Controls on late Holocene and 20th century ice shelf dynamics in northeast Greenland

在过去的三十年里，对格陵兰岛和南极洲的卫星观测显示，由于空气和海洋温度的升高，海洋末端冰川和冰架正在加速变薄。更薄的冰架支撑内陆冰的能力更弱，导致接地线后退，冰盖变薄加剧，最终导致海平面上升。这一过程被称为海洋冰盖不稳定，有可能导致冰盖迅速和不可逆转的崩塌。事实上，一些冰盖模型表明，这些过程已经在南极洲开始了。在格陵兰岛，几项研究将最近的冰架变薄/丧失和冰流加速与大西洋暖水的入侵联系起来。然而，要了解最近这条接地线对海洋强迫（和其他扰动）响应的性质和速率，需要了解这些过程在最近几个世纪是如何演变的。在南极洲，已知一些变化在卫星观测之前就已经开始，这突出表明需要了解最近（百年）的历史，以便了解不断变化的海洋性质与冰盖动力学之间的相互作用。如果我们要准确预测北大西洋地区未来冰盖的行为以及海洋的反馈，这些知识是必不可少的。该项目将探索20世纪79N冰架的演变，该冰架目前支撑着东北格陵兰冰流（NEGIS）。NEGIS从格陵兰冰盖（GrIS）的东北部分流出，包含大约1.2米的海平面当量（sle）。它的未来稳定性不仅对未来GrIS的质量平衡至关重要，而且对东北大西洋的淡水通量，特别是北大西洋深水翻转环流至关重要。从21世纪初开始，位于NEGIS （Zachariae Isstrom和79N）前面的冰架开始不稳定，但是当ZI解体时，79N保持相对稳定。一些模拟研究表明，79N的“相对”稳定性可能会持续到下个世纪，但最近的海洋学观测表明，海洋热通量和融化速率可能会增加。因此，人们非常担心北纬79度将是下一个崩解的冰架，如果发生这种情况，它将导致向海洋排放的冰量大幅增加。该项目将利用从79N冰架下方和附近收集的现有沉积物重力、盒状沉积物和湖泊沉积物岩心来重建过去~200年的冰架历史和大西洋水循环。可用的材料是在2016年和2017年收集的，作为NERC项目（气候变暖中的格陵兰岛）的一部分，该项目与德国阿尔弗雷德·韦格纳研究所合作。具体来说，学生将对关键岩心进行多代理检查，包括沉积物性质（物理性质、粒度）、地球化学指标（氧、碳同位素、XRF、粘土矿物）和微化石含量（硅藻、有孔虫）的分析，以确定冰架的存在/不存在、冰架厚度的变化和水质量特征。有孔虫和氧、碳稳定同位素分析将用于研究融水通量和靠近冰架及其下方的大西洋水的变异性。将使用放射性（Pb, Cs）和放射性碳(C)定年方法确定冰架动态变化和温水入侵的地质年代学框架。Pb分析对于建立20世纪冰架动力学年表至关重要。这些数据集的集成将使控制撤退行为的过程得以确定。成功的学生将在多代理核心分析的各个方面进行培训，并尽可能花时间在关键设施和合作实验室。此外，学生将受益于一个庞大而充满活力的极地研究社区。

Over the past three decades satellite observations over Greenland and Antarctica have revealed that marine terminating glaciers and ice shelves have been thinning at an accelerating rate in response to both increased air and ocean temperatures. Thinner ice shelves are less able to buttress inland ice, leading to grounding line retreat, increased ice sheet thinning and ultimately, sea-level rise. This process, known as the marine ice sheet instability, has the potential to drive rapid and irreversible ice sheet collapse. Indeed, some ice sheet models indicate that these processes are already underway in Antarctica. In Greenland, several studies have linked recent ice shelf thinning/loss and accelerated ice flow to the incursion of warm Atlantic water. However, understanding the nature and rate of this recent grounding line response to ocean forcing (and other perturbations) requires knowledge of how these processes have evolved over recent centuries. In Antarctica, some changes are known to have started prior to satellite observations, highlighting the need to understand the recent (centennial) history in order to understand the interplay between changing ocean properties and ice sheet dynamics. Such knowledge is essential if we are to accurately predict future ice sheet behaviour as well as the ocean feedbacks in the North Atlantic region. This project will explore the twentieth century evolution of the 79N ice shelf, which currently buttresses the Northeast Greenland ice stream (NEGIS). The NEGIS drains the northeast sector of the Greenland ice Sheet (GrIS) and contains approximately 1.2 m sea-level equivalent (sle). Its future stability is pivotal not only to future mass balance of the GrIS but also the freshwater flux to the northeast Atlantic and specifically, to the North Atlantic Deep Water overturning circulation. Starting in the early 2000s, the ice shelves that front NEGIS (Zachariae Isstrom and 79N) have started to destabilise, but while ZI has disintegrated, 79N has remained relatively stable. Some modelling studies suggest that the 'relative' stability of 79N could continue over the next century, but recent oceanographic observations have shown that ocean heat flux and melt rates maybe increasing. Thus, there is great concern that 79N will be the next ice shelf to disintegrate and if this occurs, it will result in a substantial increase in ice discharge to the ocean.This project will use existing sediment gravity, box and lake sediment cores collected from beneath and adjacent to the 79N ice shelf to reconstruct ice shelf history and Atlantic Water circulation over the last ~200 years. The available material was collected in 2016 and 2017 as part of the NERC project (Greenland in a Warmer Climate) in collaboration with the Alfred Wegener Institute in Germany. Specifically the student will employ a multi-proxy examination of key cores including analyses of sediment properties (physical properties, grain size), geochemical proxies (oxygen, carbon isotopes, XRF, clay mineral) and microfossil content (diatoms, foraminifera) to determine ice shelf presence/absence, changes in ice shelf thickness and water mass characteristics. Foraminiferal and oxygen and carbon stable isotope analyses will be used to investigate the variability in meltwater flux and Atlantic Water adjacent to and beneath the ice shelf. A geochronological framework for changing ice shelf dynamics and incursions of warm water will be determined using radiometric (Pb, Cs) and radiocarbon (C) dating methods. Pb analysis will be critical to establish a chronology twentieth century ice shelf dynamics. Integration of these datasets will enable the processes controlling retreat behaviour to be determined. The successful student will be trained in all aspect of multi-proxy core analyses and wherever possible spend time at key facilities and partner laboratories.In addition, the student will benefit from a large and energetic Polar research communities.