A3 Use Of Foraminifera Shells In Paleo Climate

A key proxy record of climate is 518O, the ratio of 18O to 16O in the shells of surface (planktonic) and bottom (benthic) dwelling foraminifera which are made of calcium carbonate, CaCO3. The 518O in the shell, measured and reported as

X 1000°/o where (18O/16O)smpl is the ratio of 18O

to 16O in the sample and (18O/16O)sd is the ratio in a standard reference, is controlled by the 518O value of the water and the temperature at which the shell formed. It turns out that 18O is increasingly enriched in CaCO3 as the calcification temperature decreases, with a 1°C decrease in temperature resulting in a 0.2o /oo increase in á18O. However, in order to use oxygen isotope ratios in foraminifera as an indicator of temperature, one must take into account the isotopic signal associated with ice volume stored on the continents and local precipitation-evaporation-runoff conditions, as we now describe.

The vapor pressure of H,J6O is higher than that of H128O, and so H216O becomes concentrated in the vapor phase (atmosphere) during evaporation of seawater. The residual water therefore becomes enriched in 18O relative to water vapor. Similar physics (that of Rayleigh distillation) results in the condensate (rain or snow) from clouds being depleted in 18O relative to 16O. Therefore as an air mass loses water, for example as it is advected to higher latitudes or altitudes, or transported into a continental interior, it becomes progressively depleted in 18O. Precipitation in polar latitudes is commonly 20-40°/oo depleted in 18O relative to average ocean water. During glacial periods, when perhaps 120 m of sea level-equivalent water is stored in polar ice sheets as highly isotopically depleted ice, the entire ocean becomes enriched in 18O by approximately 1o/oo. This whole-ocean change in 518O has been a powerful tool for reconstructing ice age cycles and paleoclimatology in general. The same principal results in regions of the ocean that receive continental runoff having low 518O values in their surface water, and those regions of the ocean that experience an excess of evaporation over precipitation to have high surface water 518O values.

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