Abstract
In this paper we conduct geochemical and colourimetric measurements of glauconite grains in micromorphological thin sections from the Middle Stone Age site of Blombos Cave, South Africa, to investigate the formation, internal structure and reworking of heat-exposed cave deposits that are related to prehistoric burning events. Controlled heating experiments were first carried out on glauconite-rich loose sediments and block samples, both of which were collected from the Blombos Cave bedrock. The control samples were then subjected to Fourier transform infrared spectrometry (FTIR), microscopic Fourier transform infrared spectrometry (micro-FTIR) and petrographic-colourimetric analyses. The control experiment shows that glauconitic minerals undergo a gradual and systematic colour change when temperatures reach higher than c. 300–400 °C, primarily due to dehydration and iron oxidation. They also undergo clear structural changes when temperatures reach higher than c. 550 °C due to dehydroxylation and mineral transformation. By assessing the nature and degree of heat-induced optical and molecular alteration in glauconitic minerals, we demonstrate how glauconite grains in thin sections can be classified by the temperature to which they were exposed (20–400 °C, >400 °C, >600 °C and >800 °C). To assess the archaeological relevance of our controlled heating experiment, we applied this glauconite classification scheme to >200 grains found in three micromorphological thin sections of a Middle Stone Age (MSA) combustion feature. These grains were individually geo-referenced within the local coordinate system of Blombos Cave, through a thin-section-based GIS mapping procedure. With improved spatial control, we were able to study both the general distribution of non-altered and heat-altered glauconite grains in their original sedimentary context, as well as to calculate heat distribution models that cover the entire sampled section. This combined geo-chemical, optical and spatio-contextual approach provides insights into more elusive aspects of MSA site structure and burning events, such as heat intensity, burning frequency, temperature distribution, internal hearth structure and post-depositional reworking. The workflow we propose may easily be implemented and adapted to other archaeological contexts and to analogous sedimentary materials that show comparable heat-induced alteration patterns.
Highlights
Encouraged by the abundance of glauconitic grains within the Blombos Cave deposits and confronted with the preservation state and complexity of the combustion features at this site, the aim of this study is to investigate a new proxy to examine site formation processes associated with prehistoric burning events, and to develop a spatio-contextual framework in which this proxy can be practically applied to oriented archaeological sediment samples
Representative micro-Fourier transform infrared spectrometry (FTIR) spectra from each control sample are shown in Fig. 7
At the same time, when temperatures reach higher than c. 600 C, our FTIR and micro-FTIR measurements show that the glauconite grains undergo clear structural changes; namely the loss of hydroxyl groups in the 3530-60 cmÀ1 region and hydroxyl groups that are in coordination with Fe3þ and Mg cations at 810 cmÀ1 and 660 cmÀ1
Summary
At some sites an intact, unlined open hearth can only be recognised by the presence of a reddened substrate, overlain by thin layers of charcoal and ash constituting a predictable trifold structure (Canti and Linford, 2000; Mentzer, 2012; Friesem et al, 2014b). As in the case of the Middle Stone Age (MSA) and Later Stone Age (LSA) layers at Blombos Cave, South Africa (Fig. 1), the identification of a structurally intact hearth versus a post-depositionally disturbed, redeposited or mixed burnt deposit can be more difficult, since the physical preservation of burnt deposits is variable and the quartzrich substrate does not rubify. Discrete hearths, combustion features and other fire-related deposits containing ash, charcoal and burnt sediments have been documented within the MSA deposits (Henshilwood et al, 2001; Henshilwood, 2005). It is hard to tell from the location and nature of macroscopic observations whether specific deposits inside the cave truly were burnt, whether they were burnt in-situ or whether they could have been reworked or redeposited
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
