It has been used for dating a variety of Pleistocene sediments, including loess. 

 For geological purposes, TL needs further refinement because most results to 

 date are considered in error, generally being too young. It does, however, 

 generally provide a good estimate of stratigraphic order. Thermoluminescence 

 dating has the best potential where clay-fired artifacts are present and has 

 promise for dating a variety of deposits of Quaternary age. 



Magnetostratigraphy or paleomagnetic dating is a geochronologic technique 

 that is used in conjunction with correlations of regional radiometric dates and 

 paleomagnetic characteristics. Because the earth's magnetic field changes 

 constantly, the magnetic characteristics of rock and sediments can be used to 

 determine an age for materials. The most dramatic changes are reversals, in 

 which the earth's polarity switches from the north to the south pole. The 

 reversals are relatively infrequent occurrences, with the last one being 

 700,000 years ago. Less dramatic secular variations of the geomagnetic field, 

 however, can also be important in helping to provide a time scale useful for 

 dating over hundreds or thousands of years by linking magnetic properties 

 with time scales established by radiometric techniques. The combination of 

 declination (the angle between true and magnetic north), inclination (the dip of 

 the earth's magnetic field), and the magnetic intensity produce a characteristic 

 paleomagnetic signature for a particular location and time. The magnetic 

 alignments can be incorporated and preserved in baked materials, in sediment 

 particles that settle out in standing water, and in cooled magma. The tech- 

 nique is most suited to lake sediments containing homogeneous particle sizes 

 and organics. This technique can be used in places where the magneto- 

 stratigraphy has been linked with radiometric dates and can be extended to 

 over 200 million years before present. 



Dendrochronology or tree ring dating can provide precise data regarding 

 minimum age of a geomorphic surface. It can also provide proxy data con- 

 cerning environmental stresses, including climatic conditions such as cold 

 temperatures and droughts. In some parts of the world, overlapping sets of 

 rings on trees have been used to construct a comprehensive environmental 

 history of the region. 



Lichenometry is the study of the establishment and development of lichen 

 to determine a relative chronology (Worsley 1981). Although used most 

 extensively for studies of glacier fluctuations, this technique also has applica- 

 tion in shoreline dating. The method involves the measurement of thallus 

 size, with increasing diameter representing increasing age. It is valid from 

 about 10 years to a few centuries before present. This measurement is often 

 conducted in the field with a ruler or with calipers. Field techniques differ, 

 although normally the largest diameters are measured. Although there has 

 been a lack of critical assessment of the technique, the majority of research 

 shows that the technique gives reasonable dates when applied to a variety of 

 environments. 



Paleoecology is the study of fossil organisms in order to reconstruct past 

 environments. Pollen analysis, or palynology, is the single most important 



Chapter 4 Laboratory Techniques and Approaches 



75 



