Age constraints on subtropical iceberg scour events

随着最后一个冰河时代之后北美冰盖的融化，巨大的冰山崩解入海，随着洋流漂过北大西洋。这些冰山中的许多向东移动，留在极地地区，但来自海底高分辨率地图的新证据，结合过去洋流的模型，表明数百个巨大的（约300米厚）冰山可能沿着美国大西洋海岸向南漂移，其中一些冰山在完全融化之前一直移动到佛罗里达群岛。这些冰山可能是被融化的冰水的巨大爆发带向南方的，这些冰水能够克服向北流动的墨西哥湾流。这一途径的发现意味着来自冰盖的新鲜融水的循环模式及其对全球气候的影响可能比以前认为的更为复杂；然而，这些事件发生的时间仍不确定。本研究的目的是通过检查该地区的海底沉积物，为冰山冲刷事件的时间提供初始年龄限制，并评估获得沿边缘向南流动的冷融水脉冲的高分辨率记录的潜力。这些数据将在理解冰川融水脉冲如何与全球环流和气候模式相互作用和影响方面迈出重要的一步。观察过去的冰盖融化事件可以为未来的变化提供关键的见解，这是特别有趣的，因为格陵兰岛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。在过去的2万到6000年间，北半球气候的许多突变被认为是由大量融水和冰山流入北大西洋的亚极地引起的。最近在亚热带纬度（35-24°N）发现的冰山冲刷现象，以及相关融水洪水的高分辨率数值模型重建表明，淡水可能会迅速到达亚热带，而不会首先使缓和大西洋经向翻转环流（AMOC）强度的亚极地深水形成区变新鲜。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，会被较咸的洋流混合而大大稀释。这种亚热带淡水化模式与当前的模式形成鲜明对比，即增加的融水和冰山排放使亚极地环流变得新鲜，如北大西洋海洋沉积物中存在的漂流碎屑（IRD）所示。对亚热带冰山冲刷的海洋沉积物岩心的分析将有助于建立一个记录通往亚热带北大西洋的融水和冰山事件的频率、持续时间和规模的古气候数据库。这些数据将提供关键的模型参数，描述陆地冰山在边缘移动的时间和条件，这些参数可用于检验冰山冲刷和IRD的存在是否代表了大尺度海洋环流的变化，还是冰山融化轨迹和位置的更局部的变化。了解融水和冰山何时被运送到亚热带地区，对于揭示高纬度淡水强迫的变化如何影响过去的气候至关重要。

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.