ES 331/767 Lecture 10
QUATERNARY CHRONOLOGY
AND DATING

James S. Aber

Table of Contents
Introduction Radiometric dating
Radiocarbon dating Fission-track dating
Cosmogenic dating Paleomagnetism
Amino-acid dating Lichenometry
Related sites References

Introduction

The Quaternary Period traditionally was thought to have started roughly one million years ago. The concept of four major glacial cycles during the Quaternary was accepted early in the 20th century and became the basis for establishing Quaternary chronology and interpretation. The traditional glacial stages were determined originally from geomorphic features, such as end moraines and river terraces, as well as from the stratigraphy of tills interbedded with peat, soil, and water-laid sediments.

The model of four Quaternary glaciations served well for more than half a century. Since the 1960s, however, much new evidence has accumulated that indicates: (1) the Quaternary began at least 1½ million years ago, perhaps more than 2 million years BP, and (2) many more than four glaciations took place during the Quaternary. The exact number of major glacial cycles is not known, but is at least ten or probably more, each lasting about 100,000 years in duration.

The beginning date of the Quaternary is the subject of continuing discussion among geologists. Officially the Tertiary-Quaternary (Pliocene-Pleistocene) boundary is set in a stratotype at Vrica, Italy, which is dated around 1.8 million years old. However, much controversy surrounds the selection of this stratotype to represent the Tertiary-Quaternary boundary. Many Quaternary scientists want to select another, older boundary at about 2.6 million years ago (Morrison and Kukla 1998); whereas others are adamant to retain the Vrica boundary at 1.8 million years old (Aubry et al. 1998). An increasing amount of evidence favors rapid buildup of glaciation in different parts of the world by 2½ million years ago (Prueher and Rea 1998). However, this debate will not be settled anytime soon.

Determining the age of Quaternary events is like timing the Olympic 100-meter sprint in comparison to the marathon of geologic time. It is feasible, but technically difficult. Millennia, centuries, and even decades are important time spans for Quaternary dating. New geophysical techniques for dating geologic materials have revolutionized Quaternary chronology. These techniques have been developed and refined during the past few decades. Among the most important are several means of radiometric dating based on carbon-14, argon-argon, chlorine-36, beryllium-10, and uranium-thorium (Hall 1995). Non-radiometric dating techniques employ paleomagnetism, amino acids, tree rings, weathering rinds, lichens, and other materials. In order to determine environmental and climatic relationships, it is critically important to accurately date Quaternary records (Nesje and Dahl 2000).

Radiometric dating

Radiometric dating depends on the well-documented assumption that each radioactive isotope decays at a specific and constant rate. The decay rate for an isotope is most easily illustrated by its half life--see Fig. 10-1. The half life is the length of time necessary for one-half of the original mass of the parent isotope to decay into daughter isotopes.

Pinitial ===> Px + Dx

Half lives of different isotopes vary enormously from less than one second to billions of years. The effective interval for geologic dating using any particular isotope is from about 1/10th half life (minimum) to 10 half lives (maximum). This interval is related to the analytical limits of laboratory measurements of radioactive isotopes. After 10 half lives, less than 1/1000th of the parent isotope still remains--too little to detect (Hall 1995). Several geologically common isotopes have half lives of appropriate lengths for dating Quaternary materials. For example, the argon-argon method has proven quite effective for dating volcanic deposits of Pleistocene eruptions--see Fig. 10-2.

Laacher See volcano, Germany. Lake occupies the volcano caldera, and hills are formed by tephra rim around the caldera. This volcano was formed during the latest eruption from the East Eifel volcanic province, near the Rhine valley of western Germany. Photo date 7/95; © by J.S. Aber.
Laacher See tephra exposed in vertical wall of a pumice mine. The tephra consists of interbedded ash and pumice. Argon-argon and radiocarbon dates indicate an eruption age of 13,000 years BP (van den Bogaard 1995). This tephra is a key stratigraphic marker across western Europe in Italy, France, Switzerland, Germany, Benelux, Denmark, Sweden and Poland. It marks the Allerød, a warm interval near the end of the last glaciation. Photo date 7/95; © by J.S. Aber.

Radiocarbon dating

Radiocarbon (14C) is undoubtedly the most important for dating latest Pleistocene and Holocene sediments and fossils.Carbon occurs in three isotopic forms: 12C (99%), 13C (1%), and 14C (< 1‰). Only 14C is radioactive; its half life is about 5600 years. 14C is produced in minute quantities in the upper atmosphere by cosmic radiation--see Fig. 10-3. Assuming constant production and constant decay of 14C, a balance is achieved in 14C content of the atmosphere. Radiocarbon is readily circulated as CO2 and is incorporated into the bodies of plants and animals as well as in surface water and ice. While living, plants and animals contain in their tissues a 14C to 12C ratio that is equal to that of the atmosphere.

Upon death, the amount of 14C in tissues of a plant or animal body begins to decline as 14C decays into nitrogen. This nitrogen normally escapes from the body and cannot be measured. The amount of remaining 14C can be measured, however, and this is the basis for the radiocarbon dating method. The method was first developed by the chemist W.F. Libby in 1950, for which he won a Nobel prize in 1960. The critical assumption for radiocarbon dating is that the dated material originally contained 14C in the same abundance as the atmosphere.

Given its half life (std. value of 5570 years), a dating range of a few centuries to around 50,000 years is possible. Older material simply has too little remaining 14C to measure accurately. Several analytical techniques are in use for radiometric dating: proportional gas counting, liquid scintillation, and negative-ion accelerator mass spectrometry (AMS)--see Fig. 10-4. During measurement of a sample's radioactivity, decay activity normally varies slightly about a mean value. Thus radiometric dates are given with a standard deviation, for example 10,000±500 years BP. This indicates the dated age lies between 9,500 and 10,500 years ago; BP means "before present" (1950).

Radiocarbon dating has proven enormously popular and useful for dating a host of geologic and archeologic materials. The best materials are considered to be organic: wood, peat, charcoal, leather, textiles, etc. Organic hard parts, such as bones, aquatic shells, bird eggshell, and teeth, are next in suitability for dating. Glacier ice, steel, and many other materials may also be dated using radiocarbon, as long as the critical assumption is met. Even mortar in ancient buildings can be dated by radiocarbon (Hale et al. 2003). On the basis of radiocarbon dating a detailed chronology is now established for Late Quaternary events.

Sand dune section on North Sea coast at Lodbjerg, northern Jylland, Denmark. Peat layer represents a time of stability, vegetation growth, and soil development; sand layers show times of wind erosion and deposition of sand dunes. Scale pole marked in 20-cm intervals. Photo date 6/79; © by J.S. Aber.
Closeup view of peat from coastal cliffs at Lodbjerg, northern Jylland, Denmark. Peat consists of compressed and partly carbonized plant fragments. Such material is often utilized for radiocarbon dating. Photo date 6/79; © by J.S. Aber.
Fossiliferous marine strata of late glacial age, Skærbæk, northern Denmark. The marine clay contains abundant shells of Cardium and Litorina. Such fossils may be used for radiocarbon dating. Photo date 6/79; ©; by J.S. Aber.

As with any dating method, complications and limitations exist for radiocarbon dating. The basic assumption is that atmospheric 14C has remained constant through time. This is certainly not true for this century. Massive burning of fossil fuel has released much 12C into the atmosphere, whereas atmospheric nuclear explosions have created much 14C. The atmospheric 14C constant used for radiocarbon dating is therefore based on wood samples from the 1800s.

Radiocarbon dating may be compared directly to tree rings dated by dendrochronology. Bristlecone pine (Pinus aristata) has the longest tree-ring record, extending back more than 8000 years, covering nearly all of the Holocene. In general, radiocarbon dates less than 3500 years BP turn out to be slightly too old, whereas dates greater than 3500 years BP are several centuries too young. This indicates that atmospheric 14C has varied in the past, presumably because of changes in the sun's or earth's magnetic fields. A tree-ring calibration now allows for correction of Holocene radiocarbon dates, however older radiocarbon dates cannot be corrected. For this reason older radiocarbon dates are not regarded as true year dates; such dates are reported as radiocarbon years.

Forest of ponderosa pine, Pine Ridge, northwestern Nebraska. Pines and other conifers are considered excellent trees for dendrochronology, especially in dry upland sites such as this. Photo date 7/80; © by J.S. Aber.
Closeup view of boring a tree-ring core from ponderosa pine, Antelope Springs Cemetery, northwestern Nebraska. The hollow pipe has been drilled into the tree by hand (red handles). A channel-wedge (brass cap) is inserted to hold wood core in tube when the drill is removed from tree. Usually two or three cores are taken from different sides of a tree. The tree is not damaged by this process. Photo date 6/80; © by J.S. Aber.
Three mounted and prepared cores from tree shown above. The outer (bark) ends are to left; center of tree is darker (heart) wood toward right. Scale in cm. Photo date 3/89; © by J.S. Aber.

Other conditions complicate radiocarbon dating. Some organisms are able to preferentially take up 12C and thus partially exclude 14C from their bodies. Living plants normally contain 3-4% less 14C than the atmosphere, which could yield age discrepancies for older samples. This photosynthesis effect can be corrected by reference to 13C, a stable isotope. Certain species of shellfish are able to exclude most 14C from construction of their shells. These species yield anomalously old dates.

Plants, animals, and organic deposits in lakes may be subject to a hard-water effect, in which carbon is derived not only from the atmosphere but also from weathering of older rocks in the lake basin. Erroneous dates that are too old result from such conditions. This is a particular problem in the northern Great Plains region, where glacial sediment contains much reworked limestone, dolostone, coal, and lignite. Many radiocarbon dates obtained from aquatic materials--shells, peat, and organic-rich soils--have proven to be 1000s of years too old. Only dates of samples obtained from dryland sites are considered valid--see Fig. 10-5.

Lake Oro, southern Saskatchewan. The lake occupies a deep kettlehole, which is surrounded by glacial sediment that contains much reworked Paleozoic dolostone and K/T lignite. Surface waters in southern Saskatchewan are quite hard and may result in anomalous radiocarbon dates for aquatic samples. Photo date 7/91; © by J.S. Aber.
Erratic boulder of Paleozoic dolostone, southern Saskatchewan. Dolostone and lignite introduce old (dead) carbon into surface and ground water in contact with glacial sediment. Scale pole marked in feet. Photo date 8/84; © by J.S. Aber.

Finally the problem of sample contamination by later events must always be taken into account. Growth of modern plant roots or chemical changes brought about by ground-water circulation may alter the 14C content of a potential sample. In spite of the pitfalls for radiocarbon dating, it is regarded as indispensable for Late Quaternary chronology and is widely used for all manner of geological, biological, and archeological sample materials. The AMS radiocarbon dating technique has become especially important for providing accurate dates in critical situations (i.e. Fleisher et al. 1999).

Fission-track dating

Fission-track dating is based on the presence of uranium isotopes 238U and 235U in appropriate sample material. 238U undergoes spontaneous fissioning with a half life of 4.5 billion years. Each fission reaction creates a tiny damage track in the surrounding material. Fission tracks can be enlarged by acid etching, so they can be identified and counted using an optical microscope. In simple terms, the number of fission-tracks in a sample is a function of age and uranium content. The method can be used both for very young materials (with high U content) and very old materials (with low U content). Volcanic rocks and tephra have proven useful for fission-track dating of Quaternary events both on land and in the ocean.

Indian Creek volcanic ash bed, southeastern Nebraska. Light-colored sediment in trench is ash and reworked ash mixed with local silt. The ash consists almost entirely of glass shards. Such ash beds are common across the Great Plains from Saskatchewan to Texas; most were derived from eruptions at Yellowstone. Volcanic ash can be dated using the fission-track method. Photo date 10/83; © by J.S. Aber.

Fission-track dating is complicated in two ways: (1) determining the uranium content of a sample and (2) fission-track annealing. Uranium content is determined by comparing the spontaneous (natural, 238U) fission tracks to induced (artificial, 235U) fission tracks--see Fig. 10-6. Inducing 235U fission tracks is done by irradiating the sample with high-energy neutrons in a nuclear reactor. The number of induced fission tracks depends on the sample's uranium content and the neutron dose. The two uranium isotopes always occur in the same ratio in nature; thus, the number of induced fission tracks is a measure of total uranium content in a sample.

Fission-track annealing takes place over time, as the minute damage tracks recrystallize and disappear. The rate of annealing depends on temperature and type of material. High temperature accelerates annealing; thus, fission-track dating gives the age since the sample has cooled below metamorphic temperatures. Glass is especially subject to annealing even at low temperature. This effect is demonstrated by the Huckleberry Ridge ash of the Yellowstone region (Naeser and Naeser 1988). Fission-track dates on glass usually fall in the 1.2 to 1.4 million years BP range; whereas, fission-track dates on zircon crystals are consistently 1.9 million years BP. Other types of dating support the 1.9 million year age for the Huckleberry Ridge ash.

Cosmogenic radiometric dating

One difficult problem in Quaternary science is dating exposed rock surfaces, such as bedrock pavements or boulder beds. An ingenious method has been developed that depends upon the accumulation of the 36Cl isotope beneath the exposed surface. This isotope is generated by the interaction of cosmic radiation with the nucleii of K, Ca, and Cl atoms within the rock. It is one of several so-called cosmogenic isotopes that have been utilized for dating rock surfaces (Cerling and Craig 1994). The basic assumptions for this dating method are that the rock surface has been continuously exposed to cosmic radiation and that all resulting 36Cl remains trapped in the rock for analysis.

The rate of production of 36Cl depends on the concentrations of the target elements (K, Ca, Cl), sample elevation, surface orientation, and geomagnetic latitude (which governs the cosmic-ray flux). The rate of accumulation of 36Cl depends on the production rate and on the rate of erosion of the rock surface, if the surface is not stable (Jackson et al. 1997).

The primary difficulties of this method are twofold--sample analysis and uncertainty of erosional history of rock surfaces. Sample analysis involves complete solution of the rock material in strong acids (HF and HNO3) followed by accelerator mass spectrometric analysis of the 36Cl/Cl ratio. These techniques require special laboratory procedures and equipment, which are expensive and not widely available. Erosional history of rock surfaces is often difficult to constrain, and so must be estimated based on geomorphic field evidence. In spite of these complications, cosmogenic dating has proven effective in situations where other types of dating were not feasible--see Figs. 10-7 & 8.

Giant erratic blocks of quartzite at Okatoks, Alberta, Canada. These erratics were transported from the Canadian Rocky Mountains by the Cordilleran ice sheet. They are part of the "foothills erratic train" that extends from Jasper southward to near the U.S.-Canada border. These erratics were successfully dated using cosmogenic isotopes--see Figs. 10-7 and 8. Photo date 8/84; © by J.S. Aber.

Paleomagnetism

Iron-bearing rocks and sediments may acquire at the time of their formation a natural remnant magnetism. This primary magnetism is aligned parallel to the existing magnetic field of the Earth. Remnant magnetism may be acquired through thermal, chemical, or depositional means. Volcanic lava, chemical sediment, and even till often preserve a stable primary remnant magnetism. Such rocks and sediments can be used as "fossil compasses" to determine the nature of the Earth's paleomagnetic field.

Thick basaltic lava flows form the resistant ledge of the waterfall, Fagifoss, Iceland. Such iron-rich volcanic rock is excellent material for paleomagnetic measurements. Its magnetic character was acquired at high temperature as the lava cooled. Photo date 8/94; © by J.S. Aber.
Iron-cemented sand and gravel exposed at Ristinge Klint, Langeland, Denmark. The orange-brown color indicates secondary iron cementation of the sand and gravel sediment. Such cement may preserve primary remnant magnetism that dates from the time the cement was chemically precipitated. Photo date 8/82; © by J.S. Aber.
Collecting paleomagnetic samples from unweathered gray till at the Independence Formation stratotype, Atchison, Kansas. These samples possess weak, but stable, natural remnant magnetism with normal polarity. Ibrahim Abdelsaheb (left) and Brian Nutter. Photo date 10/87; © by J.S. Aber.
Split core sample of subsurface till from Nemaha County, northeastern Kansas. This core sample preserves weak, but stable, natural remnant magnetism with normal polarity. Core is about 8 cm (3 inches) in diameter. Photo date 11/89; © by J.S. Aber.

The orientation of the Earth's magnetic field at any point is specified by two measurements: declination (trend) and inclination (plunge). The inclination varies from horizontal at the equator to vertical at the poles--see Fig. 10-9. The field is directed downward in the northern hemisphere and upward in the southern hemisphere at present. The magnetic field periodically reverses its polarity at intervals ranging from several 1000 to > 1 million years; during reversed polarity, a compass needle would point south.

Polarity reversals should be excellent stratigraphic markers. Reversals are global in extent, they are detectable in many kinds of rocks and sediments, and the ages of reversals can be dated by radiometric techniques. Remnant magnetism of samples is measured using either spinner or astatic magnetometers. Secondary remnant magnetism acquired long after the time of rock formation is a troublesome factor. The effects of secondary magnetism can usually be removed from samples by stepwise demagnetization. Various field tests may also be applied to determine the origin of natural remnant magnetism in samples.

A detailed record of paleomagnetic reversals is available for the Cenozoic Era--see Fig. 10-10. Positive (normal) intervals are numbered, while reversed episodes are unnumbered; the younger anomalies have been given individual names. The Neogene/Quaternary boundary is currently defined at the end of anomaly 2 (Olduvai event = 1.6 million years BP); and the Early/Middle Pleistocene boundary is set at the beginning of anomaly 1 (Brunhes epoch = 720,000 years BP). The Blake event was a short-lived reversal during the Brunhes epoch, about 123,000 to 117,000 years BP (Abrahamsen 1995). It corresponds to the Sangamon/Eemian high stand of sea level.

By itself, paleomagnetism cannot give an absolute date. However, in combination with other dating techniques it may further refine or verify the age of Quaternary sediments, rocks, and fossils.

Amino-acid dating

All living organisms contain proteins made up of amino acids. The proteins produced by organisms consist almost entirely of amino acids in the L-isomer (left-handed) configuration. Upon death, the L amino acids begin inverting to their respective D-isomer (dextral or right-handed) configurations. This process is called racemization. Each of the 20 common amino acids undergoes its own inversion from L to D forms at rates that depend mainly on temperature and secondarily on taxonomy (species). D/L ratios in fossil organic material are functions of time, temperature and species.

Most amino-acid dating work has been done using leucine or isoleucine in marine mollusks (Wehmiller et al. 1988), and aspartic acid in conifer wood has also proven useful (Rutter and Vlahos 1988). Sample material is analyzed with various gas chromatographic methods. In the case of marine mollusks, the D/L isoleucine ratio increases from nearly zero in modern shells to equilibrium (maximum) values of about 1.30±0.05. The time required to reach the equilibrium ratio depends on the effective temperature experienced by the fossil. At +10ºC, it takes about 2 million years; at -10ºC, 20 million years are required (Miller and Mangerud 1985). D/L ratios cannot be directly compared for different amino acids or between different species.

Amino-acid ratios can be used for either relative or absolute dating. Absolute dating requires calibration with radiometric techniques, such as radiocarbon dates, and knowledge of the temperature history of the fossil--see Fig. 10-11. Once such information is established for a region, amino-acid dating may be used with confidence. However, amino-acid time calibration cannot be extended beyond the area of study due to regional differences in temperature history. Amino-acid dating is particularly useful for verifying the ages of fossils that were dated with other techniques.

The aminostratigraphy of northwestern Europe has been studied in considerable detail. A regional stratigraphy of marine benthic foraminiferas is based on species with similar racemization rates. Times of high sea level and marine deposition during interglaciations are documented for late and middle Pleistocene--see Fig. 10-12. D/L ratios fall into four groups.

  1. Youngest with D/L ratios between 0.03 and 0.04. Considered to be Late Vistulian.
  2. Abundant sites with D/L ratios between 0.08 and 0.115. Considered to be Eemian.
  3. D/L ratios approximately 0.145. Considered to be Holsteinian.
  4. Oldest with D/L ratio of 0.244. Considered to be pre-Late Elsterian, possibly Cromerian.

Shelly layers in Eemian marine clay, Ristinge Klint, southern Denmark. The beds are tilted as a result of ice-push deformation. Such shell material may be utilized for amino-acid dating. Pocket watch for scale. Photo date 8/82; © by J.S. Aber.

Lichenometry

Lichenometry is a method for age dating a landform based on the rate of lichen growth on the exposed rock surfaces. Lichenometry has been employed widely for dating the ages of glacial and periglacial features in alpine and desert regions. The method depends on the increase (growth) in diameter of lichen thalli through time. On siliceous (quartz-rich) rocks, various species of Rhizocarpon are favored for lichenometry. These bright yellow-green colored lichens are easy to identify and measure in the field. Two approaches are possible for dating rock surfaces with lichenometry.

  1. Relative dating -- Average size of maximum lichen thalli can give an indication of relative age (young vs. old) when comparing different landforms. This information can be used in the absence of any other type of dating.

  2. Absolute dating -- The sizes of lichen thalli are calibrated with some independent dating method, such as radiocarbon or dendrochronology, to determine lichen growth rates (mm per year). On this basis, lichen size is an indicator for absolute age of the rock surface; however, use of a lichen growth curve is restricted to the immediate vicinity in which it was developed.

Either method is limited by the eventual growth of lichens to cover rock surfaces. When this happens, the maximum size of lichen thalli is dependent on the density of initial colonization of the surface and competition between lichens when they come into contact with each other rather than age. Lichenometry has been used widely in the Rocky Mountains for dating Holocene moraines and other rock surfaces. Luckman and Osborn (1979) and Osborn (1985) found lichenometry can provide absolute ages for relatively young (less than 500-year old) glacial deposits in the Canadian Rockies, but only relative ages for older neoglacial deposits.

The following examples demonstrate lichens on periglacial features in vicinity of Trinchera Peak, Sangre de Cristo Mountains, southern Colorado. Periglacial phenomena include inactive rock glaciers (Wallace and Lindsey 1996), as well as stone polygons, terraces, and talus streams. Lichen coverage shows considerable variation on these landforms, indicating different ages for most recent activity. The youngest periglacial forms are located on the steepest slopes. The Sangre de Cristo Mountains were subjected to a series of late Holocene cold climate intervals that resulted in cirque glacier advances and/or periglacial phenomena--see Table 10-1.

Table 10-1. Chronology of late Holocene cold climate episodes in the Sangre de Cristo Mountains of northern New Mexico.
Climatic episode Age*
Periglacial event
ca. 4900 yr BP
Cirque glaciation
ca. 3700 yr BP
Periglacial event
ca. 2800 yr BP
Little Ice Age
ca. 120 yr BP
* Approximate date in 14C years before present.
Based on Armour et al. (2002).

Jet Tilton kneels at the center of a stone polygon on the crest of Trinchera Peak, approx. 13,500 feet (4100 m) elevation. Photo date 8/01, © J.S. Aber.
Closeup view of stones in polygon on crest of Trinchera Peak. Note dense coverage of lichens. Large, lime-green lichens are Rhizocarpon. These stones have been relatively stable for several centuries. Photo date 8/01, © J.S. Aber.
Closeup view of stones in talus stream on steep slope. Note dense lichen coverage and growth of grass and shrubs. These features indicate a stable landform for some time. Photo date 8/02, © J.S. Aber.
Stone terrace on steep slope below Trinchera Peak. Many of the upturned stones bear few lichens, and Rhizocarpon thalli are small in size. This terrace probably was active during the Little Ice Age in the 19th century. Photo date 8/01, © J.S. Aber.

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