Written in the Earth

It has been a while since I promised to tell you more about climate proxies and palaeoevidence for Bond cycles in my last post. I am sure that since then you have been dying for more information on this exciting subject, so without further ado I shall prove you the fact that we are able to transfer ourselves almost a million years back in history by simply using... a drill.

To do that let's go back to the fancy word proxy we have learnt last week.

A proxy, as defined by IPCC (2001), is "a local record that is interpreted using physical or biophysical principles to represent some combination of climate-related variations back in time". What it basically means, is that when a particular material (i.e. sediment, ice or even a living organism) forms, it reflects the physical and chemical characteristics of the environment in which it has been formed - usually through the character of deposition or the rate of growth. By determining the age of the object scientists can later reconstruct the environmental conditions of the time in which it was laid down or grew. Examples of proxies include ice cores, boreholes, corals, lake/ocean sediments, tree rings and sub-fossil pollen and are nicely summarised in the picture below I found on the NOAA website.

Source: NOAA, http://www.ncdc.noaa.gov/paleo/paleo.html.

Climatic changes very often influence isotopic ratio of the analysed proxy. Do not worry if this word seems familiar, but you can't actually remember what it means... Isotopes are basically atoms of the same element with the same number of protons and electrons and but different number of neutrons, which means various isotopes will have similar charges but different masses. For elements of low atomic numbers these mass differences will allow various physical, chemical and biological processes to change the relative proportions of various isotopes (aka the isotopic ratio), which could later be used to infer about the processes that formed a particular isotopic ratio. For example, Beryllium-10, Calcium-II and Carbon-14 are the isotopes used to calculate the intensity of solar irradiance at an established epoch (Nahle, 2009).

Some other proxies may enable a direct measurement of the chemical tracers of the past: gas bubbles trapped in ice reflect the composition of the Earth's atmosphere at the time of the ice formation or the concentration of aragonite, calcite and magnesium-calcite in fossial shells of molluscs and foraminiera are analysed to determine the environmental temperature (Nahle, 2009).

In order to successfully track down Bond Cycles scientists need to recreate long term records of temperature and greenhouse gases. Ice cores, cylinders of ice drilled out of glaciers and polar ice sheets, are especially favoured by the palaeoclimatologists and are one of the most effective and widely used proxy archives. Both at high altitudes and in polar regions snow falls during an annual cycle and remains there permanently. Eventually, the layers of snow compact due to their own weight and become ice, which preserves the environmental conditions during the snowfall.

The longest ice record has been available from Antarctica due to the fact that it is mostly located over land. Lorius et al. (1985) reviewing the data from the Antarctic ice gives a special mention to the the EPICA (deepest in the world!) and Vostok cores. At the same time, the only major area in the Arctic covered with snow is Greenland. A number of drilling projects are taking place there at present, with GISP2 and NGRIP being particularly famous, although they are nowhere near as deep as the cores from the Antarctic. The location of the main coring sites is shown on the maps below:

Source: USGS National Ice Core Laboratory, http://nicl.usgs.gov/coresite.htm

Ice cores are most commonly analysed in terms of their δ¹⁸O isotopic record, which is used to infer about past temperatures and snow accumulation. Although the water in the oceans contains primarily ¹⁶O, 12% of the oxygen in the oceans has an atomic weight of 18. The lighter ¹⁶O evaporates more easily, while the heavier ¹⁸O condenses more readily as the conditions get colder. The ratio of two isotopes will vary depending on the temperature of evaporation and can be measured very accurately. Over long periods of time this ratio provides a clear indicator of the average temperature of in the region of the coring site. Moreover, water contains hydrogen isotopes ¹H and ²H (also known as deuterium), which are also used as temperature proxies (USGS, 2004). Normally, ice cores from Greenland are analysed for δ¹⁸O and those from Antarctic for δ-deuterium, although GISP2 investigators are trying to perform the δ-deuterium analysis aswell as it "will allow even finer detail about source temperature and condensation history" (GISP2, 2006).

This post was initially meant to be dedicated to the information inferred from the natural records, however I got bit too excited talking about different proxies and wrote slightly more than the intended small introductory paragraph. Therefore, not wishing to overload you with information I decided to split the big topic into two parts and the next part will focus on all the wonderful stories that have been narrated to us by the ice cores.

List of references:

GISP2 (2006) 'Ice Cores that Tell the Past' (WWW), GISP2, (http://www.gisp2.sr.unh.edu/MoreInfo/Ice_Cores_Past.html), 10/11/2012.
Lorius, C., J. Jouzel, C. Ritz, L. Merlivat, N. Barkov, Y. Korotkevich and V. Kotlyakov (1985) 'A 150,000-year Climatic Record from Antarctic Ice', Nature, 316, 591-596.
Nahle, N. (2009) 'Correlation between Total Solar Irradiance and Iron Stained Grains during the Last 420 Years' (WWW), Biology Cabinet (http://www.biocab.org/Hematite_Stained_Grains_and_TSI.html), 8/11/2012.

IPCC (2001) IPCC Third Assessment Report: Climate Change 2001, Cambridge University Press: Cambridge.
US Geological Survey (2004) 'Fundamentals of Stable Isotope Geochemistry' (WWW), USGS (http://wwwrcamnl.wr.usgs.gov/isoig/res/funda.html), 10/11/2012.