COLLABORATIVE RESEARCH: Resolving Ambiguous Exposure-Age Chronologies of Antarctic Deglaciation with Measurements of In-Situ-Produced Cosmogenic Carbon-14

该项目的总体目标是确定过去南极冰盖大小的变化对全球海平面的影响。在25,000年前的最后一个冰河时代的顶峰，海平面比现在低120米(400英尺)，因为现在成为海洋一部分的水是北美、欧亚大陆和南极洲扩大的冰川和冰盖的一部分。从那时到现在，这些陆地冰的融化和消退导致海平面上升。在这个项目中，我们的目标是提高我们对南极冰盖大小变化如何促成这一过程的理解。实现这一目标的总体战略包括：(I)探访南极洲现在未被冰覆盖的地区；(Ii)寻找地质证据，特别是岩石表面和沉积物沉积物，以表明这些地区过去曾被更厚的冰覆盖；以及(Iii)确定这些地质表面和沉积物的年龄。该项目解决了这一战略的最后部分--确定南极冰川岩石表面或沉积物的年龄--使用一种相对较新的技术，该技术涉及测量岩石表面的微量元素，这些元素是在岩石表面因冰川消退而暴露后由宇宙射线轰击产生的。通过将这种方法应用于以前访问南极洲时收集的岩石样本，可以确定过去冰盖扩张和收缩的时间。预计该项目的主要科学成果是：(1)增进对南极冰盖变化如何促进过去全球海平面上升的理解；(2)从更长期的角度更好地理解现代观测到的南极冰盖变化。这第二个结果可能会改善对未来冰盖行为的预测。该项目的其他成果包括对本科生和研究生进行个人培训，以及开发一门将在新奥尔良杜兰大学教授的关于海平面变化的新课程。新奥尔良是一座今天正在受到海平面变化影响的城市。该项目将使用从南极洲几个地点收集的现有样本中原位产生的宇宙成因碳-14的测量来解决现有末次冰川最大值中存在的主要含糊不清的问题，以提供冰盖重建。该项目之所以重要，是因为准确重建冰盖变化对于限制重建过去的海平面变化至关重要。虽然碳-14最常被用作地质计时器，因为它是在高层大气中产生的，并被并入有机材料中，但它也是在暴露于地球表面宇宙射线通量的岩石和矿物的晶格中产生的。在后一种情况下，其浓度与地表暴露的时间成正比，现场产生的碳-14的测量可以用来确定形成或暴露岩石表面的地质事件的日期，例如冰盖扩张和后退。虽然碳-14是可用于这一目的的几种痕量放射性核素之一，但它的独特之处在于，相对于冰川-间冰期变化的时间尺度，它的半衰期很短。因此，在极地地区的岩石表面在许多冰川-间冰期周期中被冰盖变化反复覆盖和暴露的情况下，碳14测量是唯一适合于准确测定最近一次冰盖进退的时间。我们的目标是利用这一特性来改善我们对南极冰盖变化的理解，这些地点位于一些关键地点，在这些地点，其他地表暴露测年方法产生了模糊的结果。在地理上，这些问题集中在南极洲的韦德尔海海湾，南极大陆的几何结构可能允许冰川-间冰期冰量发生大的变化，但现有的冰盖变化的地质记录特别模糊。此外，在已经存在独立约束的冰川消融年表的情况下，应用原位碳14测量，可能使我们能够确定上一次冰盖进动和最近一次退缩的日期。

The overall goal of this project is to determine the effect of past changes in the size of the Antarctic Ice Sheet on global sea level. At the peak of the last ice age 25,000 years ago, sea level was 120 meters (400 feet) lower than it is at present because water that is now part of the ocean was instead part of expanded glaciers and ice sheets in North America, Eurasia, and Antarctica. Between then and now, melting and retreat of this land ice caused sea level to rise. In this project, we aim to improve our understanding of how changes in the size of the Antarctic Ice Sheet contributed to this process. The overall strategy to accomplish this involves (i) visiting areas in Antarctica that are not now covered by ice; (ii) looking for geological evidence, specifically rock surface and sediment deposits, that indicates that these areas were covered by thicker ice in the past; and (iii) determining the age of these geological surfaces and deposits. This project addresses the final part of this strategy -- determining the age of Antarctic glacial rock surfaces or sediment deposits -- using a relatively new technique that involves measuring trace elements in rock surfaces that are produced by cosmic-ray bombardment after the rock surfaces are exposed by ice retreat. By applying this method to rock samples collected in previous visits to Antarctica, the timing of past expansion and contraction of the ice sheet can be determined. The main scientific outcomes expected from this project are (i) improved understanding of how Antarctic Ice Sheet changes contributed to past global sea level rise; and (ii) improved understanding of modern observed Antarctic Ice Sheet changes in a longer-term context. This second outcome will potentially improve predictions of future ice sheet behavior. Other outcomes of the project include training of individual undergraduate and graduate students, as well as the development of a new course on sea level change to be taught at Tulane University in New Orleans, a city that is being affected by sea level change today.This project will use measurements of in-situ-produced cosmogenic carbon-14 in quartz from existing samples collected at several sites in Antarctica to resolve major ambiguities in existing Last Glacial Maximum to present ice sheet reconstructions. This project is important because of the critical nature of accurate reconstructions of ice sheet change in constraining reconstructions of past sea level change. Although carbon-14 is most commonly exploited as a geochronometer through its production in the upper atmosphere and incorporation into organic materials, it is also produced within the crystal lattice of rocks and minerals that are exposed to the cosmic-ray flux at the Earth's surface. In this latter case, its concentration is proportional to the duration of surface exposure, and measurements of in-situ-produced carbon-14 can be used to date geological events that form or expose rock surfaces, for example, ice sheet expansion and retreat. Although carbon-14 is one of several trace radionuclides that can be used for this purpose, it is unique among them in that its half-life is short relative to the time scale of glacial-interglacial variations. Thus, in cases where rock surfaces in polar regions have been repeatedly covered and uncovered by ice sheet change during many glacial-interglacial cycles, carbon-14 measurements are uniquely suited to accurately dating the most recent episode of ice sheet advance and retreat. We aim to use this property to improve our understanding of Antarctic Ice Sheet change at a number of critically located sites at which other surface exposure dating methods have yielded ambiguous results. Geographically, these are focused in the Weddell Sea embayment of Antarctica, which is an area where the geometry of the Antarctic continent potentially permits large glacial-interglacial changes in ice volume but where existing geologic records of ice sheet change are particularly ambiguous. In addition, in-situ carbon-14 measurements, applied where independently constrained deglaciation chronologies already exist, can potentially allow us to date the last period of ice sheet advance as well as the most recent retreat.