Using SPIRAL (Single Pollen Isotope Ratio AnaLysis) to estimate C<sub>3</sub>- and C<sub>4</sub>-grass abundance in the paleorecord

C3 and C4 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 C3 and C4 grass reconstructions. This technique, SPIRAL (Single Pollen Isotope Ratio AnaLysis), has only been evaluated using pollen of known C3 and C4 grasses. To test the ability of SPIRAL to reproduce the abundance of C3 and C4 grasses on the landscape, we measured δ 13C values of >1500 individual grains of grass pollen isolated from the surface sediments of ten lakes in areas that span a large gradient of C3- and C4-grass abundance, as determined from vegetation surveys. Results indicate a strong positive correlation between the δ 13C-based estimates of % C4-grass pollen and the abundance of C4 grasses on the landscape. The % C4-grass pollen slightly underestimates the actual abundance of C4 grasses at sites with high proportions of C4 grasses, which can be corrected using regression analysis. Comparison of the % C4-grass pollen with C/N and δ 13C measurements of bulk organic matter illustrates the distinct advantages of grass-pollen δ 13C as a proxy for distinguishing C3 and C4 shifts within the grass family. Thus SPIRAL promises to advance our understanding of grassland ecology and evolution.

C3和C4草本植物在对环境控制的响应以及对生物地球化学过程（如水、碳和养分循环）的影响方面存在很大差异。区分这两类草本植物功能群的困难阻碍了以草本植物为主的生态系统的古生态研究。使用与同位素比率质谱仪相连的线轴式微燃烧装置对单个草本植物花粉粒进行稳定碳同位素分析，有望改进对C3和C4草本植物的重建。这种技术，即SPIRAL（单花粉同位素比率分析），仅使用已知的C3和C4草本植物的花粉进行过评估。为了测试SPIRAL重现景观中C3和C4草本植物丰度的能力，我们测量了从10个湖泊的表层沉积物中分离出的1500多个单个草本植物花粉粒的δ13C值，这些湖泊位于C3和C4草本植物丰度梯度较大的区域，其丰度是通过植被调查确定的。结果表明，基于δ13C的C4草本植物花粉百分比估计值与景观中C4草本植物的丰度之间存在很强的正相关。在C4草本植物比例较高的地点，C4草本植物花粉百分比略微低估了C4草本植物的实际丰度，这可以通过回归分析进行校正。将C4草本植物花粉百分比与大块有机物的C/N和δ13C测量值进行比较，说明了草本植物花粉δ13C作为区分草本植物家族内C3和C4转变的替代指标的明显优势。因此，SPIRAL有望增进我们对草原生态和演化的理解。