BIOMARKER SIGNATURES OF FIRE AND GRASSLAND-FOREST TRANSITIONS ASSOCIATED WITH HOMININS AT OLDVUAI GORGE, TANZANIA

Grasslands are integral to human history: they set the stage for hominid and Homo evolution in the Pliocene/Pleistocene and fostered the rise of pastoralism and agriculture in the early Holocene. C₄ ecosystems expanded globally in the middle and late Miocene, as documented by continental isotope records. Local vegetation changes were typically abrupt, and C₄- grasslands typically but not exclusively replaced forests. Rapid, repeated forest-grass shifts are observed isotope records throughout eastern Africa. For example, our prior Plio/Pleistocene biomarker and isotope records from Olduvai Gorge, Tanzania (Magill et al., 2013a; b; in review), reveal rapid shifts between moist and dry climate conditions and between C₃ and C₄ dominance of the landscape that was paced with the Earth’s axial precession cycle.

Fire, integral to grassland ecology today, disfavors forests, which re-sprout less vigorously, and fire is widely suspected as a significant agent that links abrupt shifts in vegetation to climate. Fire was likely a factor both in C₄-grassland expansion in the Miocene, and in high-frequency variations in eastern tropical Africa during the Quaternary. Materials released during biomass combustion carry organic tracers of fire activity, including the better known solid forms, such as char, black carbon, and soot. Geochemical records built from both polar compounds associated with cellulose and hemi-cellulose combustion (including, levoglucosan, mannosan, galactosan) as well as polycyclic aromatic hydrocarbons (PAH) can also track past fire activities. The taphonomy of each indicator differs. Generally, particulate phases have shorter transport trajectory than compounds borne in aerosols, and more soluble forms are less well preserved under aggressive soil conditions. Biomarker molecules offer power tools to track fire, landscape vegetation, and climate patterns of the past. Here, we compare terrestrial plant biomarkers, molecular carbon and hydrogen isotope records, and fire indicators, especially PAH, in the Lake Olduvai catchment, to evaluate the linkages among fire, climate, and ecosystem stability during hominin occupation, ca. 2 million years ago.