Heat-induced alteration of glauconitic minerals in the Middle Stone Age levels of Blombos Cave, South Africa: Implications for evaluating site structure and burning events

Magnus M. Haaland, David E. Friesem, Christopher E. Miller, Christopher S. Henshilwood

Research output: Contribution to journalArticlepeer-review

26 Scopus citations

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.

Original languageEnglish
Pages (from-to)81-100
Number of pages20
JournalJournal of Archaeological Science
Volume86
DOIs
StatePublished - Oct 2017
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2017 The Authors

Funding

We thank Panos Kritikakis for producing the thin sections, Torstein Monsen for scanning of the thin sections and Sarah Meinekat and Lucie Leierer for the preparation of bulk samples. We would also like to thank Susan Mentzer for providing assistance and helpful information regarding the FTIR procedures and measurements, and Matthew Canti and Annika Burns for providing us with a copy of their glauconite comparative database. Financial support was provided to MMH by the Meltzer Research Fund (ID: 1303 , 3718 , 7604 , 13107 ) and the Travel Fund for Doctoral Fellows at the Department of Archaeology, History, Cultural Studies and Religion, University of Bergen, Norway . The FTIR instrumentation was funded by a grant to CEM from the Deutsche Forschungsgemeinschaft ( MI 1748/1-1 ). CSH was funded by European Research Council Advanced Grant, TRACSYMBOLS No. 249587 , awarded under the FP7 program at the University of Bergen, Norway and by a National Research Foundation/Department of Science and Technology funded Chair at the University of the Witwatersrand, South Africa . The Publication fund for Open Access at the University of Bergen ( BORA #5808 ) and the Department of Archaeology, History, Cultural Studies and Religion, University of Bergen, facilitated Open Access for this article. We thank Panos Kritikakis for producing the thin sections, Torstein Monsen for scanning of the thin sections and Sarah Meinekat and Lucie Leierer for the preparation of bulk samples. We would also like to thank Susan Mentzer for providing assistance and helpful information regarding the FTIR procedures and measurements, and Matthew Canti and Annika Burns for providing us with a copy of their glauconite comparative database. Financial support was provided to MMH by the Meltzer Research Fund (ID: 1303, 3718, 7604, 13107) and the Travel Fund for Doctoral Fellows at the Department of Archaeology, History, Cultural Studies and Religion, University of Bergen, Norway. The FTIR instrumentation was funded by a grant to CEM from the Deutsche Forschungsgemeinschaft (MI 1748/1-1). CSH was funded by European Research Council Advanced Grant, TRACSYMBOLS No. 249587, awarded under the FP7 program at the University of Bergen, Norway and by a National Research Foundation/Department of Science and Technology funded Chair at the University of the Witwatersrand, South Africa. The Publication fund for Open Access at the University of Bergen (BORA #5808) and the Department of Archaeology, History, Cultural Studies and Religion, University of Bergen, facilitated Open Access for this article.

FundersFunder number
Department of Archaeology
National Research Foundation/Department of Science and Technology
Travel Fund for Doctoral Fellows
University of the Witwatersrand, Johannesburg5808
Seventh Framework Programme
European Research Council249587
College of Emergency Medicine
Deutsche ForschungsgemeinschaftMI 1748/1-1
Seventh Framework Programme
Universitetet i Bergen
Norway Grants
L. Meltzers Høyskolefond1303, 13107
National Commission on Research, Science and Technology

    Keywords

    • Colourimetry
    • Combustion feature
    • FTIR
    • Fire experiment
    • Fire proxy
    • GIS
    • Hearth
    • Micro-FTIR
    • Micromorphology
    • Middle Stone Age
    • Site structure
    • Spatial analysis

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