Paleothermometer

A paleothermometer is a methodology for determining past temperatures using a proxy found in a natural record such as a sediment, ice core, tree rings or TEX86.MAT = 1.141 +(0.306 × Pmargin), standard error ± 2.0 °C    (1) σ [ LMA ] = c P margin ( 1 − P margin ) r {displaystyle sigma lbrack { ext{LMA}} brack =c{sqrt {frac {P_{ ext{margin}}(1-P_{ ext{margin}})}{r}}}}     (2)MAT = 2.223 +(0.363 × Pmargin), standard error ± 2.0 °C    (1′) A paleothermometer is a methodology for determining past temperatures using a proxy found in a natural record such as a sediment, ice core, tree rings or TEX86. The isotopic 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 H216O, with small amounts of HD16O and H218O. In Standard Mean Ocean Water (SMOW) the ratio of D to H is 155.8×10−6 and 18O/16O 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 and a similar formula for δD. δ18O values for precipitation are always negative. The major influence on δ18O 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 δ18O 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: which is empirically calibrated from measurements of temperature and δ18O as a = 0.67‰/°C for Greenland and 0.76‰/°C 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‰/°C (Cuffey et al., 1995), implying that glacial-interglacial temperature changes were twice as large as previously believed. Magnesium (Mg) is incorporated into the calcite shells (tests) of planktic and benthic foraminifera as a trace element. Because the incorporation of Mg as an impurity in calcite is endothermic, more is incorporated into the growing crystal at higher temperatures. Therefore, a high Mg/Ca ratio implies a high temperature, although ecological factors may confound the signal. Mg has a long residence time in the ocean, and so it is possible to largely ignore the effect of changes in seawater Mg/Ca on the signal. Strontium (Sr) incorporates in coral aragonite, and it is well established that the precise Sr/Ca ratio in the coral skeleton shows an inverse correlation with the seawater temperature during its biomineralization.