The temperature dependent strontium isotope fractionation (δ88/86Sr) during calcium carbonate precipitation

Full text: In order to study the influence of stable isotope fractionation during inorganic and biologically controlled CaCO3 precipitation we have developed the analytical principles for the measurement of strontium (Sr) isotope fractionation. We have established a measurement protocol for the application on a MC-ICP-MS (AXIOM) using the common bracketing standard technique. The Sr-standard CRM NBS987 was used as reference material for all measurements and to calculate the Sr fractionation. Latter value is expressed by the δ-notation defined as: δ88/86Sr = [(88Sr/86Sr)sample / (88Sr/86Sr)standard ] * 1000 -1. A first set of experiments focused on the temperature dependency of Sr-isotope fractionation. For this purpose inorganically precipitated aragonite and calcite was prepared under controlled conditions in a temperature range from 10 to 50 oC. In addition, cultured and naturally grown corals were analyzed for their δ88/86Sr values. Repeated measurements of IAPSO seawater standard showed a mean δ88/86Sr value of 0.383 ± 0.008 (2 SEM) being the isotopically heaviest material measured so far. The first results of the inorganically precipitated aragonite and the natural corals revealed a clear temperature dependency of the δ88/86Sr values. For inorganic aragonite the slope of this correlation is about 0.0055 permil/oC. However, for naturally grown corals (Pavona clavus) a 6 fold steeper slope of 0.033 permil/oC was determined. This strong temperature dependency implies the potential to use stable Sr isotopes as a new marine (paleo)temperature proxy. (author)