QUATERNARY GEOCHRONOLOGY DATING METHODS

Sefa ŞAHİN

Aralık 2013

Dating Methods in Quaternary

1) Methods that provide direct age estimates * Radiometric methods (using isotopic decay) * Incremental methods (counting layers)

2) Methods that Establish age ‐equivalence* Through certain well dated stratigraphy markers

3) Relative Age methods* Using the relative Position Of each stratigraphy units

Radiometric Dating

This process, known as radioactive decay, is time-dependent, and if the rate of decay for a given element can be determined, then the ages of the host rocks and fossils can be established.

RadiocarbonUranium seriesPotassium Argon / Argon ArgonLuminescenceElectron spin resonanceFission track

Incremental Dating methods

DendrochronologyVarve chronologyLichenometryAnnual layers in ice

Age - equivalent dating methods•Paleomagnetism•Tephrachronology•Oxygen Isotope Chronology

Relative dating methods

•Weathering characteristics of rock surfaces•Pedogenesis•Stratigrapic relationships

Radiometric Methods

Carbon-14 (Radiocarbon) dating-Uses natural decay of C-14 organic material

K-Ar and Ar-Ar dating-Uses decay of K and Ar

Uranium series dating-Uses natural decay of Uranium isotopes

Fission track dating-Due to the radioactive marks on mineral lattice

Luminescense dating-Other mineral nearby nuclear disintegration

Cosmogenic dating-Exposure of rock surface

Radioactive decay, also known as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses energy by emitting particles of ionizing radiation.

A material that spontaneously emits this kind of radiation—which includes the emission of energetic alpha particles, beta particles, and gamma rays—is considered radioactive.

Radioactive decay

A typical radiometric dating method

Parent

Daughtertime

P:D

U Pb238 206

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.

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.

Age of deposition using ratios of intermediate daughters, 234U, 230Th.

Age range: several thousand to approximately 500,000 years, with greater precision from higher U content and younger age sample.Extremely young material can be dated given sufficient 234U (hundreds of years).

Most commonly applied to carbonates, opal, or other high-U, low-Th mineral

Uranium Series Methods

40 Potassium / 40Argon 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.

40-Argon/39-Argon Geochronology 

 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

FISSION TRACKS

Based on radiation damage (tracks) due to spontaneous fission of 238U. When combined with temperature required for annealing of the tracks in specific minerals can yield cooling ages (e.g. age of most recent uplift, pluton emplacement and cooling, etc.).

These tracks come from the spontaneous fission of 238U present in the mineral. By determining the number of tracks present on a polished surface of a grain and the amount of uranium present in the grain, it is possible to calculate how long it took to produce the number of tracks preserved.

Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses.

Fission-track dating is a relatively simple, but robust method of radiometric dating that has made a significant impact on understanding the thermal history of continental crust, the timing of volcanic events, and the source and age of different archeological artifacts.

The method involves using the number of fission events produced from the spontaneous decay of uranium-238 in common accessory minerals to date the time of rock cooling below closure temperature.

Fission tracks are sensitive to heat, and therefore the technique is useful at unraveling the thermal evolution of rocks and minerals.

Most current research using fission tracks is aimed at: a) understanding the evolution of mountain belts; b) determining the source or provenance of sediments;c) studying the thermal evolution of basins; d) determining the age of poorly dated strata; and e) dating and provenance determination of archeological artifacts

This image shows the appearance of etched fission tracks in a crystal of apatite etched for 20 seconds in 5N Nitric acid at 20°C. The tracks appear as randomly oriented dark straight lines on the surface of the mineral that also show variable focus along their length due to the fact that they are 3-dimensional structures dipping at various orientations. Very faint polishing scratches can also be seen traversing right across the surface of the grain in various directions. Each etched fission track represents a single 238U nucleus that has undergone spontaneous fission, so that counting the individual tracks is actually counting individual atoms, an extreme detection sensitivity which makes fission-track dating feasible despite the extraordinarily long half-life for this decay scheme.

The technique of fission track dating is based on the infrequent, but predictable, break-up of the most abundant isotope of uranium, 238U, in a fission reaction. The process leads to high-energy collisions between the fission fragments and neighbouring atoms and creates damage tracks or trails in the enclosing crystal lattice.

Precisely how this occurs is not completely understood, but it seems that as two positively charged fission fragments are driven apart, they strip electrons from atoms in the host lattice and, after passage of the fission fragment, a zone of positively charged ions remain which mutually repel each other.

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).

Luminescence Dating

This emission of light is known as thermoluminescence (TL) and is the basis of thermoluminescence dating. Alternatively, the electrons can be released from traps by shining a beam of light onto the sample, and again the luminescent signal is a reflection ofthe number of electrons trapped within the crystal lattice.

This is optically stimulated luminescence (OSL), and hence the technique is generally referred to as optically stimulated luminescence dating, although the abbreviated term optical dating

Luminescence Dating

 As time passes, the luminescence signal increases through exposure to the ionizing radiation and cosmic rays.

Luminescence dating is based on quantifying both the radiation dose received by a sample since its zeroing event, and the dose rate which it has experienced during the accumulation period

Electron Spin Resonance Dating

ESR Dating, sometimes referred to as Electron Paramagnetic Dating (EPR), has a greater time range than most of the other Quaternary dating methods described in this book, extending from a few thousand years to about 2 million years in the case of tooth enamel.

In general, the most important applications lie in the time range between 40 000 and 200 000 years, although a number of valuable age estimates of up to 500 000 years have also been obtained.

The basic principles of ESR dating are very similar to those for luminescence in that it is based on measurements of the trapped electrons in crystal lattices of rocks, sediments or other materials. In this case, however, the electrons are not released by heat or by light.

Rather their abundance is estimated on the basis of their paramagnetic properties. The sample is placed in a strong, steady magnetic field and exposed to high-frequency electromagnetic radiation.

The magnetic field is slowly changed and at a certain frequency the electrons (or spins) become ‘excited’ and resonate. The strength of this resonance signal can be determined using an ESR spectrometer.

When the electrons are in resonance, electromagnetic power is absorbed in direct proportion to the number of electrons present, and hence the greater the number of electrons, the greater the absorption (Aitken, 1990). The latter is a reflection of the time that has elapsed since the onset of electron trapping, and hence is a measure of age.

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).

The dating and study of annual rings in trees. The word comes from these roots

The science that uses tree rings dated to their exact year of formation to analyze temporal and spatial patterns of processes in the physical and cultural sciences.

Dendrochronology

Varve chronology

A correlation of varve series and records that together have a uniform numbering system and apply to a broader region than a varve sequence, perhaps from different lakes or valleys.

A varve chronology will grow as it incorporates series and smaller chronologies over time.

These varves were deposited in a position very distal to the receding glacier and also are from the relatively shallow flanks of Lake Vermont where less sediment was deposited during the summer than in deeper parts of the basin. As a result, winter layers are thicker here than summer layers

Lichenometry

Lichens are a symbiotic relationship between an alga and a fungus.

dating that uses lichen growth to determine the age of exposed rock, based on a presumed specific rate of increase in radial size over time.

As a relative-age technique it is based on the assumption that some index of lichen size increases over time.

Lichenometry has been used for dating many types of surfaces including raised beaches, river terraces, talus, rockfalls, trimlines , snow-avalanche activity and outwash plains .

Age: the size of the lichen thallus increases with age. assume growth begins soon after the deposit stabilized growth rate slows with age

Paleomagnetism

The Earth's magnetic north pole can change in orientation (from north to south and south to north), and has many times over the millions of years that this planet has existed. The term that refers to changes in the Earth's magnetic field in the past is paleomagnetism

Paleomagnetic and rock magnetic techniques provide means of understanding the Earth's ancient geomagnetic field by using the magnetic record contained in rocks and archeological materials.

When the magma from which igneous rocks form is still molten, iron-rich minerals can orient themselves in line with the local magnetic field in the same way that a compass needle does.

As the magma cools, the tiny iron-rich crystals are “frozen” in position, recording the orientation of the local magnetic field at that time. Iron minerals in sediments can also align themselves with the magnetic field, then become fixed as the sediment turns to rock.

The orientation of the magnetic crystals in rocks can be used to infer two pieces of information—the direction of the magnetic pole from the point where the rock formed, and the polarity (reversed or normal) of the magnetic field at the time the rock formed. 

TephrochronologyTephra is a term used to describe all of the solid material produced from a volcano during an eruption

Tephrachronology ist he study of volcanic ash deposits. Volcanic ash layers often have unique chemical and physical characteristics that can be used for correlation.

The chemical and physical characteristics of volcanic ash, select igneous minerals in the ash can be used for absolute dating

As the deposition of the tephra layers is essentially instantaneous on a geological timescale, these horizons provide distinctive and often widespread isochronous (i.e. of the same age) marker horizons that offer a valuable basis for inter-site correlation.

Each tephra horizon contains a unique geochemical fingerprint relating it to the source of the eruption.

Tephra layers are excellent time-stratigraphic markers, but, to establish a chronology, it is necessary to identify and correlate as many tephra units as possible over the widest possible area.

Sources :

- Quaternary Dating Methods – Mike Walker (2005)- USGS