Paleothermometer

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A paleothermometer is a methodology for determining past temperatures using a proxy found in a natural record such as a sediment or ice core, or tree rings.

Contents

[edit] Common paleothermometers

[edit] δ18O

Main article: d18o

The ratio of 18O to 16O, usually in foram tests or ice cores. High values mean low temperatures. Confounded by ice volume - more ice means higher δ18O values.

Ocean water is mostly {H_2}^{16}O, with small amounts of HD16O and {H_2}^{18}O. In standard mean ocean water (SMOW) the ratio of D to H is 155.8 * 10 − 6 and O-18 to O-16 is 2005 * 10 − 6. Fractionation occurs during changes between condensed and vapour phases: the vapour pressure of heavier isotopes is lower, so vapour contains relatively more of the lighter isotopes and when the vapour condenses the precipitation preferentially contains heavier isotopes. The difference from SMOW is expressed as δ18O = 1000 * ((18O / 16O) / (18O / 16O)SMOW − 1); and a similar formula for δD. δ values for precipitation are always negative. The major influence on δ is the difference between ocean temperatures where the moisture evaporated and the place where the final precipitation occurred; since ocean temperatures are relatively stable the δ value mostly reflects the temperature where precipitation occurs. Taking into account that the precipitation forms above the inversion layer, we are left with a linear relation:

δ18O = aT + b

which is empirically calibrated from measurements of temperature and δ as a = 0.67 ‰/oC for Greenland and 0.76 ‰/oC for East Antarctica. The calibration was initially done on the basis of spatial variations in temperature and it was assumed that this corresponded to temporal variations (Jouzel and Merlivat, 1984). More recently, borehole thermometry has shown that for glacial-interglacial variations, a = 0.33 ‰/oC (Cuffey et al., 1995), implying that glacial-interglacial temperature changes were twice as large as previously believed.


[edit] Mg/Ca

Magnesium (Mg) can be incorporated into the tests of bottom-dwelling foramanifera; higher temperatures make it easier to incorporate. Therefore a high Mg/Ca ratio implies a high temperature, although ecological factors may confound the signal.

[edit] Alkenones

Distributions of organic molecules in marine sediments reflect temperature.

Further information: [[alkenone unsaturation index ]] and [[TEX-86]]

[edit] Pollen distribution

Certain plants prefer certain temperatures; if their pollen is found one can work out the approximate temperature.

[edit] See also

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