Reconstructing Earth's Climate History. Kristen St. John
the oldest living trees are nearly 10 000 yr old, most trees do not live this long. For example, the average lifespan of a paper birch is 100 yr, and the average lifespan of a post oak is 250 yr. Extending the historical climate record from living trees further into the past requires correlating recognizable ring patterns in overlapping samples from living trees to dead trees (e.g. tree stumps and wooden beams in archeological sites) of the same species in the same geographic region. Why do you think it is important to use overlapping samples from trees of the same species to reconstruct past changes in regional climate?
Correlating ring patterns in overlapping samples not only extends the paleoclimate record that tree ring data can provide, but also confirms the dendrochronology (i.e. the tree ring timeline) – it helps ensure accurate ages are assigned to each layer by identifying the common pattern and flagging individual irregularities (e.g. incomplete rings, or missing rings). In addition, radiometric dating (in this case 14C dating) is commonly used in conjunction with ring counting techniques to ensure good age control. You will learn more about radiometric dating in Chapter 3.
Cave Deposits: Speleothems
Speleothems are secondary mineral deposits in caves. The term “secondary” is used because these mineral deposits are not part of the original limestone bedrock (which may be hundreds of millions of years old) that was dissolved by groundwater to form the cavities underground in the first place. The speleothems form later from water saturated in dissolved Ca2+ and CO32− ions that drips into the cave through fractures in the cave ceiling, and precipitates solid calcium carbonate (CaCO3). These mineral deposits are like the white spots and crusts that can build up around a household faucet if you have “hard” water (i.e. water with a lot of calcium carbonate dissolved in it) and a leaky faucet. The dripping water in a cave is connected to the hydrologic cycle; it was once rainwater that percolated down through the soil and became part of the groundwater system. As the water drips in a cave, it forms layers of solid mineral deposits hanging from the ceiling like icicles (stalactites) and building up on the floor (stalagmites) of the cave. While speleothems suitable for paleoclimate research are not technically cores, their natural formation shape is already cylinder or core‐like, the pristine center of which is sampled for isotopic analyses that can provide information on changes in past temperature, the water cycle, and the carbon cycle. You will work with speleothem data from South America in Chapter 3.
1 The best speleothems for paleoclimate reconstruction form when cave conditions are just right – like Goldilocks needed her porridge just right. Think about how stalagmites form on the floor of a cave:If conditions were too wet (e.g. if the cave was flooded with groundwater), how would that affect the ability of a stalagmite to form?If conditions were too dry, where no groundwater is dripping into the cave through fractures in the ceiling, how would that affect the ability of a stalagmite to form?How would evaporation conditions differ near the entrance to a cave compared to farther back in a cave? Which of these two settings would provide a more ideal location to obtain a speleothem for paleoclimate reconstruction?
2 What would a layer of hardened mud in a stalagmite imply about the environmental history of the cave? (In other words, how could the mud have gotten there?)
3 Go to the supplemental resources to watch videos and read a short article on selecting and collecting a speleothem for paleoclimate research. Make a list of the challenges of obtaining speleothems for paleoclimate research and the strategies scientists use to overcome these challenges.ChallengesSolutions
4 Like tree‐rings, radiometric dating (in this case, U‐Th isotopic dating, which you can explore more in Chapter 3) can be used to determine the age of the speleothem layers. However, unlike tree‐rings, the layers in speleothems are not necessarily annual layers. Their accumulation rate depends on the rate of water dripping into the cave.What could you infer about regional precipitation if you see that the younger layers in a stalagmite increase in thickness?Would you expect all caves globally to show the same changes in layer thickness through time? Why or why not?
Glacial Ice
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