Page 48 - Earth's Climate Past and Future

24 PART I • Framework of Climate Science

date lake sediments and other kinds of carbon-bearing little or no life-sustaining oxygen. The lack of oxygen
archives. Neutrons that constantly stream into Earth’s suppresses or eliminates bottom-dwelling organisms
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atmosphere from space convert N (nitrogen gas) to C that would otherwise obliterate the thin annual layers
(an unstable isotope of carbon). Vegetable and animal by their physical activity. Varve couplets usually result
life forms on Earth use carbon from the atmosphere to from seasonal alternations between deposition of light-
build both their hard shells and soft tissue, and a small hued mineral-rich debris and darker sediment rich in
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part of the carbon used is radioactive C. The death of organic material.
the plant or animal closes off carbon exchange with the In regions of marked seasonal variations of climate,
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atmosphere and starts the decay clock ticking. The C trees produce annual layers called tree rings (Figure
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parent decays to the N daughter, a gas that escapes to 2–9C). These rings are alternations between thick layers
the atmosphere. The amount of C that has been lost of lighter wood tissue (cellulose) formed by rapid growth
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when a sample is analyzed is measured by examining a in spring and thin, dark layers marking cessation of
stable isotope of carbon ( C) that has not been removed growth in autumn and winter. Because most individual
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by radioactive decay. Because half of the original amount trees live no more than a few hundred years, the time
of C is lost by radioactive decay every 5780 years, span over which this dating technique can be used is
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radiocarbon dating is most useful over five or six half- limited, but in some areas distinctive year-to-year varia-
lives (back to about 30,000 years ago), but in some cases tions in tree ring thickness can be used to splice records
it can be applied over the last 50,000 years or more (see from younger trees with records from older trees whose
Table 2–1). fossil trunks can still be found on the landscape.
Another technique relies on the same uranium (U) In tropical oceans, corals record seasonal changes in
decay series used to date igneous rocks (see Table 2–1) the texture of the calcite (CaCO ) incorporated in their
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but uses it in a different way to date corals. Ocean corals skeletons (Figure 2–9D). The lighter parts of the coral
incorporate a small amount of 234 U and 238 U (but no bands are laid down in summer, during intervals of fast
230 Th) from seawater into their shells (substituting it for growth, and the darker layers are laid down during win-
calcium). When the corals die, the parent ( 238 U) slowly ter, when growth slows. Individual corals dated in this
decays and produces 230 Th in the coral skeleton. In way rarely live more than a few decades or at most a few
this case, however, the daughter product ( 230 Th) is not hundred years, but older records may be spliced into
stable but radioactively decays away with a half-life of younger ones (as with tree rings).
75,000 years. Gradually the amount of 230 Th present in Correlating Records with Orbital Cycles Another
the coral moves toward a level that reflects a balance way to date climate records is to use the characteristic
between the slow decay of the parent U and the faster imprint of variations in Earth’s solar orbit in a “tuning”
loss of the daughter 230 Th. The clock provided by the exercise. Changes in Earth’s orbit around the Sun alter
Th/U ratio is useful for dating over the last few hun- the amount of solar radiation received by season and by
dred thousand years. This technique is also used for latitude. The timing of these orbital variations is known
dating stalactite and stalagmite deposits in caves. very accurately from astronomical calculations (Part III),
Counting Annual Layers Some climate repositories and the physical processes that link these orbital changes
contain annual layers that can be used to date archives to climatic responses on Earth have become reasonably
by simply counting back in time year-by-year from the well understood in recent decades. The two most
present. These annual layers form because of seasonal prominent examples are changes in the strength of low-
changes in the accumulation of distinctive materials. latitude monsoons and the cyclical growth and decay of
The most visible forms of annual layering in ice high-latitude ice sheets. Because of these relationships,
(mountain glaciers and ice sheets) are alternations climate scientists can date many of Earth’s climatic
between darker layers that contain dust blown in from responses by linking them to the well-dated external dri-
continental source regions during the dry, windy season ver provided by the orbital variations. This technique
and lighter layers marking the part of the year with lit- provides scientists with absolute dating of Earth’s cli-
tle or no dust (Figure 2–9A). These dark/light couplets matic responses over many millions of years.
form annual layers that are easily visible in the upper Internal Chronometers In specific instances, the
parts of glacial ice but are gradually stretched and techniques of counting annual layers and orbital tuning
thinned deeper in the ice, where they cannot easily be can serve a similar purpose much further back in time.
discerned. Ages of these deeper parts of the ice are usu- Even in the absence of radiometric dates of absolute age
ally estimated by methods based on models of how the (in years before the present), some climate archives con-
ice flows. tain internal chronometers with which climate scientists
Sediments in some lakes contain annual couplets can measure elapsed time (duration in years).
called varves (Figure 2–9B). These layers are particu- For example, annual varves deposited in lake sedi-
larly common in the deeper parts of lakes containing ments millions of years ago still survive today in a few