Parallel between the isotopic composition of coccolith calcite and carbon levels across Termination II: developing a new paleo-CO<sub>2</sub> probe
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Published:2022-03-07
Issue:3
Volume:18
Page:449-464
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ISSN:1814-9332
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Container-title:Climate of the Past
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language:en
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Short-container-title:Clim. Past
Author:
Godbillot Camille, Minoletti FabriceORCID, Bassinot Franck, Hermoso MichaëlORCID
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.
Funder
Agence Nationale de la Recherche Centre National de la Recherche Scientifique
Publisher
Copernicus GmbH
Subject
Paleontology,Stratigraphy,Global and Planetary Change
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