74 



Cesium-137 ( 137 Cs) is an artificial isotope, primarily produced during the 

 atmospheric testing of nuclear weapons. These tests began in the 1940's, 

 peaked in the early 1960s, and have declined since the advent of nuclear test 

 ban treaties (Wise 1980). 137 Cs is strongly absorbed onto sediment or soil 

 and has been used in studies of soil erosion and sediment accumulation in 

 wetlands, lakes, and floodplains. The timing of very recent events (post- 

 1954) and human impacts on coastal ecosystems can be improved using such 

 techniques. 



Lead-210 ( 210 Pb) is an unstable, naturally occurring isotope with a half-life 

 of just over 22 years and a dating range of 100 to 200 years (Oldfield and 

 Appleby 1984; Wise 1980). It forms as part of a decay chain from 

 Radium-226 which escapes into the atmosphere as the inert gas Radon-222. 

 The excess or unsupported 210 Pb returns to the earth as rainfall or dry fallout, 

 and can be separated from that produced by in situ decay. Applications in 

 coastal environments are limited but show good potential. This technique 

 would be of greatest value in low-energy environments and would allow 

 documentation of the timing of recent events and human impacts on coastal 

 ecosystems. 



Thorium-230/Uranium-234 ( 230 Th/ 234 U), a useful dating technique which 

 complements other methods, is applicable for dating coral sediments. The 

 technique involves comparing the relative amounts of the radioactive isotope 

 of thorium, 230 Th, with that of uranium, 234 U. Thorium-230 increases in 

 coral carbonate from zero at the death of the organism to an equilibrium with 

 Uranium-234 at 0.5 million years, allowing samples as old as middle 

 Pleistocene to be dated. 



Non-Radiometric Methods of Dating and Relative Dating 



Archival and archeological documentation can assist in understanding the 

 geologic history of coasts. Historical and social documents may contain 

 detailed descriptions of timing of major storms, of ice movements, of shore- 

 line changes, and of other catastrophic events. Historical records are most 

 useful if they correspond to a particular date or specified range of time, as do 

 newspaper reports. Archeological evidence can provide important clues for 

 assessing Holocene environmental changes. Pottery, stone tools, coins, and 

 other artifacts can be assigned ages and thus may be of assistance in dating 

 surface and subsurface deposits. If discovered in a stratigraphic sequence, 

 cultural artifacts provide a minimum age for deposits beneath and maximum 

 age for deposits above. Archeological evidence, such as buried middens, 

 inland ports, or submerged buildings, may also indicate shoreline changes and 

 sometimes can be used to estimate rates of deposition in coastal areas. For 

 example, the Holocene Mississippi River deltaic chronology was revised using 

 artifacts as indicators of the age of the deltaic surfaces (Mclntire 1958). 



Thermoluminescence (TL), a technique that is commonly practiced in 

 archeology for dating pottery, has been extended for use in geological studies. 



Chapter 4 Laboratory Techniques and Approaches 



