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

新仙女木期（YD）是地球历史上记录最充分的气候突变的例子，然而新仙女木期的基本特征仍然是个谜。格陵兰岛位于YD气候变化的中心，在冰芯中记录了强烈的冷却，但格陵兰冰盖在YD期间退缩。一个有影响力的假设认为，北大西洋的突变气候变化具有极端的季节性特征，温和的夏季可以解释YD冰川的退缩。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各个方面的影响至关重要——包括对冰盖的影响，因为夏季气温会推动表面融化。本研究将首次通过建立定量的夏季和冬季温度重建来直接评估格陵兰岛YD的季节性。这项研究还将评估8200年前格陵兰岛大寒流的季节性。有趣的是，尽管年规模比YD更短，更小，但那次事件推动了格陵兰岛周围的冰川前进。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，特别是格陵兰岛及其冰盖。在YD期间，尽管有证据表明气温极低（年），但冰盖仍在退缩，通过解决气温的季节性，本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。这项研究将测试全新世8.2 ka事件是否本质上是一个迷你YD，或者根本不同。后者可能会使YD成为未来突变气候变化的糟糕模拟，这将在间冰期世界展开。8.2 ka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很少。这项工作的另一个主要贡献是校准和推进了重建北极气候的季节性方法。这些方法需要在不同的第四纪时间尺度上解决季节性问题，为气候模式提供必要的检验。拟议的工作还将通过提供直接比较，对稳定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，这可能会从这些记录中获得新的见解。该项目将利用保存稀有的、易于确定年代的YD湖泊沉积物记录的经典地点。沉积物岩心中多种校准的、特定季节的代用物——包括昆虫和花粉组合、陆生叶蜡δ2H和水生昆虫δ18O——将相互检验和补充，并为冰芯的年度综合记录提供新的比较。这项工作将以研究小组在格陵兰校准和应用这些代用品的经验为基础。一项新的校准研究将在格陵兰一系列气候条件下验证叶蜡δ2H的季节性。未来气候突变的可能性是一个主要的社会问题。随着北大西洋环流最近和正在发生变化的证据越来越多，北大西洋次极区过去的突变与未来的关系越来越密切。本项目将培养2名博士生和至少6名本科生。除了在学年期间进行研究外，学生还将参加有指导的暑期研究机构，以建立研究和沟通技巧。为了在学术界之外分享科学并培养外联技能，学生们将与参加PI 2018年高中教师气候科学研讨会的教师合作，在研究地点附近的格陵兰岛和(b)芝加哥地区一所为3200名不同学生提供服务的高中开展外联活动。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.