| RADIODARBON DATING | THERMOLUMINESCENCE (TL) DATING | AMINO ACID DATING |
| URANIUM SERIES DATING | TEPHROCHRONOLOGY | HYDROGRAPH | REFERENCES |

RADIODARBON DATING

Background

The ¹⁴C isotope, known as radiocarbon, is continuously formed in the upper atmosphere. ¹⁴C atoms combine with oxygen to form carbon dioxide and therefore all living things contain a proportion of the radiocarbon. There is assumed to be a constant amount of radiocarbon in the atmosphere. The production of radiocarbon in the upper atmosphere is balanced by the radioactive decay of radiocarbon. All living things will contain the same proportion of ¹⁴C while living. Once an organism dies equilibrium of ¹⁴C with the atmosphere ceases and the ¹⁴C present begins to decay.

The radiocarbon dating method produces ages in radiocarbon years which must be converted (calibrated) to calendar years.

Radiocarbon dating depends on knowing how much ¹⁴C was in an organism prior to death, the decay rate, and how much ¹⁴C remains.

Dateable organic carbon materials include charcoal, wood, peat, soil humates, seeds and other small plants, algae residues, feces, and bone collagen. Dateable carbonate carbon materials include mollusk aragonite, ostracod aragonite, marl, saline mudflat dolomite, tufa, ooid aragonite, travertine calcite, charaphyte, and shells.

Limits

The upper limit of this dating method is approximately 50,000 years BP. Pitfalls of this dating technique Problems arise with inclusion of much older carbon at the time of deposition (old-carbon effect), deposition in a water body receiving effluents having been in contact with older carbonates (hard water effect), contamination with young carbon, and isotopic mass fractionation.

Use in the Bonneville basin

This dating method is by far the most widely used in Bonneville basin investigations. The following studies contain radiocarbon dating: Spencer and others (1984), McCoy (1987), Oviatt and others (1990), Thompson and others (1990), Oviatt and others (1992), and Oviatt and others (1994a).

THERMOLUMINESCENCE (TL) DATING

Background

This technique involves exposing minerals (quartz, feldspars, calcite, and clays) to heat until they emit light. Defects within the mineral crystal structure attract free electrons that are produced due to nuclear radiation exposure. By heating these crystals the electrons escape the crystal defects and migrate to another defect known as a luminescence center. This movement of free electrons results in the emission of photons which can be measured as a light signal. The amount of light obtained is related to the amount of radiation that the minerals have been exposed to in the natural environment since deposition.

In order for the TL value to be zeroed the mineral must be exposed to heat or sunlight for a long period of time. The sunlight bleaching effect does not empty all defects, but does empty the majority of them.

Optically-stimulated luminescence(OSL) is a relatively new technique within this field is of importance to determining radioactive exposure in sediments. Instead of heating the mineral this technique involves exposing it to lasers, which releases the electrons from the defects. This is useful in minerals that may have experienced brief exposure to light bleaching and thus were not zeroed well.

Limits

For most minerals used by this technique the maximum limit for reliable age determinations is about 100 Ma.

Pitfalls of this dating technique

One potential source of error involves laboratory calibration, which can vary from± 3 to 5 %. Also important is the possibility of differential bleaching in sediments deposited in water, thus giving varying TL levels.

Use in the Bonneville basin

We found essentially no information on this being done in the Bonneville basin. In the future we may want to check with Ed Haskel to see what is the status of TL or OSL research in the basin.

AMINO ACID DATING

Background

The amino acid dating method involves the ratio of L-amino acids and D-amino acids, which are different configurations of the same amino acid. These different configurations are called optical isomers. The amino acid most often used in dating methods is called isoleucine. Living organisms contain only L-isoleucine. Upon death the L-isoleucine configuration interconverts, called racemization or epimerization, with D-isoleucine until equilibrium is reached. Datable materials include shell, bone, antler, dental tissue, and wood.

Limits

Potential for dating of materials over the last 2.4 MA.

Pitfalls of this dating technique

The rate of racemization or epimerization is dependent on temperature, thus paleotemperature must be taken into account. There exist intra-shell variations which require for the same portion of each shell be analyzed in a given study.

Use in the Bonneville basin

Fair amount of work employing this method in the Bonneville basin by Oviatt and others (1994a) and McCoy (1987).

URANIUM SERIES DATING

Background

The ²³⁰Th/²³⁴U method involves using the ratio of parent to daughter present in a given material. ²³⁰Th content in waters is usually negligible and therefore not incorporated by given materials. ²³⁴U is incorporated into materials due to being soluble in the 6+ state where it combine with two oxygen. With time ²³⁰Th, the daughter product of ²³⁴U, builds up within the materials. The uranium disequilibrium series dating method is often used on such materials as speleothems, travertines, caliches, molluscs, corals, bone, teeth, lacustrine sediments, evaporites, phosphorites, and peat.

Limits

The effective range of the ²³⁰Th/²³⁴U method is 350 ka for alpha spectrometry and 500 ka for MSU.

Pitfalls of this dating technique

²³⁰Th must not be present in material prior to deposition. The system should be closed post deposition to the migration of ²³⁰Th and ²³⁴U.

Use in the Bonneville basin

This method has been employed in the Bonneville basin by Oviatt and others (1994a).

TEPHROCHRONOLOGY

Background

Tephrochronology involves the use of thin volcanic ash layers, which are deposited over exposed surfaces at the time of eruption. The layers of tephra basically represent an isochronous horizon that can be traced over a certain area depending on the size of the eruptive event. By radiocarbon dating of organic material caught in the ash, the time of tephra deposition can be constrained.

Use in the Bonneville basin

Most of the work in the Bonneville basin has involved the Pavant Butte ash (approximately 16-15.3 ka), Tabernacle Hill ash(approximately 14.5-14.3 ka), and the "Thiokol" glass (25 ka). A good review of these tephra layers is in Oviatt and Nash (1989). The method has also been employed by Oviatt and others (1994b).

THE HYDROGRAPH

The hydrograph for the Lake Bonneville represents lake level throughout the past 32,000 years, which is constrained by radiocarbon dates as well as dates from a few other dating techniques mentioned above. Several other tools can be used to examine the accuracy of the hydrograph in a relative sense. These techniques include: stable isotopes, total carbonate, aragonite/calcite ratios (see Oviatt (1997), Oviatt and others (1994b), and Spencer and others (1984) for more information pertaining to Lake Bonneville); ostracode assembleges (see Forester (1987), Spencer and others (1984), and Thompson and others (1990) for more information relating to Lake Bonneville).

REFERENCES

Forester, Richard M., 1987, Late Quaternary paleoclimate records from lacustrine ostracodes, in Ruddiman, W.F. and Wright, H.E. Jr., eds., North America and adjacent oceans during the last deglaciation: Boulder, Colorado, Geological Society of America, The Geology of North America, v. K-3, p. 261-276.

Lowe, J.J. and Walker, M.J.C., 1997, Reconstructing Quaternary Environments: Longman, Essex, 446P.

McCoy, William D.,1987, Quaternary aminostratigraphy of the Bonneville basin, western United States: Geological Society of America Bulletin, v. 98, p. 99-112.

Oviatt, Charles G., 1997, Lake Bonneville fluctuations and global climate change: Geology, v. 25, p. 155-158.

Oviatt, Charles G. and Nash, William P., 1989, Late Pleistocene basaltic ash and volcanic eruptions in the Bonneville basin, Utah: Geological Society of America Bulletin, v. 101, p. 292-303.

Oviatt, Charles G., Currey, Donald R. and Miller, David M., 1990, Age and Paleoclimatic Significance of the Stansbury Shoreline of Lake Bonneville, Northeastern Great Basin: Quaternary Research, v. 33, p. 291-304.

Oviatt, Charles G., Currey, Donald R. and Sack, Dorothy, 1992, Radiocarbon chronology of Lake Bonneville, Eastern Great Basin, USA: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 99, p. 225-241.

Oviatt, Charles G., McCoy, William D. and Nash William P., 1994a, Sequence stratigraphy of lacustrine deposits: A Quaternary example from the Bonneville basin, Utah: Geological Society of America Bulletin, v. 106, p. 133-144.

Oviatt, Charles G., Habiger, Geoff D. and Hay, James E., 1994b, Variation in the composition of Lake Bonneville marl: a potential key to lake-level fluctuations and paleoclimate: Journal of Paleolimnology, v. 11, p. 19-30.

Roberts, Neil, 1998, The Holocene: An Environmental History: Biackwell Publishers, Oxford, 316p.

Smart, P.L. and Frances, P.D., 1991, Quaternary Dating Methods - A User's Guide, Quaternary Research Association Technical Guide No. 4, 233p.

Spencer, Ronald J., Baedecker, M.J., Eugster, H.P., Forester, R.M., Goldhaber, M.B., Jones, B.F., Kelts, K., Mckenzie, J., Madsen, D.B., Rettig, S.L., Rubin, M. and Bowser, C.J., 1984, Great Salt Lake, and precursors, Utah: the last 30,00 years: Contributions to Mineralogy and Petrology, v.86, p. 321-334.

Thompson, Robert S., Toolin, Laurence J., Forester, Richard M. and Spencer, Ronald J., 1990, Accelerator-mass spectrometer (AMS) radiocarbon dating of Pleistocene lake sediments in the Great Basin: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 78, p. 301-313.