Robbins, Jessie L., Dirks, Paul H.G.M., Roberts, Eric M., Kramers, Jan D., Makhubela, Tebogo V., Hilbert-Wolf, Hannah, Elliott, Marina, Wiersma, Jelle P., Placzek, Christa J., Evans, Mary, and Berger, Lee R. (2021) Providing context to the Homo naledi fossils: Constraints on the age of sediment deposits in the Rising Star Cave, South Africa. Chemical Geology, 567. 120108.

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Abstract

Rising Star Cave in the Cradle of Humankind, South Africa, contains one of the richest hominin-bearing deposits in the world, and is the type locality for the Homo naledi fossils. This paper provides a stratigraphic and geochronological framework, within which published and future fossil finds from Rising Star Cave can be placed. Detailed mapping of flowstone horizons combined with new age constraints based on both U-Th disequilibrium and 234U/238U dates and one new OSL date help define seven periods of flowstone formation that punctuate episodes of clastic sedimentation and erosion. Clastic sediments entered the cave through an opening in the roof of the Postbox Chamber from about 600 ka onward, until the opening was choked by coarse breccia blocks, probably sometime after 180 ka. Depositional and erosional events changed the internal morphology of the cave chambers over time, and thereby changed the access route into the Dinaledi Chamber where the bulk of the H. naledi fossils were found.

Periods of pervasive flowstone formation at all levels of the cave occurred at >600 ka, ~500–400 ka, ~225–190 ka and ~110-90 ka. Additional periods of localised flowstone growth restricted to individual chambers (or parts thereof) occurred at ~300 ka, ~160 ka, ~70 ka, ~50 ka, ~30 ka, and ~10 ka. Flowstone horizons bracket sedimentary units that include a variety of sediment types that changed with time. The oldest flowstones overlie lithified mud clast breccias (LMCB), which were partly eroded before they were covered by externally derived laminated orange sands (LOS) and 500–400 ka flowstones. These flowstones and sands were removed between 290 ka and 225 ka, with sediment transported to deeper parts of the cave via erosion channels characterised by massive orange sands (MOS). In this period of time, the access route into the Dinaledi Chamber may have further changed due to the collapse of the Dragon's Back block. Deposition of laminated orange-red mud (LORM) from suspension occurred between 225 and 190 ka and temporally overlaps with widespread flowstone formation around ~225 ka and ~200 ka. The LORM deposits were largely removed from the upper chambers by ~110 ka, before the youngest group of flowstones formed in the cave; some of which are still growing today.

The U-Th ages from Rising Star Cave, combined with other dating constraints reveal age clusters of flowstone formation, which coincided with warmer interglacial or interstadial periods. The patterns recognised in Rising Star Cave overlap with periods of flowstone formation recognised in nearby caves in the Cradle of Humankind, thereby confirming a regional climatic control on flowstone growth in caves during the past 500 ky.

The new ages further constrain the minimum age of H. naledi to ~241 ka. Thus, H. naledi entered the cave between 241 ka and 335 ka, during a glacial period, at which time clastic sediments inside the cave were undergoing erosion. H. naledi would probably have entered the cave through an access point in the roof of the Postbox Chamber and made its way along a SW trending fracture towards the Dragon's Back and Dinaledi Chambers.

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