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Geological evidence for evolution and an old earth. (Evolution Symposium Abstracts).

Key Words: plate tectonics, biological evolution, sedimentary environments, fossil record, radioisotope dating

Most people are unaware of the overwhelming confirmation geology gives to biological evolution. The stratigraphic and fossil record offers strong evidence that fossil succession occurred over billions of years as predicted by evolutionary theory. The stratigraphic record consists of a sequence of sedimentary environments, each with distinctive fossil assemblages, arranged in time and space in a manner that only makes sense in light of the great unifying theories of global plate tectonics and biological evolution. The oldest known sedimentary rocks (>3900 Ma), which were discovered on the southwestern coast of Greenland, preserve a biogeochemical record of early life.

Many geological and biological events, previously thought to be unrelated, can now be explained by the theory of Plate Tectonics. Most significantly, the validity of plate tectonic theory has been confirmed by evidence based on most of the subdisciplines of physics, mathematics, and chemistry. This evidence includes (1) the fit of the continents; (2) the correlation of strata, fold belts, and mountain ranges; (3) glaciation; (4) paleontology; (5) paleomagnetism; (6) topography; (7) isotope dating; (8) sediment-core analyses; (9) deep-ocean drilling; (10) seismology; (11) heat-flow analysis; (12) acoustic bottom profiling; (13) seismic reflection profiling; (14) gravity surveys; (15) geochemistry; (16) paleobiogeography; (17) petrology; (18) structural geology; and (19) geomorphology.

Tectonic plate movements have had an enormous effect on the evolution of life, including geographic distribution, extinctions and diversity. Most geologic environments, together with their fossil assemblages, were created,

evolved and terminated as a consequence of plate tectonic movement. Plate tectonic activity influences biological evolution in terms of size and shape of the continents and ocean basins; topography of the land; morphology and depth of the sea floor; features such as mountain ranges-both oceanic and terrestrial; spreading centers; mantle plumes; ocean currents; temperature; nutrients and chemistry of sea water; climate; sedimentary environments; location, size and type of igneous activity such as emplacement of granitic batholiths; and extrusion of large igneous basaltic provinces. When continents or plates rift apart, or a land barrier emerges that formerly was open ocean, the fauna and flora are separated and new biogeographic provinces are formed--each having opportunities to evolve new species in response to the newly isolated gene pool. Such processes promote greater diversification; however, when continents collide, diversity tends to diminish.

Large igneous provinces are composed of enormous sequences of basaltic lava several kilometers thick covering millions of square kilometers of the continents and oceans. These lavas emerge from a mantle plume (hotspot), which may or may not be situated at a spreading center or plate boundary. The mantle plumes have produced up to 26 cubic kilometers of oceanic basalt each year during the last 150 million years. This would represent up to 4 billion cubic kilometers for the last 150 million years and perhaps 5 to 8 billion cubic kilometers since the end of the Precambrian. Beginning about 125 million years ago, the production rate of oceanic crust doubled for a period of about 40 million years and then leveled where it was before the episode. These eruptive episodes have significantly affected the chemistry and circulation of both the oceans and the atmosphere and had an enormous impact on the evolution of life, including regional and global extinctions, origin of new species and biological diversity.

The composite sequence of sedimentary rock on the Earth, supposedly deposited by the Noachian Flood, approaches 100 miles thick. More than a century ago, geologists figured the Earth must be at least 2 billion years old, based on reasonable rates of sedimentation. Although these sediments clearly reflect a great variety of depositional environments, complete with their associated fossil assemblages, all arranged in an orderly evolutionary sequence (faunal and floral succession), they contain no significant material characteristic of a worldwide flood. These sedimentary environments include alluvial fans, meandering river deposits, lake deposits, eolian (desert) deposits, deltaic deposits, lagoon deposits, glacial deposits, peritidal deposits, clastic shelf deposits, continental rise deposits, and pelagic sediments of the deep sea.

Carbonate rocks, which can form chemically or biochemically, constitute about 10 percent of the Earth's sedimentary rocks. Of all the rock types, limestones provide a vast amount of information and documentation of biological evolution. Not only are many limestones composed almost entirely of fossils, but, in reefs for example, the fossils are preserved in growth position complete with their fossil assemblages and environment of deposition. When warm green- house conditions and high sea level occurred in the past (Ordovician -- Devonian and Jurassic and Cretaceous), huge quantities of limestone were deposited-far greater than present amounts. Certain physical conditions important for the formation of limestone include: (1) warm tropical seas, (2) shallow water, (3) agitated water such as breaking waves, (4) light for photosynthesis; and (5) water clean and free of sediment or it will clog the gills and filter-feeding parts of the marine life that build the carbonate accumulation.

Carbonate reefs are completely generated and built up to the wave zone by the organisms that inhabit them. Fossil reef builders include stromatolites, archaeocyathids, stromatoporoids, corals, brachiopods, bryozoans, sponges and many others. Reefs represent the best examples of a paleoecological community preserved in growth position, with community after community built upon one another. In 1992 the Ocean Drilling Project drilled through 1,618 m of shallow-water limestone on Resolution Guyot in the central Pacific Ocean. At a reasonable upward growth rate of less than 1 cm/yr, it would take at least 161,800 years to grow the 1,618 m reef on Resolution Guyot.

Faunal succession is the observed chronologic sequence of life forms in the stratigraphic record through geologic time. During the early 1800s faunal succession was first conceived and used by men who were creationists and believed that all strata along with associated fossils were deposited during the Noachian Flood year. They found that each successive layer of sedimentary rock had a distinctive and characteristic fossil assemblage. It is important to be aware that, at the time, faunal succession had no evolutionary implications because this work predated Darwin's book On Origin of Species by several decades. We now know that evolution is the mechanism that causes successive changes in species so that we find sequential and nonrepeating appearances of fossils through geologic time.

During the late 1980s and 1990s, outstanding examples of macroevolution were discovered. These examples detailed the transition from fish to tetrapod, the transition of land mammal to whale, and the transition from therapod dinosaur to birds. Jennifer Clack discovered Acanthostega, a perfect transitional fossil of an early tetrapod, in Upper Devonian rocks of East Greenland. This discovery confirmed that tetrapod limbs evolved for life in the water, but happened to have the necessary foundation for evolving on land. One of the most remarkable mammalian evolutionary stories is the transition of large predatory land-dwelling Mesonychids into completely aquatic whales and dolphins. Pakicetus, an excellent transitional fossil, was discovered by Philip Gingerich in Eocene rocks of Pakistan in 1978. Ambulocetus, the "walking swimming whale" is perhaps one of the best examples of a transitional fossil ever found. It was found in the lower Eocene river sediments in Pakistan by Thewissen in 1994. The mainstream conse nsus among paleontologists is that birds evolved from small maniraptoran theropod dinosaurs. After intense study for many decades, the Archaeopteryx, discovered in the Solnhofen district of Bavaria in 1861, has no features that would preclude it from being an ancestor to birds. Many feathered dinosaurs have been discovered during the 1990s in China including Confuciusornis, a primitive bird, complete with well-preserved feathers and beak; this was the earliest beak as Archeoptetyx had a reptilian snout.

There are many methods to date rocks by radiometric or isotope methods. Common examples are potassium-argon, argon/argon, rubidium-strontium, uranium-lead, thorium-lead, radiocarbon, fission-track, thermoluminescence, and cosmogenic isotope methods. Since 1907, numerous experiments have shown that radioactive decay rates do not change through chemical or physical processes. The consistency of dating results worldwide, especially the use of different decay schemes or the use of more than one decay constant on the same rock, confirms the accuracy and reliability of age-dating technique. Multiple crosschecking methods, such as the isochron and the concordia-discordia methods, allow geologists to place a high degree of confidence in radiometric measurements. The isochron method is self-checking because it reveals whether or not the sample has performed as a closed system. The uranium-lead concordiadiscordia method is one of the most reliable dating methods available, and can be used on closed or open systems. It is particularly useful for dating old rocks with complex histories involving multiple metamorphic events and is self-checking as is the isochron method.

In addition to the self-checking isochron and concordia-discordia methods, multiple ways are generally used to cross check the reliability of age measurements: (1) repeat the analytical measurements in order to minimize analytical errors; (2) make age measurements on several samples of minerals or rocks from the same rock unit; this technique documents geological disturbances that occurred after the rock formed because different mineral species usually respond differently to heating and chemical changes; (3) the use of different decay methods on the same rock is an excellent way to check the accuracy of age results; (4) the validity of dates is further corroborated where more than one independent investigator replicates the results and more than one laboratory is used; and (5) geologic relationships also constitute an important way to evaluate radiometric data; for example, a series of age measurements on rock bodies whose relative ages are known because they are stacked one on top of another, or are arrange d in a linear sequence, should fall in the same sequence.
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Author:Maley, Terry S.
Publication:Journal of the Idaho Academy of Science
Date:Jun 1, 2001
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