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Towards a better understanding of paleoclimates in the sedimentary archive

Calcite microfossils preserved in sediments are widely used to reconstruct the Earth's past oceanographic conditions and climates. By analyzing their chemical composition, and more specifically their isotopic composition, we can reconstruct some of the conditions (temperature, acidity, dissolved CO2 concentration) under which the microorganisms that produced these biominerals lived.

Towards a better understanding of paleoclimates in the sedimentary archive

Scanning electron microscopy of two unicellular algal coccospheres

Publication date: 28/02/2017

Press, Research

Related teams :
Stable Isotope Geochemistry

Related themes : Origins

Coccoliths are calcareous discs a few micrometres in diameter produced by a group of unicellular algae living on the surface of the oceans: the coccolithophorids. These coccoliths have formed an important, even dominant, part of marine sediments since the Jurassic period. However, they are rarely used in palaeo-reconstruction because of their small size, unlike foraminifera, which can be separated under a binocular magnifying glass. Another major problem in the use of coccoliths in palaeoceanography is the biological imprint (or vital effect) that distorts the geochemical composition of the calcite and prevents access to the desired environmental conditions. Previous studies have empirically linked the expression of the vital effect to the algal growth rate, which in turn is linked to the concentration of CO2 available in the environment.

In an article recently published in the journal Nature Communications, an international team comprising researchers from Oxford University, Washington University in St Louis, the Institut de Physique du Globe de Paris and the Plymouth Marine Laboratory has succeeded in modelling the flow of carbon into and out of laboratory-grown coccolithophorid cells, and to formalise bionumerically the carbon isotopic composition (δ13C) of the calcite produced as a function of environmental parameters, as well as the intracellular exchanges of carbon and the partitioning of CO2 between the chloroplast for photosynthesis and the calcification vesicle for biomineralisation.

 

Scanning electron microscopy of two unicellular algal coccospheres cultured for this study: Coccolithus pelagicus ssp. braarudii (left) and Gephyrocapsa oceanica (right).

This new mathematical model, and the understanding of the vital effects it enables, paves the way for the study of an important component of the sedimentary archive that currently remains largely under-exploited. Measuring the δ13C of sedimentary coccoliths that it is possible to microseparate will eventually make it possible, by integrating these biogeochemical constraints, to trace the dissolved CO2 concentration of the ocean and atmosphere at the time of biomineral formation, and thus further improve our palaeocenographic toolbox.

 

Ref: H.L.O.M. McClelland, J. Bruggeman, M. Hermoso, R.E.M. Rickaby, The origin of carbon isotope vital effects in coccolith calcite: www.nature.com/articles/ncomms14511

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