Climate History & the Cryosphere

Part B: Ice Cores

Ice Core Climate Records

In order to fully understand the implications of how climate is changing today, it is important to look at historical records to see how climate has changed in the past. Current climate data collection methods, including satellite observations, only cover a very small window of Earth's long 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 all over the world. Paleoclimatologists (scientists who study past climate) make inferences based on indirect measures of proxy dataproxy data: data that paleoclimatologists gather from natural recorders of climate variability, e.g., tree rings, ice cores, fossil pollen, ocean sediments, coral and historical data, that can help extend our understanding of climate far beyond the 140 year instrumental record.—biological, geological, or chemical indicators that reflect climate conditions. For example, glacial ice is made up of layer upon layer of compacted snowfall that contains dust, pollen, gas bubbles, and other materials that give us clues about what climate was like at different times in the past.

1. Watch the video below to learn more about how scientists use ice coresice core: a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. to study past climate. Then answer the Checking In questions.
2. As you watch, take notes and write down any questions you have about ice cores.

Ice Core Data

This activity was adapted from Vostok Ice Core: Excel (Mac or PC).

Ice cores have been extracted 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 dating back to over 800,000 years ago. The dataset you will use in this activity is from a core whose record goes back about 160,000 years and includes information about the depth in meters (m) of the ice core, the "ice" and "gas" ages in thousands of years ago (kyr), concentration of carbon dioxide found in the ice bubbles in parts per million by volume (ppmv), the hydrogen isotopic ratios, δD, given in parts per thousand (permille), and dust concentration in units of 10^-9cm³g^-1.

Several different climate indicators can be measured from samples of the ice:

• dust: The amount of dust in each annual layer provides information about airborne continental dust and biological material, volcanic ash, sea salts, cosmic particles, and isotopes produced by cosmic radiation that were in the atmosphere at the time the dust was deposited in the ice. The color contrast between dust and snow also provides a visual indicator of boundaries between different ice layers.
• air bubbles: Bubbles trapped in ice cores give scientists actual samples of air from hundreds of thousands of years ago. By analyzing the composition of the air in these bubbles, we can find out what the atmosphere was like long ago.
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  Isotopic signatures during glacial and interglacial periods. Click for larger view.

  Provenance: Image courtesy of University of Michigan (http://www.globalchange.umich.edu/gctext/Inquiries/Inquiries_by_Unit/Unit_8a.htm)
  Reuse: If you wish to use this item outside this site in ways that exceed fair use (see http://fairuse.stanford.edu/) you must seek permission from its creator.
  isotopesisotope: each of two or more forms of the same element that have equal numbers of protons but different numbers of neutrons in their nuclei, and therefore have different atomic masses but not chemical properties. of water: All water molecules are made of two Hydrogen atoms and one Oxygen atom, but there are different stable isotopes of Hydrogen and Oxygen. Although most water molecules consist of two ¹H atoms and one ¹⁶O atom, sometimes water molecules form with a heavy ¹⁸O isotope, written as H₂¹⁸O, or with one ordinary Hydrogen atom replaced by a heavier Deuterium (²H) atom, written as HD¹⁶O.
  
  The maximum amount of moisture that air can hold drops with decreasing temperatures. When humid air cools, the water molecules will condensate to form precipitation. Heavier isotopes have a slightly higher tendency to condensate, so humid air gradually loses relatively more and more of the heavier water molecules (H218O or HD16O). Every time precipitation forms, the air mass becomes more depleted in heavy isotopes. During cold conditions (e.g., during winter or in a cold climatic period), the air masses arriving in over ice sheets have cooled more and have formed more precipitation, which means that the remaining vapor is more depleted in heavy isotopes. Deuterium depletion (δD) therefore, can be used as a proxy for temperature.

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 (δD), 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.

Part 1: 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 details of which are beyond the scope of this unit). 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.

1. Download the Vostok ice core data (Excel 2007 (.xlsx) 18kB Aug6 18) and open the spreadsheet in Excel. If you are unfamiliar with Excel, go here for tips and tutorials or use the program's Help function to assist you.
2. Plot both the ice age and the gas age as a function of depth on the same graph.

   Checking In

    

   1. What is the gas age at a depth of 500 meters?

      22.27 years
      

       

      2,227 years
      

       

      22,270 years Correct! The gas age at 500 m is 22.27 kyr, which is equal to 22, 270 years.
      

       

      222,700 years
      

       
   2. At what depth in the ice core is the ice age closest to 100,000 years?

      270 m
      

      1430 m You got it! The ice age is 100.38 kyr at 1430 m.
      

      1460 m
      
   3. How much younger is a bubble of gas than the ice that surrounds it, at a depth of 250 meters?

      2520 years That's right! The ice age at 250 m is 9.31 kyr (9310 years) and the gas age is 6.79 kyr (6790 years), so the difference is 2520 years.
      

      252 years
      

      25.2 years
      

      2.52 years
      
       
    

Part 2: δD as a proxy for temperature

Next you will calculate the temperature based on the isotopic composition of the water in the ice core.

1. Insert a blank column into the table to the right of the delta-deuterium column (δD). Label the new column Temperature (degrees C).
2. Calculate the temperature at the Vostok station based on the following formula describing the empirical relationship between temperature and deuterium concentration. *Be sure to save your work!*
   
   Temperature (°C) = -55.5 + (δD + 440) / 6
3. Plot your calculated temperature vs. ice age. 

Part 3: CO₂ and Dust

1. Plot CO₂ as a function of gas age. Compare it to the graph you made of temperature vs. ice age. How closely does the plot of CO₂ resemble that of temperature?
2. Now make a plot of dust as a function of ice age. Compare this to the temperature vs. ice age plot; how well do the changes in dust concentration correlate with the temperature changes?

Optional Extension: Tropical Ice Cores

Although most of the planet's glacial ice is found near the poles, there are also many mountain glaciers near the equator that can provide scientists with different clues about Earth's climate history, particularly about climate events like El Niño and monsoons, which don't occur near the poles.

1. Read Background Essay: Tropical Ice Cores Measure Climate (Acrobat (PDF) 46kB Jul4 11).
2. Watch the video segment below about glaciologist Lonnie Thompson and his research of tropical mountain glaciers. Then answer the Checking In questions.