GRE Reading Comprehension

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The determination of the age of KNM-ER 1470. a humanoid skull, would add greatly to our knowledge of mammalian evolution. Anthropologists originally dated the habilis skull at 3 million years old. This age seemed unlikely because it was older than the age of any known aus- tralophithecines. which are presumed to be the habilis's ancestor. Further attempts to date the skull have led to speculative results.

An elemental property of all living things is that they contain a certain portion of their carbon as the radioactive isotope carbon-14. Carbon-14 is created when solar radiation blasts nuclei in the upper atmosphere, in turn producing neutrons that bombard nitrogen-14 at lower altitudes, turning it into carbon-14. All living things maintain an equilibrium of carbon-14 as they exchange carbon with their surrounding atmosphere. Presuming the rate of production to be constant, the activity of a sample can be compared to the equilibrium activity of living matter, and thus the age can be calculated. However, carbon-14 decays at a half-life of 5730 years, limiting age determinations to the order of 50.000 years. This time frame can be extended to perhaps 100,000 years using accelerator tech-niques. Even so. at these ages carbon dating is increasingly unreliable as a result of changes in the carbon-isotope mix. Over the last century, the burning of fossil fuels, which have no carbon-14 content, have had a diluting effect on the atmospheric carbon-14. As a countervailing effect, atmospheric testing of nuclear weapons in the 1950s may well have doubled the atmosphere's carbon-14 content.

Other radiometric dating methods, using relative concentrations of parent-daughter products in radio decay changes of other elements, such as argon, may prove to be of greater benefit for dating such ancient samples as habilis. However, the assumption that the decay rates of these isotopes have always been constant would first have to be substantiated.

Question List: 1 2 3 4

Consider each of the choices separately and select all that apply.

The author suggests that the burning of fossil fuels has had which of the following effects on the efficacy of carbon dating techniques?

  • A It may increase the carbon-isotope mix of the object being dated.
  • B It may make items subjected to carbon dating appear to have died later than is the case.
  • C It may tilt the fragile equilibrium activity of living matter.

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