Thursday, March 16, 2006

 

The GISP2 ice core and the age of the earth

Jared at LDS Science Review has a great post about ice cores, and the reasons why those cores indicate the earth is more than a few thousand years old. In the comments on that post Rob Osborn claimed that the The Lost Squadron indicated that the GISP2 ice core, drilled in Greenland and recording ~110,000 years of snow accumulation according to conventional scientists, could only be a few thousand years old (a claim made by several young earth creationists including Michael Oard, Larry Vardiman, and Kent Hovind). In his original post Jared linked to a paper by Paul Seelyfrom the ASA that discussed the GISP2 ice cores, as well as reasons why YEC authors are wrong about the implications of the Lost Squadron. It's a great paper, and well worth reading. I want to spend a bit of time going over Seely's paper.

The GISP2 core

The GISP2 core was drilled through the Greenland Ice Sheet near its summit, and is more than 3000 m long. Similar to varves, there are seasonal variations recorded in the ice, and by counting these seasonal variations it's possible to date the core. The seasonal variations are determined using a variety of independent techniques.

Variations in ice morphology

In Greenland the sun only shines during the summer. This results in the formation of hoar, which is a coarse grained low-density type of snow. The snow in the winter is fine grained and high density. So one year is equivalent to one layer of hoar plus one layer of regular snow (there's probably a more technical term than "regular snow", but not being a glaciologist I don't know it). The upper portion GISP2 core has around 12000 of these pairs. These pairs aren't visible in the deeper portion of the core, because as older snow is buried by younger snow it is compacted, and eventually this compaction destroys the summer and winter types of ice crystals. Other techniques are required to date the older, deeper portions of the core.

Seasonal variations in dust
The wind in Greenland blows most strongly in the late winter and early spring, and at those times more dust is deposited over the snow. So one year of snow deposition is equal to one high dust and one low dust pair.

Dating of volcanic ash
Similar to more mundane dust, ash from volcanic eruptions can be spread over Greenland by wind. The ages of the eruptions can be determined using one or more of the techniques discussed here, and then those ages can be compared to ages from known eruptions.

Seasonal variations in electrical conductivity
Nitric acid is produced in the atmosphere during the summer, but not in the winter. The presence of this nitric acid makes the summer layers more electrically conductive than the winter layers. One year equals one high conductivity layer plus one low conductivity layer.

Seasonal variations in oxygen isotopes
Evaporation affects oxygen isotopic composition. Winter snow is isotopically lighter than summer snow. So one year of deposition is also equal to one isotopically light layer plus one isotopically heavy layer.

Other techniques

This link lists all of the techniques that were used to date the GISP2 core. They list several that Seely didn't discuss, and since I'm not familiar with them I'm not going to write about them at this point. I'll try to look up the references listed on that site at some point.

All of these independent techniques indicate that the GISP2 core is far older than 6-10,000 years, which is a serious problem for YECs.

The Lost Squadron

During World War II a squadron of P-38 fighters and B-17 bombers crash landed near the coast of Greenland. 50 years after that a group of people returned to the crash site and salvaged one of the planes, which was buried beneath 268 ft of ice. According to one of the salvagers there were also hundreds of layers in the ice. This caused some YECs to argue 1) that the 3000+ m of ice from the GISP2 site could have been deposited much more quickly than originally thought, and 2) the layers observed in the GISP2 core weren't necessarily due to seasonal variations – they might be due to shorter term warming and cooling cycles. There are a few reasons why those claims don't hold up.

The depth of ice – the amount of annual snowfall at the Lost Squadron site (near the coast) is much greater than the GISP2 site (far inland near the summit of the ice sheet). The annual snowfall at the Lost Squadron site is around 7 ft per year. So 268 ft of snow in 50 years isn't unusual for that site. The amount of annual snowfall at the GISP2 site is much lower (around 1 ft per year). Using the amount of snowfall accumulation at the Lost Squadron site to infer the rate of snow accumulation at the GISP2 site is wildly inappropriate. That would be like using the amount of rainfall on the west side of the Cascade Mountains in Oregon to make predications about the amount of annual rainfall in Arizona.

The layering from the Lost Squadron site -- The Lost Squadron site should contain at least 100 layers (50 years with 2 layers per year), so some number of layers should be expected at that site. More importantly, the glaciologists who study ice cores do not simply assume that all layers are due only to seasonal variations. Layers can be caused by nonseasonal events like melting, but those layers look quite different from the winter and summer layers. The summer layers are coarse grained, low density snow with many large air bubbles; the winter layers are finer grained and lower density, while the melt layers are glassy and almost bubble free. Here is a partial record of melt features in the GISP2 core. Clearly it is possible to distinguish summer, winter, and melt layers. If YECs want to refute the age of the GISP2 core they need to do more than to point out that the ice at the Lost Squadron site had a lot of layers. They need to 1) demonstrate that those layers have the same characteristics (ice morphologies) as the layers from the GISP2 site – in other words they need to show that the layers at the Lost Squadron site aren't melt layers, and 2) show that the Lost Squadron layers have the same variations in dust concentration, conductivity, etc., that is seen at the GISP2 site.

Comments:
I'm glad you found my post worth expanding on.

I picked up Richard Alley's "The Two Mile Time Machine" from my local library--it's an easy read and is interesting. I didn't know, for example, that there are such things as ice chemists. In the context of glaciology it makes sense--I just never thought of it before.
 
It is amazing the amount of information that can be pulled out of ice cores. They can even compare the relative amounts of stable Oxygen isotopes (O16 and O18, I think) and determine the average global temperature at the time the ice formed.
 
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