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 (Sr-87/Sr-86) 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 (delta Sr-88/86(sw)), which we find to be sensitive to imbalances in the Sr input and output fluxes. This delta Sr-88/86(sw) record varies from similar to 0.25 parts per thousand to similar to 0.60 parts per thousand (vs. SRM987) with a mean of similar to 0.37 parts per thousand. The fractionation factor between modern seawater and skeletal calcite Delta Sr-88/86(cc-sw), based on the analysis of 13 modern brachiopods (mean delta Sr-88/86 of 0.176 +/- 0.016 parts per thousand, 2 standard deviations (s.d.)), is -0.21 parts per thousand and was found to be independent of species, water temperature, and habitat location. Overall, the Phanerozoic delta Sr-88/86(sw) record is positively correlated with the Ca isotope record (delta Ca-44/40(sw)), but not with the radiogenic Sr isotope record ((Sr-87/Sr-86)(sw)). A new numerical modeling approach, which considers both delta Sr-88/86(sw) and (Sr-87/Sr-86)(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 -4.7 x 10(10) mol/Myr and an input of +2.3 x 10(10) mol/Myr with a mean of -1.6 x 10(10) mol/Myr. On time scales in excess of 100 Myrs 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 20 Myrs 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 delta Sr-88/86(sw)-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 similar to 1 to similar to 20 Myrs. (C) 2013 Elsevier Ltd. All rights reserved.