HR: 09:30h
AN: PP21A-07    [Abstracts]
TI: Combining Radiogenic Strontium (⁸⁷Sr/⁸⁶Sr) and Stable Strontium Isotope (δ^88/86Sr) Fractionation to Balance the Ocean Strontium Budget
AU: * Eisenhauer, A
EM: aeisenhauer@ifm-geomar.de
AU: Fietzke, J
EM: jfietzke@ifm-geomar.de
AU: Liebetrau, V
EM: vliebetrau@ifm-geomar.de
AB: 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: δ^88/86Sr=((⁸⁸Sr/⁸⁶Sr)_sample/(⁸⁸Sr/⁸⁶Sr)_NBS987-1)*1000, where δ^88/86Sr_NBS987=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 δ^88/86Sr 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 (δ^88/86Sr) 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.
DE: 1030 Geochemical cycles (0330)
DE: 1039 Alteration and weathering processes (3617)
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting