Characterising Luminescence Stratigraphies in the Mount Elgon Caves (Chepnaylil Rockshelter and Kiptogot Cave)

Kinnaird, T.C. et al. (2014) Characterising Luminescence Stratigraphies in the Mount Elgon Caves (Chepnaylil Rockshelter and Kiptogot Cave). Technical Report. SUERC, University of Glasgow. (Unpublished)

Abstract

This report provides sediment-luminescence stratigraphies for key sedimentary sequences in the Chepnyalil and Kiptogot rockshelters, Mount Elgon, Kenya. Recent excavations in the two aforementioned caves by the National Museum of Kenya have unearthed lithic tool assemblages, which resemble both Middle Stone Age (MSA) and Late Stone Age (LSA) technologies (e.g. levallois and discoi core reduction methods) and typologies (retouched points, bladelets, geometrics; Kiura, 2012). It should be noted that younger assemblages are also found. If the sediment-OSL chronologies reported herein confirm the MSA and LSA typological classification, and by association the implied occupation of the caves, then this provides the first evidence of MSA occupation in this topographically elevated region of East Africa. Furthermore, as environmental proxies indicate that the late Pleistocene to early Holocene was marked by increase aridity, it suggests an important link between the environment and human adaptations, as people moved from lowlands to higher elevation ecosystems. The above observations all stress the need for a detailed sediment-OSL chronology to determine TPQ and TAQ for occupational phases in the Mount Elgon caves, specifically at Chepnyalil and Kiptogot. T. Kinnaird visited the Mount Elgon caves in October 2013 to collect samples for OSL profiling and dating. During fieldwork portable OSL equipment, in combination with field spectrometry, was used to appraise luminescence stratigraphies, and identify the key units for OSL dating. The field objective was to obtain luminescence proxy information from sediment stratigraphies at both Chepnyalil and Kiptogot, and assess this data against existing chronological data (including sediment depositional ages; 15-23 ka, and radiocarbon ages; 3rd - 15th century AD) to determine if maxima/trends in the profiles were suggestive of a prolonged environmental history, indicative of a multiphase occupational history. Sediment-OSL stratigraphies were generated for 8 profiles - 5 from Chepnyalil rockshelter and 3 from Kiptogot cave. The significance of these profiles is that they encompass strata (i) enclosing the lithic MSA?/LSA? typologies (profiles 1 and 2, Chepnyalil), and dated to 15-23 ka (profiles 1 and 2, Chepnyalil), and (iii) strata enclosing the younger occupational materials (profiles 3 and 4, Chepnyalil). Intriguingly, the field data did distinguish each stratigraphic unit, emphasising distinct luminescence packages/or units, and an overall progression in signal intensities with depth, consistent with at least a fourfold model of sediment accumulation. Moreover, through reference to the independent age controls, it was possible to assign discrete packages of sediment in the lower cave at Chepnyalil (Leopards Cave) and in the Kiptogot Cave to temporal units on the basis of their luminescence proxies. Field gamma dose rates were high, between 1.1 and 1.7 mGy a-1, implying that high environmental dose rates would be encountered in the dating samples. Laboratory profiling confirmed the correlations suggested in the field profiling dataset. Furthermore, the laboratory profiling results from the lower and upper units (i and ii, above) at Chepnyalil and Kiptogot, indicated a significant step in stored dose values across this lithological boundary (2-3 Gy vs 18 Gy and above), consistent with a notable unconformity, and the hypothesis that the caves have a prolonged, complex history of occupation. In addition, the calibrated luminescence screening results, indicated the dating samples which could be dated using the conventional quartz SAR approach (the fast component of the quartz OSL signal will register stored doses up to 75-80 Gy, which in the Chepnyalil/Kiptogot environmental settings, is equivalent to 15-20ka), and those that would require higher temperature quartz approaches, and/or, post-IR elevated temperature IRSL protocols on K feldspar. Interestingly, the range in apparent stored dose estimates obtained for the stratigraphies at Chepnyalil and Kiptogot, in this environmental setting, corresponds to luminescence ages between 200 yrs BP and 25 ka BP, providing further confirmation of a prolonged environmental chronology, and suggesting a complex, multiphase occupational history. Finally, quartz SAR OSL analyses, and preliminary K feldspar post-IR elevated temperature IRSL measurements, provide the means to construct sediment-OSL chronologies for the stratigraphies at Chepnyalil and Kiptogot. It must be noted that full quantitative luminescence dating was restricted to five samples. Four out of the five samples produced finite quartz SAR ages - 5.8 ± 0.4 ka (SUTL2613), 6.7 ± 0.4 ka (SUTL2620), 0.19 ± 0.01 ka (AD 1830 ± 10, SUTL2626) and 4.6 ± 0.3 ka (SUTL2627). The fifth sample (SUTL2612) returned normalised OSL signals larger than the saturation limit of the dose response curve. A minimum age estimate of > 20 ka is suggested for this sample. The depositional ages reported here for Chepnyalil (SUTL2612 and 2613) verify that the lower units (in the position of profile 2), enclosing the lithic assemblages which resemble MSA and LSA technologies, do contain strata of LSA age (the LSA is subdivided into the Terminal Pleistocene, 40-12 ka, early Holocene, 12-8ka, middle Holocene, 8-4.5ka and late Holocene, 4.5-2ka). In contrast, the depositional ages from Kiptogot cave (SUTL2626 and 2627), do not yield LSA-equivalent ages; however, both the field and laboratory profiling datasets, do suggest that the sediment-OSL chronologies extend into the Pleistocene. At present work is underway to characterise the stability of the post-IR IRSL signals over prolonged periods of accumulation/storage; once complete, the report will be updated to include a post-IR IRSL age for SUTL2612. Interestingly, the sediment-luminescence profiles, generated in the field and subsequently calibrated for sensitivities/age in the laboratory, together with the full quantitative luminescence dating analyses, have all verified a prolonged environmental temporal record for the sediments at Chepnyalil and Kiptogot, consistent with a complex, muliti-period occupation of the caves.

• Item Type: Research Reports or Papers (Technical Report)
• Status: Unpublished
• Glasgow Author(s) Enlighten ID: Kinnaird, Dr Timothy and Sanderson, Professor David
• Authors: Kinnaird, T.C., Kinyanjui, R.N., Ndiema, E.K., Powles, P., Powles, S.H., Powles, C., Walukana, E.M., Watene, P., Jovanelly, T., Kiura, P., Sanderson, D., and Spencer, J. Q.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Publisher: SUERC, University of Glasgow