LIGHT STABLE ISOTOPES OF OSTRICH EGGSHELLS PROVIDE SITE-SPECIFIC PALEOENVIRONMENTAL RECORDS DURING THE MIDDLE-TO-LATER STONE AGE TRANSITION IN EASTERN AFRICA

The Middle to Later Stone Age (MSA-LSA) transition in Eastern Africa (variably ~60-30 ka) has been linked to cooler, drier environments with greater short-term variability, but precisely dated local environmental records from archaeological contexts are needed to test this hypothesis. Ostrich eggshell (OES) fragments are commonly found in African archaeological sequences, are amenable to ¹⁴C and U-Th dating, and their stable carbon (δ¹³C values) and nitrogen (δ¹⁵N values) isotopes track local vegetation and mean annual precipitation (MAP), respectively. OES oxygen isotopes (δ¹⁸O values) may be a proxy for evapotranspiration if δ¹⁸O_MAP can be evaluated independently. We present δ¹³C, δ¹⁵N, and δ¹⁸O values for >80 OES from two rockshelters with archaeological sequences that record the MSA-LSA transition, Kisese II (Tanzania, ~46.2 – 4 ka) and GvJm22 Lukenya Hill (Kenya, >46 ka – 1.2 ka). These are furthermore well dated sequences based on 55 ¹⁴C and 7 U-Th burial ages on OES. At Lukenya Hill and Kisese II, dietary δ¹³C values range from -17.8‰ to -25.7‰ and -16.6‰ to -26.1‰, respectively, and estimated MAP ranges from ~310 – 900 mm/a and ~280 – 1000 mm/a, respectively, with more C₄-rich flora during periods of lower MAP. δ¹⁸O values imply evapotranspiration varied with local floral composition and/or MAP. Mean δ¹⁵N values (median N_OES = 7) at Lukenya Hill show precipitation increased progressively over time: it was arid (~350 – 500 mm/a) during the local MSA-LSA transition ~46 – 26 ka, and wettest (~750 – 850 mm/a) in the African Humid Period ~14-4 ka. In contrast, at Kisese II mean δ¹⁵N values (median N_OES = 5) imply that the driest (~500 – 740 mm/a) conditions at ~46 – 44 ka were wetter than the coincident arid period at Lukenya Hill, and wettest conditions at Kisese II (~650 – 900 mm/a) occur during the local MSA-LSA transition at ~36 – 34 ka and the Last Glacial Maximum ~24 – 21 ka. Geographic differences between local environments separated by only ~300 m elevation, 3° latitude, and 350 km distance (e.g., latitude, topography, dynamic controls on precipitation) likely explain the observed site-specific rainfall differences leading to contrasting paleoenvironments during their respective MSA-LSA transitions.