Wednesday, August 6, 2008
Exhibit Hall CD, Midwest Airlines Center
David Nelson, University of South Alabama, Mobile, AL, Feng Sheng Hu, Department of Plant Biology, Department of Geology, and Program in Ecology, Evolution, and Conservation Biology, University of Illinois at Urbana-Champaign, Urbana, IL, Dirk Verschuren, Department of Biology, Ghent University, Gent, Belgium and Ann Pearson, Earth and Planetary Sciences, Harvard University, Cambridge, MA
Background/Question/Methods Grass-dominated ecosystems cover about one-third of Earth’s land surface, influence key biogeochemical processes, and serve as major food sources for humans and herbivores. One of the major challenges in studying paleorecords of grasslands is that pollen assemblages are uninformative about C3 and C4 grass variations because grass pollen generally cannot be morphologically identified below the family level. We report here (1) a novel technique for analyzing the stable carbon isotope composition of individual grass-pollen grains using a spooling-wire microcombustion device interfaced with an isotope-ratio mass spectrometer, and (2) an application of this technique to investigate C4-grass responses to changes in pCO2, temperature, and moisture over the last ~25,000 years at Lake Challa, East Africa.
Results/Conclusions
Our results indicate that SPIRAL reliably distinguishes pollen from C3 and C4 grasses from herbarium specimens. A strong positive correlation also exists between the estimated proportion of C4-grass pollen in surface lake sediments and the abundance of C4 grasses in modern vegetation at ten sites across North America. Preliminary data from sediment core samples from Lake Challa show that C4-grass abundance fluctuates greatly, having a generally smaller contribution before ~9,000 yr BP and a generally higher abundance since then. Low pCO2 during the last glacial did not lead to the highest values of C4-grass abundance. This contrasts with bulk-sediment and leaf-wax δ13C-based records from the region. One plausible explanation is that leaf-wax δ13C reflects the abundance of C3 and C4 grasses plus non-grass C3 and C4 plants, whereas the SPIRAL unambiguously indicates grasses. Leaf-wax δ13C may overestimate C4-grass abundance because aquatic sedges (many of which are C4) were more abundant and trees (all C3) were less abundant during the last glacial than during the Holocene. Our results also suggest that C4 grasses were neither favored by severe drought (~20,000-15,000 yr BP) nor by wet conditions (~11,000-9,000 yr BP). C4-grass abundance was generally low during the last glacial-interglacial transition, which contrasts with the expectation of steadily increasing C4-grass abundance that should have occurred if temperature were the main driver of C3- and C4-grass proportions. We hypothesize that generally greater C4-grass abundance during the Holocene than during the last glacial resulted from more active fire regimes during the Holocene and that the effects of pCO2 and climate were manifest indirectly via changes in fuel loads.
