POLLEN AND SPORES — LEOPOLD AND SOOTT 311 



and those in the unknown category (not shown) represent a total 

 of only 10 percent of the 1950 pollen rain. 



From studies of this type it can be seen that the pollen rain of an 

 area reflects the local vegetation only in a general way, and that the 

 representation of wind-pollinated species is better than that of in- 

 sect-pollinated species. The amount of total pollen rain may vary 

 from year to year depending on fluctuations in weather affecting 

 blooming and pollen dispersal, but during a 10-year period the per- 

 cent composition of the total sample remains somewhat similar for 

 a given station. 



FACTORS IN DEPOSITION 



Although the aerial pollen rain settles out at random as a fine 

 dust on land and water, pollen does not persist in numbers at or near 

 the soil surface owing to prolonged exposure to oxygen and to alter- 

 nate wet and dry conditions. Water-laid sediments that remain wet 

 for long periods of time and that are relatively deficient in oxygen 

 provide the conditions under which pollen is best preserved. Many 

 lakes and quiet lagoons have a low dissolved-oxygen content in their 

 deep-water layers and particularly at and below the mud-water inter- 

 face ( Vallenty ne, 1957) ; consequently, these environments, along 

 with acid peat bogs, furnish extensive areas for the accumulation of 

 pollen. 



Because the pollen rain is progressively less in the seaward direc- 

 tion, sedimentary environments in which pollen concentrations can 

 be found are limited to near-shore sites in marine and lacustrine 

 waters. Pollen and spores from modern marine sediments are often 

 associated with microalgae, diatoms, and other forms of oceanic 

 plankton; conversely, assemblages from fresh-water sediments often 

 include typically fresh-water microorganisms. Evidence of this sort 

 provides the paleontologist with a way to recognize the environment 

 of deposition of a fossil sample. 



Pollen and spores are of silt size and are readily eroded with the 

 sediment in which they are imbedded. Modern fluvial erosion of 

 Tertiary pollen-bearing rocks, followed by transport and deposition 

 of the Tertiary pollen in a modern stream terrace, is not uncommon. 

 The situation can be recognized from the resulting mixture of extinct 

 or ecologically displaced pollen types with a modern assemblage. 



Redeposition of pollen has been observed in sediments originating 

 during periods of rapid erosion by glacial meltwater streams drain- 

 ing ice masses that eroded older, pollen-bearing beds (Iversen, 1936). 

 Redeposited pollen does not seem to be important in most highly 

 organic sediments such as peats, coals, and black muds, but the paly- 

 nologist must be constantly on the alert for it. 



451800—58 21 



