Paper No. 5
Presentation Time: 2:45 PM
HOW FIRE, PCO2, AND CLIMATE CONTROL C3/C4 VARIATION: A LATE-QUATERNARY PERSPECTIVE FROM EQUATORIAL EAST AFRICA
Grass-dominated ecosystems cover one-third of Earth’s land surface, influence key biogeochemical processes, and serve as major food sources. A challenge in studying the response of grasslands to environmental change in paleorecords is that grass pollen generally has low taxonomic specificity, which means that pollen assemblages are uninformative about C3 and C4 grass variations. To overcome this difficulty, we have developed a novel technique for analyzing the stable carbon isotopic composition of individual grass-pollen grains (SPIRAL). We conducted SPIRAL and charcoal analyses of lake sediments to assess how fire and climate affected C4-grass abundance during the late Quaternary in equatorial East Africa. Fire enacts a strong influence on the maintenance of modern tropical grasslands where C4-grasses are most abundant. Thus fire has been suggested as a main driver for the Miocene expansion of C4-grasslands. SPIRAL data from Lake Challa (3.3° S, 37.7° E, 840 m a.s.l.) indicate that C4-grass abundance in the Mt. Kilimanjaro area averaged 45 + 25% during the last 25,000 yr BP. Charcoal accumulation rates (i.e., fire) had no significant relationship with C4-grass abundance throughout the record. Temperature/pCO2 showed the strongest relationships with C4 abundance with more C4 grass when pCO2 and temperatures were lower during the last glaciation, generally agreeing with bulk-sediment and leaf-wax δ13C records from the lake. However, C4 estimates are ~30-40% lower from SPIRAL than from leaf-wax δ13C during the late Pleistocene while converging during much of the Holocene. The disparity probably results from differences in the carbon isotope source (i.e., pollen vs. leaf wax) and dispersal area of the substrates for these proxies. Moisture seasonality also appears to be a strong positive control, with C4 grasses favored by long seasonal droughts. Strikingly, C4-grass pollen abundance has reached its highest levels (>80%) of the past 25,000 years only within the last millennium. This recent rise in C4 grasses cannot be easily attributed to any changes in the proxy climate records from the same region.