Basic Earth Science and Society - Basic Research Opportunities in Earth Science - page 17

in the solar system. 13 Sequences of sedimentary rocks record events on time
scales ranging from the subannual to billions of years. Metamorphic rocks found
in ancient continental regions yield radiometric ages of up to 4 billion years, and
they document processes active when the Earth was still fresh from a Hadean
period of planetary formation and bombardment. Meteorites extend this record
back to the earliest events in the condensation of the solar nebula, 4.56 billion
years ago, and minute samples of cosmic dust have been identified that push this
chronicle even further back to the actual manufacturing of the chemical elements
in earlier generations of stars.
The methods available for reconstructing the history of the Earth and its
parent nebula have been greatly extended and improved. Entirely new techniques
are now available from previously inaccessible isotopic systems; examples
include the use of tungsten isotopes to constrain the timing of the segregation of
the Earth's metal core and osmium isotopes to date refractory mantle samples.
Augmented capabilities have also come from substantial improvements to old
techniques, such as recalibration of the carbon-14 method, and the use of
accelerator mass spectrometry to extend its temporal resolution and reduce the
requisite sample size.
State-of-the art analytical techniques promise to define much more precisely
the timing, duration, and lateral extent of “extreme events,” which include major
magmatic eruptions, large bolide impacts, unusual excursions in global climate,
and collapses and reversals of the Earth's magnetic field. During these rare
occurrences, conditions at the Earth's surface have greatly exceeded their usual
range, and they have therefore exerted a disproportionate influence on the
evolution of the planet and its biosphere. Absolute dates on the ages of individual
units within geological formations can now be obtained from the uranium-lead
and potassium-argon systems with sufficient precision (fractions of a percent) to
estimate the duration of the Cambrian “explosion” (the sudden first appearance of
macroscopic, skeleton-bearing life), the great mass extinction at the Permo-
Triassic boundary, and the huge outpourings of magma (millions of cubic
kilometers) in the form of flood basalts that have occurred at irregular intervals
throughout Earth history.
Extreme events of short duration can be difficult to decipher because only a
small fraction of history is preserved in the geological record. What happened
must be inferred from fragmentary evidence, such as the sequence of sedimentary
deposits (floods, mudflows, major storms), juxtapositioning
13 Almost all of the techniques for deciphering and interpreting extraterrestrial samples
are common to the investigation of geological materials, and most NSF support for these
activities falls under the Earth Science Division. For the purposes of this report, the
disciplinary domains of planetary science and cosmochemistry are considered an integral
part of Earth science.