Geochronometry

Geochronometry is the measurement of geological time to produce a numerical time-scale (not `absolute', as there is always a margin of error).  It applies geochronological methods, especially radiometric dating.  The geochronological scale is a periodic scale using the year as a basic unit.  Apparent ages obtained in geochronometry are referred to as radiometric or isotope dates.  For older rocks, multiple annual units are normally written in thousands of years (ka) or million years (ma); Holocene and Pleistocene dates are normally quoted in years before 1950 (years BP (before present)) or more recently have been quoted as b2k (i.e. before the year 2000). Note that although the duration of an interval is normally expressed differently from its age, there is no international ‘standard’: we recommend ky and my.

Rank terms of geological time (eon, era, period, epoch and age) may be used for geochronometrical units when such terms are formalised (cf. chronostratigraphy).

Decay schemes that can be used for geochronology have to fulfil several criteria;  they have to have an isotope with a long enough half life to be useful over the period of geological time and the half life has to be known accurately.  In addition, the element has to exist in sufficient quantity in the rocks and minerals under study to be extracted and analysed.  There are now many different isotope decay schemes in use for geochronological purposes and, because of varying chemical and mineral stability during geological events, complex geological histories can be deduced by targeting problems with a suitable geochronometer.  It is important to know what event or process is under scrutiny and then to choose an appropriate geochronological tool. Good descriptions of techniques and their applications relavant to Quaternary problems can be found in Walker (2005).

The two techniques most commonly used by Quaternary stratigraphers are radio carbon dating  (14C),  which is applied to any materials containing sufficient organic carbon, and 40Ar-39Ar of potassium-bearing minerals, because both these methods can provide high precision ages. 

Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 (14C) to estimate the age of carbon-bearing materials up to about 50 ka.  Uncalibrated radiocarbon ages are usually reported in 14C years before present (BP), i.e. 1950. Such  ages can be calibrated to give calendar dates. When plants fix atmospheric carbon dioxide (CO2) into organic material during photosynthesis they incorporate a quantity of 14C that approximately matches the level of this isotope in the atmosphere (a small difference occurs because of isotope fractionation, but this is corrected after laboratory analysis. After plants die or they are consumed by other organisms the 14C fraction of this organic material declines at a fixed exponential rate due to the radioactive decay of 14C. Comparing the remaining 14C fraction of a sample to that expected from atmospheric 14C allows the age of the sample to be estimated.

The 40Ar-39Ar method is based on the decay of potassium to the inert gas argon which becomes physically trapped in the crystal lattice on formation.  A reliable age is dependent upon the argon being held in place in substantial parts of the crystal.  The commonly used step heating method, which involves progressive degassing of the samples up to melting point and analysis of the argon from each step, provides a way of looking at argon loss from different parts of the lattice and enables well-preserved parts of the crystal yielding crystallization ages to be distinguished from those which have suffered argon loss.

U- and Th-series offer a group of isotopes that constitute magnifying 'lenses' into recent temporal dimensions of Earth System processes. Whereas mass spectrometry (MS) measurements of 238U-234U-230Th and 235U-231Pa disequilibria give access to time ranges varying between about a million of years to hundreds of thousand years, MS or counting methods of shorter-lived daughter isotopes (e.g., 226Ra, 210Pb, 234Th, 228Th, 228Ra) inform on time scales varying from 50 ka (226Ra-230Th 'pseudo-concordias'), 10 ka (226Ra-excess method), 100 a (210Pb-226Ra '"pseudo-Concordia' or 210Pb-excess method), 30 a (228Ra-excess), 10 a (228Th-228Ra-232Th disequilibria) and up to 3 months (234Th-excess). U-series isotopes, and especially the sequence 238U-234U-230Th revealed essential in validating the astronomical theory of climate through the dating of high interglacial sea levels and provided the means to calibrate the radiocarbon time scale into "calendar years". – (after: Hillaire-Marcel, C. 2009. From deep-sea to coastal zones: methods and techniques for studying paleoenvironments. IOP Publishing IOP Conference Series: Earth and Environmental Science 5. 012008 doi:10.1088/1755-1307/5/1).

Luminescence dating is a method of determining how long ago minerals were last exposed to daylight. It is increasingly widely used by Quaternary geologists and archaeologists to date events.  The most commonly used technique is optically stimulated luminescence dating (OSL dating).  All sediments and soils contain trace amounts of radioactive isotopes including uranium, thorium, rubidium and potassium. These slowly decay over time and the ionising radiation they produce is absorbed by other constituents of the soil sediments such as quartz and feldspar. The resulting radiation damage within these minerals remains as structurally unstable electron traps within the mineral grains. Stimulating samples using either blue, green or infrared light causes a luminescence signal to be emitted as the stored unstable electron energy is released, the intensity of which varies depending on the amount of radiation absorbed during burial. The radiation damage accumulates at a rate over time determined by the amount of radioactive elements in the sample. Exposure to sunlight resets the luminescence signal and so the time period since the soil was buried can be calculated.

Cosmogenic radionuclide dating. The Earth is constantly bombarded by primary cosmic rays, high-energy protons and alpha particles. These particles interact with atoms in atmospheric gases, producing a cascade of secondary particles that may interact and reduce their energies in many reactions as they pass through the atmosphere. By the time the cosmic ray cascade reaches the Earth's surface it is primarily composed of neutrons. When one of these particles strikes an atom it can dislodge one or more protons and/or neutrons from that atom, producing a different element or a different isotope of the original element. In rock and other materials of similar density, most of the cosmic ray flux is absorbed within the first metre of exposed material in reactions that produce new isotopes called cosmogenic nuclides.  Using certain cosmogenic radionuclides, it is possible to date how long a particular surface has been exposed, how long a certain piece of material has been buried, or how rapidly a location or drainage basin is eroding. The basic principle is that these radionuclides are produced at a known rate, and also decay at a known rate. Accordingly, by measuring the concentration of these cosmogenic nuclides in a rock sample, and accounting for the flux of the cosmic rays and the half-life of the nuclide, it is possible to estimate how long the sample has been exposed to the cosmic rays. Rates of nuclide production must be estimated in order to date a rock sample.  The excess relative to natural abundance of cosmogenic nuclides in a rock sample is usually measured by means of accelerator mass spectrometry. The two most frequently measured cosmogenic nuclides are 10Be and 26Al. The parent isotopes are the most abundant of these elements, and are common in crustal material, whereas the radioactive daughter nuclei are not commonly produced by other processes. . Each of these nuclides is produced at a different rate. 36Cl nuclides are also measured to date surface rocks. This isotope may be produced by cosmic ray spallation of calcium or potassium. In contrast cosmogenic noble gases, especially 3He, require less intensive purification and a simple sector-field mass spectrometer. Thus cosmogenic noble gases offer the advantage of faster and less expensive data acquisition.

Amino-acid geochronometry is a technique used to estimate the relative age of a fossil specimen. This technique relates changes in amino-acid molecules to the time elapsed since they were formed.  All biological tissues contain amino-acids. All amino-acids except glycine (the simplest) are optically active, having an asymmetric carbon atom. This results in the amino-acid can have two different configurations, 'D' (dextrorotary) or 'L' (laevorotary) which are mirror images of each other. With a few important exceptions, living organisms keep all their amino-acids in the "L" configuration. When an organism dies, control over the configuration of the amino-acids ceases, and the ratio of D to L moves from a value near zero towards an equilibrium value near 1, a process called racemisation. Thus, measuring the ratio of D to L in a sample enables one to estimate how long ago the specimen died.

The main application of geochronology in stratigraphy is the calibration of the time-scale.  This requires the combination of well-defined stratigraphical units interbedded with material suitable for radiometric dating.  Volcanic ashes and their altered bentonite equivalents represent short-lived eruptions.  They are laterally extensive and cross facies boundaries, thus providing excellent time planes within the stratigraphical record.  Although the original volcanic glass has usually been converted to clay, crystalline igneous minerals are commonly preserved.  U-Th dating of carbonate cements, teeth and shell, and 40Ar-39Ar dating of micas and sanidine from such deposits have provided some of the most precise calibration of the time-scale in recent years. There are several other geochronological methods available.