Controls on calcium isotope fractionation in cultured planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera

Specimens of two species of planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera, were grown under controlled laboratory conditions at a range of temperatures (18–31 °C), salinities (32–44 psu) and pH levels (7.9–8.4). The shells were examined for their calcium isotope compositions (δ44/40Ca) and strontium to calcium ratios (Sr/Ca) using Thermal Ionization Mass Spectrometry and Inductively Coupled Plasma Mass Spectrometry. Although the total variation in δ44/40Ca (∼0.3‰) in the studied species is on the same order as the external reproducibility, the data set reveals some apparent trends that are controlled by more than one environmental parameter. There is a well-defined inverse linear relationship between δ44/40Ca and Sr/Ca in all experiments, suggesting similar controls on these proxies in foraminiferal calcite independent of species. Analogous to recent results from inorganically precipitated calcite, we suggest that Ca isotope fractionation and Sr partitioning in planktic foraminifera are mainly controlled by precipitation kinetics. This postulation provides us with a unique tool to calculate precipitation rates and draws support from the observation that Sr/Ca ratios are positively correlated with average growth rates. At 25 °C water temperature, precipitation rates in G. siphonifera and G. ruber are calculated to be on the order of 2000 and 3000 μmol/m2/h, respectively. The lower δ44/40Ca observed at ⩾29 °C in both species is consistent with increased precipitation rates at high water temperatures. Salinity response of δ44/40Ca (and Sr/Ca) in G. siphonifera implies that this species has the highest precipitation rates at the salinity of its natural habitat, whereas increasing salinities appear to trigger higher precipitation rates in G. ruber. Isotope effects that cannot be explained by precipitation rate in planktic foraminifera can be explained by a biological control, related to a vacuolar pathway for supply of ions during biomineralization and a pH regulation mechanism in these vacuoles. In case of an additional pathway via cross-membrane transport, supplying light Ca for calcification, the δ44/40Ca of the reservoir is constrained as −0.2‰ relative to seawater. Using a Rayleigh distillation model, we calculate that calcification occurs in a semi-open system, where less than half of the Ca supplied by vacuolization is utilized for calcite precipitation. Our findings are relevant for interpreting paleo-proxy data on δ44/40Ca and Sr/Ca in foraminifera as well as understanding their biomineralization processe

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

Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: III. Impact of salinity/ionic strength

In order to apply Sr/Ca and 44Ca/40Ca fractionation during calcium carbonate (CaCO3) formation as a proxy to reconstruct paleo-environments, it is essential to evaluate the impact of various environmental factors. In this study, a CO2 diffusion technique was used to crystallize inorganic calcite from aqueous solutions at different ionic strength/salinity by the addition of NaCl at 25 °C. Results show that the discrimination of Sr2+ versus Ca2+ during calcite formation is mainly controlled by precipitation rate (R in μmol/m2/h) and is weakly influenced by ionic strength/salinity. In analogy to Sr incorporation, 44Ca/40Ca fractionation during precipitation of calcite is weakly influenced by ionic strength/salinity too. At 25 °C the calcium isotope fractionation between calcite and aqueous calcium ions (Δ44/40Cacalcite-aq = δ44/40Cacalcite − δ44/40Caaq) correlates inversely to log R values for all experiments. In addition, an inverse relationship between Δ44/40Cacalcite-aq and log DSr, which is independent of temperature, precipitation rate, and aqueous (Sr/Ca)aq ratio, is not affected by ionic strength/salinity either. Considering the log DSr and Δ44/40Cacalcite-aq relationship, Sr/Ca and δ44/40Cacalcite values of precipitated calcite can be used as an excellent multi-proxy approach to reconstruct environmental conditions (e.g., temperature, precipitation rate) of calcite growth and diagenetic alteration

Conditions of Mytilus edulis extracellular body fluids and shell composition in a pH-treatment experiment: Acid-base status, trace elements and delta B-11

Mytilus edulis were cultured for 3 months under six different seawater pCO(2) levels ranging from 380 to 4000 mu atm. Specimen were taken from Kiel Fjord (Western Baltic Sea, Germany) which is a habitat with high and variable seawater pCO(2) and related shifts in carbonate system speciation (e. g., low pH and low CaCO3 saturation state). Hemolymph (HL) and extrapallial fluid (EPF) samples were analyzed for pH and total dissolved inorganic carbon (C-T) to calculate pCO(2) and [HCO3-]. A second experiment was conducted for 2 months with three different pCO(2) levels (380, 1400 and 4000 mu atm). Boron isotopes (delta B-11) were investigated by LA-MC-ICP-MS (Laser Ablation-Multicollector-Inductively Coupled Plasma-Mass Spectrometry) in shell portions precipitated during experimental treatment time. Additionally, elemental ratios (B/Ca, Mg/Ca and Sr/Ca) in the EPF of specimen from the second experiment were measured via ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry). Extracellular pH was not significantly different in HL and EPF but systematically lower than ambient water pH. This is due to high extracellular pCO(2) values, a prerequisite for metabolic CO2 excretion. No accumulation of extracellular [HCO3-] was measured. Elemental ratios (B/Ca, Mg/Ca and Sr/Ca) in the EPF increased slightly with pH which is in accordance with increasing growth and calcification rates at higher seawater pH values. Boron isotope ratios were highly variable between different individuals but also within single shells. This corresponds to a high individual variability in fluid B/Ca ratios and may be due to high boron concentrations in the organic parts of the shell. The mean delta B-11 value shows no trend with pH but appears to represent internal pH (EPF) rather than ambient water pH