Linking grassland fire-regime shifts to climatic variation of the past 25,000 years: Charcoal records from two east African lakes

Background/Question/Methods: In African grasslands and savannas, fire plays a key role in vegetational dynamics and biogeochemical cycling. Only a few fire records in Africa extend into the Last Glacial Maximum, and even fewer exist in Africa’s vast equatorial grassland. We report here two macro-charcoal (particles >180 µm) records spanning the last 25 ka (thousand years before present) from two lakes in East Africa. Lake Challa is a low-elevation (840 m) crater lake surrounded by open grassland near Mt. Kilimanjaro, and Lake Rutundu is a high-elevation (3000 m) crater lake in the ericaceous belt of Mt. Kenya. To assess the effects of climatic and vegetational changes on fire regimes, we compare charcoal accumulation rates (CHAR) with (1) proxy records of vegetation and moisture from the same sites, and (2) high-latitude climatic records (i.e., GISP2 d¹⁸O) shown to be strongly linked to sub-millennial climate variability at lower latitudes. Together these data provide an excellent opportunity to identify the dominant climatic and vegetational controls on the African grassland fire regime.

Results/Conclusions: CHAR in grasslands indicates shifts in factors such as fuel availability, fuel moisture, and/or vegetational type. CHAR remains low (<2.9 pieces/cm²/yr) between 25-14 ka at Rutundu and is higher and more variable (0.8-13.5 pieces/cm²/yr) at Challa. However, pollen assemblages indicate that open grassland prevailed near both lakes at this time. These patterns suggest greater and more variable biomass burning at Challa probably because of greater moisture availability and warmer temperatures leading to higher plant productivity. CHAR in both lakes increase in parallel with an increase in regional moisture availability after ~16 ka. The convergence in the CHAR records likely represents an increase in fuel availability as a result of a shift to closed bushed grassland, and thus increased plant biomass, around Rutundu. CHAR reaches its maximum values in both lakes ~10.5 ka and then generally decreases as East Africa grew drier towards the present. The CHAR records also reveal fire-regime shifts coincident with abrupt changes during the last glacial-interglacial transition. For example, CHAR in both lakes decline at 12.9 ka and then rise rapidly at 11.5 ka in concert with fluctuating moisture levels associated with Younger Dryas (YD) aridity. At Challa CHAR fluctuations coincide with centennial-scale events in the GISP2 d¹⁸O record, including the Intra-Allerod Cold Period (~13.1 ka) and the Pre-Boreal Oscillation (~11.4 ka). These results suggest a connection between northern hemisphere climate variability and fire regimes in East African grasslands.