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

Wesleyan University, Middletown CT

Investigators

Abstract

Beuning; ATM 9912052 This 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.