Abstract
Abstract. The response of coccolithophore calcification to ocean acidification has been studied in culture experiments as well as in present and past oceans. The response, however, is different between species and strains, and for the relatively small carbonate chemistry changes observed in natural environments, a uniform response of the entire coccolithophore community has not been documented so far. Moreover, previous palaeo-studies basically focus on changes in coccolith weight due to increasing CO2 and the resulting changes in the carbonate system, and only few studies focus on 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 decreasing 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 atmospheric CO2, provides the conditions for such a comprehensive analysis. For an analysis on changes in major components of Holocene coccolithophores under natural conditions, 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 (Emiliania huxleyi and Gephyrocapsa) coccoliths decreased at the Azores (Geofar KF 16) from around 7 to 6 pg, but increased at the Rockall Plateau (ODP site 980) from around 6 to 8 pg, and at the Vøring Plateau (MD08-3192) from 7 to 10 pg. The 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 not only 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.
Highlights
With the increasing anthropogenic influence on the carbon cycle by CO2 emissions into the atmosphere over the past 250 yr, seawater carbonate chemistry changed due to the uptake of atmospheric CO2 into the ocean (Doney et al, 2009)
Different processes lead to the observed strong variability in mean coccolith weight of the dominant coccolithophore family Noelaerhabdaceae during rather stable CO2 conditions of the pre-industrial Holocene in the North Atlantic (Table 1)
The recently debated negative influence of decreasing carbonate ion concentration on coccolithophore calcification could not be detected for a small decrease of 10 to 45 μmol kg−1 SW
Summary
With the increasing anthropogenic influence on the carbon cycle by CO2 emissions into the atmosphere over the past 250 yr, seawater carbonate chemistry changed due to the uptake of atmospheric CO2 into the ocean (Doney et al, 2009). One result of the uptake is a decrease in seawater pH, a process known as ocean acidification (Feely et al, 2004; Raven et al, 2005; IPCC, 2007). Among other calcifying plankton groups, interesting are coccolithophores, which are one of the main producers of calcite in the oceans.
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