Paleodistributions of C3 and C4 Grasses in Tropical East Africa- Climatic Implications

这项研究提出了一种新的方法，通过对沉积岩心中提取的化石草和花粉进行碳同位素分析，重建C3和C4草分布的过去变化。这种方法的优点是，C3和C4植物很容易通过它们的碳同位素特征来区分，而草花粉忠实地记录了寄主植物的碳同位素特征。因此，过去C3植物与C4植物的转换将被记录在草的花粉同位素记录中。由于草花粉记录的漂移将与冰芯对二氧化碳的估计结合使用，以提供与湿度无关的、受限制的热带LGM大陆温度变化估计。工作计划包括对来自热带东非的已确认的现代牧草的花粉进行分析，以测试新的方法，并确认草花粉碳同位素信号的完整性。对乌干达西部四个湖泊的表层沉积物样本进行分析，结合对为这些盆地提供花粉的低地环境中草物种丰度的半定量估计，将提供必要的现代类比，以与化石价值进行比较。过去气候变化对草原群落的影响将通过分析从至少一个火山口湖采集的岩芯中保存的最后一次冰川沉积物样本中保存的草花粉来评估。拟议研究的结果将完善对热带东非在末次盛冰期期间冷却的估计，并将通过提供古生态数据来改进对从末次盛冰期至今陆地碳库变化的模型预测，从而有助于全球碳收支估计。

Beuning; ATM 9912052This study proposes a new methodology to reconstruct past changes in C3 vs C4 grass distributions by carbon isotopic analysis of fossil grass and pollen extracted from sediment cores. The advantage of this methodology is that C3 and C4 plants are easily distinguished by their carbon isotopic signatures, and grass pollen faithfully records the carbon isotopic signature of the host plant. Therefore, past shifts C3 vs. C4 plants will be recorded by the grass pollen isotopic record. As shifts recorded by the grass pollen will be used in conjunction with ice core estimates of pCO2 to provide moisture-independent, constrained estimates of LGM continental temperature change in the tropics. The work plan includes analysis of pollen from identified modern grasses from tropical East Africa to test mew methodologies and confirm integrity of the grass pollen carbon isotopic signal. Analysis of surface sediment samples from four lakes in western Uganda in conjunction with semi-quantitative estimates of grass species abundance in the lowland environments contributing pollen to these basins will provide necessary modem analogs for comparison with fossil values. Impacts of past climate change on the grassland communities will be assessed through analysis of grass pollen preserved in Last Glacial sediment samples from a core collected in at least one crater lake. Results of the proposed research will refine estimates of tropical East African cooling during the LGM and will contribute to global carbon budget estimates by providing paleoecological data to improve model predictions of change in terrestrial carbon reservoirs from the LGM to present day.