Extending and Refining Radiation Exposure Dating Methods

扩展和完善辐射暴露测年方法

• 批准号: RGPIN-2014-05136
• 负责人: Rink, William
• 金额: $ 2.19万
• 依托单位: McMaster University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2016
• 资助国家: 加拿大
• 起止时间: 2016-01-01 至 2017-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=611462
• 关键词: Extending; Refining; Radiation; Exposure; Dating

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.

在过去的10年里，革命性的新测年方法被用于这项研究，以极大地扩展我们1）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革的时间表。电子自旋共振光学测年法（ESROD）是一种装在沙粒里的时钟，可以让我们在过去的3-4百万年里保持时间。它的工作原理是在原子水平上转移电荷，这些电荷在光照下被耗尽，在黑暗中被埋葬。一种相关的方法称为光学发光测年（OSL），以同样的方式工作，但只能使用到大约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40，000年前。OSL方法是加拿大的一项创新，于1985年在西蒙弗雷泽大学发明，而ESROD方法已成为一种经过验证的方法，通过麦克马斯特大学的努力，我们的小组于2007年发表。 这两种技术的主要区别是ESROD需要持续数天的光照来将时钟重置为零，而OSL只需要几分钟就可以做到这一点。这导致我们可以使用这两种方法研究非常不同类型的环境。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光线下。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，这使得它对考古遗址的年代测定非常有用。只有五年历史的矿床可以用光释光测年，而放射性碳测年只能从几百年前开始使用。此外，碳不需要存在使用OSL。 该项目的一个方面是集中在与ESROD和OSL测年海滩砂序列，逐渐沉积，但在多个事件，在过去的几百万年在格鲁吉亚美国。古老的海滩被埋在地表之下，但可以通过使用与海岸线平行数百公里的相等海拔的陆地表面的线索来发现。被掩埋的海滩的图像是使用地面穿透雷达制作的，该雷达向下发送雷达，然后在几分之一秒后测量反射波。这些结果将帮助我们了解海岸线是如何以及何时发展到现在的形状的。这很重要，因为它将显示陆地如何对海平面变化作出反应。 该项目的另一方面将使用OSL来帮助了解穴居生物如何重新分配考古遗址形成期间和之后沉积的沙粒。这一过程称为生物扰动作用，它只是埋藏后扰动的一种形式。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定年的遗址，由于谷物的重新分配，OSL年龄可能太老，考古学家正在失去这种新方法在他们的遗址中工作的信心。在北美，这些更古老的OSL年龄引发了关于前克洛维斯假说的巨大争论，该假说表明人类到达北美的时间比普遍接受的大约13，000年前要早得多。在我的一项研究合作之后，它指出了蚂蚁向上移动谷物的可能性，在这里进行了新的研究，以进一步确定这个问题的细节和规模，使用OSL在各种各样的考古遗址中与穴居生物。将通过包括不同年龄和环境背景的地点来研究问题的范围。