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

Program in Ecology and Evolutionary Biology, University of Illinois, 265 Morrill Hall, 505 S. Goodwin Avenue, Urbana IL 61801, USA, dmnelson{at}life.uiuc.edu

Feng Sheng Hu

Program in Ecology and Evolutionary Biology, University of Illinois, 265 Morrill Hall, 505 S. Goodwin Avenue, Urbana IL 61801, USA, Department of Plant Biology, University of Illinois, 265 Morrill Hall, 505 S. Goodwin Avenue, Urbana IL 61801, USA, Department of Geology, University of Illinois, 245 Natural History Building, 1301 W. Green Street, Urbana IL 61801, USA, fshu{at}life.uiuc.edu

Robert H. Michener

Department of Biology, Stable Isotope Laboratory, Boston University, 5 Cummington Street, Boston MA 02215, USA

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 ¹³C(¹³C_p) of Poaceae (the grass family) as a proxy indicator for palaeoecological studies. On average ¹³C_p differs by 13- between modern C₃ (-22.6 to - 26.8) and C₄ (-9.2 to -17.7) grass species. ¹³C_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. ¹³C_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 ¹³C_p-based estimates of C₄ plant abundance were lower than charcoal ¹³C-based estimates, probably reflecting different source areas of pollen and charcoal. Grass ¹³C_p also revealed greater submillennial-scale variability in C₃ and C₄ abundance than charcoal ¹³C. These results suggest that grass ¹³C_p can provide palaeoenvironmental information not available from other proxy indicators.

Key Words: Stable-carbon isotope • 13C • Poaceae • C3 and C4 grasses • prairie • palaeoecology • pollen • palynology • modern analogue

The Holocene, Vol. 16, No. 6, 819-825 (2006)
DOI: 10.1191/0959683606hol974rp


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