A biogeochemical perspective on stable isotope

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A biogeochemical perspective on stable isotope fractionation in marine
carbonate with implications for palaeoceanographic reconstructions during
the Meso-Cenozoic
Michael Hermoso
NERC Research Fellow, University of Oxford
The majority of palaeoceanographic reconstructions are derived from stable isotope
analyses made on marine biocarbonates. Since the Jurassic, pelagic sediments have
predominantly had a biological origin, mainly assigned to calcareous plankton (coccoliths
and foraminifera). As a consequence, primary palaeoceanographic signals, including sea
surface temperatures or palaeoproductivity recorded in the 18O and 13C of carbonate are
obscured by the physiology of these organisms, a concept referred as to the vital effect. The
coccolithophores, unicellular photosynthetic algae, have played a major role in the carbon
cycle since the Jurassic, and their biominerals, the coccoliths, are the greatest contributor to
sedimentary particles, which form the carbonate archive.
In this seminar, I will present the main findings of my NERC-funded research, which have
been aimed at removing the “vital effect” component in coccolith calcite with a view to
generating more reliable isotopic signals during the Cenozoic.
Through laboratory culture experiments, extant coccolithophore species can be subjected to
perturbation experiments (i.e. changing pH, alkalinity, temperature, light etc.) in order to
develop a mechanistic understanding of stable isotope fractionation in coccoliths. Unlike the
for first interpretations made in the ‘80s stating that the vital effect can be regarded as a
species-specific constant, I demonstrate that the magnitude of stable isotope fractionation
in biogenic calcite changes according to ambient conditions, opening the door of their
utilisation in terms of palaeoenvironmental proxy. The biogeochemical dataset I have
developed, along with existing work on foraminifera, will enable a more meaningful
deciphering of the sedimentary archive. Transferring laboratory calibrations into the
sedimentary coccoliths is now possible by using an innovative technique that permits the
separation of near-monotaxic assemblages from sediments. Taken together, this combined
approach will bring new constrains on the physico-chemistry of the oceans during the MesoCenozoic. Additionally, I will illustrate how the geochemistry of coccoliths can overcome
current limitations in this field, particularly with regards to the evolution of sea surface
temperatures during hyperthermal events when foraminiferal data are typically unreliable.
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