Page 347 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 11 Climate Change 311
(for example, whether they are made up of dust, sand, or ancient coal deposits) to complex and costly laboratory analyses of rock samples (for example, mass spectrom- etry analyzes the chemical makeup of single molecules from rock and other materials). Fossils of animal and plant material preserved within rock layers also provide important climate clues; for example, fossils of tropical plants indicate warmer climate conditions; fossils of ocean-dwelling creatures indicate ancient marine envi- ronments. The beds of coal that we rely on today for en- ergy were formed of organic matter from plants that grew in warm, wet tropical and temperate climate conditions about 325 million years ago.
Climate reconstructions spanning millions of years show that Earth’s climate has cycled between periods that were colder and warmer than today. An extended period of cold (not a single brief cold spell), in some cases last- ing several million years, is known as an ice age, or gla- cial age. An ice age is a time of generally cold climate that includes one or more glacials (glacial periods, character- ised by glacial advance) interrupted by brief warm periods known as interglacials. The most recent ice age is known as the Pleistocene Epoch, discussed in Chapter 17, from about 2.5 million years ago to about 11700 years ago. The geologic time scale, which places the Pleistocene within the context of Earth’s 4.6-billion-year history, is presented in Chapter 12, Figure 12.1.
Methods for Long-Term Climate
Reconstruction
Some paleoclimatic techniques yield long-term records that span hundreds of thousands to millions of years. Such records come from cores drilled into ocean-bottom sediments or into the thickest ice sheets on Earth. Once cores are extracted, layers containing fossils, air bubbles, particulates, and other materials provide information about past climates.
The basis for long-term climate reconstruction is isotope analysis, a technique that uses the atomic structure of chemical elements, specifically the rela- tive amounts of their isotopes, to identify the chemical composition of past oceans and ice masses. Using this knowledge, scientists can reconstruct temperature con- ditions. Remember that the nuclei of atoms of a given chemical element, such as oxygen, always contain the same number of protons but can differ in the number of neutrons. Each number of neutrons found in the nucleus represents a different isotope of that element. Different isotopes have slightly different masses and therefore slightly different physical properties.
Oxygen Isotope Analysis An oxygen atom contains 8 protons, but may have 8, 9, or 10 neutrons. The atomic weight of oxygen, which is approximately equal to the number of protons and neutrons combined, may there- fore vary from 16 atomic mass units (“light” oxygen) to 18 (“heavy” oxygen). Oxygen-16, or 16O, is the most com- mon isotope found in nature, making up 99.76% of all
oxygen atoms. Oxygen-18, or 18O, comprises only about 0.20% of all oxygen atoms.
We learned in Chapter 7 that water, H2O, is made up of two hydrogen atoms and one oxygen atom. Both the 16O and 18O isotopes occur in water molecules. If the water contains “light” oxygen (16O), it evaporates more easily but condenses less easily. The opposite is true for water containing “heavy” oxygen (18O), which evaporates less easily, but condenses more easily. These property differences affect where each of the isotopes is more likely to accumulate within Earth’s vast water cycle. As a result, the relative amount, or ratio, of heavy to light oxygen isotopes (18O/16O) in water varies with climate; in particular, with temperature. By comparing the isotope ratio with an accepted standard, scientists can deter- mine to what degree the water is enriched or depleted in 18O relative to 16O.
Since 16O evaporates more easily, over time the at- mosphere becomes relatively rich in “light” oxygen. As this water vapour moves toward the poles, enrichment with 16O continues, and eventually this water vapour condenses and falls to the ground as snow, accumulating in glaciers and ice sheets (Figure 11.6). At the same time, the oceans become relatively rich in 18O—partly as a re- sult of 16O evaporating at a greater rate and partly from 18O condensing and precipitating at a greater rate once it enters the atmosphere.
During periods of colder temperatures, when “light” oxygen is locked up in snow and ice in the polar re- gions, “heavy” oxygen concentrations are highest in the oceans (Figure 11.7a). During warmer periods, when snow and ice melt returns 16O to the oceans, the concen- tration of 18O in the oceans becomes relatively less—the isotope ratio is essentially in balance (Figure 11.7b). The result is that higher levels of “heavy” oxygen (a higher ratio of 18O/16O) in ocean water indicates colder climates (more water is tied up in snow and ice), whereas lower levels of “heavy” oxygen (a lower ratio of 18O/16O) in the oceans indicates warmer climates (melting glaciers and ice sheets).
Ocean Sediment Cores Oxygen isotopes are found not only in water molecules but also in calcium carbon- ate (CaCO3), the primary component of the exoskeletons, or shells, of marine microorganisms called foraminifera. To d a y, f l o a t i n g ( p l a n k t o n i c ) o r b o t t o m - d w e l l i n g ( b e n - thic) foraminifera are some of the world’s most abundant shelled marine organisms, living in a variety of envi- ronments from the equator to the poles. Upon the death of the organism, foraminifera shells accumulate on the ocean bottom and build up in layers of sediment. By ex- tracting a core of these ocean-floor sediments and com- paring the ratio of oxygen isotopes in the CaCO3 shells, scientists can determine the isotope ratio of seawater at the time the shells were formed. Foraminifera shells with a high 18O/16O ratio were formed during cold periods; those with low ratios were formed during warm periods. In an ocean sediment core, shells accumulate in layers that reflect these temperature conditions.