Lab: Vostok Ice Core
Map showing Antarctica and location of Vostok Ice Core. Colors indicate elevation.
I. Introduction
Snow falling in the polar regions of the earth (e.g. Greenland and Antarctica) sometimes is preserved as annual layers within the ice sheets, provided that they are not destroyed by flow of the ice. These annual layers provide a record of the earth's climate that reaches back as much as 200,000 years.
Several different climate indicators can be measured from samples of the ice:
· The amount of dust in each annual layer is indicative of the environment at the time that the dust was deposited. Various kinds of fallout from the atmosphere, including airborne continental dust and biological material, volcanic debris, sea salts, cosmic particles, and isotopes produced by cosmic radiation, are deposited on the ice sheet surface along with the snow, thus mixing with the snow and also acting as a distinctive barrier between different ice layers.
· The composition of bubbles of air trapped in the ice is a measure of the composition of the atmosphere in ancient times. With increasing pressure from subsequent snow deposition on an ice cap or glacier, the snow becomes compacted and, consequently, air is trapped within the deposited layer. This entrapment of air occurs essentially with no differentiation of the atmospheric gas components. However, carbon dioxide has different chemical properties from other atmospheric gases, thus, the carbon dioxide concentration in the air-filled spaces might be affected by interaction with the ice itself or with trapped impurities.
· The isotopic composition of water, and in particular the concentration of the heavy isotope of oxygen, 18O, relative to 16O, as well as 2H (deuterium) relative to 1H, is indicative of the temperatures of the environment. During cold periods, the concentration of less volatile 2H (18O) in the ice is lower than during warm periods. The reason for this is that at lower temperature, the moisture has been removed from the atmosphere to a larger degree resulting in an increased depletion of the heavier isotopes.
The Vostok core was drilled in East Antarctica, at the Soviet station Vostok from an altitude of 3488 m, and has a total length of 2083 m. Ice samples have been analyzed with respect to isotopic content in 2H, dust, and methane and carbon dioxide trapped in air bubbles. The profiles of 2H, methane, and carbon dioxide concentrations behave in a similar way with respect to depth in the core, showing a short interglacial stage, the Holocene, at the top, a long glacial stage below, and the last interglacial stage near the bottom of the core. The record goes back in time about 160,000 years.
For more information on these topics, read the through this lab on the University of Michigan website: http://www.globalchange.umich.edu/gctext/Inquiries/Inquiries_by_Unit/Unit_8a.htm.
Remember that this is just another resource- the questions asked there are not the same as what we are asking you!
II. Lab Instructions
A. Gas Age vs. Ice Age
Age is calculated in two different ways within an ice core. The ice age is calculated from an analysis of annual layers in the top part of the core, and using an ice flow model for the bottom part. The gas age data accounts for the fact that gas is only trapped in the ice at a depth well below the surface where the pores close up.
Task 1: Transfer the Vostok ice core data to Excel, and save it as an Excel workbook. Plot both the ice age and the gas age as a function of depth on the same graph. Describe the graph and answer the following questions:What are the units of both age scales?
· What are the depths of the shallowest and deepest data points? For the rest of the lab, assume that the most shallow ice core measurements represent the environmental conditions in the 18th century before the Industrial Revolution.
· Does age increase or decrease down the core? Why?
· Why do the two age curves differ?
· How much younger is a bubble of gas than the ice that surrounds it, at a depth of 250 meters?
· Is the thickness of an annual layer of ice smallest at the top or bottom of the core? Why?
· Note that if you look carefully at the plot you can see that the curve changes slope between the top and the bottom of the core. Why do you think that this happens?
B. δD as a proxy for temperature
Task 2: Next you will calculate the temperature based on the isotopic composition of the ice. Insert a blank column into the table to the right of the delta-deuterium column (δD). Isotopic ratios are used to model temperature. Calculate the temperature at Vostok based on the following formula describing the empirical relationship between temperature and deuterium concentration:
Temperature (deg-C) = -55.5 + (δD + 440) / 6
*Be sure to save your work!*
Now plot your calculated temperature vs. ice age. How reliable do you consider this paleoclimate record to be? (Hint: think about some of the uncertainties in the age models and compare the age of the Last Glacial Maximum (LGM) to what scientists consider the age to be today.) How long ago did the maximum temperature occur? How long ago did the minimum temperature occur? How do these temperature compare to the current temperature at current average Vostok temperature?
C. CO2, CH4, and Dust
Task 3: Plot CO2 as a function of gas age. How closely does the plot of CO2 resemble that of temperature? Now plot CO2 against temperature. Add a trendline and record the R2 value. Based on your work in previous labs, do you think this correlation is significant?
Task 4: Make the same plots for CH4. Is CO2 or CH4 more closely correlated with temperature? Why do you think that is?
Now make a plot of dust as a function of ice age. Compare this to the temperature plot; how well do the changes in dust concentration correlate with the temperature changes?
Task 5: The present atmospheric CO2 and CH4 concentrations are 375 ppmv and 1,700 ppbv, respectively, according to IPCC (2001). Calculate the changes in CO2 and CH4 concentrations between the last glacial maximum and the 18th century, and between the 18th century and today. Why were CO2, CH4, and dust concentrations different during the glacial time as compared to the 18th century?
Task 6: Insert today's CO2 concentration (use the IPCC value given above) into the linear regression equation from question #4 to determine what the past relationship between CO2 and temperature predicts that today's temperature should be at Vostok. How does your calculation compare with the known value?
D. Trends
You can calculate the rate of change of temperature (degrees/ka) by subtracting one temperature from the next oldest and dividing by the ice-age difference. That is:
(younger temp - older temp)/(older ice age - younger ice age) = rate of change (positive for warming, negative for cooling)
Recall that younger means shallower here.
Task 7: Calculate the warming rate for the entire time series and plot warming rate vs. ice age. What is the most rapid warming that occurred during the deglaciation that began around 15,000-20,000 years ago? How do the rates of warming that you've calculated compare with the current temperature data?
Task 8: Now calculate the rate of change in greenhouse gas concentrations (CO2 and CH4) versus gas age for this time period. How do these rates of change compare with the change in carbon dioxide and methane concentrations that has occurred over the past 100 years?
Task 9: Note that there were two major warming events representing two deglaciations in the Vostok calculated temperature data. Then look at how CO2 and CH4 change during those deglaciation periods. From the data provided in this lab, can you tell which changes first, temperature or greenhouse gas (CO2, CH4) concentrations? Why is this important?
III. Lab Report Instructions
Write a lab report (as per the Lab Report Format) summarizing the major findings of your investigation. In addition, address the following questions:
1. How did conditions during the last glacial maximum (around 20,000 years ago) differ from today's conditions?
2. How do the glacial/interglacial changes in temperature, carbon dioxide, and methane compare to the changes since the 18th century?
3. Why are these ice core paleoclimate records so important to our understanding and prediction of climate change?
IV. Contributor
J. Chapellaz, Laboratoire de Glaciologie et Geophysique de l'Environment, Grenoble
V. Optional Reading List
· Dansgaard, W., H.B. Clausen, N. Gundestrup, C. U. Hammer, S. J. Johnsen, P. M. Kristinsdottir, and N. Reeh, A new Greenland deep ice core, Science, 218, 1273-1277, 1992.
· Lorius C., J. Jouzel and D. Raynaud, The ice core record: past archive of the climate and signpost to the future, In: Antarctica and Environmental Change, Oxford University Press: New York, 27-34, 1993.
· Oeschger H., B. Stauffer, R. Finkel, and C. C. Langway, Variations of the CO2\ concentration of occluded air and of anions and dust in polar ice, In: The carbon cycle and atmospheric CO2: Natural variation Archean to present (eds. E. T. Sundquist and W. S. Broecker), Geophysical Monograph 32, American Geophysical Union, Washington, DC, 1985.
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Created by Stephanie Pfirman, Barnard College and the Columbia/Barnard Earth’s Environmental Systems: Climate team.
Climate Change & the Cryosphere
Simulation of annual average global warming by the year 2060. Click on image to enlarge. Image Credit: NASA.
Introduction
According to the Intergovernmental Panel on Climate Change (IPCC), the average surface temperature of the Earth has risen by more than one half of a degree Celsius over the last century. As small as this value may sound, it is a legitimate cause for concern. For comparison, today's average global temperature is only 5 degrees Celsius warmer than it was during the last ice age. Of course, Earth's climate goes through natural cycles. However, many scientists believe that human activities—primarily fossil fuel burning— which increase atmospheric carbon dioxide levels and average global temperatures, are pushing climate to a tipping point. If temperatures continue to climb, there could be extreme changes in weather patterns and sea level, which could potentially have drastic effects on human health, wildlife habitats and ecosystems worldwide.
Part A: Ice Core Climate Records
Climate change refers to long-term changes in weather and environmental patterns. In order to fully understand how climate is changing and whether or not current trends are significant, it is important to look at historical records. Current climate data collection methods, including satellite observations, only cover a very small window of Earth's history with respect to climate change time scales. Luckily, clues to past climatic conditions, dating hundreds of thousands of years back in time, are recorded in glacial ice. Scientists can analyze differences in ice layers and the chemicals trapped in air bubbles in the ice to piece together the planet's climate history.
19 cm long section of Greenland Ice Sheet Project 2 ice core from 1855 meters showing annual layer structure illuminated from below by a fiber optic source. Section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers. Image source: Wikimedia Commons.
1. Walk through the American Museum of Natural History's Ice Core Interactive to learn about how ice cores are used to study past climate.
Checking In
Using the information in the Ice Core Interactive to help you, answer the following questions about ice cores.
o What do the different layers of an ice core represent?
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Each layer represents a yearly cycle of snow accumulation.
o How do scientists determine the ages of ice core layers?
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Scientists use carbon dating of organic material trapped in the core to determine the ages of the layers.
o What does the ratio of Oxygen-16 to Oxygen-18 isotopes in an ice core's water molecules reveal?
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The ratio of O-16 to O-18 is related to the air temperature of the cloud that deposited the snow, so it can be used to determine the yearly air temperature over the core drilling site.
o How do scientists use ice cores to figure out what gases were present in the atmosphere in the past?
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Scientists melt and crush samples of the ice core and collect the air that was trapped in them to analyze the atmospheric gases that were present at the time of the snowfall for that layer of the core.
Ice cores have been taken from many locations around the world, primarily in Greenland and Antarctica. One of the deepest cores ever drilled was at the Vostok station in Antarctica, which includes ice from as far back as 420,000 years ago.
2. Examine the plot below of Vostok ice core data. NOTE: in the plot, ppm stands for parts per million; ppmv stands for parts per million by volume; and Δ means change.
Graph of CO2(Green graph), temperature (Blue graph), and dust concentration (Red graph) measured from the Vostok, Antarctica ice core as reported by Petit et al., 1999. Data source: Petit J.R., Jouzel J., Raynaud D.,Barkov N.I.,Barnola J.M., Basile I., Bender M., Chappellaz J., DavisJ., Delaygue G., Delmotte M., Kotlyakov V.M., Legrand M., Lipenkov V.,Lorius C., Pépin L., Ritz C., Saltzman E., Stievenard M. (1999). , Nature, 399: 429-436.
Stop and Think
1: Based on the Vostok ice core data plot above, how would you describe the relationship between temperature change (blue line) and atmospheric CO2 concentration (green line)? Explain why you think this relationship exists.