Age-depth model for uppermost Ndutu Beds constrains Middle Stone Age technology and climate-induced paleoenvironmental changes at Olduvai Gorge (Tanzania).

Olduvai Gorge in northern Tanzania is part of a globally important archeological and paleoanthropological World Heritage Site location critical to our understanding of modern human evolution. The Ndutu Beds in the upper part of the geological sequence at Olduvai Gorge represent the oldest unit to yield modern Homo sapiens skeletal material and Middle Stone Age technology. However, the timing of the deposition of the Ndutu Beds is poorly constrained at present, which limits our understanding of the paleoenvironments critical for contextualizing H. sapiens and related technologies in the Olduvai Basin. Using a suite of 15 luminescence ages of sedimentary core samples, combined with Bayesian statistics, this study provides a new higher-resolution age-depth model for the deposition of the uppermost Upper Ndutu and Naisiuiu Beds cored by the Olduvai Gorge Coring Project. The luminescence and modeled ages are presented as ±1 σ uncertainties. The Ndutu Beds intersected by the Olduvai Gorge Coring Project cores are dated to between 117.1 ± 17.9 and 45.3 ± 4.2 ka (between 125.9 ± 26.5 and 45.8 ± 8.2 ka modeled ages), while a probable overlying layer of Naisiusiu Beds dates to 23.7 ± 10.9 to 12.1 ± 1.7 ka (25.7 ± 18.9 ka and 12.0 ± 3.4 ka modeled age). Time-averaged accretion rates are derived during this time: (1) initially low rates (<5 cm ka-1) from the bottom of the core at 117.1 ± 17.9 ka up to 95.3 ± 11.1 ka (125.9 ± 26.5 to 95.5 ± 23.3 ka modeled ages); (2) the middle section spanning between 95.3 ± 11.1 and 62.7 ± 5.7 ka (95.5 ± 23.3 to 61.9 ± 10.4 ka modeled ages) with mean rates above 15 cm ka-1; and (3) the last 62.7 ± 5.7 ka (61.9 ± 10.4 ka modeled age) where the accretion rate reduces to below 5 cm ka-1. This reduction can be explained by the evolution of the gorge system that was likely driven by subsidence of the Olbalbal depression and changes in climate, particularly precipitation and resulting lake and base level changes. Older Upper Ndutu and Lower Ndutu Beds are contained within proto-gorges within the modern gorge system.

Novel luminescence diagnosis of storm deposition across intertidal environments.

Salt marshes provide valuable nature-based, low-cost defences protecting against coastal flooding and erosion. Storm sedimentation can improve the resilience of salt marshes to accelerating rates of sea-level rise, which poses a threat to salt marsh survival worldwide. It is therefore important to be able to accurately detect the frequency of storm activity in longer-term sediment records to quantify how storms contribute to salt marsh resilience. Luminescence is able to infer how long mineral grains were exposed to sunlight prior to burial (e.g., the presence or absence of sediment processing). This study used sediment cores collected from the Ribble Estuary, North West England, to show that luminescence properties of sand-sized K-feldspar grains can diagnose the differential modes of deposition across intertidal settings (i.e., sandflat, mudflat and salt marsh) in longer-term sediment records by detecting the variability in sediment bleaching potential between settings (i.e., sediment exposure to sunlight), thus establishing a framework for the interpretation of luminescence properties of intertidal sediments. It then used modern sediment samples collected before and after a storm event to show how such properties can diagnose changes in sediment processing (i.e., bleaching potential) of mudflat sediments caused by storm activity, despite no changes in sediment composition being recorded by geochemical and particle size distribution analyses. This new luminescence approach can be applied to longer-term sediment records to reveal (and date) changes in the environment of deposition and/or depositional dynamics where there is no obvious stratigraphic evidence of such.

Ice-stream demise dynamically conditioned by trough shape and bed strength.

Ice sheet mass loss is currently dominated by fast-flowing glaciers (ice streams) terminating in the ocean as ice shelves and resting on beds below sea level. The factors controlling ice-stream flow and retreat over longer time scales (>100 years), especially the role of three-dimensional bed shape and bed strength, remain major uncertainties. We focus on a former ice stream where trough shape and bed substrate are known, or can be defined, to reconstruct ice-stream retreat history and grounding-line movements over 15 millennia since the Last Glacial Maximum. We identify a major behavioral step change around 18,500 to 16,000 years ago-out of tune with external forcing factors-associated with the collapse of floating ice sectors and rapid ice-front retreat. We attribute this step change to a marked geological transition from a soft/weak bed to a hard/strong bed coincident with a change in trough geometry. Both these factors conditioned and ultimately hastened ice-stream demise.