The strontium stable isotope composition of global rivers and the implications for the marine δ 88 Sr record

Variations in the 87 Sr/ 86 Sr isotopic composition of the oceans are frequently used as a monitor of long-term changes in the flux of continental and hydrothermal material to the oceans. Such records cannot provide a complete interpretation of changing oceanic inputs, however, as it is not possible to resolve different source inputs from weathering fluxes. This issue can be addressed through the measurement of both the radiogenic ( 87 Sr/ 86 Sr) and stable (δ 88 Sr) strontium isotopic compositions using double-spike TIMS, providing additional compositional space in which the weathering flux and source compositional variations can be resolved. Riverine δ 88 Sr values from several continental settings have been shown to vary from 0.24 ‰ to 0.42 ‰, bracketing the hydrothermal δ 88 Sr composition of 0.27 ‰ as well as that of modern seawater, 0.39 ‰. Carbonates, which are the dominant marine Sr sink, have δ 88 Sr compositions that are ~0.20 ‰ lighter than seawater. These δ 88 Sr values may therefore support 87 Sr/ 86 Sr evidence that oceanic Sr inputs and outputs are currently in a non-steady state. It has recently been postulated that the marine Sr system may have attained steady state during glacial episodes of enhanced carbonate weathering. However, to evaluate this model requires a better understanding of riverine δ 88 Sr and 87 Sr/ 86 Sr variations within different continental settings. This study therefore presents new δ 88 Sr and 87 Sr/ 86 Sr isotopic data for a suite of global rivers that account for >40 % of the global Sr flux to the oceans. In addition to providing a more thorough overview of the modern δ 88 Sr marine input, the new data compares δ 88 Sr values from silicate and carbonate drainage basins as well as between wet and dry seasonal flow regimes. The δ 88 Sr values for these samples are also evaluated against previously measured δ 26 Mg and δ 44 Ca compositions, providing an indication of the sensitivity of the stable Sr isotope system to fractionation during weathering processes.