Heritage Cases

Our main objective in analyzing the samples is to identify glass shards of volcanic origin and then by use their geochemistry to locate the source volcano. Shards are formed during a violent volcanic eruption and carried many thousands of kilometers in the eruption plume. Deposition of shards is almost instantaneous (in terms of geological time) so their deposits form a distinct time line that can be used to determine the age date of the enclosing sediments. We expect to find shards from volcanic eruptions in South America as well as Indonesia at Klasies River Main Site. These shards will be used to test existing age models for this archaeological site and provide an independent dating technique for the Klasies River Mains Site sediments. We have also collected cryptotephra samples from the site of Diepkloof (CASEID 9891) and Pinnacle Point PP5-6. Each sample that was collected was from a single well-studied stratigraphic profile within the main excavation area of the site, spanning the basal Upper Member deposits, the RockFall Member, and ending in the upper SAS deposits. Each sample contains ~30 g of sediment. All samples were mapped with a digital total station and photographed. The samples are now housed in the Munro House on the ground of the Dias Museum, Mossel Bay, where the SACP4 project (PI: Curtis Marean) is based. We propose to ship the samples to the University of Nevada, Las Vegas for processing. It is not possible to process the samples in South Africa because the separation of distal shards from sediment requires special care and specialized techniques, which are only available in a few labs worldwide. We will employ a modified procedure to process extremely low abundance (ELA) cryptotephra that was developed at Oxford University under the guidance of Dr. Christine Lane. Processing techniques were refined and further developed in the cryptotephra laboratory at the University of Nevada Las Vegas (UNLV). The methodology for processing the samples is as follows: Each sample will be weighed to 1 g and placed in 50 mL centrifuge tubes. A 10 % HCl acid solution is then added to dissolve carbonates. The samples are subsequently sieved at 80 and 20 μm. The 80-20 μm sieve portion is further processed using LMT (Lithium Metatungstate) heavy liquid at densities of 1.95 g/cm3 and 2.55 g/cm3 to separate the glass shards and other low-density grains. Samples are centrifuged twice at each density for 15 min at 2500 rpm. After cleaning, the separate (>1.95 but < 2.55 g/cm3) containing shards is mounted on petrographic slides using Epothin epoxy. The slides are scanned to count the shards using a petrographic microscope fitted with a mechanical stage. This count is reported as shards per gram of dried sediment (s/g) and plotted against vertical position in the sediment column to construct shard frequency diagrams. Slides with identified shards were ground and polished for geochemical analysis. Major element analysis is done by electron probe microanalysis (EPMA) at UNLV. Trace elements were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Michigan State University and by secondary ionization mass spectrometry (SIMS) at Arizona State University. Samples are totally consumed during processing and analysis.