Page 352 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

316 part II The Water, Weather, and Climate Systems
protons in this isotope cause it to decay into a different atom, 14N, or nitrogen-14. The rate of decay is constant and is measured as a half-life, or the time it takes for half of a sample to decay. The half-life of 14C is 5730 years. This decay rate can be used to date plant material, a tech- nique known as radiocarbon dating.
As an example, pollen is a plant material found in ice and lake sediments and is often dated using radiocarbon dating techniques. Since land plants use carbon from the air (specifically, carbon dioxide in photosynthesis), they contain 14C in some amount, as does their pollen. As time passes, this radioactive carbon decays: after 5730 years, half of it will be gone, and eventually all of it will be gone. The amount of 14C in the pollen can tell scientists
(a) A cross section of a tree trunk shows growth rings; trees generally add one ring each year.
how long ago it was alive. Radioactive isotopes are useful for dating organic material with ages up to about 50000 years before the present.
Lake Cores The sediments at the bottom of glacial lakes provide a record of climate change extending back as far as 50000 years. Annual layers of lake sediments, called varves, contain pollen, charcoal, and fossils that can be dated using carbon isotopes. The layers are drilled to pro- duce lake sediment cores similar to deep-ocean and ice cores. Materials in the layers reflect variations in rain- fall, rates of sediment accumulation, and algal growth, all of which can be used as a proxy for climate.
Tree Rings Most trees outside of the tropics add a growth ring of new wood beneath their bark each year. This ring is easily observed in a cross section of the tree trunk, or in a core sample analyzed in a laboratory (Figure 11.12). A year’s growth includes the formation of earlywood (usually lighter in colour with large-diameter cells) and latewood (darker with small-diameter cells). The width of the growth ring indicates the climatic conditions: wider rings suggest favourable growth conditions, and nar- rower rings suggest harsher conditions or stress to the tree (often related to moisture or temperature). If a tree- ring chronology can be established for a region, involv- ing cross-correlations among a number of trees, then this technique can be effective for assessing climatic condi- tions in the recent past. The dating of tree rings by these methods is dendrochronology; the study of past climates using tree rings is dendroclimatology.
To use tree rings as a climate proxy, dendroclimatol- ogists compare tree-ring chronologies with local climate records. These correlations are then used to estimate relationships between tree growth and climate, which in some cases can yield a continuous record over hun- dreds or even thousands of years. Long-lived species are
Harsh Bark growing
conditions
(b) The size and character of annual
growth rings in a core sample indicating Pith growing conditions.
▲Figure 11.12 Tree rings and a tree core sample.
[(a) Dietrich rose/getty images. (b) after D.e. kitchen, Global Cli- mate Change, Figure 5.42, page 149, © 2013 by Pearson educa- tion, inc. reprinted and electronically reproduced by permission of Pearson education, inc., Upper Saddle river, new Jersey.]
Good growing conditions
Annual growth rings
Earlywood Latewood