branch of paleoecology for the late Pleistocene and Holocene. Uses of 

 paJeoecological tools include: (a) the establishment of relative chronologies 

 and indirect dating by means of correlation with other dated sequences; 

 (b) characterization of depositional environments at or near the sampling site 

 since certain species and combinations of species are adapted to certain 

 conditions; (c) reconstruction of the paleoenvironmental and paleoclimatic 

 conditions; (d) establishment of human-induced transformations of the vegeta- 

 tion and land use regime (Oldfield 1981). 



The use of weathering and coating indices for relative age dating in 

 geomorphology is rapidly increasing. Using laboratory microscopes, samples 

 are calibrated with those of known age and similar chemistry for each geo- 

 graphic area. One such method, obsidian hydration dating, is based on the 

 reaction of the surface of obsidian with water from the air or soil, which 

 produces a rind whose thickness increases with time (Pierce, Obradovich, and 

 Friedman 1976). Rock varnish-cation ratio dating is used primarily in deserts, 

 where rocks develop a coating (Dorn 1983). One study used dated graffiti to 

 determine the rates of erosion and weathering in sandstone cliffs (Emery 

 1941). 



Varve chronology may be useful in quiescent or low energy basins where 

 thin laminae of clay and silt are deposited. In glaciated coastal areas, the thin 

 layers or varves are usually annual deposits. The sequences of successive 

 graded layers can be discerned visually. Color variations occur because 

 usually the winter season deposits have a higher organic material content. 

 The result is alternating light-colored, gray-brown sediment layers and dark- 

 colored organic layers. 1 Varve chronology rarely extends beyond about 

 7,000 years. 



A major limitation of varve chronology is the fact that in the marine 

 environment, annual varves are usually only preserved in anoxic basins, where 

 a lack of oxygen causes a dearth of bottom-dwelling animals. Otherwise, 

 mollusks, worms, fish, and crustaceans thoroughly rework the seafloor. This 

 reworking, known as bioturbation, thoroughly destroys near-surface 

 microstructure in most of the shallow-water portions of the world's oceans. 

 Examples of anoxic basins include portions of the Black Sea and Saanich inlet 

 in British Columbia. The latter receives an annual input of clays from the 

 Fraser River. Yearly variations in the discharge of the Fraser River's spring 

 freshet cause changes in the varve thicknesses. 



Physical Models 



The use of physical models can be invaluable in understanding how 

 geomorphic variability occurs in coastal areas. Physical modelling provides 



76 



In freshwater lakes, varves are caused by clay-silt deposition cycles. The silt settles out in 

 spring and summer, and the clay in fall and winter. 



Chapter 4 Laboratory Techniques and Approaches 



