Stable Strontium Isotope (δ88/86Sr) Fractionation in the Marine Realm: A Pilot Study

The determination of the isotopic composition of natural substances is an important field of research within isotope geochemistry. Especially the investigation of the alkaline earth element strontium (Sr) plays an important role in geological and geochemical research. In order to quantify the degree of natural stable Sr isotope fractionation a double spike technique was developed in the frame of this study. This technique allows the precise determination of natural Sr isotope fractionation without normalizing the 87Sr/86Sr to a fixed 88Sr/86Sr ratio in order to correct for instrumental mass fractionation. Variations in the stable Sr isotope ratio are presented in the common δ-notation in per mill [‰] deviation from standard material NIST SRM 987 (δ88/86Sr[‰]=((88Sr/86Sr)sample/(88Sr/86Sr)standard–1)∙1000). Measurements were carried out at the IFMGEOMAR in Kiel using a thermal ionization mass spectrometer (TIMS). Long term measurements of the coral standard JCp-1 and the seawater standard IAPSO resulted in δ88/86Sr=0.194±0.025‰ and δ88/86Sr=0.389±0.026‰ (2SD), respectively. This corresponds to an improvement of measurement precision of at least a factor of 2 when compared to multi collector inductively coupled plasma mass spectrometer (MC-ICP-MS) measurements using bracketing standard (FIETZKE and EISENHAUER, 2006). The precise determination of natural Sr isotope fractionation adds a new dimension to the well established radiogenic Sr isotope system. Seawater and marine carbonates show significant differences in their stable Sr isotopic composition which were not accessible by applying the radiogenic 87Sr/86Sr ratio alone. In order to constrain glacial/interglacial changes in the marine Sr budget the isotope composition of modern seawater and modern marine biogenic carbonates are compared with the corresponding values of river waters and hydrothermal solutions in a triple isotope plot (δ88/86Sr vs. 87Sr/86Sr). The Sr sources (87Sr/86Sr ~ 0.7106±0.0008, δ88/86Sr ~ 0.31±0.01‰) show a heavier isotopic composition compared to marine carbonates (87Sr/86Sr ~ 0.70926±0.00002, δ88/86Sr ~ 0.21±0.02‰), representing the main Sr sink. This reflects isotopic disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs was achieved during the last glacial maximum (10-30 kyr before present). This can be explained by invoking three times higher Sr inputs from a uniquely “glacial” source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the “weathering peak” hypothesis that invokes enhanced Sr inputs resulting from weathering of post-glacial abundant finegrained material left exposed by the retreating ice masses (VANCE et al., 2009). Furthermore, the temperature dependency of δ88/86Sr in cultured and temperature controlled (21°C to 29°C) warm water corals (Acropora sp.) was investigated. A strict linear trend like reported by (FIETZKE and EISENHAUER, 2006; RUGGEBERG et al., 2008) could not be confirmed in this study. Our measurements rather revealed a nonlinear relationship between temperature and δ88/86Sr (δ88/86Sr=0.001∙T2 – 0.039∙T + 0.692, r2=0.47) whereas the Sr/Ca ratio shows the expected linear trend. Moreover, we determined δ88/86Sr-, δ18O- and Sr/Ca-ratios of a fossil (15 kyr B.P.) Porites sp. coral originating from Tahiti (French-Polynesia). The Sr/Ca as well as the isotope ratios shows a similar seasonal variability. Fossil Porites sp. (δ88/86Srmean=0.205±0.017‰, 2SEM) and recent Porites sp. represented in this study by the coral standard JCp-1 (δ88/86SrJCp-1=0.194±0.009‰, 2SEM) show connatural mean δ88/86Sr values. The average δ88/86Sr is obviously not affected by enhanced weathering and elevated Sr fluxes from exposed shelves during glacial times like it is the case for Sr/Ca elemental ratios. Therefore, stable Sr isotope fractionation can potentially serve as independent and unbiased parameter for reconstructing paleo-sea-surface-temperatures.