Geochronology

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Geochronology is the science of determining the age of rock formations and their associated geological events. Geochronology is important in the geosciences because it allows the quantification of the changes that occur across the landscape such as depositional timing, paleogeography, basin development, sediment provenance, and much more. There are many different dating methods that can be used to determine the age of rocks, fossils, and sediments, and the advancement of modern technology is allowing faster determination of more accurate age measurements. The ages can be determined either absolutely using radioactive isotopes or relatively using dating methods such as index fossils, global stable isotopic trends, and paleomagnetism.

Geochronology, biostratigraphy, and chronostratigraphy are all closely related disciplines and are commonly applied towards the same problems. Biostratigraphy is only concerned with assigning a sedimentary sequence to a particular geological period based on the fossil assemblages present within the rock. Chronostratigraphy is similar to biostratigraphy, but it attempts to assign an absolute age for a particular fossil assemblage. Geochronological units are referred to as periods of time when chronostratigraphic units are only referred to in their geological context[1].

Absolute dating methods

Radiometric Dating

A photograph of a zircon crystal. This mineral is a commonly used mineral for U-Pb isotopic analysis as zircons are ubiquitous in clastic sedimentary layers

Ages of minerals can be determined by measuring the number of radiogenic daughter isotopes compared to the parent isotopes. The process is accomplished by analyzing an individual mineral grain, such as a zircon crystal, using equipment such as a laser ICP-MS or SIMS.Often multiple decay systems can be analyzed in the same study to yield more thorough results. Different isotopes will have unique half-life decay periods, so an isotope that decays slowly can be used to analyze samples hundreds of millions of years old and an isotope such as carbon-14 which can be applied to samples younger than 60,000 years. Some geochemical isotopic methods include Rb-Sr, Re-Os, U-Pb, K-Ar, Ar-Ar, La-Hf.

Cosmogenic Nuclide Geochronology

Cosmogenic nuclide geochronology, sometimes known as "surface exposure dating", can be used to estimate the length of time sediment has been exposed at the Earth's surface[2]. It is accomplished by either analyzing a single sample, such as a large boulder that has been exposed at the surface or by analyzing a depth profile into a hill/mound.The six most commonly used cosmogenic isotopes include: 14C, 36 Cl, 26Al,3 He,21Ne, and 10 Be [3].It can be used for rock that has been exposed between 10 and 30 million years.

Fission Track Dating

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This is a photograph of fission tracks left in a mineral grain. The analysis of the length and number of tracks allows for absolute dating of a mineral.

Any minerals that contain uranium can be dated using fission track dating which can be accomplished by inspecting crystal makings that are left by the spontaneous fission of Uranium-238 impurities[4]. Fission track dating can be accomplished by inspecting a crystal to track markings that are left by the spontaneous fission of uranium-238 impurities[5]. The tracks left by fission fragments are used to date the time the rock was cooling below closure temperature. This method of dating is especially useful in explaining the thermal history of a deposit or mineral grain.

Relative dating methods

Biostratigraphy

Biostratigraphy is a relative dating method that relies on the comparison of fossil assemblages of sedimentary rocks and assigning them to an interval of time when the fossils are known to have existed. In biostratigraphy, a fossil assemblage is compared with fossils of similar physical characteristics, and by knowing both the age and spatial extent a particular species is found a relative age can be assigned[6]. A large database containing a large collection of fossil and paleontological reports around the world can be found on the USGS website.

Paleomagnetism

File:Oceanic.Stripe.Magnetic.Anomalies.Scheme.fr.gif
A schematic figure showing polarity reversals that occur along the ocean floor. Reversals can be mapped on the ocean floor through magnetic surveys, and by knowing the time of reversals a relative date can be assigned and spreading rates can be calculated

Paleomagnetism utilizes the reversals of Earth's magnetic field through time, and by analyzing the magnetic changes recorded in sedimentary and volcanic sequences a time-scale can be assigned. Geomagnetic reversals are preserved on the ocean floor as stripes spreading out from seafloor spreading centers which can provide a time-scale.

Magnetostratigraphy

Another correlation that can be made between sedimentary sequences are the changes in the sediment magnetization direction which are left in place after deposition. Magnetostratigraphy is similar to paleomagnatism, except it relies on the orientation of minerals on a much smaller scale. In the field, magnetostratigraphy requires an orientated core to be drilled that can be later analyzed for magnetic mineral orientations. Depending on sediment accumulation rates, magnetic reversals occur as bands through the core that can be correlated with known magnetic reversals through geologic time[7].

Chemostratigraphy

Chemostratigraphy relies on stable isotope trends that change through geologic time and can be correlated between sedimentary deposits. The isotopic signatures can be found within sediment of any lithology or sediment age. Some of the isotopes that can be correlated include carbon, sulfur, strontium, and oxygen. Chemostratigraphy is useful at all scales from cuttings to outcrop samples and can be applied to relatively small sample sizes.

Luminescence Dating

Luminescence dating is accomplished through the analysis of light that is observed from various minerals and sediment. There are several methods covered under luminescence dating including thermoluminescence, cathodoluminescence, and optically stimulated luminescence. Thermoluminescence can be used to date sediment that has undergone some level of lithification before being exhumed and recharged by naturally occurring UV light. This recharge can be measured and compared to the latent luminescent signal.[8]

File:Geomagnetic late cenozoic.png
Geomagnetic polarity reversals through the late Cenozoic Normal polarity in black and reverse polarity in white. Figures such as these are used to correlate mineral ornetations taken from cores.
Three stages of thermoluminescence that are present within a quartz grain.

References

External links and further reading