Ocean-ice Interaction in the Ross Sea during Past Warm Periods

格陵兰岛和南极洲的两个极地冰盖所含的冰足以使全球海平面上升约65米。总共有58米相当于海平面的冰被束缚在南极洲，其中约4米位于特别容易受到环境变化影响的南极西部地区。这些地区的冰位于海平面以下几百米的床上，这意味着冰与变暖的海洋直接接触。在这种情况下，冰盖不仅通过更高的大气温度和表面融化从上而下融化，而且还通过与温暖的海水接触从下而上融化。最近对西南极洲大部分海洋终冻冰消失的速度的估计从几百年到几千年不等。距离这个范围的下限只有几代人的时间了，而且在二氧化碳排放不减、大气二氧化碳浓度增加和导致全球变暖的情况下，这种情况越来越有可能发生。我们可以详细观察冰盖如何对持续的环境变化作出反应。但我们无法亲眼观察到的是，当全球温度比今天高1度、2度、4度甚至8度时，冰会发生什么变化。作为地球科学家，我们喜欢侦探小说。我们可以利用海底沉积的泥浆来弄清楚南极冰盖对变暖气候的反应。就像老式的录音机一样，这种泥浆保存了过去时代的环境信号，那时地球的气候比现在更温暖，大气中的二氧化碳含量也比现在高。2018年1月至3月，国际海洋探索计划（ippc）第374号科考队将起航，我们想要恢复这样的纪录。我在探险队中的任务是观察泥土中的化学指纹。每一块被上面的冰盖压碎并从大陆运到海洋的岩石都有一个典型的指纹，就像每个人都有一个非常特殊的DNA一样。我们可以用这个指纹来破译这些物质在大陆上的位置，这反过来又可以告诉我们刮掉泥浆的冰缘的位置。你可以想象，在更温暖的时代，那个位置和今天不一样。随着环境条件的变化而变化的气候系统的另一部分是海洋如何在全球范围内循环和运输热量和碳。我们都习惯于认为墨西哥湾流给北欧海岸带来温暖的海水。在南方，在巨大的南极大陆旁边，另一个过程开始起作用：地表水的冻结和温暖的水与寒冷的浮动冰架的相互作用产生了非常咸、冷和稠密的水，这些水下沉到海底。这样的水域被称为南极海底水域，占全球海洋的30-40%。在一个变暖的世界里，情况会是一样的吗？还是说，南极冰盖的退缩也会对全球海洋环流产生重大影响？作为地球科学侦探，我们可以通过观察泥土的化学指纹来解决另一个问题。IODP远征374将是一个收集独特材料的好机会，并在一个更温暖的世界中推进我们对极地冰盖和海洋环流的基本理解——这个世界可能看起来就像我们正在走向的世界。

The two polar ice sheets in Greenland and Antarctica contain enough ice to raise global sea levels by ~65 metre. In total, 58 metre of sea level equivalent ice is tied up in Antarctica, of which ~4m is located in West Antarctic areas that are particularly vulnerable to environmental change. These are areas where the ice rests on a bed several hundred meters below sea level, meaning the ice is in direct contact with the warming ocean. In such a set up the ice sheet is not just melting from the top down by means of higher atmospheric temperatures and surface melting, but also from the bottom up, via contact with warmer ocean water. Recent estimates on how fast West Antarctica may lose most of its ocean terminating ice range from a few hundred to a few thousand years into the future. The lower end of this range is just a few generations away, and is something that becomes increasingly likely under unabated carbon dioxide emissions, increased atmospheric carbon dioxide concentrations and resulting global warming. We can make detailed observations on how the ice sheet reacts to ongoing environmental change. But what we can't observe with our own eyes is what will happen to the ice when global temperature are 1, 2, 4 or even 8 degrees warmer than today. As Earth Scientists we love our detective stories. Figuring out the response of the Antarctic ice sheets to warmer climates is something we can do by using the mud deposited at the bottom of the ocean. Just like an old-fashioned tape recorder, this mud preserves an environmental signal from times in the past, where earth's climate was warmer and atmospheric carbon dioxide levels were higher than today. It is such records that we want to recover when we set sail on Expedition 374 of the International Ocean Discovery Program in January 2018 to March 2018. My role on the expedition will be to look at the chemical fingerprint in the mud. Every rock that gets crushed by the overlying ice sheet and transported from the continent to the ocean has a typical fingerprint, just like every human being has a very specific DNA. We can use this fingerprint to decipher where the material was located on the continent, which in turn can tell us about the location of the ice margin that scrapes off the mud. You can imagine that back in times of greater warmth that location was not the same as it is today. Another part of the climate system that changes with changing environmental conditions is how the oceans circulate and transport heat and carbon around the globe. We are all used to thinking about the Gulf Stream that brings warm waters to the shores of Northern Europe. Down south, next to the giant Antarctic continent, another process comes into play: Freezing of surface waters and interaction of warmer water with cold floating ice shelves creates very salty, cold and dense water, which sinks to the bottom of the ocean. Such waters are called Antarctic Bottom Waters and fill up 30-40% of our global oceans. Will this be the same in a warming world? Or will the retreat of the Antarctic ice sheets also have a major impact on global ocean circulation? Another question we can tackle as Earth Science detectives by looking at the chemical fingerprint of mud. IODP Expedition 374 is going to be a great opportunity to collect unique material and advance our basic understanding of polar ice sheets and ocean circulation in a warmer world -a world which may just look like the one we are headed towards.