Abstract
Since the beginning of the industrial revolution, massive release of CO2 has affected both global climate and ocean chemistry. To predict future impacts, mankind relies on numerical modeling of the Earth system. To test whether such models reliably describe climate and ocean change as a function of increasing atmospheric pCO2,
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such models are verified using reconstructions of past natural change. These reconstructions rely on so-called proxies, which relate measurable variables in the fossil record to physical and chemical environmental parameters, such as temperature and salinity. Several of these proxies are based on the incorporation of trace and minor elements into test carbonates of Foraminifera. Although foraminiferal element incorporation provides an important tool for the reconstruction of past oceans, many aspects involved in the uptake of these elements are unknown. Controlled growth experiments and detailed analyses of living specimens, collected from their natural environment, were used to unravel the processes involved in foraminiferal biomineralization and the effects of physical and chemical ocean parameters on the chemistry of foraminiferal tests. From laboratory culture experiments using the planktonic foraminifer Globigerinoides sacculifer it is concluded that a change of 4 salinity units is equivalent to a 1C bias on Mg/Ca-based temperatures. Salinity constitutes the overriding control on Mg incorporation within the range of calcite saturation state of the seawater (Ω) studied (5.25 to 6.50) at a constant temperature of 26 C. In contrast, Ω constitutes the main control on foraminiferal Sr incorporation (0.10 mmol/mol per 100 Ω mol/kg rise in [CO32-]), whereas salinity has no influence on the incorporation of this trace element. From a second set of laboratory experiments with benthic foraminifera (Ammonia tepida and Heterostegina depressa), it is shown that Ω is not directly responsible for the changes observed in Mg incorporation into the calcite of these benthic species. These results also indicate that ions involved in biomineralization (Ca2+ and CO32-) are taken up through two separate cell-transport mechanisms. The planktonic foraminifer Globigerinoides sacculifer was grown under controlled laboratory conditions to determine the effect of physiological processes (vital effect) on Mg and Sr incorporation. The inter-individual variability observed in Mg incorporation contributes 2-3C to the apparent temperature variance. The variability due to ontogeny corresponds to a decrease of ~1C in Mg/Ca-based temperature per chamber (from the oldest to the newest). A significant ontogenetic effect on Mg incorporation can potentially offset Mg/Ca-based temperature reconstructions. Therefore, a new empirical Mg/Ca-temperature equation for whole foraminiferal tests is proposed: Mg/Ca =(0.55-0.0001*MSD)e0.089*T (MSD=maximum shell diameter). Specimens of deep-sea benthic foraminifera Cibicidoides pachyderma were collected from core-top samples from the Bay of Biscay and their tests chemical composition was analysed. Inter and intra-individual variations of the element composition of the tests reflect changes in chemistry of bottom and pore-water. The Ba and Mn test concentrations indicate vertical migration within the top centimetres of the sediment. The intra-individual variability in Mg/Ca ratios cannot be explained by migration alone, whereas changes in Ba and Mn concentrations in the pore-water are more directly reflected in the test carbonate.
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