Читать книгу Reconstructing Earth's Climate History - Kristen St. John - Страница 21

1 Where does glacial ice (i.e. glaciers or ice sheets) exist today?

2 Go to the supplemental resources to watch videos and read a short article on the ice core drilling process. Make a list of the challenges of obtaining ice cores for paleoclimate research and the strategies scientists use to overcome these challenges.ChallengesSolutions

FIGURE 1.6. Piece of an Antarctic ice core showing trapped air bubbles.

(Source: Photo credit: Oregon State University, media release 9‐11‐08, http://oregonstate.edu/dept/ncs/newsarch/2008/Sep08/icecore.html).

Bubbles of ancient air (Figure 1.6) found in glacial ice are unique and valuable indicators of past climate. Unlike most other climate indicators, which indirectly record climate parameters, the trapped air in glacial ice is a direct measure of atmospheric gases (e.g. CO₂ and CH₄) of the past. As snow recrystallizes into ice below the surface of a glacier, air is trapped in the pore spaces between ice crystals. The pore spaces are eventually closed off from the atmosphere by continued accumulation of new snow, and by the recrystallization and fusing of individual ice crystals from layers of snow to firn (compacted snow) to ice. Because the pore spaces are open to the atmosphere until the ice forms, the age of the gases in the pore spaces is younger than the age of the surrounding ice. Trapped gas comprises 10–15% of the volume of glacial ice at the “bubble close‐off depth” (i.e., the depth of the firn–ice transition) (Bender et al., 1997).

1 Is the age of the trapped gas older than, younger than, or the same age as the ice that is trapping it?

2 Examine Figure 1.7, which shows a lab where ice cores samples are analyzed, and read the following brief description of the sample preparation and gas analysis process:Ice samples were cut with a bandsaw in a cold room (at about −15 °C) as close as possible to the center of the core in order to avoid surface contamination. Gas extraction and measurements were performed by crushing the ice sample (~40 g) under vacuum in a stainless‐steel container without melting it, expanding the gas released during the crushing in a pre‐evacuated sampling loop, and analyzing the CO2 concentrations by gas chromatography. The analytical system, except for the stainless‐steel container in which the ice was crushed, was calibrated for each ice sample measurement with a standard mixture of CO2 in nitrogen and oxygen. (text modified from: http://cdiac.ornl.gov/trends/co2/vostok.html)Identify some specific conditions and methods from the above description and propose why these are necessary to produce accurate and precise gas concentration data from the ice core:FIGURE 1.7. Class 100 Clean Room at Byrd Polar and Climate Research Center. Class 100 means there are less than 100 particles (diameter > 0.5 μm) per cubic foot of air.(Source: Photo from: http://bprc.osu.edu/Icecore/facilities.html#AnalyticalFacilities).

3 There are hundreds of analytical labs around the world that are capable of measuring CO2 and CH4 gas concentrations from ice core samples. How could investigators ensure that the results from different labs are comparable?

4 In 1992, the European Greenland Ice Core Project (GRIP) drilled down 3029 m to the base of the Greenland Ice Sheet at Summit, Greenland (72°N, 38°W). A year later, the U.S. Greenland Ice Sheet Project 2 (GISP2) completed drilling of a companion record through the ice sheet 30 km to the west. What value might there be in obtaining two parallel ice core records so close together?

5 The upper 2788 m of the GRIP ice core contains a Greenland paleoclimate record of the last 110 000 yr. The European Project for Ice Coring in Antarctica (EPICA) recovered the deepest and oldest ice record to date at Dome C, Antarctica. This 3270.2 m‐long ice core contains a paleoclimate record of the last 740 000 yr.Calculate the average ice accumulation rates (cm/yr) for the GRIP and EPICA ice cores. Show your work, including the conversion from meters to centimeters.Which has a higher average ice accumulation rate: GRIP or EPICA? Circle your answer.Which record spans a greater period of Earth history: GRIP or EPICA? Circle your answer.Which has a higher average resolution: GRIP or EPICA? Explain your reasoning.