Appendix S1

OSL dating provides an estimate of the time elapsed since luminescent minerals, such as quartz, were last exposed to sunlight (Huntley et al., 1985; Aitken, 1998; Lian and Roberts, 2006; Jacobs and Roberts, 2007). Buried grains are exposed to an ionizing radiation flux (the 'dose rate') from the surrounding sediments and from cosmic rays, resulting in the absorption of energy and storage of trapped charge. The corresponding 'equivalent dose' (De) can be estimated using the OSL signal, and the burial age then obtained by dividing the De by the environmental dose rate. Table S1 lists the data used to determine the dose rates, De values and burial ages of the two sediment samples collected from Aybut Auwal (AYB1-OSL1 and AYB1-OSL2).