Response of the coccolithophores Emiliania huxleyi and Coccolithus braarudii to changing seawater Mg~(2+) and Ca~(2+) concentrations: Mg/Ca, Sr/Ca ratios and δ~(44/40)Ca, δ~(26/24)Mg of coccolith calcite

摘要

Calcium and magnesium concentrations in seawater have varied over geological time scales. On short time scales, variations in the major ion composition of seawater influences coccolithophorid physiology and the chemistry of biogenically produced coccoliths. Validation of those changes via controlled laboratory experiments is a crucial step in applying coccolithophorid based paleoproxies for the reconstruction of past environmental conditions. Therefore, we examined the response of two species of coccolithophores, Emiliania huxleyi and Coccolithus braarudii, to changes in the seawater Mg/Ca ratio (≈0.5 to 10mol/mol) by either manipulating the magnesium or calcium concentration under controlled laboratory conditions. Concurrently, seawater Sr/Ca ratios were also modified (≈2 to 40mmol/mol), while keeping salinity constant at 35. The physiological response was monitored by measurements of the cell growth rate as well as the production rates of particulate inorganic and organic carbon, and chlorophyll a. Additionally, coccolithophorid calcite was analyzed for its elemental composition (Sr/Ca and Mg/Ca) as well as isotope fractionation of calcium and magnesium (Δ~(44/40)Ca and Δ~(26/24)Mg). Our results reveal that physiological rates were substantially influenced by changes in seawater calcium rather than magnesium concentration within the range estimated to have occurred over the past 250million years when coccolithophores appear in the fossil record. All physiological rates of E. huxleyi decreased at a calcium concentration above 25mmolL~(-1), whereas C. braarudii displayed a higher tolerance to increased seawater calcium concentrations. Partition coefficient of Sr was calculated as 0.36±0.04 (±2σ) independent of species. Partition coefficient of Mg~(2+) increased with increasing seawater Ca~(2+) concentrations in both coccolithophore species. Calcium isotope fractionation was constant at 1.1±0.1‰ (±2σ) and not altered by changes in seawater Mg/Ca ratio. There is a well-defined inverse linear relationship between calcium isotope fractionation and partition coefficient of Sr~(2+) in all experiments, suggesting similar controls on both proxies in the investigated species. Magnesium isotope ratios were relatively stable for seawater Mg/Ca ratios ranging from 1 to 5, with a higher degree of fractionation in Emiliania huxleyi (by ≈0.2‰ in Δ~(26/24)Mg). Although Mg/Ca ratios in the calcite of coccolithophores and foraminifera are similar, the former have considerably higher Δ~(26/24)Mg (by >+3‰), presumably due to differences in calcification mechanisms between the two taxa. These observations suggest, a physiological control over magnesium elemental and isotopic fractionation during the process of calcification in coccolithophores.