Dating methods for climate change

Reliable dating methods permit scientists to describe past climate change quantitatively and to establish connections between known astronomical cycles and climate cycles.

Background

Because the geological and climatological history of Earth began long before recorded history, scientific dating methods are necessary to determine when many climatic events occurred. For example, such methods could be used to determine when glacial deposits formed or when a boulder was dropped on top of those deposits by a melting glacier.

Primordial Isotopes

When the Earth formed, it inherited an inventory of radioactive elements that have been decaying ever since. The for a particular isotope can be determined by measuring the rate at which disintegrations occur in a sample of known mass. Half-lives are calculated from decay constants. Known half-lives of radioactive enable scientists to determine the age of some objects that contain those isotopes.

For example, water moving through the ground will often dissolve small amounts of uranium. A stalagmite may form from this water in a cave as the water evaporates, incorporating any uranium 238 (U²³⁸) present. The uranium will decay to produce thorium 234 (Th²³⁴). Thorium is insoluble in water, so it can be assumed that the stalagmite initially contained no thorium. Thorium, too, is radioactive, and may decay into U²³⁴, which decays to Th²³⁰, which is also radioactive. Using the decay constants and the amounts of U²³⁸, U²³⁴, and Th²³⁰ present in a specimen, scientists can calculate how long it has been since the uranium came out of solution. This technique is limited to ages less than 500,000 years.

Cosmogenic Isotopes

Cosmic rays are subatomic particles traveling at very high velocities. When they strike the nucleus of an atom, they can eliminate nucleons, altering the identity of the atom. An atom of nitrogen 14 (N¹⁴), for instance, might become carbon 14 (C¹⁴), or atoms of silicon or oxygen might become beryllium 10 (Be¹⁰) or aluminum 26 (Al²⁶). C¹⁴, Be¹⁰, and Al²⁶ are all radioactive, and their decay constants are known, so they provide a means of dating organic material and the surfaces of boulders.

On Earth, C¹⁴ is generally created only as a result of cosmic ray bombardment in the atmosphere, so only atmospheric carbon replenishes its C¹⁴ level. Nonatmospheric C¹⁴ decays over time without replenishment. An organism will interchange carbon with the atmosphere while it is alive, maintaining a relatively constant ratio of C¹⁴ to carbon 12 (C¹²), but once it dies that interchange will cease and the ratio will decrease. By assuming a historically constant ratio of C¹⁴ to C¹² in the atmosphere (and thus in living organisms) and by comparing that ratio to the ratio in a sample of tissue from a deceased organism, it is possible to determine how many half-lives of C¹⁴ have passed since the organism died. The assumption of a constant ratio is known to be invalid, but it will produce the same errors in all samples, giving the same results for samples of the same age. If the goal of analysis is to compare different samples with one another and there is little need for actual calendar years, samples’ ages are often reported in C¹⁴ years.

To convert results accurately to calendar years, corrections are made using calibration curves derived from other dating techniques such as may produce different calibrated ages from the same C¹⁴ age. The effective limit of this technique is about forty-five thousand years.

Cosmic rays also cause reactions in the outer layers of quartz-rich rocks. Be¹⁰ and Al²⁶ accumulate in these layers at small but relatively constant rates. These isotopes are produced slowly, at a rate of about 100 atoms per gram of rock per year, requiring accelerator mass spectrometry (AMS) techniques to detect them. do not penetrate solids by more than a few meters, so the exposure age of a surface can reveal when melted away above that surface.

Nonradiometric Methods

DENDROCHRONOLOGY. Dendrochronology is a method for determining the age of wood by counting and examining annual tree rings. The thickness of a given ring in a tree is determined by environmental factors obtaining during the year in which the ring was formed. Such factors as temperature and rainfall affect the rate of growth and overall health of trees. As a result, patterns of ring thickness in trees that were alive at the same time in the same area tend to resemble one another. Matching patterns of ring thickness between trees of known and unknown age can thus provide evidence that the trees were alive at the same time. The reliability of this method has been extended back to about ten thousand years.

VARVES. Just as trees have annual growth cycles, so do sediments deposited in lakes in regions near glaciers. In the summer, rains bring coarse sediments into the lake. In the winter, fine clays have time to settle out. The banded sediments that result from this seasonal alternation are called varves. Just as with tree rings, patterns of thick and thin layers can be correlated in different varved sequences. Some sequences cover more than thirteen thousand years.

LICHENOMETRY. Lichens grow at fairly constant rates in a given area. In a given area, rocks covered by larger lichens have surfaces that have been exposed longer than have the surfaces of rocks covered by smaller lichens of the same type. By calibrating measurements using tombstones and other objects of know age, the absolute exposure age of lichen-covered surfaces can be estimated.

Context

Understanding climate change requires knowledge of Earth’s climatological history, which in turn requires methods capable of dating events of climatic significance over the last few million years. As technology has improved, the precision and accuracy of these methods has increased dramatically, and the size of the samples required for accurate dating has decreased by orders of magnitude.

Scientists looking at isotope-ratio records in marine sediment cores have sometimes found that different radiometric techniques indicate different dates for the same climatic excursion. As the climatic excursions were found to be global and strongly correlated with known astronomical cycles, it became possible to determine their age with greater accuracy, validating some results over others. This correlation with astronomical cycles could also be used to calibrate methods, just as counting was used to calibrate C¹⁴ dating methods. As methods developed, it became possible to date specific geologic and climatic events, such as the encroachment or retreat of ice from a given area, the speed of uplift of a surface, or the rate of development of a valley.

Key Concepts

• cosmogenic isotope: an isotope—possibly radioactive—produced when a cosmic ray strikes the nucleus of an atom
• decay constant: a measure of how radioactive an isotope is, determined with a Geiger counter
• half-life: the time needed for half of a quantity of a radioactive isotope to decay; it is calculated from the decay constant, not measured directly
• isotopes: variants of an element that are chemically identical but have different atomic mass numbers and vary in radioactivity
• primordial isotope: an isotope that has been present on Earth since the planet formed 4.5 billion years ago
• varve: an annual layer in a sediment, usually the result of seasonal variation in inputs

Bibliography

Cremeens, David L., and John P. Hart, eds. Geoarchaeology of Landscapes in the Glaciated Northeast: Proceedings of a Symposium Held at the New York Natural History Conference 6. Albany: State University of New York: State Education Deptartment, 2003.

Finnell-Gudwien, Grace. "1 Dating Method, 3 Ecosystems, Many Clues to Pace of Climate Change." Medill Reports Chicago, 2022, news.medill.northwestern.edu/chicago/1-dating-method-3-ecosystems-many-clues-to-pace-of-climate-change/. Accessed 21 Dec. 2024.

Ruddiman, William F. Earth’s Climate Past and Future. 2d ed. New York: W. H. Freeman, 2008.

Thurber, David L., et al. “Uranium-Series Ages of Pacific Atoll Coral.” Science 149, no. 3679 (1965): 55-58.