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 sharks 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.

Younger Older

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

Faunal Correlation at Boxgrove, Sussex, England

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

Radiocarbon Dating Radiocarbon is an unstable isotope. An isotope is a version of an atom that varies in mass. C 14 6 Mass number (total number of nucleons) Atomic number (just the number of protons)

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.

14 CO 2

Decay Curve of Radiocarbon % of original radioisotope remaining Half Lives Years B.P ,460 17, ,920 28,650 34,360 40,110 45, % 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.

Dr. Rainer Berger in the UCLA Radiocarbon Lab

In the early 1960s, 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.

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.

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.

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

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

Dating techniques used at Boxgrove, Sussex, England

Oxygen Isotopes Oxygen occurs in two common isotopic forms: 16 O and 18 O. They are commonly found in water molecules: H 2 O.

16 o 18 O CaC 18 O 3

During a cold period water rich in 16 O becomes locked into ice sheets. As a consequence, sea water becomes richer in 18 O. This oxygen isotope shows up in the shells of shellfish, whose shells are composed of calcium carbonate, CaCO 3.CaCO 16 O