Eisenhauer, Anton , Fietzke, Jan and Liebetrau, Volker (2007) Combining Radiogenic Strontium (87Sr/86Sr) and Stable Strontium Isotope (δ88/86Sr) Fractionation to Balance the Ocean Strontium. [Paper] In: AGU Fall Meeting 2007. , 10.-14.12, San Francisco, USA . EOS .

Abstract

The stable 88Sr/86Sr-ratio has traditionally been considered to be a constant value in order to correct instrumental mass fractionation during measurement of the radiogenic strontium isotope ratio (87Sr/86Sr). However, recent high precision measurements (Fietzke and Eisenhauer, 2006) showed that the 88Sr/86Sr ratio of marine carbonates and certain silicates (deSouza et al., 2007) are not constant, but rather variable and temperature dependent. Variations of the 88Sr/86Sr-ratios are expressed in the usual δ-notation: δ88/86Sr=((88Sr/86Sr)sample/(88Sr/86Sr)NBS987-1)*1000, where δ88/86SrNBS987=0. The recent measurements also showed that the presently accepted (88Sr/86Sr)-value for seawater differs by about ~0.4 ‰ from the 88Sr/86Sr)NBS987 value. This in turn implies that the 87Sr/86Sr isotope ratio in seawater not corrected for natural isotope fractionation is also significantly different from its normalized value (87Sr/86Sr=0.70916). Correcting the 87Sr/86Sr ratio of seawater results in a 87Sr/86Sr value of about ~0.70930. First results from continental rocks and minerals indicate that they are significantly lighter than seawater by about 0.3 ‰ and presumably become fractionated due to dissolution, precipitation of secondary mineral phases and biological utilization ((deSouza et al., 2007), (Halicz et al., 2007)). The combination of δ88/86Sr and 87Sr/86Sr data provides a unique solution for three-component mixing processes and a way of distinguishing the different sources that supply Sr to seawater. Latter mixing has to account for the characteristic radiogenic ingrowth of the 87Sr/86Sr ratio as well for the natural fractionation related to chemical weathering, temperature and other yet not known effects. This may help to elucidate the link between continental weathering and atmospheric pCO2 in more detail on geological time scales. deSouza G., Reynolds B., and Bourdon B. (2007) Evidence for Stable Strontium Isotope Fractionation during Chemical Weathering. 17th Goldschmidt Conference. Fietzke J. and Eisenhauer A. (2006) Determination of temperature-dependent stable strontium isotope (δ88/86Sr) fractionation via bracketing standard MC-ICP-MS. Geochmistry, Geophysics, Geosystems 7(8), doi:10.1029/2006GC001243. Halicz L., I. Segal, N. Fruchter, B. Lazar, and Stein M. (2007) 86Sr/88Sr Ratio by ICP-MS-MC as a New Tracer of Terrestrial Geochemical Processes. 17th Goldschmidt Conference 2007

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