Strontium stable isotope variations and the marine strontium cycle

Coupled stable and radiogenic strontium (Sr) isotopes potentially provide key information on the present-day Sr cycle, on variations in the flux and composition of material delivered by continental weathering and hydrothermal exchange at mid-ocean ridges, and on changes in carbonate productivity over time. This study presents high-precision stable (and radiogenic) Sr isotope data (obtained using double-spike TIMs) for rivers, MOR hydrothermal fluids, seawater and preliminary data for a Quaternary marine foraminiferal record. Riverine data, for both basaltic and granitic catchments, indicates that a significant fractionation of Sr stable isotopes may occur during weathering. This fractionation is largely due to the formation of secondary weathering minerals, and results in Sr stable isotope compositions that are both lighter and heavier than seawater, with an overall range of some 1 per mil. High-temperature hydrothermal fluids are slightly lighter than seawater, conistent with mixing between MOR basalt/peridotite sourced fluids and ambient seawater. Data for a Quaternary marine foraminiferal record indicate a very small shift in the Sr stable isotope composition of seawater over this interval, which has implications for changes in the flux and composition of material delivered by continental weathering. These results indicate that, while chronostratigraphy based on shifts in the normalised radiogenic Sr isotope ratio remains a robust technique, information on Sr fluxes into and out of the oceans, and their variation over time (i.e. the cause of the radiogenic Sr variations) are only revealed through reconstruction of both stable and radiogenic marine Sr records.