Page 353 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 11 Climate Change 317
the most useful for dendroclimatological studies. For example, bristlecone pine in the western United States are some of the oldest organisms on Earth, reaching up to 5000 years in age. Evidence from tree rings in the U.S. Southwest is particularly important for assessing the magnitude of the present drought in that region.
Speleothems Limestone is a sedimentary rock that is easily dissolved by water (rocks and minerals are dis- cussed in Chapter 12). Natural chemical processes at work on limestone surfaces often form caves and under- ground rivers, producing a landscape of karst topography (discussed in Chapter 14). Within caves and caverns are calcium carbonate (CaCO3) mineral deposits called spe- leothems that take thousands of years to form. Speleo- thems include stalactites, which grow downward from a cave roof, and stalagmites, which grow upward from the cave floor. Speleothems form as water drips or seeps from the rock and subsequently evaporates, leaving behind a residue of CaCO3 that builds up over time (Figure 11.13).
The rate of growth of speleothems depends on sev- eral environmental factors, including the amount of rain- water percolating through the rocks that form the cave, its acidity, and the temperature and humidity conditions in the cave. Like trees, speleothems have growth rings whose size and properties reflect the environmental con- ditions present when they formed, and that can be dated using uranium isotopes. These growth rings also contain isotopes of oxygen and carbon, whose ratios indicate tem- perature and the amount of rainfall.
Scientists have correlated speleothem ring chro- nologies with temperature patterns in New Zealand and Russia (especially in Siberia), and with temperature and
precipitation in the U.S. Southwest, among many other places. Research conducted in northern Yukon suggested that the presence of permafrost restricts speleothem growth. Some speleothem chronologies date to 350000 years ago and are often combined with other paleocli- matic data to corroborate evidence of climate change.
Corals Like the shelled marine organisms found in ocean sediment cores, corals are marine inverte- brates with a body called a polyp that extracts calcium carbonate from seawater and then excretes it to form a calcium carbonate exoskeleton. These skeletons ac- cumulate over time in warm, tropical oceans, form- ing coral reefs (see discussion in Chapter 16). X-rays of core samples extracted from coral reefs reveal seasonal growth bands similar to those of trees, yielding infor- mation as to the water chemistry at the time the exoskel- etons were formed (Figure 11.14). Climatic data covering hundreds of years can be obtained this way. Although the process damages polyps living at the surface of the drill site, it does not damage the reef, and drill holes are recolonized by polyps within a few years.
Earth’s Short-Term Climate History
The Pleistocene Epoch, Earth’s most recent period of re- peated glaciations, began 2.5 million years ago. The last glacial period lasted from about 110000 years ago to about 11700 years ago, with the last glacial maximum (LGM), the time when ice extent in the last glacial period was greatest, occurring about 20000 years ago. In Chapter 17, changes to Earth’s landscapes during this time are dis- cussed, and Figure 17.25 shows the extent of glaciation
(a) Speleothems in Royal Cave Buchanan, Victoria, Australia.
(b) Growth bands in a speleothem cross section.
▲Figure 11.13 Speleothems in a cavern and in cross section. [(a) Chris Howes/Wild Places Photography/alamy. (b) Pauline Treble, australian nuclear Science and Technology Organisation.]