312 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



The quantity of pollen and spores in a sediment sample is deter- 

 mined by the relationship between the density of the pollen rain and 

 the rate of accumulation of the sediment. Maximum concentrations 

 of pollen are produced by dense pollen rains in combination with 

 slow deposition of sediment. Original low density of pollen in a 

 deposit can result from either a light pollen rain or from dilution of 

 a heavy pollen rain by rapidly accumulating sediments. Examples 

 of these relations are shown diagrammatically in figure 5. Bars in 

 the lower part of the figure show the probable rate of annual sedi- 

 ment accumulation ; those in the upper part form a record on a loga- 

 rithmic scale of the number of pollen grains per gram (dry weight) 

 of sediment. 



Rich in pollen are samples of lake peat and lake clay from Durham, 

 Conn, (on right, fig. 5). It has been determined by carbon-14 dating 

 that these sediments were deposited very slowly, perhaps at the rate 

 of only one millimeter per year. The rich pollen flora contained in 

 these sediments indicates that at the time of deposition well-developed 

 coniferous forests grew near the lake. In all probability the annual 

 pollen rain was like that of coniferous forests now growing in central 

 New England, perhaps 70,000 grains per square centimeter per year. 



A sample of varved (laminated) glacial clay from Hartford, Conn., 

 poor in pollen (fig. 4), is laminated in a manner that indicates rapid 

 sedimentation. The varves, which are 10 millimeters apart, may mark 

 increments of sediment that accumulated annually or oftener. Al- 

 though the pollen rain may have been less than the present fallout at 

 that site, the low pollen content in this sediment seems to have been 

 due primarily to dilution of the pollen by rapid sedimentation. 



The most sterile sediment among these examples is a modern la- 

 goonal mud from Kapingamarangi Atoll in the South Pacific. This 

 sample, along with 10 others from different depths, contains 25 or 

 fewer pollen grains per gram of sediment (McKee et al., MS.). The 

 total land area of the atoll islands that surround this lagoon is less 

 than one square mile, and does not support sufficient vegetation to 

 furnish large quantities of pollen locally. In spite of the slow rate 

 of sediment accumulation at these sampling stations (McKee et al., 

 MS.), the pollen density remains low owing to the limited numbers 

 of source plants. 



POLLEN- AND SPORE-BEARING ROCKS AND THEIR LABORATORY 



TREATMENT 



Unweathered sediments originating in reducing environments are 

 most apt to contain pollen; these include marine and fresh-water 

 shales, limestones, siltstones, coals, peats, and lignites. High organic 

 content, usually manifested by dark color, is often an indication of 



