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

Plants using the C₃ and C₄ 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 δ¹³C(δ¹³C_p) of Poaceae (the grass family) as a proxy indicator for palaeoecological studies. On average δ¹³C_p differs by ∼ 13- between modern C₃ (-22.6 to - 26.8) and C₄ (-9.2 to -17.7) grass species. δ¹³C_p is 1.2-3.7 more negative for modern grass pollen treated with the same protocol as for fossil samples than for untreated modern samples. δ¹³C_p ranges from -20.1 to -25.4 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 C₃ and C₄ grass abundance around this lake. Both C₃ and C₄ grasses expanded relative to Ambrosia and Artemisia with the decline of aridity from 8000 to 4000 BP. C₃ grasses were generally more abundant than C₄ grasses throughout the middle Holocene, suggesting the presence of mixed-grass prairie around West Olaf Lake. The grass δ¹³C_p-based estimates of C₄ plant abundance were lower than charcoal δ¹³C-based estimates, probably reflecting different source areas of pollen and charcoal. Grass δ¹³C_p also revealed greater submillennial-scale variability in C₃ and C₄ abundance than charcoal δ¹³C. These results suggest that grass δ¹³C_p can provide palaeoenvironmental information not available from other proxy indicators.