The site of Blombos Cave (BBC) is well known for archaeological remains that have advanced our understanding of the development of modern human behaviour during the Middle Stone Age (MSA). Occupation of the cave occurred against a backdrop of landscape-scale environmental and sedimentary processes that provide the broader context for finer-scale interpretations of the site-formation history and archaeological patterns detected in the cave deposits. Aeolian and palaeosol sequences are abundant in the vicinity of BBC and these provide a partial view of the past landscapes available to the inhabitants of the cave. An important extension to the palaeo-landscape around BBC currently lies submerged on the Agulhas Bank, as sea levels were lower than at present for the entire period of human occupation of BBC.
In this paper, we revisit the optically stimulated luminescence (OSL) chronology for the full sequence of sediment deposition inside BBC, increasing the number of dated samples to a total of 40 and revising the period of MSA occupation to between 97.7 ± 7.6 and 71.0 ± 5.7 ka (uncertainties at 95.4% probability). We describe the geological successions at four main areas around BBC, estimate the time of sediment deposition using OSL, and describe and interpret three seismic profiles on the Agulhas Bank, offshore of BBC. By correlating these onshore and offshore geological sequences with the sedimentary deposits inside BBC, we place the archaeological record within a landscape-scale chrono-stratigraphic framework to examine how environmental changes may have regulated the presence or absence of humans in the cave and surrounding terrain between about 100 and 70 ka.
The southern Cape coast of South Africa hosts a number of important archaeological cave sites, including Die Kelders (Marean et al., 2000), Klipdrift Shelter (Henshilwood et al., 2014), Blombos Cave (Henshilwood et al., 2001a), Pinnacle Point Site 13B (Marean, 2010) and Pinnacle Point Site 5e6 (Smith et al., 2018) (Fig. 1). All these sites are located along the leading edges of different halfheart embayments that make them effective sediment traps. The trapped sediments preserve a record of the associated depositional processes that have also played a dominant role in shaping the stratigraphic sequences of these sites, forming the backdrop against which traces of human occupation are exposed (e.g., Karkanas et al., 2015). Since these depositional processes are landscape scale processes, stratigraphic interpretation of the sediments inside these cave sites should be made based on geological and geomorphological contextual knowledge of the wider landscape.
Aeolian deposits are common features along the south Cape coast, often in association with interbedded rhizomorph-bearing palaeosols. Aeolian derived sediment is also an important component of many of the cave deposits. As the source of aeolian sediment is linked to the exposure of sandy coastal areas on the adjacent continental shelf, sedimentation rates have changed over time due to changes in sea level (e.g., Bateman et al., 2004). Aeolian deposition could be interpreted as periods of landscape instability and change, whereas palaeosols mark breaks in dune sedimentation and only form on dunes after their colonisation and stabilisation by vegetation. Sand layers can be easily observed inside caves, but palaeosols rarely extend into caves, making it more difficult to recognise periods of landscape stability in cave sediment records.
Blombos Cave (BBC) provides an excellent site to investigate the relationship between the sediments found inside and outside a cave in this region. Thick aeolian and thinner palaeosol sequences are preserved up to 50 m above modern sea level around BBC. The cave is situated ~100 m from the Indian Ocean shore and ~34.5 m above mean sea level (amsl), so the deposits are protected from the
erosive action of storm surges and waves. It was also sealed for most of the last ~70 ka by a sand dune that covered the cliff face, became cemented and protected the site thereafter. This ensured that the Middle Stone Age (MSA) deposits were conserved in an almost pristine condition, with excellent preservation of its in situ, anthropogenically deposited faunal and malacological remains.
The MSA sedimentary layers in BBC (Fig. 2AeF) have been dated to between ~100 and 70 ka ago, placing occupation within the moderate interglacial conditions of Marine Isotope Stage (MIS) 5d to 5a and during the transition to moderate glacial conditions near the start of MIS 4 (Jones, 2001; Henshilwood et al., 2002, 2004, 2011; Jacobs et al., 2003a,b, 2006, 2013; Tribolo et al., 2006). Over this period, relative sea-level fluctuated between about 18 m and 76 m below mean sea level (bmsl) (Waelbroeck et al., 2002), exposing approximately 2.3e32.5 km of the now-submerged Agulhas Bank (Fisher et al., 2010). A large part of the terrestrial landscape between ~100 and 70 ka ago is now underwater, and the relationship to the onshore sedimentary records, including those trapped in BBC, is not well known. To better understand and interpret the site formation processes at BBC, and the history of human occupation and concurrent environmental changes, requires knowledge of both the onshore and offshore sedimentary records in the vicinity of the cave, the timing of changes in these records, and the correlation or interplay between them.
In this study, we revisit the chronology of the sedimentary deposits inside BBC. We map, describe and date the onshore Pleistocene geological deposits near the cave, and document and describe three seismic transects (two perpendicular to the coast and another parallel to the coast) across the Agulhas Bank. We then relate the onshore Pleistocene sequences and sedimentary deposits found within BBC to place the cave deposits within the landscape-scale chrono-stratigraphic framework developed in this study. Linking these two records sheds light on how long-term environmental and landscape changes have regulated the absence and presence ofhumans in the cave and surrounding terrain.