Stable-carbon isotope composition of Poaceae pollen:: an assessment for reconstructing C3 and C4 grass abundance

Plants using the C-3 and C-4 photosynthetic pathways differ in carbon-isotope composition, and this difference offers a means to estimate the relative abundance of these two functional groups in the palaeorecord. We report here results of a study aiming to evaluate pollen delta C-13 (delta C-13(P)) of Poaceae (the grass family) as a proxy indicator for palaeoecological studies. On average delta C-13(P) differs by similar to 13 parts per thousand between modern C-3 (-22.6 to -26.8 parts per thousand) and C-4 (-9.2 to -17.7 parts per thousand) grass species. delta C-13(P) is 1.2-3.7 parts per thousand more negative for modern grass pollen treated with the same protocol as for fossil samples than for untreated modern samples. delta C-13(P) ranges from -20.1 to -25.4 parts per thousand for grass pollen in the middle-Holocene sediments from West Olaf Lake, located near the modern tallgrass prairie-forest ecotone in western Minnesota. We applied a two end-member mixing model to estimate fluctuations in C-3 and C-4 grass abundance around this lake. Both C-3 and C-4 grasses expanded relative to Ambrosia and Artemisia with the decline of aridity from 8000 to 4000 BP. C-3 grasses were generally more abundant than C-4 grasses throughout the middle Holocene, suggesting the presence of mixed-grass prairie around West Olaf Lake. The grass delta C-13(P)-based estimates Of C-4 plant abundance were lower than charcoal delta C-13-based estimates, probably reflecting different source areas of pollen and charcoal. Grass delta C-13(P) also revealed greater submillennial-scale variability in C-3 and C-4 abundance than charcoal delta C-13. These results suggest that grass delta C-13(P) can provide palaeoenvironmental information not available from other proxy indicators.