -0.33 to -0.47%o. The temporal trend of & delta;53Cr follows that of the regional pelagic sedimentation rate, suggesting that primary productivity may have played a role in shaping the & delta;53Cr signals recorded in the hydrogenetic Fe-Mn crusts. Our data show that non-O2 factors may influence & delta;53Cr values recorded in Fe-rich sedimentary archives deposited in a largely oxic ocean, thus adding more complexities to the Cr isotope paleoproxy.

Automated summary

Stable chromium isotope composition is widely used as a paleo-redox proxy, but its correlation with dissolved O2 concentrations in modern seawater suggests that non-O2 factors may affect marine Cr cycling. New δ53Cr data from hydrogenetic Fe-Mn crusts in the western and central-north Pacific Ocean suggest that trivalent Cr is the major species incorporated from seawater. The δ53Cr values of the crusts decrease from late Eocene to middle Miocene, followed by an increase to modern values, possibly influenced by primary productivity.