Stable-carbon isotope composition of Poaceae pollen: an assessment for reconstructing C 3 and C 4 grass abundance David M. Nelson, 1 * Feng Sheng Hu 1,2,3 * and Robert H. Michener 4 ( 1 Program in Ecology and Evolutionary Biology, University of Illinois, 265 Morrill Hall, 505 S. Goodwin Avenue, Urbana IL 61801, USA; 2 Department of Plant Biology, Universityof Illinois, 265 Morrill Hall, 505 S. Goodwin Avenue, Urbana IL 61801, USA; 3 Department of Geology, University of Illinois, 245 Natural History Building, 1301 W. Green Street, Urbana IL 61801, USA; 4 Department of Biology, Stable Isotope Laboratory, Boston University, 5 Cummington Street, Boston MA 02215, USA) Received 25 October 2005; revised manuscript accepted 23 February 2006 Abstract: 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 d 13 C(d 13 C p ) of Poaceae (the grass family) as a proxy indicator for palaeoecological studies. On average d 13 C p differs by /13 between modern C 3 ( / 22.6 to / 26.8) and C 4 ( /9.2 to / 17.7) grass species. d 13 C 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. d 13 C 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 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 d 13 C p -based estimates of C 4 plant abundance were lower than charcoal d 13 C-based estimates, probably reflecting different source areas of pollen and charcoal. Grass d 13 C p also revealed greater submillennial-scale variability in C 3 and C 4 abundance than charcoal d 13 C. These results suggest that grass d 13 C p can provide palaeoenvironmental information not available from other proxy indicators. Key words: Stable-carbon isotope, d 13 C, Poaceae, C 3 and C 4 grasses, prairie, palaeoecology, pollen, palynology, modern analogue. Introduction Grass-dominated communities occupy a significant portion of Earth’s land surface (Saugier and Roy, 2000) and play a key role in global biogeochemical cycles (eg, Knapp et al., 2002). In these communities, C 3 and C 4 plants respond differently to variation in environmental factors, such as temperature, moisture availability and atmospheric CO 2 concentrations. For example, C 4 plants are physiologically more adapted to moisture deficits than C 3 plants, and C 3 plants will probably fare better than C 4 plants as atmospheric CO 2 concentrations rise (Long, 1999; Sage et al., 1999). These two functional groups possess distinct carbon-isotope compositions primarily because they use different enzymes for initial carbon fixation: ribulose-1,5-bisphosphate for C 3 plants and phosphoenolpyr- uvate for C 4 plants. Ribulose-1,5-bisphosphate discriminates more strongly against 13 C than phosphoenolpyruvate (O’Leary, 1981), causing C 3 plants to be more depleted in 13 C than C 4 plants (Cerling, 1999). To estimate past fluctua- tions in the relative abundance of C 3 and C 4 plants, recent studies relied on d 13 C values from a variety of substrates, *Authors for correspondence (e-mail: dmnelson@life.uiuc.edu, fshu@ life.uiuc.edu) The Holocene 16,6 (2006) pp. 819  825 # 2006 SAGE Publications 10.1191/0959683606hol974rp