Age constraints on subtropical iceberg scour events

亚热带冰山冲刷事件的年龄限制

• 批准号: 1558994
• 负责人: Jenna Hill
• 金额: $ 7.94万
• 依托单位: Coastal Carolina University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-10-01 至 2020-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1558994&HistoricalAwards=false
• 关键词: Age; constraints; subtropical; iceberg; scour

As the ice sheets across North America melted following the last ice age, huge icebergs calved off into the sea, drifting in ocean currents across the North Atlantic. Many of these icebergs travelled east, remaining in polar regions, but new evidence from high-resolution maps of the seafloor, combined with modeling of past ocean currents, suggests hundreds of massive (~300m thick) icebergs likely drifted south along the Atlantic coast of the U.S., with some traveling all the way to the Florida Keys before completely melting. These icebergs would have been carried south by huge outburst floods of melting ice water that were able overcome the northward flow of the Gulf Stream current. The discovery of this pathway implies the circulation patterns of fresh meltwater from ice sheets and the subsequent impact on global climate may be more complex than previously thought; yet the timing of these events remains uncertain. The goal of this research is to provide initial age constraints on the timing of the iceberg scour events by examining seafloor sediment from this region and assess the potential to obtain high resolution records of cold, meltwater pulses flowing south along the margin. These data will present an important step forward in understanding how glacial meltwater pulses interact with and impact global circulation and climate patterns. Looking to ice sheet melting events in the past can provide critical insight to future changes, which is of particular interest as modern ice sheets in Greenland and Antarctica continue to melt and the impacts of shifts in global climate are felt around the world.Many abrupt shifts in Northern Hemisphere climate during the last 20,000 to 6,000 years are thought to have been triggered by the discharge of large volumes of meltwater and icebergs to the subpolar North Atlantic. The recent discovery of iceberg scours at subtropical latitudes (35-24°N), along with high-resolution numerical model reconstructions of associated meltwater floods, suggests that freshwater rapidly would have reached the subtropics without initially freshening the subpolar deepwater formation regions that moderate the strength of the Atlantic Meridional Overturning Circulation (AMOC). Freshwater associated with these events would have been significantly diluted by mixing with the saltier current before being advected northward to the subpolar gyre by the Gulf Stream. This pattern of subtropical freshening contrasts with the current paradigm that increased meltwater and iceberg discharge freshened the subpolar gyre, as shown by the presence of ice-rafted detritus (IRD) in marine sediments in the northern Atlantic. The analysis of marine sediment cores from the subtropical iceberg scours will help initiate the compilation of a paleoclimate database recording the frequency, duration and magnitude of meltwater and iceberg events routed to the subtropical North Atlantic. These data will provide critical model parameters describing the timing and conditions for grounded iceberg transport across the margin that can be used to examine whether the presence iceberg scours and IRD represent changes in large-scale ocean circulation, or more localized changes in the trajectory and location of melting icebergs. Understanding when meltwater and icebergs were transported to the subtropics is vital for unraveling how changes in high-latitude freshwater forcing may have influenced past climate.

随着北美冰盖在上一个冰河时代之后融化，巨大的冰山崩解落入大海，随洋流漂流穿过北大西洋。其中许多冰山向东移动，仍然位于极地地区，但来自高分辨率海底地图的新证据，结合对过去洋流的模拟，表明数百座巨大(约300米厚)的冰山可能沿着美国大西洋海岸向南漂移，其中一些冰山一直漂流到佛罗里达群岛，然后完全融化。这些冰山可能会被大量融化的冰水洪水带到南方，这些洪水能够克服墨西哥湾流向北的流动。这一途径的发现意味着来自冰盖的新鲜融化水的循环模式以及随后对全球气候的影响可能比之前想象的更复杂；然而这些事件的发生时间仍然不确定。这项研究的目的是通过检查该区域的海底沉积物，提供对冰山冲刷事件时间的初步年龄限制，并评估获得沿边缘向南流动的寒冷、融化的水脉冲的高分辨率记录的可能性。这些数据将在理解冰川融化水脉冲如何与全球环流和气候模式相互作用和影响方面向前迈出重要的一步。回顾过去的冰盖融化事件可以为未来的变化提供关键的洞察，随着格陵兰和南极洲的现代冰盖继续融化，全球气候变化的影响在世界各地都能感受到，这一点特别有趣。过去20,000到6,000年间，北半球气候的许多突然变化被认为是由大量融化的水和冰山排放到亚极地北大西洋引发的。最近在亚热带纬度(35-24°N)发现的冰山冲刷以及相关融水洪水的高分辨率数值模式重建表明，如果没有最初淡化亚极深水形成区，使大西洋子午线翻转环流(AMOC)的强度减弱，淡水就会迅速到达亚热带。与这些事件相关的淡水在被墨西哥湾流向北平流到亚极地环流之前，会因与较咸的洋流混合而被显著稀释。这种亚热带的清新模式与目前的模式形成了鲜明对比，目前的模式是增加融水和冰山排放使亚极地环流变得新鲜，北大西洋海洋沉积物中存在冰筏碎屑(IRD)就表明了这一点。对亚热带冰山冲刷形成的海洋沉积物岩心的分析将有助于开始编制一个古气候数据库，记录流向亚热带北大西洋的融水和冰山事件的频率、持续时间和强度。这些数据将提供关键的模型参数，描述冰山穿越边缘搁浅的时间和条件，可用于检查冰山冲刷和IRD的存在是否代表大规模海洋环流的变化，或者冰山融化轨迹和位置的更局部变化。了解融化的水和冰山何时被输送到亚热带地区，对于解开高纬度淡水强迫的变化可能如何影响过去的气候至关重要。