Determination of 87Sr/86Sr*-δ88/86Sr Values of the Oceans Sr Sources and Sinks to Balance the Global Sr Cycle

With the application of the Sr double spike TIMS technique (KRABBENHOEFT et al., 2009) we are now entering a new dimension in Sr isotope geochemistry by the simultaneous measurement of paired 87Sr/86Sr*-δ88/86Sr values of geological samples. The most important advantage of using paired 87Sr/86Sr*-δ88/86Sr values is that now a complete balance of the oceans Sr budget can be calculated including Sr input and output values. With the normalization to a fixed 88Sr/86Sr=8.375209 ratio to correct for mass dependent fractionation during TIMS measurement any natural Strontium (Sr) isotopic fractionation in 88Sr/86Sr is ignored and important additional information are lost. A first study performed with a MC-ICP-MS (FIETZKE and EISENHAUER, 2006) showed significant fractionation between the IAPSO seawater standard and the SRM987 carbonate standard in the δ88/86Sr value. In order to provide a Sr isotope balance for the global ocean we collected paired 87Sr/86Sr*-δ88/86Sr values of a set of river waters samples (87Sr/86Sr*=0.713902(9) - δ88/86Sr=0.300(13)), hydrothermal fluids (87Sr/86Sr*=0.704518(8) - δ88/86Sr=0.253(15) , major marine carbonate producers (foraminifera, coccolithophores, corals) and seawater and present it in a 3-isotope-plot. Rivers and mid ocean ridges represent the main Sr sources to the ocean while marine carbonates are representing the major Sr sink. The major Sr output corresponds to the Sr incorporated by the major marine calcifiers (87Sr/86Sr*=0.709312(9) - δ88/86Sr=0.240). The offset between the Sr input and the Sr output was determined to be ~ 0.50 ‰ in its δ88/86Sr indicates that modern ocean is apparently not in steady state with respect to Sr. Weathering of young carbonates on the shelfes during sea level low stands possibly can shift the δ88/86Sr of rivers from its recent value of 0.300(24) to 0.23‰ to equilibrate in- and output.