Extending and Refining Radiation Exposure Dating Methods

在过去的10年里，革命性的新测年方法得到了充分的改进，在这项研究中，我们极大地扩展了我们的方式：1)记录过去海平面变化对海滩的影响；2)建立人类历史上技术变化的时间表。电子自旋共振光学测年法（ESROD）是沙粒内的一个时钟，可以让我们记录过去3-4百万年的时间。它的工作原理是在原子水平上电荷的变化，这些电荷在光照下被排出，在黑暗中被埋藏。另一种相关的方法称为光学发光测年法（OSL），其工作原理与此相同，但只能用于大约20万年前。通过这项研究，这两种方法正在取代一些更原始的测年方法，这些方法很难用于这些问题，或者无法深入到大约4万年前的时间。OSL方法是加拿大的一项创新，于1985年由西蒙弗雷泽大学发明，而ESROD方法通过麦克马斯特大学的努力已成为一种经过验证的方法，由我们的团队于2007年发布。

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.