Abstract
Abstract. Beyond the pCO2 records provided by ice core measurements, the quantification of atmospheric CO2 concentrations and changes thereof relies on proxy data, the development of which represents a foremost challenge in paleoceanography. In the paleoceanographic toolbox, the coccolithophores occupy a notable place, as the magnitude of the carbon isotopic fractionation between ambient CO2 and a type of organic compounds that these photosynthetic microalgae synthesize (the alkenones) represents a relatively robust proxy to reconstruct past atmospheric CO2 concentrations during the Cenozoic. The isotopic composition of coeval calcite biominerals found in the sediments and also produced by the coccolithophores (the coccoliths) have been found to record an ambient CO2 signal through culture and sediment analyses. These studies have, however, not yet formalized a transfer function that quantitatively ties the isotopic composition of coccolith calcite to the concentrations of aqueous CO2 and, ultimately, to atmospheric CO2 levels. Here, we make use of a microseparation protocol to compare the isotopic response of two size-restricted coccolith assemblages from the North Atlantic to changes in surface ocean CO2 during Termination II (ca. 130–140 ka). Performing paired measurements of the isotopic composition (δ13C and δ18O) of relatively large and small coccoliths provides an isotopic offset that can be designated as a “differential vital effect”. We find that the evolution of this offset follows that of aqueous CO2 concentrations computed from the ice core CO2 curve and an independent temperature signal. We interpret this biogeochemical feature to be the result of converging carbon fixation strategies between large and small cells as the degree of carbon limitation for cellular growth decreases across the deglaciation. We are therefore able to outline a first-order trend between the coccolith differential vital effects and aqueous CO2 in the range of Quaternary CO2 concentrations. Although this study would benefit from further constraints on the other controls at play on coccolith geochemistry (growth rate, air–sea gas exchange, etc.), this test of the drivers of coccolith Δδ13C and Δδ18O in natural conditions is a new step in the development of a coccolith paleo-CO2 probe.
Highlights
Reconstructing the changes in atmospheric pCO2 levels over geological timescales has been one of the major challenges in paleoenvironmental research for the last 40 years (Neftel et al, 1982; Broecker, 1982; Pagani, 2013)
There is compelling evidence that coccoliths are isotopically mineralized from HCO3− ions that are pumped from the intracellular carbon pool into the coccolith vesicle (Nimer et al, 1994; Brownlee and Taylor, 2004)
The paleobiogeochemical records presented in this study throw light on the nature of the factors forcing both the isotopic composition of the coccoliths and the magnitude of the vital effects in relatively small versus large coccoliths
Summary
Reconstructing the changes in atmospheric pCO2 levels over geological timescales has been one of the major challenges in paleoenvironmental research for the last 40 years (Neftel et al, 1982; Broecker, 1982; Pagani, 2013). Other proxies alternatively derive the levels of aqueous CO2 from a set of parameters of ocean carbonate chemistry This is the case of the foraminifera boron isotope proxy, which aims at reconstructing values for surface ocean pH (Sanyal et al, 1995; Foster, 2008) and can further be used to derive surface ocean CO2 (Shao et al, 2019). The 45 Myr record of atmospheric pCO2 (Zhang et al, 2013) and its 66 Myr update (Rae et al, 2021) are a flagship outcome of the εp and boron proxies in the published literature. They have yielded invaluable insights into the radiative forcing caused by a decline in pCO2, and its role in the global Cenozoic cooling
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