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Climate proxy (changes)

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Idea

A climate proxy is a preserved physical characteristic of the past that is correlated with some feature of the Earth’s climate. Climate proxies let scientists reconstruct the climate conditions that prevailed during the long period before reliable human records of climate began in the 1880s.

For example, oxygen isotopes reflect changes in both temperature and ice volume. δ 18\delta^{18}O, δ 13\delta^{13}C and species assemblage are long established proxies.

Physico-chemical basis of the isotopic temperature scale

An old, but freely accessible paper that gives a discussion of the physico-chemical basis of the isotopic temperature scale of δ 18\delta^{18}O is:

An edited summary of one of its sections:

Molecular energy can be divided into translational, vibrational, rotational and electronic energy. Various isotopes tend to concentrate to different extents in different compounds, so as to produce the maximum decrease in free energy. Except for hydrogen isotopes, at ordinary temperatures and above, only vibrational energy affects the isotopic distributions.

Example, equilibrium reaction for hydration of carbon dioxide:

CO 2+H 2O(equilibriumreactionarrow)H 2CO 3 CO_2 + H_2O (equilibrium reaction arrow) H_2 CO_3

For oxygen, the isotopes are as follows: 1:500 is an O-18, 1:2700 is an O-17. Therefore, for the compound (CO 2H 2O)(CO_2 - H_2O) 1:166 will contain an O-18. The distribution of this O-18 over CO 2CO_2 wrt H 2OH_2O is not simply 2:1 - it is greater for carbon dioxide. The reason is that the total vibrational energy for the couple (CO 2H 2O)(CO_2 - H_2O) with one O-18 in CO 2CO_2 is smaller than the alternative, that the O-18 is in the water.

We define the fractionation factor aa of species X wrt species Y:

a=O18/O16(speciesX)O18/O16(speciesY) a = \frac{O-18/O-16 (species X) }{O-18/O-16 (species Y)}

At zero degrees, the fractionation factor between carbon dioxide and water is 1.045.

With increasing temperature, the decrease of free energy becomes less important and the aa will tend to unity.

Another example, for calcium carbonate, there is a variation in the fractionation factor (calcium carbonate with respect to water) of 0.2 per mil per degree centigrade. Thus the temperature at which the calcium carbonate was originally deposited is determined by the fractionation factor of the calcium carbonate in the deposits. In practice, the δ 18O\delta^18 O is measured, this is the per mil difference delta between the fractionation factor in the sample and in a fixed standard.

δ=1000a(sample)a(standard)a(standard) \delta=1000 \frac{a(sample) - a(standard)}{a(standard)}

The δ 18O\delta^18 O also depends on the isotopic composition of the original water from which the sediments were deposited. The variation is caused because the vapour pressure of water with O-16 is higher than of water with O-18. Therefore, more water with O-16 evaporates from the tropical and subtropical evaporation belts of the oceans, and travels poleward. Polar ocean water is depleted in the O-18/O-16 ratio. The bottom water in the deep seas also has a lower ratio than average (because of circulation from the polar regions). In addition, there are probably also variations during geological times.

Reference needed for how the ice volume affects the isotopic ratio (as mentioned in the Idea section).

References

A discussion of a few of the most famous climate proxies can be found here:

Here is a review of “new” proxies:

and here is a review of “old” proxies:

  • G. Wefer, W. H. Berger, J. Bijma, and G. Fischer, Clues to ocean history: a brief overview of proxies, Use of proxies in paleoceanography - Examples from the South Atlantic (G Fischer, G Wefer, eds) Springer, Berlin, 1999, pp. 1-68.

Some more information about Isotope geochemistry