Collaborative Research: Reconstructing East Antarctica’s Past Response to Climate using Subglacial Precipitates

在过去的一个世纪里，气候科学通过对极地冰芯、深海沉积物和洞穴洞穴等各种地质档案的化学和同位素表征，建立了对地球冰期循环的广泛记录。这些气候档案提供了一幅关于冰河时代周期以及由这些重大气候节律引发的相关全球海平面大幅波动的深刻图景。然而，这些记录仍然提供了有限的洞察力，即在过去的温暖气候时期，地球冰盖是如何或哪些导致海平面上升的。这对我们的现代世界特别重要：南极冰盖目前是世界上最大的淡水水库，如果完全融化，将使全球海平面上升60米(200英尺)以上。然而，南极冰盖对温暖气候的敏感性的地质记录尤其有限，而且很难获得，因为比现代冰盖几何形状更小的直接记录仍然隐藏在覆盖该大陆的一英里厚的冰层之下。因此，目前尚不清楚这些冰盖在多大程度上导致了过去气候变暖期间海平面的上升，也不清楚它将如何应对预期的近期气候变暖。在这项拟议的研究中，我们试图开发冰下化学沉淀物--在冰盖下发现的湖泊中形成的矿物--作为气候档案，记录南极冰盖对过去气候变化的反应。这些冰下矿物形成在冰下积累了超过10万年，记录了随着气候变化而发生的冰下化学和同位素性质的变化。最终，这些样本被冰盖侵蚀，并被转移到南极冰缘，在那里收集并提供研究。这项研究将利用先进的地球化学、同位素和地质年代学技术来记录南极洲冰盖过去对温暖气候时期的反应，直接为了解南极洲将如何应对未来变暖的努力提供信息。为了改善对全球变暖的海平面预测，人们把重点放在了确定地球过去暖期的温度、海平面和极地冰盖的范围上。然而，由于南极冰盖(AIS)对气候的敏感性较差，在重建过去和未来海平面方面存在很大的不确定性。这一研究项目旨在开发利用冰下降水作为南极冰盖(AIS)过去对气候变化的反应的档案。来自南极东部的冰下沉淀形成于南极冰层下的水体中，这样做为AIS如何应对气候变化提供了一种全新而独特的衡量标准。在对这些沉淀物的初步检查中，我们发现了多个样品，其中包括持续时间长达数十万年的旋回蛋白石和方解石。我们对这些样品的初步地球化学特征表明，所观察到的矿物学变化是由于冰下水成分在同位素和化学成分不同的水域之间的周期性变化造成的。蛋白石形成水减少(Ce*1和高Fe/Mn)，并表现出高的234U/238U组成，类似于在AIS外围发现的咸水地下水卤水。更确切地说，方解石形成水是氧化的，其δ18O成分与贫化的极地高原(-50‰)的成分一致。~(234)U-~(230)Te测年可以建立描述这些矿物学和成分随时间变化的可靠的时间序列。这些时间序列与其他南极气候记录(如冰芯记录)的比较表明，方解石的形成事件与当地温度的千年尺度变化或“南极同位素最大值”相一致，后者代表南半球暖期，导致大西洋经向过流环流增加。最终，这个项目寻求开发一个全面的模型，说明温盐循环的变化如何引起冰川学反应，进而引起冰川下水的组成和降水档案中记录的矿物学阶段的变化。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Over the past century, climate science has constructed an extensive record of Earth’s ice age cycles through the chemical and isotopic characterization of various geologic archives such as polar ice cores, deep-ocean sediments, and cave speleothems. These climatic archives provide an insightful picture of ice age cycles and of the related large global sea level fluctuations triggered by these significant climate rhythms. However, such records still provide limited insight as to how or which of Earth’s ice sheets contributed to higher sea levels during past warm climate periods. This is of particular importance for our modern world: the Antarctic ice sheet is currently the world’s largest freshwater reservoir, which, if completely melted, would raise the global sea level by over 60 meters (200 feet). Yet, geologic records of Antarctic ice sheet sensitivity to warm climates are particularly limited and difficult to obtain, because the direct records of ice sheet geometry smaller than the modern one are still buried beneath the mile-thick ice covering the continent. Therefore, it remains unclear how much this ice sheet contributed to past sea level rise during warm climate periods or how it will respond to the anticipated near-future climate warming. In the proposed research we seek to develop sub-ice chemical precipitates—minerals that form in lakes found beneath the ice sheet—as a climatic archive, one that records how the Antarctic ice sheet responded to past climatic change. These sub-ice mineral formations accumulated beneath the ice for over a hundred thousand years, recording the changes in chemical and isotopic subglacial properties that occur in response to climate change. Eventually these samples were eroded by the ice sheet and moved to the Antarctic ice margin where they were collected and made available to study. This research will utilize advanced geochemical, isotopic and geochronologic techniques to develop record of the Antarctica ice sheet’s past response to warm climate periods, directly informing efforts to understand how Antarctica will response to future warming. Efforts to improve sea level forecasting on a warming planet have focused on determining the temperature, sea level and extent of polar ice sheets during Earth’s past warm periods. Large uncertainties, however, in reconstructions of past and future sea levels, result from the poorly constrained climate sensitivity of the Antarctic Ice sheet (AIS). This research project aims to develop the use of subglacial precipitates as an archive the Antarctic ice sheet (AIS) past response to climate change. The subglacial precipitates from East Antarctica form in water bodies beneath Antarctic ice and in doing so provide an entirely new and unique measure of how the AIS responds to climate change. In preliminary examination of these precipitates, we identified multiple samples consisting of cyclic opal and calcite that spans hundreds of thousands of years in duration. Our preliminary geochemical characterization of these samples indicates that the observed mineralogic changes result from a cyclic change in subglacial water compositions between isotopically and chemically distinct waters. Opal-forming waters are reduced (Ce* 1 and high Fe/Mn) and exhibit elevated 234U/238U compositions similar to the saline groundwater brines found at the periphery of the AIS. Calcite-forming waters, are rather, oxidized and exhibit δ18O compositions consistent with derivation from the depleted polar plateau ( -50 ‰). 234U-230Th dates permit construction of a robust timeseries describing these mineralogic and compositional changes through time. Comparisons of these time series with other Antarctic climate records (e.g., ice core records) reveal that calcite forming events align with millennial scale changes in local temperature or “Antarctic isotopic maximums”, which represent Southern Hemisphere warm periods resulting in increased Atlantic Meridional overturing circulation. Ultimately, this project seeks to develop a comprehensive model as to how changes in the thermohaline cycle induce a glaciologic response which in turn induces a change in the composition of subglacial waters and the mineralogic phase recorded within the precipitate archive.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.