Reassessing the stable (delta Sr-88/86) and radiogenic (Sr-87/Sr-86) strontium isotopic composition of marine inputs

摘要

The stable strontium isotope system (delta Sr-88/86) 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 delta Sr-88/86(seawater) a robust understanding of delta Sr-88/86(input) is required. Surprisingly only a limited amount of delta Sr-88/86 data currently exists for rivers and hydrothermal fluids, thus this study assesses the variability of delta Sr-88/86 and Sr-87/Sr-86 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 Krabbenhoft et al. (2010), and reveal flux-weighted delta Sr-88/86(riverine) and Sr-87/Sr-86(riverine) compositions of 0.32 parts per thousand and 0.71299 respectively. The hydrothermal fluids analysed in this study are consistent with an end-member delta Sr-88/86(hydrothermal) composition that is the same as the oceanic crust at similar to 0.24 parts per thousand, although three samples that display delta Sr-88/86 compositions offset from the seawater-hydrothermal mixing trend suggest that the precipitation of alteration phases such as anhydrite may drive delta Sr-88/86(hydrothermal) to higher values. Porewater fluids obtained from sediment cores in the Atlantic and Pacific Oceans have delta Sr-88/86 compositions within error of seawater (0.39 parts per thousand), implying that the diagenetic flux of Sr may not significantly affect the delta Sr-88/86 composition of seawater. Continental loess samples have delta Sr-88/86 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 delta Sr-88/86 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 delta Sr-88/86(input) and Sr-87/Sr-86(input) compositions of 0.32 parts per thousand 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 Sr-87/Sr-86 ratio of seawater. Our results confirm that, providing a diagenetically robust proxy can be found, the delta Sr-88/86 and Sr-87/Sr-86 isotope systems should provide a useful proxy for investigating changes in the marine carbonate system through time. (C) 2015 Elsevier Ltd. All rights reserved.