Changes in coccolith calcification under stable atmospheric CO2

Coccolith calcification is known to respond to ocean acidification in culture experiments as well as in present and past oceans. Previous studies basically focus on changes in coccolith weight due to increasing CO2 and the resulting changes in the carbonate system but pay little attention to the influence of other environmental factors. In order to untangle changes in coccolithophore calcification due to environmental factors such as temperature and/or productivity from changes caused by increasing pCO2 and carbonate ion concentration we here present a study on coccolith calcification from the Holocene North Atlantic Ocean. The pre-industrial Holocene with its predominantly stable carbonate system provides the conditions for such a comprehensive analysis. For a realistic analysis on changes in major components of Holocene coccolithophores, the family Noelaerhabdaceae was selected, which constitutes the main part of the assemblage in the North Atlantic. Records of average coccolith weights from three Holocene sediment cores along a North–South transect in the North Atlantic were analysed. During the Holocene mean weight (and therefore calcification) of Noelaerhabdaceae (E. huxleyi and Gephyrocapsa) coccoliths decreases at the Azores (Geofar KF 16) from around 7 to 5.5 pg, but increases at the Rockall Plateau (ODP Site 980) from around 6 to 8 pg and at the Vøring Plateau (MD08-3192) from 7 to 10.5 pg. This amplitude of average weight variability is within the range of glacial/interglacial changes that were interpreted to be an effect of decreasing carbonate ion concentration. By comparison with SEM assemblage counts, we show that weight changes are partly due to variations in the coccolithophore assemblage, but also an effect of a change in calcification and/or morphotype variability within single species. Our results indicate that there is no single key factor responsible for the observed changes in coccolith weight. A major increase in coccolith weight occurs during a slight decrease in carbonate ion concentration in the Late Holocene at the Rockall Plateau and Vøring Plateau. Here, more favourable productivity conditions apparently lead to an increase in coccolith weight, either due to the capability of coccolithophore species, especially E. huxleyi, to adapt to decreasing carbonate ion concentration, or due to a shift towards heavier calcifying morphotypes.

Bauke C., Meier K. J. S., Kinkel H. & Baumann K.-H., 2013. Changes in coccolith calcification under stable atmospheric CO2. Biogeosciences Discussions 10: 9415-9450. Article.

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