Extending and Refining Radiation Exposure Dating Methods

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Revolutionary new dating methods fully refined in the last 10 years are used in this research to greatly expand the way we 1) document changes to our beaches as a result of past sea level change, and 2) establish timelines of technological change in human history. The method of electron spin resonance optical dating (ESROD) is a clock inside grains of sand that allows us to keep time over the last 3-4 million years. It works by shifts in electrical charge at the atomic level, which are drained by light exposure and gained by burial in the dark. A related method called optical luminescence dating (OSL) works the same way, but can only be used out to about 200,000 years ago. Through this research, both methods are replacing some more primitive dating methods which are hard to use for these problems, or which cannot reach into deep time beyond about 40,000 years ago. The OSL method is a Canadian innovation, invented at Simon Fraser University in 1985, while the ESROD method has become a proven method through efforts at McMaster University published by our group in 2007. The main difference in the two techniques is the ESROD requires light exposures that last many days to reset the clock to zero, while only minutes are required to do the same for OSL. This leads to the very different types of environments we can study with the two methods. In the first case, generally only coastal zones, where grains of sand are repeatedly exposed to light before burial are of use. Similar deposits of sand that are younger can also be studied from these environments with OSL. The great advantage of OSL lies in its sensitivity to light, and rapid response during burial, which makes it very useful for dating archaeological sites. Deposits that are only five years old can be dated with OSL, while radiocarbon dating can only be used starting a few hundred years ago. Moreover, carbon doesn’t need to be present to use OSL. One aspect of this project is focused on dating with ESROD and OSL a sequence of beach sands that were deposited gradually, but in multiple episodes, over the last several million years in Georgia USA. The old beaches are buried below the surface but can be found by using clues from the land surfaces of equal elevation that run for hundreds of kilometers parallel to the shoreline. Images of buried beaches are made using ground penetrating radar, which sends radar downward and then measures the reflected waves a fraction of a second later. The results will help us to understand how and when the coastline grew to its present configuration. This is important because it will show how the land responded to sea level change. The other aspect of the project will use OSL to help understand how burrowing organisms redistribute sand grains that are deposited during and after archaeological sites are formed. This process is called bioturbation and is but one form of post-burial disturbance. This is needed because sites without radiocarbon, or too old to be dated with radiocarbon, have OSL ages that can be too old as a result of this redistribution of grains, and archaeologists are losing faith that this new method can work in their sites. In North America, these older OSL ages have fueled a huge debate about the Pre-Clovis hypothesis, which suggests humans reached North American much earlier than the commonly accepted time of about 13,000 years ago. Following one of my research collaborations which points to the possibility that ants are moving the grains upward, new research is here undertaken to determine further the details and the magnitude of this problem using OSL in wide variety of archaeological sites with burrowing organisms. The scope of the problem will be studied by including sites of differing age and environmental context.