Planktonic foraminifera shells record seasonal temperature changes

When planktonic foraminifera calcify, the magnesium to calcium (Mg/Ca) ratios in their shells are sensitive to water temperature. This has enabled paleoceanographers to use Mg/Ca ratios measured from multiple fossil foraminifera shells in sediments as a proxy for average water temperatures in the past. Now Haarmann et al. show that some foraminifera Mg/Ca ratios can also be used to determine seasonal variations in temperature. The authors suggest that because planktonic foraminifera calcify over a period of a few weeks to months, the Mg/Ca ratios in single specimens could capture seasonal temperature variations. They tested this in samples of three different foraminifera species collected off the coast of northwestern Africa, where there is strong seasonal sea surface temperature variability. The researchers found that one of the three species, Globigerinoides ruber (pink), did show strong variations in Mg/Ca that tracked seasonal temperature changes and thus could potentially be used to reconstruct seasonality in the near and distant past. (Paleoceanography, doi:10.1029/2010PA002091, 2011)