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变化对全球海平面的影响。在25000年前最后一个冰河时代的高峰时期，海平面比现在低120米（400英尺），因为现在属于海洋的水是北美、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，陆地冰的融化和退缩导致海平面上升。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进这一过程的理解。实现这一目标的总体战略包括：(1)访问南极洲目前未被冰覆盖的地区；寻找地质证据，特别是岩石表面和沉积物沉积物，这些证据表明这些地区过去被较厚的冰覆盖；（三）确定这些地质表面和矿床的年龄。该项目解决了该策略的最后一部分——确定南极冰川岩石表面或沉积物沉积物的年龄——使用一种相对较新的技术，包括测量岩石表面的微量元素，这些元素是在岩石表面因冰退缩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。本项目预期的主要科学成果有：(i)提高对南极冰盖变化如何导致过去全球海平面上升的认识；（ii）提高对现代观测到的南极冰盖长期变化的认识。第二个结果将有可能改善对未来冰盖行为的预测。该项目的其他成果包括培训本科生和研究生，以及开发一门关于海平面变化的新课程，该课程将在新奥尔良的杜兰大学（Tulane University）教授，新奥尔良目前正受到海平面变化的影响。该项目将使用在南极几个地点收集的现有样品中测量石英中原位产生的宇宙成因碳14，以解决现有末次盛冰期的主要模糊问题，以呈现冰盖重建。这个项目很重要，因为精确重建冰盖变化对限制过去海平面变化的重建具有关键性质。虽然碳-14通过其在高层大气中的产生和与有机物质的结合而最常被用作地球时计，但它也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。在后一种情况下，它的浓度与地表暴露的持续时间成正比，对原位产生的碳-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年代，例如，冰盖的扩张和退缩。虽然碳-14是可用于这一目的的几种微量放射性核素之一，但它的独特之处在于，相对于冰期-间冰期变化的时间尺度，它的半衰期很短。因此，在许多冰期-间冰期循环期间，极地地区的岩石表面被冰盖变化反复覆盖和暴露的情况下，碳-14的测量方法是唯一适合于精确测定最近冰盖前进和后退时期的方法。我们的目标是利用这一特性来提高我们对南极冰盖变化的理解，在一些关键地点，其他表面暴露测年方法已经产生了模糊的结果。在地理上，这些研究集中在南极洲的威德尔海湾内，这是一个南极大陆的几何形状可能允许冰量发生大的冰期-间冰期变化的地区，但现有的冰盖变化地质记录特别模糊。此外，原位碳14测量，应用于已经存在独立约束的冰川消融年表，可能使我们能够确定冰盖最后一个时期的前进和最近的退缩。

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.