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Geochronology is the science of dating and determining the time sequence of events in the history of the Earth. This web page provides an overview of selected geochronology methods used by USGS scientists. New dating methods are invented all the time, however, most have practical limitations. Geologic research and mapping requires the determinations of the ages and composition of rocks. A geologic map or report typically is only a summary of investigations that frequently involve the collecting and processing of hundreds of rock samples, followed by the evaluation and interpretation of data from a variety of analytical techniques.

Relative Dating Methods

A relative age is the age of a fossil organism, rock, or geologic feature or event defined relative to other organisms, rocks, or features or events rather than in terms of years. Tradition paleontological and biostratigraphic correlation methods are still perhaps the most common relative dating methods used by geologists. More modern correlation technologies include use of marine stable isotope records, paleomagnetic dating, tephrachronology, geomorphological methods, sedimentation characteristics, and other geochemical and radiometric methods. Relatively young deposits can be sometimes dated using tree rings, varved-lake sediments, coral growth patterns, and other methods.

Paleontology and Biostratigraphy

Paleontology is the study of life in past geologic periods (fossil plants and animals), incorporating knowledge of an organism's phylogeny, relationships to existing organisms, and correlation to an established chronology of Earth History. Paleontology is limited to the study of sedimentary deposits where fossils are preserved, but can be used in establish relative ages of nearby igneous intrusion, faults, and other geologic features. With the cumulative experience of centuries of paleontological research, the chronology of many fossil species are well established in context of both geologic time and distribution. Biostratigraphy is the science of correlation of sedimentary units base on the identifiable fossils they contain. Paleontologists examine fossils of all kinds, but micropaleontology (the study of microscopic organisms) is perhaps the most useful method of dating because the remains of tiny organisms tend to be better preserved, more widely distributed, and may provide more precise age determinations than larger shells or bone material. Palynologists separate pollen from sediments for correlation and paleoenvironmental reconstructions. Typically, paleontological information is used in conjunction with other methods of relative or absolute age dating.

The most important tools for paleontologists are collections of fossils and paleontological reports (with fossil plates for identification) from other locations in the region or around the world. Micropaleontologists and palynologists work with microscopes or scanning electron microscopes (SEM). Paleontologists frequently work in conjunction with other scientists utilizing any number of other geochronology methods.

Technical details for techniques, equipment, and contacts for paleontological investigations conducted by the USGS can be found on the following link:

Paleontological analyses

Correlation Methods in Geochronology

Like fossils, the chemical and physical characteristics of rocks, minerals, and organic materials can be used for correlation. Selected examples of correlation geochronology methods used by USGS scientists include:

Paleomagnetic Dating - Under certain conditions, a record of the orientation of the Earth's magnetic field is preserved in rocks and sediments. Paleomagnetic dating is based on correlation of measurements derived from oriented samples to established records of variations of the Earth's magnetic field through time. Paleomagnetism can be used in conjunction with other correlation or dating methods to establish the age or rocks or to decipher changes in a rock's orientation through time. In Menlo Park, contact: Dwayne Champion for more information about the paleomagnetic lab.

Technical details regarding paleomagnetism techniques, equipment and contacts at the USGS can be found on the following website link:

Paleomagnetism analyses

Tephrochronology is the study of volcanic ash deposits. Volcanic ash layers often have unique chemical and physical characteristics that can be used for correlation. Great volcanic eruptions in the Western United States in the geologic past produced airfall deposits that have been recognized as far away as the East Coast. The USGS maintains a tephrochronology laboratory in Menlo Park, CA. In addition to the chemical and physical characteristics of volcanic ash, select igneous minerals in the ash can be used for absolute dating (discussed below). For more information, contact Andrei Sarna-Wojcicki.

87-Strontium/86-Strontium Geochronology - With modern isotope separation equipment, the content of selected elemental isotopes can now be measured in concentrations to parts-per-million to parts-per-billion and beyond. The 87-Sr/86-Sr geochronology method involves extracting these isotopes from fossil shell material (only several milligrams of sample are necessary for X-ray fluorescence spectroscopy). The ratio of these two isotopes derived from a sample is compared with a database of known samples to determine relative ages. The basic science behind this method is that calcareous shell material incorporates the two strontium isotopes in the same ratio that occurs in seawater at the time the organism was alive. At different times in Earth's history, the relative abundance of these two isotopes in seawater gradually changed through time (such as during the Permian, the Late Cretaceous, and parts of the Tertiary). A relative age of the original shell can be established by comparing the strontium isotope ratio of the shell material to published data for the time periods where this method is usable. The method is most effective when used in conjunction with other dating methods. In Menlo Park, contact Robert Fleck.

Stable Isotope Records - Stable isotope data derived from mineral and biological materials can provide a variety of insights into environmental conditions (past and present), and can be used in geochronology and correlation. Oxygen isotopes (18-O/16-O) are widely used in correlation of Quaternary marine sediments. Oxygen isotope concentrations in mollusk shell and calcareous algal material normalize with seawater while the organisms are alive. During periods of glaciation, large volumes of 16-O become trapped in glacial ice, enriching ocean water in the heavier oxygen isotope. As a result, oxygen isotope data extracted from shell-bearing sediments can provide information about cycles of glaciation (and climate change), and can be used for relative dating. To a lesser degree, other stable isotopes are used for correlation (such as 13C/12C and 36-S/34-S).

Numeric Dating Methods

Numeric dating involves methods of determining the geologic age of a fossil, rock, or geologic feature or event given in units of time, usually years Numeric dating (also called absolute dating) establishes the ages of samples using radiometric or isotopic methods, and by other means. Most absolute dating methods rely on extraction and sampling of radiogenic elements and their by-products of decay. Below is a list of absolute dating methods use by USGS scientists.

40-Potassium/40-Argon Geochronology - 40-K/39-Ar geochronology is one of the most widely used absolute-dating methods. The method relies on samples rich in mineral grains containing potassium, typically an igneous volcanic rock rich in sanidine feldspar. 40-K undergoes natural radiogenic decay through time (converting to argon-40 at a known rate). As the potassium gradually decays to argon, the naturally inert gas accumulates, confined within the mineral crystal lattice. As a result, the ratio of 40-K to 40-Ar derived from mineral grains is compared with the known rate of radiogenic decay of 40-K. By eliminating possible sources of error, this absolute dating method can be used of on selected rock samples typically ranging in ages from ~10,000 years on back in time to billions of years. The USGS maintains a 40-K/39-Ar laboratory (sample extraction equipment and mass spectrometer) in Menlo Park, CA. For more information, contact Robert Fleck.

40-Argon/39-Argon Geochronology - The USGS also maintains an 40-Ar/39-Ar laboratory in Menlo Park, CA. Samples are irradiated before testing (neutron activation). 40-Ar/39-Ar methodology overcomes many of the problems intrinsic to the 40-K/39-Ar, primarily the potential for radiogenic argon to escape form minerals and/or rocks due to thermal processes (igneous or metamorphic) over time. This absolute dating method can be used on selected rock samples typically ranging in ages from ~10,000 years on back in time to billions of years. For more information, contact Robert Fleck.

Uranium Series Methods - This methodology involves the measurement of isotopes of uranium (238-U and 235-U), thorium (232-Th), and certain members of their daughter nuclides. Uranium series geochronology is typically used to date authigenic minerals in sediments or fossils, but has also been used to date speleothems, calcite veins, rock varnishes, salts, and other materials. Time determinations "windows" vary for the different radiogenic isotopes, but applications are typically used for Quaternary deposits.

Lead-210 Geochronology - Lead-210 (210-Pb) geochronology is an isotopic age determination method that is based on the radioactive decay of 222-Rn (radon) and 210-Pb. The radiogenic half-life of 210-Pb is in the range of 22.3 years, making it an ideal tracer for late Quaternary geochronology. As radioactive radon gas in the atmosphere decays it produced the solid lead isotope (210-Pb) that immediately accumulates in the environment. Once trapped in a "closed system environment" (such as rocks, minerals, or organic remains buried in the Earth), 210-Pb eventually decays to stable 206-Pb. The success of this method is contingent on scientific criteria (or assumptions) related to the "closed system environment" and other factors. For more information, contact Robert Fleck.

Radiocarbon Geochronology - This isotopic method has been in use since the late 1950s and has greatest utility for the study of late Quaternary deposits containing organic residues. The radioactive half-life of 14-C is 5,730 years (as it decays to 12-C). The radioactive form of carbon is generated by cosmic ray interactions with atmospheric nitrogen and oxygen, and possibly a trace from earth and ocean matter. 14-C is incorporated into all organic mater through respiration. Therefore, living tissue is in equilibrium with carbon isotope concentrations in the biosphere. When an organism dies and some of its organic material becomes isolated from bacterial decay, it no longer takes in "fresh" carbon and the progressive decline in the 14-C isotope begins. With modern analytical sensitivity, radiocarbon geochronology is suitable for materials dating back to about 50,000 years. However, certain considerations in data are necessary because the relative concentration of radiogenic carbon in the biosphere has varied slightly through time.

Radiogenic Methods

Fission-Track Geochronology - Zircons, apatite, volcanic glass, and other minerals accumulate physical damage trails left by nueclei expelled during fission decay of trace uranium-238. Fission-track geochronology is most useful for volcanics with zircons or glass for materials that fall in the range of late Tertiary to late Precambrian. The method is suitable for samples that have not experienced annealing temperature effects. In fact, as is the case with apatite samples, the annealing effect can be used to target "unroofing" rates by erosion in mountain belts. This is because apatite crystals will not preserve radiogenic fission tracks until they rise into the temperature/pressure range within about 3 kilometers of the surface. Fission-track methodology is conducted on target grains that have been cut, polished, and chemically etched (to make the particle tracks visible), with examination conducted with a scanning-electron microscope or a high power optical microscope.

Luminescence Geochronology - Optical stimulated luminescence (OSL) and thermoluminscence (TL) are dating methods that involve the analysis of the optical properties of minerals exposed to environmental radiation. Radiation damage to the crystal lattice of mineral grains produces defects or "electron traps." These "traps" can be stimulated to produce measurable luminescence. However, these traps are highly sensitive to both sunlight and/or heat. Rocks exposed even to a few hours of sunlight can loose their luminescent properties.As a result, the measurable luminescence a rock produces is directly proportional to the amount of radiation exposure since the time of burial. The OSL and TL methods are suitable for studying terrestrial deposits up to about 500,000 years (late Quaternary).

More technical summaries of selected geochronology and geochemical tracer methods, techniques, equipment, and USGS contacts can be found at these links:

Geochemistry and tracer studies

A list of analytical laboratory facilities maintained by the USGS (including techniques, equipment, and contact information) can be found at this link:

Chemical analyses

Selected reference resource for this page:

Noller, J. S., Soweres, J. M., and Lettis, W. R. (eds.), 2002, Quaternary Geochronology: Methods and Applications, Washington, DC: American Geophysical Union, AGU Reference Shelf 4, 582 p.



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