Chronostratigraphy II:

But first: Outcrop du jour:

Several other methods operate along the same general principles as radiometric dating.

Thermoluminescence:

Suppose we want to the age of a sample since it was last heated.

  • The elements uranium and thorium in minerals, like zircon, quartz, and fluorite (right) decay to produce alpha particles. Their energy can elevate electrons into states where they can move about the lattice, but can get trapped in imperfections of the crystal lattice that form local dips in electrical potential. If this goes on long enough, ultimately the crystal can become saturated. Since this is a background process the accumulation of alpha particles can be used to constrain the age of ambient minerals. Heat or exposure to bright light will release the trapped particles' energy as light, producing luminescence, which can be quantified. If the minerals are pristine, one can expose them to heat and measure their luminescence to get an age. Because heating/illumination releases the trapped energy, it effectively "resets" a sample's thermoluminescence clock, making thermoluminecence a useful method for determining when a crystal was last heated or illuminated.
  • Range: 5,000 - 300,000 yrs. Saturation of the crystal sets upper limit on recoverable ages.
  • Measurements must be calibrated for different materials.

    Fission track dating

  • α particles resulting from the decay of 238U make tracks (holes) in crystals as they escape (10-20μm long). These tracks can be thought of as if they were the "daughter products" of radioactive decay and can be used for dating provided: So: Fission tracks close when the crystal is heated to modest temperatures (74-200ºC depending on the mineral). Thus, fission track dating provides the sample's age since its last episode of heating, when the crystal experienced closure - the immobilization of its crystal lattice.
  • Provides estimates of heating events in the crystal's history, including burial and tectonic uplift. The precise information is a function of the mineral involved. Apatite, whose fission tracks close at temperatures as low as 74º C gives the age of last cooling. Zircon, whose tracks close at higher temperatures, records something closer to the crystal's actual age.
  • Commonly applied to zircon and apatite.
  • Range 100,000yrs-2MA

    Cosmogenic nuclide dating

    This method allows us to infer the length of time for which a specimen has been sitting on an exposed surface. Consider:


    Amino acid racemization

    A method for determining the relative age of biological samples, based on the deterioration or racemization of amino acids.


    Tephrochronology

    Here we apply the concepts of chemostratigraphy to datable material.


    Some methods involve simply counting seasonal units that display some kind of identifiable secular variability in thickness or chemical composition.

    Varves and ice cores

  • Lake sediments that record seasonal variations enabling years to be counted. Within a basin, patterns of variation between seasons can be correlated.
  • While physically different, ice cores can be employed similarly. The deep cores from Greenland and Antarctic ice date back 180,000 yrs.

    Dendrochronology

    Here, the seasonal units in question are layers of wood laid down in growing trees.


    Astrochronology

  • Milankovitch cycles: In the 1920s, the Yugoslavian meteorologist Milutin Milankovitch realized The Earth's movement through space is subject to three kinds of cycles:

    Solar forcing: The sum of the effects of these cycles gives the general tendency for glaciers to form. Note: Solar forcings are different at different latitudes and in different hemispheres.

  • Solar forcings can stimulate positive feedback processes that result in global climate changes, tipping climate systems into glacial and interglacial modes. These cycles can be seen in records of ice and sediments.
  • Milankovitch cycles can be tracked in carbonates, deep sea, and lake sedimentary packages.
  • Range as far back as 10 ma.

    Chronostratigraphy: The web of correlation

    Establishing the time relationships among geologic units by means of integrated methods including:

    Considerations and caveats:

    Results can vary within a method (eg. Radiometric dating), requiring: