Stable carbon isotope composition of Poaceae pollen and its potential in paleovegetational reconstructions

Stable carbon isotope differences between ecologically distinct groups of Poaceae (C-3 and C-4 photosynthetic groups) provide a means of isotopically subdividing grass pollen in paleovegetation studies. We examined the isotopic composition of bulk grass plant tissue, untreated pollen, and chemically treated pollen, from several C-3 and C-4 grass species. Based on our data, untreated pollen is isotopically similar to the host plant from which it is derived, although small, random differences between plants and pollen occur. Methods of pollen concentration involving carbon-bearing compounds can alter the isotopic composition of recovered pollen, and in some cases, make pollen from different grass types isotopically indistinguishable. We conclude that the isotopic composition of physically separated Poaceae pollen should be an important means of determining the proportion of C-3/C-4 grasses as long a carbon-bearing chemicals are not used in sample preparation. The carbon isotope composition of pollen should provide a new means of determining paleoclimatic conditions in grassland environments and aid in identifying the origin of the C-4 photosynthetic pathway in the geologic past. (C) 1997 Elsevier Science B.V.