Cosmogenic surface exposure dating of southern Laurentide ice retreat and eastward Lake Agassiz routing: relationship to the Younger Dryas cold event

美国俄勒冈州立大学地球、海洋和大气科学学院安德斯·e·卡尔森新仙女木事件（YD）是在12,900-11,700年前的最后一次冰川消融期间发生的一次突然的气候变化事件，该事件使北大西洋地区降温。自一个多世纪前被确认以来，YD的起因一直备受争议。YD气候变冷可能是由墨西哥湾流/北大西洋洋流强度减弱引起的，但是什么导致了海洋环流的减弱？这个项目将检验关于YD成因的主要假设之一；从劳伦泰德冰原（LIS）向东排放的融水使海洋表面变得新鲜，减缓了海洋环流。鉴于人们对气候系统临界点的认识日益提高，该项目的研究结果将有助于深入了解当前和未来北极淡水预算变化的可能影响。特别是，北极向北大西洋的淡水流量的微小变化能否驱动海洋环流的重大变化，从而导致北大西洋变冷？研究结果还将对考古学和地理生物学领域产生影响，因为它们将有助于评估更新世大型哺乳动物灭绝和早期北美人适应的潜在原因，引起广泛关注。过去的气候突变事件是如何影响人类的？本项目将资助一名博士后，在冰川地质学、地质年代学、地球化学和古气候学等领域进行培训。地球科学领域代表性不足的群体将优先招聘博士后研究员。由于目前古气候学的长期职位数量减少，对新博士的支持是一个重大的更广泛的影响。该项目的目标是确定东部阿加西斯湖出口的开放是否发生在YD开始时，这将支持这种路线导致YD的假设。南部LIS边缘的北退和东部冰川阿加西斯湖出口的开放将直接与苏必利尔湖（东部出口）西海岸的巨石10-Be表面暴露年龄相对应。如果一个或多个东部的阿加西湖出口在YD开始时开放，那么东向径流可能提供了淡水强迫，推动了大西洋经向翻转强度的减少。东部出口的后期开放表明，西北阿加西湖路线或其他强迫是YD的触发因素。解决典型气候突变事件的原因将对导致气候系统非线性的潜在机制产生重要影响，并将为评估气候模式对淡水强迫的敏感性提供关键目标。同样，10-Be年代学可以很容易地纳入冰川系统模型，以模拟美国冰川的最后一次消冰。研究结果将通过出版物和在国际会议上的发言向科学界传播。卡尔森将继续写科普文章，这将纳入该研究项目的结果。

Cosmogenic surface exposure dating of southern Laurentide ice retreat and eastward Lake Agassiz routing: relationship to the Younger Dryas cold eventAnders E. Carlson College of Earth, Ocean, and Atmospheric Sciences, Oregon State UniversityThe Younger Dryas (YD) was an abrupt climate change event during the last deglaciation that cooled the North Atlantic region 12,900-11,700 years ago. Ever since its identification over a century ago, the cause of the YD has been widely debated. YD climate cooling was likely caused by a reduction in the strength of the Gulf Stream/North Atlantic current, but what caused this weakening of ocean circulation? This project will test one of the leading hypotheses as to the cause of the YD; that eastward meltwater discharge from the Laurentide Ice Sheet (LIS) freshened the ocean surface and slowed ocean circulation. Given the heightened awareness over tipping points in the climate system, the findings of this project will provide insight into the possible effects of current and future changes in the Arctic freshwater budget. In particular, can only small changes in the Arctic freshwater discharge to the North Atlantic drive significant changes in ocean circulation leading to North Atlantic cooling? Results will also have implications for the field of archeology and geobiology as they will help assess potential underlying causes of Pleistocene large mammal extinction and adaptations of early North Americans, garnering popular attention. How have past abrupt climate change events affected humans? This project will support a postdoctoral researcher, who will be trained in the field of glacial geology, geochronology, geochemistry and paleoclimatology. Under-represented groups in the geosciences will take precedent in recruitment of the postdoctoral researcher. Because of the reduced number of permanent positions in paleoclimatology at present, support of recent PhDs is a significant broader impact. The goal of this project is to determine if the opening of eastern Lake Agassiz outlets occurred at the start of the YD, which would support the hypothesis that this routing caused the YD. The northward retreat of the southern LIS margin and opening of the eastern glacial Lake Agassiz outlets will be directly dated with boulder 10-Be surface exposure ages along the western shores of Lake Superior (the eastern outlet). If one or more of the eastern Lake Agassiz outlets opened at the start of the YD, then eastward runoff routing likely provided the freshwater forcing that drove the reduction in Atlantic meridional overturning strength. A later opening of the eastern outlets would suggest that northwest Lake Agassiz routing or another forcing was the trigger of the YD. Resolving the cause of the canonical abrupt climate event will have important implications on the underlying mechanisms that give rise to nonlinearities in the climate system and will provide a critical target for assessing climate model sensitivity to freshwater forcing. Likewise, the 10-Be chronology can be readily incorporated into glacial systems models that are simulating the last deglaciation of the LIS. Results will be disseminated to the scientific community through publications and presentations at international meetings. Carlson will continue to write popular science pieces, which will incorporate the results of this research project.