Ice Core Dating
[Last Update: January 3, 1995]
I. Methods of Dating Ice Cores A. Counting of Annual Layers 1. Temperature Dependent 2. Irradiation Dependent B. Using Pre-Determined Ages as Markers 1. Previously Measured Ice-Cores 2. Oceanic Cores 3. Volcanic Eruptions 4. Ph Balances 5. Paleoclimatic Comparison C. Radioactive Dating of Gaseous Inclusions D. Ice Flow Calculations II. The Vostok Ice-Core A. How It Was Collected B. Experimental Methodology C. Results III. Conclusions A. Minimum Age of the Earth B. Worlds in Collision? IV. References
Of the four distinct methods for determining the ages of ice cores, the first three are direct experimental tests and the fourth rests on somewhat uncertain theories.
The basis of this method lies with looking for items that vary with the seasons in a consistent manner. Of these are items that depend on the temperature (colder in the winter and warmer in the summer) and solar irradience (less irradience in winter and more in summer). Once such markers of seasonal variations are found, they can be used to find the number of years that the ice-core accumulated over. This process is analagous to the counting of tree rings. A major disadvantage of these types of dating is that they are extremely time consuming.
Of the temperature dependent markers the most important is the ratio of 18O to 16O. The water molecules composed of H2(18O) evaporate less rapidly and condense more readily then water molecules composed of H2(16O). Thus, water evaporating from the ocean it starts off H2(18O) poor. As the water vapor travels towards the poles it becomes increasingly poorer in H2(18O) since the heavier molecules tend to precipitate out first. This depletion is a temperature dependent process so in winter the precipitation is more enriched in H2(16O) than is the case in the summer. Thus, each annual layer starts 18O rich, becomes 18O poor, and ends up 18O rich.
This process also depends on the relative temperatures of different years, which allows comparison with paleoclimatic data. For similar reasons the ratio of deuterium to hydrogen acts the same way.
The major disadvantage of this dating method is that isotopes tend to diffuse as time proceeds.
Of the irradiation dependent markers the two most important are 10Be and 36Cl. Both of these isotopes are produced by cosmic rays and solar irradiation impinging on the upper atmosphere, and both are quickly washed from the atmosphere by precipitation. By comparing the ratios of these isotopes to their nonradioactive counterparts (i.e. 9Be and 35Cl) one can determine the season of the year the precipitation occurred. Thus each annual layer starts 10Be and 36Cl poor, becomes 10Be and 36Cl rich, and then becomes poor again.
CORRECTION: I really mucked this one up. Although what is said above is true, this is an exceedingly minor effect. Both 10Be and 36Cl are formed as charged ions in the ionosphere. The Earth's magnetic field then traps them, with only a slight "leakage" of the isotopes to the lower atmosphere. The amount of "leakage" depends on the height of the ionosophere, which changes primarily in response to the Solar cycle, with periods of maximum solar activity corresponding to the highest extent of the ionosphere.
It should be noted that the 10Be/9Be ratios for some ice cores have been compared with the known solar cycle and are in excellent agreement with what is known (accurately showing the time of the European Little Ice Age, which corresponded with a remarkably low amount of solar activity).
The major disadvantage of this dating method is that these isotopes also tend to diffuse over time.
In these methods, one uses the age of previously determined markers to determine the age of various points in the ice-core. The major advantage of these methods is that they can be completed relatively quickly. The major disadvantage is that if the predetermined age markers are incorrect than the age assigned to the ice-core will also be incorrect.
In this method one compares certain inclusions in a ice-core whose age has been determined with a seperate method to similar inclusions in an ice-core of a still undetermined age. These inclusions are typically ash from volcanic eruptions and acidic layers.
The major disadvantage of this method is that one must have a previously age-dated ice core to start with.
In this method one compares certain inclusions in dated ocean cores with related inclusions found in the ice-core of a still undetermined age. Examples of such inclusions are a decrease (or increase) in temperature over a period of years that can be determined from flora and fauna found in the oceanic core and a decrease (increase) in the 18O enrichment over this same period of years. Another example is volcanic ash.
ADDITION: R. Hyde has posted separately some of the relationships between ocean core data and their astronomical causes. These are the primary "inclusions" that are compared. I apologize for my use of nondescript terminology here.
The major disadvantages of this method are that one must compare different signatures of climatic change that correspond to the same event and that one is not certain of the lag times (if any) between oceanic reactions and glacial reactions to the same climatic changes.
After the eruption of volcanoes, the volcanic ash and chemicals are washed out of the atmosphere by precipitation. These eruptions leave a distinct marker within the snow which washed the atmosphere. We can then use recorded volcanic eruptions to calibrate the age of the ice-core. Since volcanic ash is a common atmospheric constituent after an eruption, this is a nice signature to use in comparing calibrated time data and an ice-core of undetermined age. Another signature of volcanism is acidity.
The major diasadvantage of this method is that one must previously know the date of the eruption which is usually not the case. Furthermore the alkaline precipitants of the ice ages limits this measure to approximately 8000 BC.
One unique marker of periods of glaciation is that precipitation during the ice ages are markedly alkaline. This is due to the fact that the ice ages tied up a large quantity of the available water thus exposing a larger portion of the continental shelves. From these shelves huge clouds of alkaline dusts (primarily CaCO3) were blown across the landscape.
The major disadvantage of this method is that it gives only very approximate age ranges (i.e. this ice was laid down during the ice age). Furthermore, the lag time between the onset of glaciation and increased alkalinity are uncertain.
In this method, one compares long range climatic changes (e.g. ice ages and interglacial warmings) with markers (such as the 18O/16O ratios) found within the ice-cores.
In this method one melts a quantity of glacial material from a given depth, collects the gases that were trapped inside and use standard 14C and 36Cl dating.
The major disadvantage of this method is that a huge amount of ice must be melted to gather the requisite quantity of gases.
In this method, one measures the length of the ice core and calculates how many years it must have taken for a glacier of that thickness to form.
This is the most inaccurate of the methods used for dating ice-cores. First one must calculate how the thickness of the annual layer changes with depth. After this one must make some assumptions of the original thickness of the annual layer to be dated (i.e. the amount of precipitation that fell on the area in a year).
To demonstrate the methods used in dating ice-cores I will use the Vostok ice-core as an example because I found plenty of literature on it and because it is an Antarctic ice-core which was what the original post was about.
The Vostok Ice-Core was collected in East Antarctica by the Russian Antarctic expedition. The Vostok Ice-Core is 2,083 meters long and was collected in two portions: 1) 0 - 950 m in 1970-1974, 2) 950 - 2083 m in 1982-1983. The total depth of the ice sheet from which the core was collected is approximately 3,700 meters.
The ice core was sliced into 1.5-2.0 meter segments. A discontinuous series sampled every 25 meters and a continuous series from 1,406 to 2,803 meters were then sent in solid form to Grenoble, France for further analysis.
At Grenoble the ice was put into clean stainless steel containers. The samples were crushed and then melted with the gases given off collected and saved for further analysis. The melt water was tested for chemical composition and then electrolysised.
The methods used in the determination of the ages include 18O/16O isotopic analysis , independent ice-flow calculations , comparison with other ice cores , paleoclimatic comparison , comparison with deep sea cores , 10Be/9Be isotopic analysis , deuterium/hydrogen isotopic analysis , comparison with marine climatic record , CO2 correspondances between dated ice-cores  and CO2 correspondances with dated oceanic cores .
The results determined from these various samples were consistent between the continuous and discontinuous slices within the sections that overlapped. They were also consistent with Greenland ice-cores, other Antarctic ice-cores, dated volcanic records, deep sea cores, and paleoclimatic evidence.
While unable to provide specific dates (within a millenia), the analysis show definate evidence of the the last two ice ages. Using the methods listed above the bottom of the ice-core was laid down 160,000 +- 15,000 years ago. It should be noted that all of the methods listed above were consistent with the above results.
In this section I will provide a brief review of how the ice-core data effects both the age of the earth question and the Velikovskian catastrophism.
NOTE: This original post was written at a time when both Bob Bales and Ted Holden were frequent posters to talk.origins. Bob Bales has argued that the age of the Earth is about 50,000 years, and you are probably aware that Ted Holden is a proponent of the Velikovskian Catastrophism. Thus, these conclusions are reader specific.
From the data gathered from the Vostok ice-core indicates that the minimum age of the earth is 160,000 +- 15,000 years. Furthermore there exists approximately 33% of additional ice below the core sample which would hold a disproportionate number of years due to thinning of the ice layers under the tremendous pressure of the ice above it.
To maintain an age for the earth of 50,000 years, one would need to describe a mechanism that allows more than 2 false ice layers to form per year. It should be noted that one also needs to describe why this mechanism has ceased to function in historic times since the Vostok ice-core demonstrates a number of the historically recorded volcanism at the correct periods of time.
ADDITION: "To the list of things excluded, you can add miles-high tides or floods. (Velikovsky and the Noachian deluge). Such a mass of water would have provided sufficient buoyancy to float the polar caps off their beds. No way to drop them exactly back onto their original location, or to regrow them. (In fact, the Greenland ice cap would not regrow under modern (last 10 ky) climatic conditions.)" --Bob Grumbine email@example.com
The Vostok ice-core shows no effects of catastrophic geological changes. By this I mean no petroleum, no vermin, no weird Venus gasses, no red snow, no manna in amongst the layers. Also no evidence for rapid rotational changes in the earth, no floods, no major asteroid bombardments. Finally, there is absolutely positively fur-darn-tootin no evidence of the earth ever having occupied any position in the solar system other than that which it holds now.
When I went to look for references on the dating of ice-cores, I decided to follow a simple philosophy...as simple as scientifically possible. I chose to do this to demonstrate that there is no excuse for someone to make the blatantly ignorant attack that Ted made when answering Sue Bishop's original post on ice-core data.
NOTE: Ted originally claimed that the Antarctic ice cores resulted from lots of snow, not lots of years.
The above sections on the Vostok ice-core was taken from references 1-4. The general information on dating methods comes from references 5-8. The last two references are about Greenland ice-cores, and are included for further reading pleasure. Reference , if you can find it, is an exceptionally lucid piece of scientific writing (even though it was a dissertation).
 C. Lorius et al., NATURE 316 (1985) 591-596.
 F. Yiou et al., NATURE 316 (1985) 616-617.
 J. Jouzel et al., NATURE 329 (1987) 403-408.
 J.M. Barnola et al., NATURE 329 (1987) 408-414.
 van Nostrands' SCIENTIFIC DICTIONARY
 THE ENCYCLOPEDIA OF SCIENCE AND TECHNOLOGY
 E. Wolff, GEOGRAPHICAL MAGAZINE 59 (1987) 73-77.
 Julie M. Palais OCEANUS 29 (Winter 86/87) 55-60.
 W. Dansgaard et al., SCIENCE 218 (1982) 1273-1277.
 C.U. Hammer et al., NATURE 288 (1980) 230-235.
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