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Geologic Investigations



Scenario for a Warmer World

By Richard Z. Poore

Human activities have resulted in significant increases of carbon dioxide and other radiatively active (greenhouse) trace gases in the Earth's atmosphere. Scientists agree that this increase in greenhouse gases will result in a global warming over the next few decades to centuries. Scientists are uncertain, however, about the magnitude of the global warming and how the effects of the warming, including changes in temperature, precipitation, winds, and ocean currents, will be distributed over the Earth's surface.

Determining the regional effects and impacts of future global warming represents major challenges to the scientific community. Anticipating the effects of climatic change is more than an intriguing scientific puzzle. Changing climate will affect the Nation's lands and waters and will cause, for example, substantial changes in agriculture and energy use. Thus, better understanding of future climatic change is important for policy makers at all governmental levels.

Studies of past climates, called paleoclimates, are one important approach that scientists can use to estimate how the Earth's environment will change with global warming. The Earth's climate is dynamic and has varied naturally through geologic time from conditions that were both significantly warmer and colder than modern conditions. Information on past climates and how climate changed in the past can provide valuable clues about how climatic change will occur in the near future and what the effect of that change will be on the Earth's environment and on life on Earth. The climate change program of the U.S. Geological Survey has started a major study of the Pliocene (a period between about 5.2 and 1.6 million years ago) as a "scenario for a time of greater warmth." The Pliocene Epoch was selected because it is the most recent interval for which we have evidence for climates that were substantially warmer than modern or near-modern climates. The study of past intervals during which climates were substantially warmer than modern climates is useful in predicting the effects of global warming. In fact, recent studies of air bubbles trapped in ice cores from Antarctica document that atmospheric carbon dioxide has varied in the geologic past and that those variations correlate with major climate change. Thus, although the exact reasons for past climatic warmings and future global warming are not likely to be identical, past warm intervals may have been caused, in part, by a carbon dioxide greenhouse effect. Even without knowing the exact causes or forcing functions, the study of past warm intervals provides scenarios for greater warmth. These scenarios then can be used to test the ability of computer-generated general circulation models, to simulate the effects of global warming, and to provide an empirical data base to determine whether regional responses to warming are similar for all warm intervals regardless of the magnitude or cause of warming.

Reconstruction of past climatic conditions requires two basic types of data: environmental conditions and geologic age. Past environmental conditions are determined primarily through paleontologic and isotopic studies, which are methods for reconstructing past climates.

Most plants and animals are sensitive to factors such as temperature and available moisture. Similarly, isotopic compositions of fossil shells and some mineral deposits are, in part, controlled by temperature and salinity conditions in which the shells or deposits were formed. Thus, mapping past distribution of fossil remains of plants and animals and analyzing isotopic composition of fossil shells and mineral deposits can be used to delineate past climatic conditions. Other studies, including geochemistry of sediments and sedimentary structures, are also useful for delineating past climatic conditions. In addition to information on past environmental conditions, one needs to know the geologic age that the fossils or deposits represent in order to construct a history of climate change or to reconstruct a discrete time interval. A wide variety of techniques, including biochronology, isotope stratigraphy, radiometric dating, paleomagnetism, and tephrochronology, are used to date materials that are used in climatic studies.

The major objective of the USGS effort is to construct a map summarizing climatic conditions during one or more warm intervals within the Pliocene. An example of this type of mapping, as shown in figure 1, contrasts distribution of assemblages of ostracodes in modern and Pliocene age marine sediments along the continental margin of the Eastern

United States. Ostracodes are Crustacea that are very sensitive to environmental conditions such as temperature. The figure shows that, during the Pliocene, ostracode assemblages representing tropical, subtropical, and mild temperate climatic zones shifted northward along the eastern seaboard reflecting the increased temperature of waters along the coast. Sea level was also higher during the Pliocene because the total amount of water stored in continental glaciers was reduced, reflecting higher mean global temperature. The exact amount of sealevel rise cannot, however, be directly determined from figure 1 because the position of the Pliocene shoreline has been modified by local movements of the Earth's surface since Pliocene time.

In addition to including intervals when global climate was warm, the Pliocene is attractive for study because Pliocene flora and fauna are very similar to

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modern flora and fauna; Pliocene records are widespread and easily accessible in both continental and marine settings, and most Pliocene records are relatively well preserved. All these features make it easier to quantify environmental information and to develop regional and even global patterns of past climatic data. Estimated winter sea-surface temperature (SST) in the North Atlantic Ocean during part of the Pliocene, as shown in figure 2, is an example of how paleontological studies can help to quantify conditions of past climates. The temperature estimates are based on analysis of the relative abundances of microfossils found in sediments from a deep-sea core. Knowledge of the modern distribution of the same or similar microfossils is used to translate variations in the abundances of microfossil taxa in the core into estimates

of Pliocene surface-water temperatures. Figure 2 shows that sea-surface temperatures were both warmer and cooler than modern conditions during the interval between 4.6 and 2.3 million years ago. In some cases the SST's were more than 3 °C above modern. Several intervals of higher SST identified on figure 2, such as the interval centered around 3.0 million years ago, are being evaluated as candidates for a global climate reconstruction representing a scenario for greater warmth. Such glimpses into the past may well give scientists an excellent view into the future of global warming—its extent and its effects—on the Earth's environment.

The USGS study of Pliocene climates is an international effort. The study includes formal cooperation with Soviet scientists as part of a U.S.-U.S.S.R. Bilateral Agreement on Environmental Protection. In addition climatic researchers from a variety of institutes in the United States, Japan, the United Kingdom, and Canada are collaborating with USGS scientists on one or more parts of the Pliocene study.

Dating Rocks and Minerals Using Lasers and Nuclear Reactors

By G. Brent Dalrymple

During the past decade, a variety of microanalytical techniques have been developed and applied to a broad spectrum of geological problems. None has proved more dramatic and revolutionary, however, than the application of a combination of lasers and nuclear reactors to the more precise measurement of geologic time by using the decay of naturally occurring potassium (40K), which has a half life of 1.25 billion years, to the inert gas argon (40Ar).

The use of nuclear reactors for measuring geologic time is not new. It was introduced in the mid-1960's as a variation of conventional potassiumargon (K-Ar) dating and is known as the 40Ar-39Ar method. Potassium is found in most rock-forming minerals. The half-life of its radioactive isotope, 40K, is such that measurable quantities of argon, the stable "daughter" product, have accumulated in potassium-bearing minerals of nearly all ages. Because of this prevalence across geologic time, the amounts of potassium and argon isotopes can be measured accurately.

In the conventional method, adopted in the 1950's, the potassium and argon are measured quantitatively in separate procedures. The argon is measured bv isotope dilution by using an inert-gas mass spectrometer. The potassium is measured by one of several standard chemical techniques, usually flame photometry. These separately acquired K and Ar data are then mathematically expressed in an equation that relates radioactive decay to geologic time. This method of dating has broader applicability than any other dating method. It has been and continues to be used to date samples of igneous, metamorphic, and even some sedimentary rocks—as old as 4.5 billion years (the age of Earth) and as young as a few thousand years—but the methods are time consuming and typically require a sample of a gram or more in weight.

The 40Ar-39Ar method has

some significantly advantageous features....

In the newer 40Ar-39Ar method of K-Ar dating, the sample is irradiated with fast neutrons in the core of a nuclear research reactor, such as the Training Research Isotope General Atomic reactor (TRIGA) used by the USGS in Denver, Colo. The neutrons convert a fraction of an isotope of potassium (S9K) to an isotope of argon ( yAr). After irradiation, the argon is extracted by melting the sample with induction heating or a resistance furnace in an ultra-high vacuum and analyzed, as in the conventional technique, by using a mass spectrometer. Because the ratio of WK to 40K is constant in all natural materials, the 39Ar made from 39K can be used as a "proxy" for the 40K, and the

age can be calculated directly from the

Ar/ Ar ratio; a separate potassium measurement is unnecessary.

The 40Ar-39Ar method has some significantly advantageous features over the conventional method of K-Ar dating. First, it is more precise because the 40Ar/39Ar ratio can be measured more precisely than the conventional 40Ar/40K ratio, which involves the quantitative measurement of isotopes of different elements. Second, smaller samples are required, usually a few tens to a few hundreds of milligrams. Third, the newer method is considerably faster and requires only an hour or two once the samples have been irradiated in the reactor. Finally, the argon can be released from the sample in increments at successive temperatures, in what is called an incremental heating experiment. This results in a series of ages known as an age spectrum, which contains useful information about the original age and thermal history of the sample. The incremental heating method is especially useful for samples that have been thermally or chemically disturbed since they first formed because it reveals that the sample has been disturbed and often allows recovery of the crystallization age despite the disturbance.

The joining of a continuous, highpowered laser with a very sensitive mass spectrometer has resulted in substantial and important improvements in the Ar39Ar dating method. The USGS is the third group in the world to develop a continuous laser system (fig. 3). The first such system was developed by the University of Toronto; the second system was developed by Princeton University.

The continuous laser system has several advantages compared to other methods of 40Ar- 9Ar dating. The most significant advantage is the ability to determine the age of a single crystal—as small as 0.000001 grams for older material. This not only saves considerable effort in mineral separation but also permits dating of rare materials and of rock units that may contain contaminating crystals from older units. A second advantage is speed. Because the samples are small, the heating and cleanup times are short, and a complete age analysis typically takes only about 15 to 20 minutes. A third advantage is that the analyzing system

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