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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, ice core, tree rings or TEX86.

Contents

Common paleothermometers

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δ18O

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 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 δ18O = 1000 * ((18O / 16O) / (18O / 16O)SMOW − 1); 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:

δ18O = aT + b

which is empirically calibrated from measurements of temperature and δ18O 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.

Mg/Ca

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

Alkenones

Distributions of organic molecules in marine sediments reflect temperature.

Pollen distribution

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

13C-18O bonds in carbonates

There is a slight thermodynamic tendency for heavy isotopes to form bonds with each other, in excess of what would be expected from a stochastic or random distribution of the same concentration of isotopes. The excess is greatest at low temperature (see Van 't Hoff equation), with the isotopic distribution becoming more randomized at higher temperature. Along with the closely related phenomenon of equilibrium isotope fractionation, this effect arises from differences in zero point energy among isotopologues. Carbonate minerals like calcite contain CO32– groups that can be converted to CO2 gas by reaction with concentrated phosphoric acid. The CO2 gas is analyzed with a mass spectrometer, to determine the abundances of isotopologues. The parameter Δ47 is the measured difference in concentration between isotopologues with a mass of 47 u (as compared to 44) in a sample and a hypothetical sample with the same bulk isotopic composition, but a stochastic distribution of heavy isotopes. Lab experiments, quantum mechanical calculations, and natural samples (with known crystallization temperatures) all indicate that Δ47 is correlated to the inverse square of temperature. Thus Δ47 measurements provide an estimation of the temperature at which a carbonate formed. 13C-18O paleothermometry does not require prior knowledge of the concentration of 18O in the water (which the δ18O method does). This allows the 13C-18O paleothermometer to be applied to some samples, including freshwater carbonates and very old rocks, with less ambiguity than other isotope-based methods. The method is presently limited by the very low concentration of isotopologues of mass 47 or higher in CO2 produced from natural carbonates, and by the scarcity of instruments with appropriate detector arrays and sensitivities. The study of these types of isotopic ordering reactions in nature is often called "clumped-isotope" geochemistry. [1]

References

  1. ^ Eiler JM (2007). ""Clumped-isotope" geochemistry – The study of naturally-occurring, multiply substituted isotopologues". Earth and Planetary Letters 262: 309–327. doi:10.1016/j.epsl.2007.08.020.  

See also


Redirecting to Paleothermometer


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