Pollen analyses form the data base for many types of inferences ranging from sequential changes in past environments to lifestyles and diets of prehistoric human populations. In all cases, interpretation of pollen data must account for those factors that may have influenced the composition of the original pollen rain, and also for those factors that may have altered the composition of the pollen assemblage following deposition.
During the last 50 years palynologists have learned that there are many complex factors that determine the original composition of the pollen rain in an arid region. These include mode of pollination, differences in pollen production, differential dispersion patterns, and the size, weight, and aerodynamic ability of pollen types to remain airborne (see Chaps. 3, 13, 14, and other related chapters in this volume). Following deposition, other factors influence eventual loss or recovery of specific pollen types. These factors include pollen redeposition, the chemical composition of a pollen grain's exine, its morphological shape and types of surface ornamentation, and its susceptibility to various types of degradation processes including those from mechanical, chemical, or biological agents (Bryant, 1978, 1988; Bryant & Holloway, 1983; Hollo way, 1989; O'Rourke, 1990). In this chapter we focus on the post depositional degradation processes.
One of the first agents that can affect pollen grains is mechanical degradation. After pollen is released from its source, it can become abraded or broken during the transportation phase. These alterations can result from impact or from changes in the natural environment.