Strontium isotope fractionation of planktic foraminifera and inorganic calcite

We have investigated the strontium isotope fractionation (Delta Sr-88/86(carb-aq)) between inorganic calcite and aqueous Sr2+ ions by precipitation experiments at a constant temperature of 25 degrees C and precipitation rates (R) ranging from 10(2.3) to 10(4.2) mu mol/m(2)/h. Strontium isotope ratios were measured using the Sr-87-Sr-84 double spike technique. It was found that strontium isotope fractionation in these calcites is strongly dependent on the precipitation rate: Delta Sr-88/86(carb-aq) = -0.08 * log(R[mu mol/m(2)/h]) + 0.08 The measured delta Sr-88/86 values are significantly correlated with previously measured delta Ca-44/40 and Sr/Ca values of the same calcite samples: Delta Sr-88/86(carb-aq) = +0.18 * Delta Ca-44/40(carb-aq) - 0.01 Delta Sr-88/86(carb-aq) = -1.5*K-d(Sr) - 0.03 The slope of Sr-88/Sr-86 versus Ca-44/Ca-40 fractionation is 0.18 +/- 0.04 and compatible with a kinetic fractionation during dehydration of the strontium and calcium ions, but not with isotope fractionation in a diffusive boundary layer. Using published equilibrium Delta Ca-44/40(carb-aq) and K-d(Sr) values we estimate the equilibrium isotope fractionation of strontium to be very close to zero (Delta Sr-88/86(eq(carb-aq)) = -0.01 +/- 0.06 parts per thousand). This estimate is confirmed by strontium isotope values of natural inorganic calcites that precipitated very slowly in basalts of the ocean crust. The results from the inorganic calcites are used to explain strontium isotope fractionation of planktic foraminifera. Specimens of two warm water species (Globigerinoides ruber and Globigerinoides sacculifer) were picked from the Holocene section of a Caribbean sediment core. We found no significant difference in delta Sr-88/86 between the two species. In addition, G. ruber specimens from Marine Isotope Stage 2 in the same core show delta Sr-88/86 values identical to the Holocene specimens. The strontium isotopes of both foraminifera species are strongly fractionated (Delta Sr-88/86(carb-aq) = -0.248 +/- 0.005 parts per thousand) when compared to published data of other major marine calcifiers. Applying the results from the inorganic precipitation experiments we find that the strong foraminiferal strontium isotope fractionation can be explained by calcification in a largely open system at high precipitation rates, comparable in magnitude to rates known from scleractinian reef corals. This interpretation is in good agreement with the kinetic calcification model for planktic foraminifera by Kisakurek et al. (2011), which was based on calcium isotopes and elemental Sr/Ca ratios. (C) 2012 Elsevier Ltd. All rights reserved.