Author: Matt Brinkman (brinkman@edseq1.llnl.gov) Title: Ice-core Dating FAQ Outline I. Met
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Author: Matt Brinkman (brinkman@edseq1.llnl.gov)
Title: Ice-core Dating FAQ
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Outline
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
A. Method of Collecting
B. References
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I. Methods of Dating Ice Cores
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.
I.A. Counting of Annual Layers
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.
I.A.1. Temperature Dependent
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
poor becomes 18O rich and then ends up 18O poor again.
CORRECTION: Since Antaractica is below the equator, January is a summer
month in Antaractica. Thus, the last sentence should read
"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 (see I.B.5). 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.
I.A.2 Irradiation Dependent Markers
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 36Be
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.
I.B. Using Predetermined Ages as markers
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.
I.B.1. Peviously Measured Ice-Cores
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 (see I.B.3) and acidic layers.
The major disadvantage of this method is that one must have a previously
age-dated ice core to start with.
I.B.2. Oceanic Cores
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
I.B.3. Volcanic Eruptions
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 (I.B.4) limits this measure to approximately 8000
BC.
I.B.4. Ph Balances
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.
I.B.5 Paleoclimatic Comparisons
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.
I.C. Radioactive Dating of Gaseous Inclusions.
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.
I.D. Ice Flow Calculations
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).
II. The Vostok Ice-Core
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.
II.A How It Was Collected
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 ~ 3,700 meters.
II.B. Experimental Methodology
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 [1], independent ice-flow calculations [1], comparison with other ice
cores [1], paleoclimatic comparison [1], comparison with deep sea cores [1],
10Be/9Be isotopic analysis [2], deuterium/hydrogen isotopic analysis [3],
comparison with marine climatic record [3], CO2 correspondances between dated
ice-cores [4] and CO2 correspondances with dated oceanic cores [4].
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.
II.C. Results
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.
III. Conclusions
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 ~50,000 years, and you are probably
aware that Ted Holden is a proponent of the Velikovskian Catastrophism.
Thus, these conclusions are reader specific.
III.A. Minimum Age of the Earth
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 ~ 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 rmg3@psuvm.psu.edu
III.B. Worlds in Collision
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.
IV. References
A brief note on the references I used.
IV.A. Methods of Collecting
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
lotsa snow, not lotsa 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 [8], if you can find it, is an exceptionally
lucid piece of scientific writing (even though it was a dissertation).
IV.B. References
[1] C. Lorius et al., NATURE 316 (1985) 591-596.
[2] F. Yiou et al., NATURE 316 (1985) 616-617.
[3] J. Jouzel et al., NATURE 329 (1987) 403-408.
[4] J.M. Barnola et al., NATURE 329 (1987) 408-414.
[5] van Nostrands' SCIENTIFIC DICTIONARY
[6] THE ENCYCLOPEDIA OF SCIENCE AND TECHNOLOGY
[7] E. Wolff, GEOGRAPHICAL MAGAZINE 59 (1987) 73-77.
[8] Julie M. Palais OCEANUS 29 (Winter 86/87) 55-60.
[9] W. Dansgaard et al., SCIENCE 218 (1982) 1273-1277.
[10] C.U. Hammer et al., NATURE 288 (1980) 230-235.
E-Mail Fredric L. Rice / The Skeptic Tank