Question

Is the thickness of an annual layer of ice smallest at the top or bottom of the core? Why? And why is this important to acknowledge?

explain why the ice age and gas ages are different.

Answer 1

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 data—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.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.
• isotopes 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.

• Gas is trapped in polar ice sheets at ~50–120 m below the surface and is therefore younger than the surrounding ice. Firn densification models are used to evaluate this ice age-gas age difference (Δage) in the past. However, such models need to be validated by data, in particular for periods colder than present day on the East Antarctic plateau. Here we bring new constraints to test a firn densification model applied to the EPICA Dome C (EDC) site for the last 50 kyr, by linking the EDC ice core to the EPICA Dronning Maud Land (EDML) ice core, both in the ice phase (using volcanic horizons) and in the gas phase (using rapid methane variations). We also use the structured 10Be peak, occurring 41 kyr before present (BP) and due to the low geomagnetic field associated with the Laschamp event, to experimentally estimate the Δage during this event. Our results seem to reveal an overestimate of the Δage by the firn densification model during the last glacial period at EDC. Tests with different accumulation rates and temperature scenarios do not entirely resolve this discrepancy. Although the exact reasons for the Δage overestimate at the two EPICA sites remain unknown at this stage, we conclude that current densification model simulations have deficits under glacial climatic conditions. Whatever the cause of the Δage overestimate, our finding suggests that the phase relationship between CO2 and EDC temperature previously inferred for the start of the last deglaciation (lag of CO2 by 800±600 yr) seems to be overestimated.