Using SPIRAL (Single Pollen Isotope Ratio AnaLysis) to estimate C3- and C4-grass abundance in the paleorecord

C(3) and C(4) grasses differ greatly in their responses to environmental controls and influences on biogeochemical processes (e.g. water, carbon, and nutrient cycling). Difficulties in distinguishing between these two functional groups of grasses have hindered paleoecological studies of grass-dominated ecosystems. Stable carbon isotopic analysis of individual grains of grass pollen using a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer holds promise for improving C(3) and C(4) grass reconstructions. This technique, SPIRAL (Single Pollen Isotope Ratio AnaLysis), has only been evaluated using pollen of known C(3) and C(4) grasses. To test the ability of SPIRAL to reproduce the abundance of C(3) and C(4) grasses on the landscape, we measured delta(13)C values of > 1500 individual grains of grass pollen isolated from the surface sediments of ten takes in areas that span a large gradient of C(3)- and C(4)-grass abundance, as determined from vegetation surveys. Results indicate a strong positive correlation between the delta(13)C-based estimates of % C(4)-grass pollen and the abundance of C(4) grasses on the landscape. The % C(4)-grass pollen slightly underestimates the actual abundance of C(4) grasses at sites with high proportions of C(4) grasses, which can be corrected using regression analysis. Comparison of the % C(4)-grass pollen with C/N and delta(13)C measurements of bulk organic matter illustrates the distinct advantages of grass-pollen delta(13)C as a proxy for distinguishing C(3) and C(4) shifts within the grass family. Thus SPIRAL promises to advance our understanding of grassland ecology and evolution. (C) 2008 Elsevier B.V. All rights reserved.