A new clumped isotope-temperature calibration of cultured coccoliths under different pCO2 and temperature conditions

Carbonate clumped isotope thermometry, based on the temperature-dependence of clumping of ¹³C and ¹⁸O in the carbonate molecule (Δ₄₇) is a promising tool for paleoclimate reconstruction. In the last few years many discrepancies among Δ₄₇-temperature calibrations have been resolved across the range of relevant paleoclimate temperatures (Meinicke et al., 2020; Anderson et al., 2021). However, there might be other environmental effects on biogenic carbonates from parameters such as the pCO₂ and growth rates of the organisms that are still unresolved. We provide a new assessment of the temperature dependence of clumped isotopes in laboratory grown biogenic carbonate at well-constrained experimental conditions, with results from three species of coccolithophores across a growth temperature range of 6-27°C. The three cultured species cover a range of growth rates, growth conditions and species-specific carbon and oxygen vital effects. Because variations in pCO₂ and media carbon chemistry are known to trigger vital effects in carbon and oxygen isotopes in coccoliths, we decoupled the temperature solubility effect on CO₂ by manipulating culture CO₂ independently. Three pCO₂ levels at reduced, present day and elevated levels; 200, 400 and 1000 ppm respectively, were kept constant for at least two different temperatures through a continuous culturing set-up. Our new multi-parameter comparison, using updated standardization approaches, provides a critical test of previous conclusions (Katz et al., 2017) that coccolithophore clumped isotopes show little to no vital effects and are close to abiotic equilibrium. Thus, we have performed the first calibration of coccolith calcite and clumped isotopes combining different temperature and pCO₂ conditions.

References:

Anderson, N. T., J. R. Kelson, S. Kele, M. Daëron, M. Bonifacie, J. Horita, T. J. Mackey, et al. 2021. "A Unified Clumped Isotope Thermometer Calibration (0.5–1,100°C) Using Carbonate‐Based Standardization." Geophysical Research Letters 48 (7).

Katz, A., M. Bonifacie, M. Hermoso, P. Cartigny, D. Calmels. 2017. “Laboratory-grown coccoliths exhibit no vital effect in clumped isotope (Δ47) composition on a range of geologically relevant temperatures.” Geochimica et Cosmochimica Acta 208: 335-353.

Meinicke, N., S.L. Ho, B. Hannisdal, D. Nürnberg, A. Tripati, R. Schiebel, and A.N. Meckler. 2020. "A robust calibration of the clumped isotopes to temperature relationship for foraminifers." Geochimica et Cosmochimica Acta 270: 160-183.