How do ice cores preserve temperature records?
Cores drilled through the icecaps in Greenland and Antarctica are our best records of the climate over the last 800,000 years. The best cores literally have 1 band of ice per year, so the ice in each core can be precisely dated. The chemistry of the ice can then show whether there were glaciers present or not….but how do geochemists do that?
Interpreting these records is done in part using isotopes of oxygen and hydrogen. Isotopes are atoms that have the same number of protons and electrons but different numbers of neutrons. For example, hydrogen has 0 neutrons and deuterium has 1 neutron, giving deuterium twice the mass of normal hydrogen.
For the most part, different isotopes of one element behave the same. Deuterium will still bond with oxygen to make water just like hydrogen will. However, there are some very subtle properties that vary between the different isotopes. For example, when water evaporates, heavier isotopes prefer to stay in the liquid rather than going into the gas.
This effect is so small that the units on isotope plots often are this funny “per mille” unit – permille is 10 times smaller than a percent. On a plot like this one, negative values mean that the measured sample is missing the heavy isotope relative to the standard, which is usually seawater.
Both hydrogen and oxygen have heavy isotopes and glacial ice is very low in those heavy isotopes relative to ocean water today (hence the very negative numbers), but as you can see there’s also a pattern. The glacial ice gets more heavy hydrogen and heavy oxygen at the same time, and 120,000 years ago there was a measureable spike in both.
What does it mean for snow at the poles to have more heavy isotopes? That’s how we turn this record into information about temperature.
The ratio of the heavy and light isotopes in water vapor evaporated from the ocean depends on the temperature of the oceans. If the oceans are cooler, any water that evaporates is missing even more of its heavy isotopes.
On top of that, the colder the planet is, the less humid it is. When the atmosphere is less humid, it’s easier for any water that does evaporate to rain out at low latitudes, so even less moisture makes it to the poles. Again the same rule applies – heavy isotopes go into the liquid, so by the time those last slivers of moisture make it to the poles, they’re missing even more of the heavy isotopes. Other factors, such as the temperature of the atmosphere where the snow forms and the altitude the snow forms at, play in as well, and all those can be calibrated and measured.
Overall, on this plot, the more negative the values are, the greater the volume of the ice sheets and the colder the planet.
When the values spike upwards, we see evidence that the ice sheets shrank, including dumping of extra sediment in the oceans from icebergs breaking off the edges. When we take this type of data back in time, we can see that the Earth has been locked in a cyclic pattern – large glaciers advance, grow for about 100,000 years with a few breakups in-between, then break up fully and start reforming after a few thousand years. The chemistry of the H2O locked in ice cores therefore tells us the story of how glaciers advanced and retreated over the past 800,00 years.
By understanding these changes as recorded by these cores of ice, we can begin to ask the all-important question for scientists…”Why did this happen”, and then start to assess what that means for changes happening today.
-JBB
Image credit: http://bit.ly/1ISmarl
Read more: http://bit.ly/1LXIaE8 http://cdiac.ornl.gov/trends/co2/ice_core_isotopes.html