Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 5 6 2008 10.5194/bg-5-1651-2008 http://www.biogeosciences.net/5/1651/2008/ http://www.biogeosciences.net/5/1651/2008/bg-5-1651-2008.html http://www.biogeosciences.net/5/1651/2008/bg-5-1651-2008.pdf 1651 1655 2008-12-10 Evidence for a multi-species coccolith volume change over the past two centuries: understanding a potential ocean acidification response P. R. Halloran I. R. Hall E. Colmenero-Hidalgo R. E. M. Rickaby Department of Earth Sciences, University of Oxford, Oxford, UK Department of Earth and Ocean Sciences, Cardiff University, Cardiff, UK Departamento de Geología, Facultad de Ciencias, Universidad de Salamanca, Salamanca, Spain now at: Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB, UK Major questions surround the species-specific nature of coccolithophore calcification in response to rising atmospheric CO₂ levels. Here we present CaCO₃ particle volume distribution data from the coccolith size-fraction of a rapidly accumulating North Atlantic sediment core. Without direct volume measurements on coccoliths produced by individual coccolithophore species, and knowledge of organic, as well as inorganic carbon production, it is not possible to state conclusively the coccolithophore calcification change at this site. However, by analysing the size distribution of CaCO₃ particles in the less than 10 μm sediment fraction, we demonstrate a changing particle volume since the late 20th Century consistent with an increase in the mass of coccoliths produced by the larger coccolithophore species, and potentially a decrease in mass of coccoliths produced by the smaller species, present at this location. This finding has significant implications for the realistic representation of an assemblage-wide coccolithophore CO₂-calcification response in numerical models. 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