Seasonality of Abrupt Climate Change over Greenland: Direct Tests for the Younger Dryas and 8.2 ka event using Paleolimnology

年轻的仙女木(YD)是地球历史上记录最好的突然气候变化的例子，但YD的基本特征仍然是谜。格陵兰岛位于YD气候变化的震中，冰芯记录到强烈的冷却，但尽管如此，格陵兰冰盖在YD期间消退。一个有影响力的假说表明，北大西洋气候突变的特点是极端的季节性，温和的夏季解释了与直觉相反的YD冰川消退。解决季节性问题对于理解过去和未来气候变化对北极系统所有方面--包括冰盖--的影响至关重要，因为夏季的温度会导致地表融化。这项研究将通过开发定量的夏季和冬季温度重建，首次直接评估格陵兰岛上YD的季节性。这项研究还将评估8200年前格陵兰主要寒流的季节性。有趣的是，尽管这一事件的年震级比YD短，也比YD小，但它推动了格陵兰周围的冰川运动。这项研究将阐明气候突变如何影响北极/亚北极北大西洋地区，特别是格陵兰及其冰盖。通过解决YD期间气温的季节性，这项研究还将澄清降水和海洋温度在推动过去冰盖行为方面的竞争作用，并对未来冰盖质量损失的预测产生影响。这项研究将检验全新世8.2ka事件本质上是一个小型的YD事件，还是根本不同。后者可能会使YD成为未来突然气候变化的糟糕类比，气候变化将在间冰期世界展开。8.2ka事件被描述为测试气候模型的理想目标，但优化此类测试所需的相关陆地气候变化的定量重建却很少见。这项工作的另一个主要贡献是校准和改进了重建北极气候的特定季节方法。需要这种方法来解决不同第四纪时间尺度上的季节性问题，为气候模型提供必要的测试。拟议的工作还将通过提供直接比较，对稳定的同位素记录和格陵兰冰芯的古温度重建进行更复杂的解释。一旦这些标志性记录捕捉到的气候变化的季节性得到澄清，这可能会产生新的见解。该项目将利用保存稀有的、容易确定日期的YD湖沉积物记录的经典遗址。沉积物岩芯中多种经过校准的、特定于季节的指标--包括昆虫和花粉组合、陆生叶蜡δ2H和水生昆虫δ18O--将相互测试和补充，并与冰芯的年度综合记录进行新的比较。这项工作将建立在研究团队在格陵兰岛校准和应用这些代理的经验的基础上。一项新的校准研究将验证叶蜡δ2H在格陵兰一系列气候中的季节性。未来突然气候变化的可能性是一个主要的社会问题。随着北大西洋环流近期和持续变化的证据的积累，北大西洋次极地过去的突然变化与未来越来越相关。该项目将培养两名博士生和至少六名本科生。除了在学年进行研究外，学生还将参加有导师的暑期研究机构，以培养研究和沟通技能。为了分享学术界以外的科学并磨练外展技能，学生们将开展外展活动(A)在研究地点附近的格陵兰岛，(B)在芝加哥地区的一所高中，与参加PI 2018年高中教师气候科学研讨会的教师合作，为3200名不同人口的学生提供服务。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The Younger Dryas (YD) is the best documented example of abrupt climate change in Earth’s history, yet fundamental traits of the YD remain enigmatic. Greenland sat at the epicenter of YD climate change, with intense cooling registered in ice cores, but nevertheless the Greenland Ice Sheet retreated during the YD. An influential hypothesis suggests that North Atlantic abrupt climate changes were characterized by extreme seasonality, with mild summers that explain counterintuitive YD glacier retreat. Resolving seasonality is crucial to understanding the impacts of past and future climate change on all aspects of the Arctic system – including the ice sheet, because summer temperatures drive surface melt. This study will directly assess seasonality of the YD on Greenland for the first time by developing quantitative summer and winter temperature reconstructions. This study will also assess seasonality of the major cold snap over Greenland 8,200 years ago. Intriguingly, despite being shorter and smaller in annual magnitude than the YD, that event drove glacier advances around Greenland.This research will clarify how abrupt climate change affects the Arctic/subarctic North Atlantic region, especially Greenland and its ice sheet. By resolving seasonality of air temperatures during the YD, when the ice sheet retreated despite evidence for extremely cold (annual) air temperatures, this study will also clarify the competing roles of precipitation and ocean temperatures in driving past ice sheet behavior – with implications for projections of future ice sheet mass loss. This study will test whether the Holocene 8.2 ka event was essentially a mini YD, or fundamentally different. The latter could make the YD a poor analog for future abrupt climate change, which would unfold in an interglacial world. The 8.2 ka event has been described as an ideal target for testing climate models, but the quantitative reconstructions of associated terrestrial climate changes needed to optimize such tests are rare.Another major contribution of this work is calibration and advancement of season-specific methods for reconstructing arctic climate. Such methods are needed to address questions about seasonality on diverse Quaternary timescales, providing essential tests for climate models. The proposed work will also enable more sophisticated interpretations of stable isotope records and paleotemperature reconstructions from the Greenland ice cores by providing direct comparisons. This may yield new insights from those iconic records, once seasonality of the climate changes they capture is clarified.This project will utilize classic sites that preserve rare, readily dateable YD lake sediment records. Multiple calibrated, season-specific proxies in sediment cores – including insect and pollen assemblages, terrestrial leaf wax δ2H, and aquatic insect δ18O – will test and complement one another and provide new comparisons to annually integrated records from ice cores. This work will build upon the research team’s experience calibrating and applying these proxies in Greenland. A new calibration study will verify the seasonality of leaf wax δ2H in a range of Greenland climates.The possibility of future abrupt climate change is a major societal concern. As evidence accumulates for recent and ongoing changes in North Atlantic Ocean circulation, past abrupt changes in the subpolar North Atlantic are increasingly relevant to the future. This project will train two Ph.D. students and at least six undergraduates. In addition to conducting research during the academic year, students will participate in mentored summer research institutes to build research and communication skills. To share science beyond academia and hone outreach skills, students will conduct outreach (a) in Greenland near the study site, and (b) in a Chicago area high school serving a diverse population of 3200 students, in partnership with teachers who participated in the PI’s 2018 climate science workshop for high school teachers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.