Heritage Cases

1. Objective of the study Established chronometers do not fully meet the needs of archaeologists and paleoanthropologists conducting research in Middle Stone Age (MSA) archaeological contexts. While single-grain quartz optically-stimulated luminescence (OSL) dating has become one of the principal chronological techniques applied to >50 ka archaeological sections, it is desirable to assess the chronologies of archaeological sites using an independent technique. Comprehensive OSL dating is being carried out at Blombos Cave as part of the SapienCE project. We propose to conduct coordinated U-Th dating of ostrich eggshell at this site to provide a detailed comparison between OSL and U-Th dates. Such a comparison will help to assess the accuracy of both techniques. A further benefit of the proposed research is that U-Th dating of ostrich eggshells provides ages with significantly smaller uncertainties than OSL ages, facilitating more precise comparison of the ages of archaeological milestones at Blombos with, for example, paleoclimate records and the archaeology at other sites. 2. Material and Methods U-Th burial dating of a limited number of pilot samples (n=4) of ostrich eggshells from layers spanning the vertical extent of excavations at Blombos Cave has produced encouraging results. The U-Th ages are self-consistent and preserve stratigraphic order, and in some layers, the U-Th and OSL ages agree. In deeper layers, however, the OSL and U-Th ages are discordant. To resolve such discrepancies, working in collaboration with Drs. C. S. Henshilwood, K. L. van Niekerk and S. Armitage of the SapienCE project, we propose to extend the application of U-Th dating of ostrich eggshell at Blombos Cave. Accordingly, we request permission to export a suite of ostrich eggshell fragments for U-Th isotopic dating via analysis by ICP-MS (inductively coupled plasma mass spectrometry). Analyses will include: (1) profiling the uranium and common thorium concentrations across eggshells by laser ablation ICP-MS, (2) analyzing U and Th isotopic compositions at selected positions along the uranium concentration gradients by ICP-MS analyses of selectively abraded eggshells in solution mode, and (3) determining U-Th burial ages for the eggshells from the analytical data. These analyses, which constitute a technique termed "230Th/U burial dating of ostrich eggshell" (e.g., Sharp et al. 2019) will be conducted under the direction of W. Sharp at the Berkeley Geochronology Center, Berkeley, U.S.A. Specifically, we request permission to export 16 ostrich eggshell fragments from Blombos Cave in order to carry out systematic dating throughout the sequence, including ostrich eggshell fragments excavated in 2019, via U-Th on eggshells. The requested ostrich eggshell fragments are selected from horizons with existing OSL ages or those that are currently being dated by OSL by Dr. Simon Armitage (Royal Holloway U. and Sapiens Centre of Excellence, U. of Bergen). The requested ostrich eggshell fragments from Blombos Cave will allow us to test whether U-Th ages: 1) are reproducible within the deeper Blombos layers, 2) preserve stratigraphic order (that is, become older downward, as expected), and 3) are concordant with OSL ages in close stratigraphic association, particularly in the deeper layers of the excavation. Many of the requested ostrich eggshell fragments have been examined by W. Sharp to ensure that they are suitable for U-Th dating. The dating process is destructive but requires ostrich eggshell fragments that are only about 7 to 9 cm2 in size. Fragments for dating are carefully photo-documented prior to dating. Any unused fragments or unused portions of larger fragments will be returned to the Blombos collections. 4. State duration of the analysis; expected results and its importance We expect to complete the analyses within one year of receiving the materials at our laboratory. The results of the study will be reported to Prof. C. S. Henshilwood and Dr. K. L. van Niekerk and in collaboration with them will be published in a refereed journal. The U-Th ages of ostrich eggshell will help to provide more reliable and precise ages for the archaeological record at Blombos Cave. They will also likely inform future dating efforts of other archaeological sequences in South Africa and elsewhere. 5. State why the research cannot be carried out in South Africa The proposed analyses cannot be done in South Africa because some of the analytical techniques that are required to perform U-Th burial dating of ostrich eggshell, such as laser ablation ICP-MS analyses of ppb levels of U and Th, are not available in South Africa. W. Sharp has developed a novel approach to U-Th dating of OES using instrumentation, analytical protocols, and data analysis techniques at the Berkeley Geochronology Center (Sharp et al. 2019). Hence, the proposed researchers are uniquely suited to performing these analyses.