Reassessing the stable (δ88/86Sr) and radiogenic (87Sr/86Sr) strontium isotopic composition of marine inputs

Abstract

The stable strontium isotope system (δ88/86Sr) has recently been suggested to be a suitable proxy for determining variations in the strength of the marine carbonate system, the principal output flux of oceanic Sr. However, in order to be able to interpret carbonate-driven variations in δ88/86Srseawater a robust understanding of δ88/86Srinput is required. Surprisingly only a limited amount of δ88/86Sr data currently exists for rivers and hydrothermal fluids, thus this study assesses the variability of δ88/86Sr and 87Sr/86Sr in global rivers, hydrothermal fluids and porewaters, as well as minor marine Sr sources such as continental dust, rainwater and glacial ice. Our analyses broadly confirm the findings of Krabbenhöft et al. (2010), and reveal flux-weighted δ88/86Srriverine and 87Sr/86Srriverine compositions of 0.32‰ and 0.71299 respectively. The hydrothermal fluids analysed in this study are consistent with an end-member δ88/86Srhydrothermal composition that is the same as the oceanic crust at ∼0.24‰, although three samples that display δ88/86Sr compositions offset from the seawater-hydrothermal mixing trend suggest that the precipitation of alteration phases such as anhydrite may drive δ88/86Srhydrothermal to higher values. Porewater fluids obtained from sediment cores in the Atlantic and Pacific Oceans have δ88/86Sr compositions within error of seawater (0.39‰), implying that the diagenetic flux of Sr may not significantly affect the δ88/86Sr composition of seawater. Continental loess samples have δ88/86Sr compositions that are consistently lighter than, or equal to, terrestrial silicates, with their tendency to lower values thought to reflect the preferential removal of heavier Sr isotopes into solution during weathering. Finally, rainwater and glacial ice samples have δ88/86Sr compositions that are also isotopically lighter than their associated water sources, a factor that may be attributed to interaction with isotopically light loess and additional Sr contributions from the bedrock. Together, the principal marine inputs define flux-weighted oceanic δ88/86Srinput and 87Sr/86Srinput compositions of 0.32‰ and 0.71161. These values are consistent with an elevated supply of riverine Sr to the oceans due to increased post-glacial weathering, but require the enhanced weathering of exposed carbonate shelves during glacial periods or significant changes in the rate of carbonate burial to match observed changes in the 87Sr/86Sr ratio of seawater. Our results confirm that, providing a diagenetically robust proxy can be found, the δ88/86Sr and 87Sr/86Sr isotope systems should provide a useful proxy for investigating changes in the marine carbonate system through time.