Stable isotopes ancient temperatures and dead oceans

In 1947, Harold C. Urey, a geochemist at the University of Chicago, made an astounding and far-reaching prediction: that stable isotopes could be used to determine ancient temperatures. As it turns out, he had come upon one of the most important tools available to earth historians. Stable isotopes have proven invaluable in reconstructions of ancient oceans, climates, ecosystems, physiologies, and a host of other subjects. Like other scientific techniques, the principle is relatively simple; however; in practice, the methods are complex and require sophisticated techniques and equipment

Like unstable isotopes, stable isotopes occur naturally. In the case of oxygen, while 99.763% of all oxygen is l60,0.191 % of all oxygen is the stable isotope l8O.The rest is l70, another rare stable isotope of oxygen. In the case of carbon, 98.89% of all carbon is the stable isotope l2C; I.I I % of all carbon are the other isotopes of carbon: l3C and l4C.

The critical point is that the minor weight differences in the isotopes cause slight differences in their behavior during chemical reactions, or during natural physical processes such as evaporation, precipitation, or dissolution (the process of being dissolved in a solution). Such differences in behavior are called fractionation, and control how the isotopes separate from each other as a reaction or physical process takes place.

Urey was aware of the fractionation of stable isotopes and observed that, in the case of oxygen, fractionation varied with temperature. For example, in a situation in which calcium carbonate (CaC03) was precipitated from seawater; if all other variables (e.g., concentration of salts in the solution) were held constant and only the temperature at which precipitation took place was varied, the ratio of 80:l60 would vary predictably.Thus, by studying that ratio, an estimate of the temperature of precipitation could be obtained.The miniscule amounts of stable isotope could be weighed on an instrument called a mass spectrometer; and the ancient temperature could be calculated.

In the past 30 years, a great deal of experimental research with stable isotopes and mass spectrometers has enabled scientists to predict temperatures of precipitation from ,80:l60 ratios.The technique is of extreme interest because, for example, stable oxygen isotopes in a CaC03 shell secreted by a clam that lived on the sea-floor millions of years ago can theoretically be used to provide a clue to temperatures of the water in which the clam was living. Indeed, because clams grow shells throughout the warm seasons, it is possible in certain instances to deduce seasonal temperature fluctuations that occurred millions of years ago: the fossilized shell of the clam records through its stable isotope composition the ancient seasonal temperature fluctuations that it experienced during its life.The method is potentially applicable to any organism that secretes a CaC03 shell, as well as to vertebrates, which have stable isotopic oxygen incorporated into their bones in the form of phosphate, For example, if indeed dinosaurs were "warm-blooded," their stable isotopic oxygen ratios should show this. Not surprisingly, this subject is revisited in greater detail in Chapter 15, in which dinosaur "warm-bloodedness" is discussed.

In the intervening years since the original stable oxygen isotope fractionation-temperature relationship was uncovered, stable isotopes have been put to a variety of uses. Fluctuations in l3C:l2C have been used to record intervals of increased atmospheric C02. Also, they have been used to record productivity - the amount of biological activity in an ecosystem - by serving as an indicator of the amount of organic carbon moving through an ecosystem.The flux - or cycling - of organic carbon through an ecosystem is a measure of its activity.Thus it was by studying the l3C:l2C ratios from ocean sediments deposited at the very end of the Cretaceous that oceanographers discovered that the oceans went virtually dead at that time: isotopic carbon recorded an astounding plunge in the flux of organic carbon, which was interpreted as a severe drop in the total productivity of the world's oceans. This apocalyptic event, the infamous "Strangelove Ocean," is covered in greater detail in Chapter 18, when the extinction of the dinosaurs is examined.

is some evidence for Late Jurassic aridity in the form of various evapor-ites deposits. Likewise, Upper Jurassic terrestrial oxidized sediments and caliche deposits in North America suggest that there, to be sure, the Late Jurassic was was marked by at least seasonally arid conditions.

All indications, however, are that the Jurassic was a time of warm equable climates, with higher average global temperatures and less seasonality than we currently experience. It appears that this was in large part a function of high eustatic sea levels and vast flooded areas on the continents.

Cretaceous

Paleoclimates in the Cretaceous are somewhat better understood than those of the preceding periods. During the first half of the Cretaceous at least, global temperatures remained warm and equable. The poles continued to be free from ice, and the first half of the Cretaceous saw far less seasonality than we see today. This means that, although equatorial temperatures were approximately equivalent to modern ones, the temperatures at the poles were somewhat warmer. Temperatures at the Cretaceous poles have been estimated at 0-15 °C, which means that the temperature differerence between the poles and the equator was only between 17 and 26 °C, considerably less than the approximately 41 °C of the modern earth.

More than one culprit bears the responsibility for this climate. The Cretaceous was a time of increased global tectonic activity and associated high volcanic activity. An increase in tectonic activity is associated with increased rates of oceanic spreading, which in turn involves elevated spreading ridges. Raised spreading ridges would have displaced more oceanic water onto the continents and, indeed, there is good evidence for extensive epeiric seas during the Cretaceous. That there was an increase in the atmosphere of carbon dioxide (C02) gas during Cretaceous times has been established using 13C. This has been attributed an increase in volcanism related to increased tectonic activity. It turns out that the amount of C02 in the atmosphere can be correlated with the amount of 13C isotope present in organic material preserved from the Cretaceous. Increased amounts of C02 in the Cretaceous atmosphere meant that the Cretaceous atmosphere tended to absorb more heat (long-wavelength radiation from earth), warming climates globally. These of course are similar to the now-notorious "greenhouse" conditions5 with which the modern earth is threatened.

So the first half of the Cretaceous was synergistic: tectonism caused increased atmospheric C02 and decreased the volume of the ocean basins, which in turn increased the area of epeiric seas. The seas thus stabilized climates already warmed by enhanced absorption of heat in the

5 The increase in C02 in the modern atmosphere (and consequent global warming) is attributable to anthropogenic (human-originated) combustion of all types, especially automobile exhausts, and not volcanism. Since Earth has already undergone an experimental flirtation with greenhouse conditions, the Cretaceous provides insights into how our modern world will respond to such conditions.

atmosphere. A 2.3% increase over today's level of mean global absorbed radiation has been hypothesized. This means that the Cretaceous earth, because of its "greenhouse" atmosphere and abundance of water, retained 2.3% more heat than does the modern earth. And, because of its extensive water masses, heat was not so quickly released during cold seasons; indeed, the seasons themselves were modified. Tropical and subtropical climates have been reconstructed for latitudes as high as 70° Sand45° N.

The last 30 million years of the Cretaceous produced a mild deterioration of these equable conditions of the mid-Cretaceous. A pronounced withdrawal of the seas took place, and evidence exists of more pronounced seasonality. Stable isotopes again provide important evidence of greater fluctuations in temperatures; however, this time they are

Figure 2.11. The carbonate shell of a modern planktonic (free-swimming) foraminifen Globorotalia menardii.The long dimension is 0.750 mm. (Photograph courtesy of S. L. D'Hondt.)

aided by information from an unexpected source: leaf margin and vein patterns. In the modern world, such patterns can be closely correlated with temperature and moisture. Once this indicator was "calibrated" in the present - that is, once the patterns are correlated with modern temperature and moisture levels - leaf margin and venation patterns could be used to infer previous temperatures and amounts of moisture.

Another important indicator of temperature are single-celled, shell-bearing organisms called foraminifera that live in the oceans (Figure 2.11). The shapes of the shells of foraminifera can be correlated with a relatively narrow range of temperatures, and thus ancient representatives of the group can provide an indication of water temperatures in the past. However foraminifera serve double duty; because their shells and made of calcium carbonate (which contains both carbon and oxygen), the shells are suitable for stable carbon and stable oxygen isotopic analyses.6

The Cretaceous was surely a world much different from our own. In its first half, warm, equable climates dominated the period. The second half, however, was marked by well-documented climatic changes, in which seasonality was increased and the equator-to-pole temperature gradient became more like that which we presently experience.

Important readings Arthur, M. A., Anderson, T. F„ Kaplan, I. R„ Veizer, J. and Land, L. S. 1983.

Stable Isotopes in Sedimentary Geology. SEPM Short Course no. 10,432pp. Barron, E. J. 1983. A warm, equable Cretaceous: the nature of the problem. Earth Science Reviews, 19, 305-338. Barron, E. J. 1987. Cretaceous plate tectonic reconstructions. Palaeo-

geography, Palaeoclimatology, Palaeoecology, 59, 3-29. Berry, W. B. N. 1987. Growth of a Prehistoric Time Scale Based on Organic

Evolution. Blackwell Scientific Publications, Boston, MA, 202pp. Crowley, T.J. and North, G. R. 1992. Paleoclimatology. Oxford Monographs in Geology and Geophysics no. 18. Oxford University Press, New York, 339pp.

Dott, R. H„ Jr and Batten, R. L. 1988. Evolution of the Earth. McGraw-Hill

Book Company, New York, 120pp. Faure, G. 1991. Principles and Applications of Inorganic Geochemistry.

Macmillan Publishing Company, New York, 626pp. Frakes, L. A. 1979. Climates Through Geologic Time. Elsevier Scientific

Publishing Company, New York, 310pp. Frazier, W. J. and Schwimmer, D. R. 1987. Regional Stratigraphy of North

America. Plenum Press, New York, 719pp. Lillegraven, J. A., Kraus, M. J. and Bown, T. M. 1979. Paleogeography of the world of the Mesozoic. In Lillegraven, J. A., Kielan-Jaworoska, Z. and Clemens, W. A., Jr (eds.), Mesozoic Mammals, the First Two-Thirds of Mammalian History. University of California Press, Berkeley, CA, pp. 277-308.

6 Free-swimming (planktonic) foraminifera first make an appearance in Cretaceous oceans. Because they were (and are) geographically widespread but have relatively short species' durations, they are also superb biostratigraphic indicators for late Mesozoic and Cenozoic marine sediments.

Lütgens, F. K. and Tarbuck, E. J. 1989. The Atmosphere: an Introduction to

Meteorology. Prentice Hall, Englewood Cliffs, NJ, 491pp. Robinson, P. L. 1973. Palaeoclimatology and continental drift. In Tarling, D. H. and Runcorn, S. K. (eds.), Implications of Continental Drift to the Earth Sciences, vol. I. NATO Advanced Study Institute, Academic Press, New York, pp. 449-474. Ross, M. I. 1992. Paleogeographic Information System/Mac Version 1.3. Paleomap Project Progress Report no. 9. University of Texas at Arlington, 32pp.

Walker, R. G. and James, N. P. 1992. Fades Models, Response to Sea Level

Change. Geological Association of Canada, St Johns, NL, 409pp. Wilson, J. T. (ed.) 1970. Continents Adrift: Readings from Scientific American. W. H. Freeman Company, San Francisco, 172pp.

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