Discussion
Which one of the following stable isotopic ratio is used for estimation of palaeotemperature of sea water ?
Answer: B
Palaeotemperature of sea water is estimated from the \(^{18}\)O/\(^{16}\)O ratio.
When calcium carbonate shells (of foraminifera, for example) precipitate from sea water, the fractionation of oxygen isotopes between water and carbonate depends on temperature: colder water gives carbonate richer in \(^{18}\)O. Measuring \(\delta^{18}\)O in fossil shells, corrected for the isotopic composition of the sea water, therefore gives the water temperature. This was Urey's discovery and underlies the marine oxygen isotope stages.
The carbon ratio tracks productivity and the carbon cycle, \(^{87}\)Sr/\(^{86}\)Sr is used for stratigraphic dating and weathering history, and the nitrogen ratio traces nitrogen cycling and food webs.
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