Extending and Refining Radiation Exposure Dating Methods

这项研究使用了在过去10年中全面改进的革命性新测年方法，以极大地扩展我们记录过去海平面变化导致的海滩变化的方式，以及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(ESROD)方法是沙粒中的时钟，它可以让我们记录过去300-400万年的时间。它的工作原理是原子水平上的电荷转移，这些电荷在光照下被耗尽，通过在黑暗中掩埋而获得。一种被称为光学发光测年(OSL)的相关方法也是这样工作的，但只能在大约20万年前使用。通过这项研究，这两种方法都在取代一些更原始的测年方法，这些方法很难用来解决这些问题，或者无法深入到大约4万年前的更深层次。OSL方法是加拿大的一项创新，由西蒙·弗雷泽大学于1985年发明，而ESROD方法通过我们小组在2007年发表的麦克马斯特大学的努力，已经成为一种经过验证的方法。 这两种技术的主要不同之处在于，ESROD需要持续数天的光照才能将时钟重新设置为零，而OSL只需几分钟就能完成同样的操作。这导致了我们可以用这两种方法研究的环境类型截然不同。在第一种情况下，通常只有沿海地区，在那里沙粒在埋葬前反复暴露在阳光下是有用的。用OSL也可以从这些环境中研究更年轻的类似沙子沉积。OSL的最大优势在于它对光的敏感性，以及在埋葬过程中的快速反应，这使得它在考古遗址的年代测定中非常有用。只有五年的矿床可以用OSL测年，而放射性碳测年只能从几百年前开始使用。此外，使用OSL并不需要碳的存在。 该项目的一个方面是用ESROD和OSL测年，这是美国佐治亚州在过去数百万年里逐渐沉积但分多次沉积的一系列海滩沙子。旧海滩埋在地表以下，但可以通过与海岸线平行数百公里的等海拔陆地表面的线索来发现。被掩埋的海滩的图像是使用探地雷达拍摄的，该雷达向下发送雷达，然后在几分之一秒后测量反射的海浪。这些结果将有助于我们了解海岸线是如何以及何时发展到目前的形态的。这一点很重要，因为它将显示陆地对海平面变化的反应。 该项目的另一个方面将使用OSL来帮助了解在考古遗址形成期间和之后沉积的沙粒是如何重新分布的。这一过程被称为生物扰动，只是埋葬后扰动的一种形式。这是必要的，因为没有放射性碳的遗址，或者太老而不能用放射性碳测定年代的遗址，由于颗粒的重新分布，OSL年龄可能太老了，考古学家正在失去对这种新方法在他们的遗址奏效的信心。在北美，这些年龄较大的OSL年龄引发了关于克洛维斯之前的假说的激烈辩论，该假说表明人类到达北美的时间比人们普遍接受的大约13,000年前的时间要早得多。在我的一项研究合作指出蚂蚁向上移动颗粒的可能性之后，这里进行了新的研究，以进一步确定在各种具有洞穴生物的考古遗址中使用OSL来进一步确定这个问题的细节和严重程度。将通过包括不同年龄和环境背景的遗址来研究问题的范围。

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.