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.
- Archeologists use the method on items like fired pottery.
- Sedimentologists have used thermoluminsecence as a method for tracking the age of burial and exhumation of sediments. (E.G. migration rates of different sizes of beach and river sands.)
Fission track dating
- They can be counted
- The concentration of 238U in the sample is known.
- Fission tracks are physically revealed by chemical etching, then counted under a microscope.
- The concentration of 238U is determined by placing the specimen in a nuclear reactor along with a calibrated standard material and bombarding it with neutrons, inducing the formation of new fission tracks. The ratio of the concentration of 238U to the concentration of fission tracks provides an estimate of the mineral's age.
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:
- The earth is constantly bombarded by cosmic rays, which create radioactive element during nuclear collisions (e.g., 14C).
- They do this with many elements, creating nuclides like 10Be, 26Al, and 39Cl. These nuclides appear in MANY different mineral types (e.g., olivine, quartz, calcite).
- As long as these minerals are exposed at the surface, they will collect cosmogenic elements until reaching a balance between the acquisition and decay rates.
- Estimates are more precise if multiple elements are cross-compared. This technique is good for calculating uplift and erosion rates.
- Range: 10,000 - 300,000. Saturation of the crystal sets upper limit on recoverable ages.
Amino acid racemization
A method for determining the relative age of biological samples, based on the deterioration or racemization of amino acids.
- Biological amino acids all have the same chirality or "handedness" - they spiral to the left.
- This spiraling, or chirality, requires active maintenance on the part of the organism. When it dies, changes through molecular kinetics cause amino acids randomly to switch to a right-handed configuration.
- In principle, one could measure the chirality of amino acids in a biological sample (mollusk shell, vertebrate bone, wood, etc.) to determine its age.
- Alas, the rate of racemization is very sensitive to:
- characteristics of the enclosing matrix
- Typically, for a given site and geologic interval, amino-acid dates must be calibrated using more reliable indicators like 14C.
- This technique's advantage is that it can be used on a wider range of biologic samples than 14C (wood only) so is good for dating shells and uplift terraces, although the uncertainties are often large.
- Range: 10,000 - 100,000 yrs.
Here we apply the concepts of chemostratigraphy to datable material.
- Ash layers represent a single geologically instantaneous event that can be correlated with datable deposits of crystalline igneous rock, even when the ash, itself, contains no crystals amenable to radiometric dating.
- Ash often spreads globally if the eruption is large enough
- Each ash layer has a unique chemical signature:
- Trace element abundances
- stable isotope ratios, etc.
- Range 0-2MA. Upper limit typically results from diagenetic alteration of the ash. Eg. Bishop Tuff (0.78MA) and Mazama ash (6000 yrs)
Some methods involve simply counting seasonal units that display some kind of identifiable secular variability in thickness or chemical composition.
Varves and ice cores
Here, the seasonal units in question are layers of wood laid down in growing trees.
- Annual variability in tree ring widths has been used to create a global record that extends back to 8000 yrs.
- Also provides info on temperature, runoff, precipitation, and soil moisture.
- Orbital eccentricity: The orbit around the Sun is an ellipse that changes shape (becoming more and less circular) in a cycle of 100,000 years.
- Axial inclination: The axis of rotation is tilted. The angle of tilt varies from 21.5 deg. to 24.5 deg. in a cycle of 41,000.
- Axial precession: The axis of rotation wobbles around an axis like that of a toy top. So, today the axis points toward Polaris, the north star, but in earlier times, it didn't. One full precessional wobble takes 23,000 years.
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.
- Unique insolation character associated with a given period of time in Milankovitch cycles control ice cover, eustatic sea level, and accommodation space.
- Weathering rates, sediment supply, ocean circulation, and sediment accumulation change in response to these cycles.
Chronostratigraphy: The web of correlation
Establishing the time relationships among geologic units by means of integrated methods including:
- Irreversible processes that operate continuously in one direction: biostratigraphy and geochronology
- Cyclic processes (pattern recognition and placement):
- sequence stratigraphy
- seismic stratigraphy
Considerations and caveats:
- Precision - repeatability of measurements. We can assess this more easily than we can accuracy - the degree to which they approach the unattainable ideal of "truth."
- Difficult to obtain a numeric value for some methods, like biostratigraphy. Avoidance of circularity. E.G.:
- The primitive Triassic Ichthyosaur Thaisaurus Mazin et. al 1991 came from poorly constrained sediments of Thailand that were assumed to be of Early Triassic age because of the presence of a primitive ichthyosaur.
- A biostratigrapher 1995 employs Thaisaurus as an index taxon for the Olenekian stage (Early Triassic) based on the above presumption.
- Resolution - ability to discriminate between two closely spaced events in geologic time. Radiometric methods lose resolution with increasing age because of increasing margins of error. Magnetostratigraphic methods don't.
- precise calibration
- ground truthing with multiple methods.