Geochronology and Climate Proxies

In order to determine when significant past climatic events occurred and changes shaped the Earth, climatologists must have a way to determine the geologic time period during which these events took place. They must be able to build an accurate climate time line. This chapter focuses on the various methods scientists use today to determine these specific intervals. It will look at both radiometric and nonradiogenic dating methods. It will also explore the importance of climate resolution and the dating techniques used as well as the concept of climate proxies, what they are, how they are used, and what they can reveal about past climate.

radiometric dating techniques

When talking about dating objects and determining when a particular event took place or a specific climate existed on Earth, it is important to understand the difference between relative age and absolute age, because both are often referred to when discussing past climate.

Relative dating is simply determining if something is older or younger than something else. It does not provide an exact numerical age, only a comparable ranking. This follows what geologists refer to as the law of superposition, which simply means that when rock formations are formed, the oldest layers are on the bottom and the younger layers are above. Each layer of rock is younger than the layer it is sitting upon. The exception to this rule is if an area of existing normal-layered rock has been subjected to severe folding and faulting and its layers have been upended and overturned relative to each other.

When examining sedimentary layers, the relative age refers to the age of the deposition, not the actual date of the material in the formation, because they are not the same thing. Unlike igneous rock, which cools after formation and can be dated radiometrically, sedimentary rocks are composed of the weathered sediment of other parent material (other rocks). Therefore, the actual sediments may be much older than the sedimentary formation itself. These layers are given relative ages when they can be associated in the formation with other datable rocks, such as igneous rocks. For instance, if a layer of sedimentary rock is sandwiched between an igneous layer 50,000 years old and another 52,000 years old, then the sedimentary layer must be between 50,000 and 52,000 years old.

Absolute dating is the dating technique that allows ages to be assigned to samples in terms of the number of years before the present, based on a specific timescale. Absolute dating can be achieved in a number of ways. For example, it can be calculated based on natural annual cycles of trees, lakes, and glaciers; radioactive sequences; historical records; and trapped electrons.

Many types of physical evidence exist that can give climatologists clues as to what climate was like in the past. It can be seen in rock formations. If a layer of sedimentary rock exists in a presently arid landscape, it provides a clue that at one time there was an ocean or lake at the location. If a geologist can determine how old a rock formation is, then climate scientists will have a better understanding of what conditions were like at that site during that time. Likewise, if fossils from coral are found at polar latitudes, it tells scientists that at one time that location had been much warmer. If the plant remains of palm trees are found at a far northern latitude, it would indicate a warmer polar climate at one time. If the palm can be dated in a laboratory, then a date can be assigned to that warm period, increasing scientists' knowledge of the climate sequence.

One commonly used dating technique is radiometric dating, also called radioactive dating. In nature, rocks are made of many individual crystals, and each crystal is made up of different chemical elements, such as iron, magnesium, calcium, and sodium. All the Earth's known elements are listed in the periodic table (see p. 167). In nature, most of these elements are stable, meaning that they do not change over time. There are a few elements, however, that are not stable—they do change. In these particular elements, some of the atoms eventually transform from one element into a completely different element through a process called radioactive decay. The original element is referred to as the parent element. The element that the parent changes, or decays, into is called the daughter element. The aspect of this radioactive decay process that makes it such a useful tool is that the decay occurs at a predictable, known rate.

Radioactive decay works something like a clock or an hourglass. As soon as the clock begins ticking, the particular item can be analyzed in a lab and its age calculated. The decay occurs at a known rate, called the decay constant. This is the measurement of how much parent element decays to daughter element over a specific time period. It is this specific rate of decay that works like a clock. It is a special clock, however. Its time line is measured by a unit called a half-life.

One half-life is the amount of time it takes for half of the parent element to decay to the daughter element. This means that the first halflife reduces the parent element to half its original amount. The second half-life reduces the remaining parent element to half of that half (or one quarter of the original amount). The next half-life has half of that remaining amount decay over the same time interval (or one-eighth of the original amount), and so forth. The half-life time interval remains constant, but each time only half of the remaining parent material decays. By the tenth half-life interval, less than one-thousandth of the

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