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; RÜGGEBERG 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

Determination of radiogenic and stable strontium isotope ratios (87Sr/86Sr; δ88/86Sr) by thermal ionization mass spectrometry applying an 87Sr/84Sr double spike

Recent findings of natural strontium isotope fractionation have opened up a new field of research in non-traditional stable isotope geochemistry. While previous studies were based on data obtained by MC-ICP-MS we here present a novel approach combining thermal ionization mass spectrometry (TIMS) with the use of an 87Sr/84Sr double spike (DS). Our results for the IAPSO sea water and JCp-1 coral standards, respectively, are in accord with previously published data. The strontium isotope composition of the IAPSO sea water standard was determined as δ88/86Sr = 0.386(5)‰ (δ values relative to the SRM987), 87Sr/86Sr* = 0.709312(9) n = 10 and a corresponding conventionally normalized 87Sr/86Sr = 0.709168(7) (all uncertainties 2SEM). For the JCp-1 coral standard we obtained δ88/86Sr = 0.197(8)‰, 87Sr/86Sr* = 0.709237(2) and 87Sr/86Sr = 0.709164(5) n = 3. We show that by applying this DS-TIMS method the precision is improved by at least a factor of 2–3 when compared to MC-ICP-MS

Constraining the marine strontium budget with natural strontium isotope fractionations (⁸⁷Sr/⁸⁶Sr*, δ^88/86Sr) of carbonates, hydrothermal solutions and river waters

We present strontium (Sr) isotope ratios that, unlike traditional 87Sr/86Sr data, are not normalized to a fixed 88Sr/86Sr ratio of 8.375209 (defined as δ88/86Sr = 0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a 87Sr–84Sr double spike. This technique yields two independent ratios for 87Sr/86Sr and 88Sr/86Sr that are reported as (87Sr/86Sr*) and (δ88/86Sr), respectively. The difference between the traditional radiogenic (87Sr/86Sr normalized to 88Sr/86Sr = 8.375209) and the new 87Sr/86Sr* values reflect natural mass-dependent isotope fractionation. In order to constrain glacial/interglacial changes in the marine Sr budget we compare the isotope composition of modern seawater ((87Sr/86Sr*, δ88/86Sr)Seawater) and modern marine biogenic carbonates ((87Sr/86Sr*, δ88/86Sr)Carbonates) with the corresponding values of river waters ((87Sr/86Sr*, δ88/86Sr)River) and hydrothermal solutions ((87Sr/86Sr*, δ88/86Sr)HydEnd) in a triple isotope plot. The measured (87Sr/86Sr*, δ88/86Sr)River values of selected rivers that together account for not, vert, similar18% of the global Sr discharge yield a Sr flux-weighted mean of (0.7114(8), 0.315(8)‰). The average (87Sr/86Sr*, δ88/86Sr)HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)‰). In contrast, the (87Sr/86Sr*, δ88/86Sr)Carbonates values representing the marine Sr output are (0.70926(2), 0.21(2)‰). We estimate the modern Sr isotope composition of the sources at (0.7106(8), 0.310(8)‰). The difference between the estimated (87Sr/86Sr*, δ88/86Sr)input and (87Sr/86Sr*, δ88/86Sr)output values reflects isotope disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs during the last glacial maximum (10–30 ka before present) 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 exposure of abundant fine-grained material

The Phanerozoic δ88/86Sr Record of Seawater: New Constraints on Past Changes in Oceanic Carbonate Fluxes

The isotopic composition of Phanerozoic marine sediments provides important information about changes in seawater chemistry. In particular, the radiogenic strontium isotope (87Sr/86Sr) system is a powerful tool for constraining plate tectonic processes and their influence on atmospheric CO2 concentrations. However, the 87Sr/86Sr isotope ratio of seawater is not sensitive to temporal changes in the marine strontium (Sr) output flux, which is primarily controlled by the burial of calcium carbonate (CaCO3) at the ocean floor. The Sr budget of the Phanerozoic ocean, including the associated changes in the amount of CaCO3 burial, is therefore only poorly constrained. Here, we present the first stable isotope record of Sr for Phanerozoic skeletal carbonates, and by inference for Phanerozoic seawater (δ88/86Srsw), which we find to be sensitive to imbalances in the Sr input and output fluxes. This δ88/86Srsw record varies from ∼0.25‰ to ∼0.60‰ (vs. SRM987) with a mean of ∼0.37‰. The fractionation factor between modern seawater and skeletal calcite Δ88/86Srcc-sw, based on the analysis of 13 modern brachiopods (mean δ88/86Sr of 0.176±0.016‰, 2 standard deviations (s.d.)), is -0.21‰ and was found to be independent of species, water temperature, and habitat location. Overall, the Phanerozoic δ88/86Srsw record is positively correlated with the Ca isotope record (δ44/40Casw), but not with the radiogenic Sr isotope record ((87Sr/86Sr)sw). A new numerical modeling approach, which considers both δ88/86Srsw and (87Sr/86Sr)sw, yields improved estimates for Phanerozoic fluxes and concentrations for seawater Sr. The oceanic net carbonate flux of Sr (F(Sr)carb) varied between an output of -4.7x1010mol/Myr and an input of +2.3x1010mol/Myr with a mean of -1.6x1010mol/Myr. On time scales in excess of 100Myrs the F(Sr)carb is proposed to have been controlled by the relative importance of calcium carbonate precipitates during the “aragonite” and “calcite” sea episodes. On time scales less than 20Myrs the F(Sr)carb seems to be controlled by variable combinations of carbonate burial rate, shelf carbonate weathering and recrystallization, ocean acidification, and ocean anoxia. In particular, the Permian/Triassic transition is marked by a prominent positive δ88/86Srsw-peak that reflects a significantly enhanced burial flux of Sr and carbonate, likely driven by bacterial sulfate reduction (BSR) and the related alkalinity production in deeper anoxic waters. We also argue that the residence time of Sr in the Phanerozoic ocean ranged from ∼1Myrs to ∼20Myrs