The age of deposits

We can say that a change in climate (e.g. the nights became colder) led to a change in the archaeological material (e.g. more use of fires) if we know that both things occurred at the same time. Consequently, understanding the age of our records of climate change and archaeology is a theme in SapienCE research.

Timing is everything

One of the reasons that the Middle Stone Age sites excavated by SapienCE fascinate us is that the archaeology is so old. For example, the equipment used to make vivid red paint, excavated a Blombos Cave, is about 100,000 years old. To put that age in context, the paint “toolkit” is more than twenty times as old as the Great Pyramid of Giza in Egypt. We know this because of the work of “chronologists”, people who calculate the age of ancient materials. To make the best use of the information that we produce in SapienCE, it is essential that we know the age of the material that we study. This applies both to our archaeological excavations and to the records of past climates that we produce. Within any particular dataset, ages allow us to estimate the timing and rates of any changes that we observe.

However, when studying datasets from different sources (e.g. climate records from an ocean core and archaeology from a cave), ages allow us to make comparisons. Chronology is therefore vital for linking the archaeological and climate components of SapienCE, allowing us to test ideas about how past climate might have affected human behaviour. However, no single method is sufficient for determining the age of all of our material. Within SapienCE we use two main methods, called luminescence and uranium-series dating respectively, to calculate the age of our samples.

Luminescence dating

Luminescence dating measures the amount of time that has passed since sand grains were last exposed to sunlight. Small quantities of sand were constantly being blown into archaeological sites while they were occupied by early humans, and over time these sands became buried and shielded from sunlight. We can therefore date the Middle Stone Age materials at our sites by luminescence dating sand extracted from the sediments in which they were buried. We are also experimenting with using luminescence dating to determine the age of ocean cores. Uranium-series measurements allow us to date the formation of new carbonate minerals, such as stalagmites in caves, and ostrich eggshells. Stalagmites from the De Hoop Nature Reserve are being analysed by SapienCE team members to produce detailed records of climate during Middle Stone Age times, whilst dating ostrich eggshells from Blombos Cave and Klipdrift Shelter add important extra detail to the luminescence dating at these sites.