Equilibrium Fractionation of Stable Strontium Isotopes During Adsorption to Mn Oxides

Understanding the stable strontium isotope fractionation between seawater and ferromanganese crusts/nodules is crucial for reconstructing seawater Sr isotopic history. This study explores stable Sr isotope fractionation during adsorption to manganese oxides, as preliminary experiments indicated their higher Sr adsorption capacity compared to iron oxides. We observed that equilibrium in concentration and isotopic composition can be achieved within a day. The lighter isotope (86Sr) is preferentially adsorbed, with the fractionation extent showing no significant change with respect to reaction time, pH, or Sr concentration under low ionic strength conditions (-0.14 parts per thousand). Equilibrium fractionation processes, rather than Rayleigh fractionation, more accurately represent the isotopic composition trends of dissolved and adsorbed Sr. Experiments in synthetic seawater demonstrated greater Sr isotope fractionation (-0.20 parts per thousand) compared to low ionic-strength solutions. This fractionation during adsorption by manganese oxides in synthetic seawater is consistent with the isotopic discrepancy between seawater and ferromanganese nodules, suggesting that Mn oxide adsorption is the primary mechanism for Sr accumulation in ferromanganese deposits. These results imply that ferromanganese deposit Sr isotopic compositions could be proxies for reconstructing seawater Sr isotopic history, which is influenced by carbonate burial and continental weathering, reflecting the global carbon cycle changes. Strontium isotope records in seawater can inform us about the history of continental weathering and marine carbonate burial. To reconstruct these records from marine sediments, we need consistent Sr isotope fractionation between seawater and the sediments. Our experiments focused on manganese oxide minerals in ferromanganese deposits, which are potential carriers for this purpose. Findings indicate that stable Sr isotopes undergo equilibrium fractionation during adsorption by manganese oxides, and this process in synthetic seawater matches the Sr isotopic offset found between seawater and natural ferromanganese nodules. Consequently, the stable Sr isotopic composition of ferromanganese deposits may consistently differ from that of seawater. This suggests the potential of ferromanganese deposits to reconstruct the historical stable Sr isotopic composition of seawater. Lighter strontium isotopes are preferentially adsorbed by manganese oxides Fractionation magnitude is consistent across varying reaction times, pH levels, and strontium concentrations The stable strontium isotopic offsets between seawater and ferromanganese nodules align with those observed in adsorption experiments