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Dating

Dating. Stratigraphy. As early as 1668 in De solido intra solidum naturaliter… the Danish scholar Niels Stensen (renamed Nicolaus Steno in college) argued that seashells and shark’s teeth found in rocks far from the sea were the remains of ancient animals.

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Dating

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  1. Dating

  2. Stratigraphy • As early as 1668 in De solido intra solidum naturaliter…the Danish scholar Niels Stensen (renamed Nicolaus Steno in college) argued that seashells and shark’s teeth found in rocks far from the sea were the remains of ancient animals. • In the same work he recognized the fact that geological strata represent past depositional events. The vertical order of strata represented a sequence of deposition. This is the “priniciple of superposition.”

  3. Younger Older

  4. Faunal Correlation A fossil with an extensive geographical distribution and relatively short span of existence on earth can be used toestablish the contemporaneity of strata that are spatially remote. A fossil used in this way is called an index fossil.

  5. Faunal Correlation at Boxgrove, Sussex, England

  6. Paleomagnetism • Lava contains large quantities of iron. • As lava cools the iron crystallizes, and becomes magnetized through exposure to the earth’s magnetic fields. • The earth’s magnetic polarity of that time thereby becomes locked into the rock. • As we can date igneous rock by 40K/40Ar, we have become aware that the earth’s magnetic field periodically reverses itself.

  7. Radiocarbon Dating • Radiocarbon is an unstable isotope. An isotope is a version of an atom that varies in mass. C14 Mass number (total number of nucleons) 6 Atomic number (just the number of protons)

  8. A radiocarbon atom has the same number of nucleons as Nitrogen, but the ratio of neutrons to protons is 8/6 = 14. There would be only 6 electrons.

  9. 14CO2

  10. Decay Curve of Radiocarbon 100 75 50 25 0 % of original radioisotope remaining 12.5% Half Lives 1 2 3 4 5 6 7 8 Years B.P. 5730 11,460 17, 190 22,920 28,650 34,360 40,110 45,840 Decay: the reversion of a radioisotope to a stable form. Half-life: the amount of time it takes for one half of a quantity of a radioisotope to decay.

  11. Dr. Rainer Berger in the UCLA Radiocarbon Lab. 1986.

  12. In the early 1960’s, older radiocarbon dates (before 1200 BC) were shown to be inaccurate. The inaccuracy was due to the fact that the production of radiocarbon in the atmosphere varied year in, year out. • Hans Suess (the real Dr. Suess) found a way to correct the problems by measuring the amount of yearly radiocarbon variation in the tree rings of the bristlecone pine growing in California. He then constructed a calibration curve.

  13. The calibration of dates these days is done with a computer program. Raw radiocarbon dates are indicated by a small bc, and calbrated dates by BC in capital letters.

  14. Argon-Argon • Postassium contains a large amount of Potassium 39. • When Potassium is irradiated and fused, it gives off both Argon 39 and Argon 40 gasses. The amount of Argon 40 represents the length of time that has transpired since the mineral potassium had been created.

  15. Carl Swisher of UC Berkeley went to Indonesia to test this technique on fossils that had been excavated between the 1930’s and 1960’s.

  16. A small sample extracted from the pumice matrix inside a child’s cranium, excavated in 1936, at Mojokerto, Java, dated to 1.81 mya.

  17. Dating techniques used at Boxgrove, Sussex, England

  18. Oxygen Isotopes Oxygen occurs in two common isotopic forms: 16O and 18O. They are commonly found in water molecules: H2O.

  19. 16o 18O CaC18O3

  20. 16O • During a cold period water rich in 16O becomes locked into ice sheets. • As a consequence, sea water becomes richer in 18O. • This oxygen isotope shows up in the shells of shellfish, whose shells are composed of calcium carbonate, CaCO3.

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