CLIMATOLOGY: A Calcium Thermometer

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Science  06 Oct 2000:
Vol. 290, Issue 5489, pp. 15b

Sea surface temperature (SST) influences many of the biological, chemical, and physical processes that connect ocean and atmosphere, and paleoclimatologists have used a host of different proxies to infer SSTs during the Quaternary. Considerable disagreement exists, however, because most methods register other factors in addition to temperature, such as glacial ice volume, salinity, interspecies metabolic differences, and possible variations in the chemical composition of the oceans.

The calcium isotopic ratios of foraminifera are a potentially valuable and robust thermometer because, by involving only one element, they are not affected by many of the chemical and biological processes that can complicate the interpretation of other temperature proxies. Nägler et al. report calcium isotope ratios from a single species of foram, Globigerinoides sacculifer, cultured at different temperatures in the laboratory, and analyses from an equatorial East Atlantic sediment core. Their experiments show a clear dependence of 44Ca/40Ca on temperature, which they then apply to their marine core samples to produce a history consistent with a Mg/Ca-based temperature reconstruction of the past 240,000 years. They find that the SST at their core location increased by between 2 and 4°C during the last deglaciation. — HJS

Geochem. Geophys. Geosys. 1, 2000GC000091 (2000).

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