Luminescence and radiocarbon dating the Naisiusiu Beds type section and timing of the Middle Stone Age/Later Stone Age transition at Olduvai Gorge, Tanzania

The Olduvai Gorge Naisiusiu Beds and Ndutu Beds are significant for understanding the cultural and biological evolution of Homo sapiens. However, the timing and span of deposition of these beds is poorly understood. We present a chronology based on luminescence dates for sedimentary drill core samples and one ostrich eggshell (OES) radiocarbon date from the Naisiusiu type section outcrop. The Naisiusiu Beds type section rests on the Olduvai protogorge floor, on a post-Ndutu incisional surface that eroded through older Olduvai Beds (∼80 m) into the top of the Bed I Basalt. The estimated minimum thickness of the Naisiusiu Beds within the gorge is >13.3 m. Three lithostratigraphic units were identified in the 9.2-m-thick type section: The lower unit comprises alternating fluvial/hyperconcentrated flow sediments and yields an OES date of 49,728 ± 1378 cal year BP; the middle unit is composed of predominantly fluvial sediments containing Later Stone Age (LSA) artifacts and faunal remains, dating between 34.2 ± 2.8 ka and 24.0 ± 2.0 ka; and the upper unit comprises volcaniclastic hyperconcentrated flow deposits with interstratified carbonate-cemented surfaces, dating between 24.0 ± 2.0 ka and 19.6 ± 1.6 ka. Slower accretion rates of the middle unit (7.3 cm/ka) are associated with repeated fluvial cutting and filling. Upper Naisiusiu stacks of hyperconcentrated flows and intervening hiatuses have faster accretion rates (54.9 cm/ka). The OES radiocarbon date of 49,728 ± 1378 cal year BP (modeled age = 49,498 ± 2385 BP) from the top part of the lower unit indicates that the type section extends back to >50 ka. The LSA assemblage is associated with sediments dated to 34.2 ± 2.8 ka using luminescence. A date of 62.7 ± 5.6 ka on the youngest Middle Stone Age occurrence in the underlying Ndutu Beds at Type Locality 26 provides a maximum age for the Middle Stone Age/LSA transition at the Olduvai Gorge.