The distance travelled by air-borne pollen and spores depends greatly on their size, weight, sculpture and on atmospheric conditions. They are most frequently found about 350-650 m above the land surface during the day, but many sink to the surface at night or are brought down by rainfall. Under favourable conditions pollen grains have been known to drift for at least 1750 km, but about 99% tend to settle within 1 km of the source. Only a very small proportion ever reaches the oceans by aerial dispersal.
Once the pollen grains or spores have settled, they stand a chance of entering the fossil record, either by falling directly into bogs, swamps or lakes, or by being washed into them and into rivers, estuaries and seas. By this stage the pollen record has already been filtered by differential dispersal in the air and may now undergo a similar filtering in water. For example, size sorting across the continental shelf can occur; large miospores, pollen grains and megaspores will tend to settle out in rivers, estuaries, deltas or shallow shelf areas, whereas small miospores and pollen grains may settle out in outer shelf and oceanic conditions. Those which are not buried in reducing sediments will tend to become oxidized and may ultimately be destroyed.
Spores and pollen may suffer several cycles of reworking and redeposition, leading to some confusion in the fossil record. Experienced palynologists detect these reworked forms by differences in preservation (e.g. colour, corrosion, abrasion and fragmentation), ecological or stratigraphical inconsistencies and associated evidence for reworking.
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