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

合作研究：通过测量原位产生的宇宙成因碳 14 来解决南极冰川消融的模糊暴露年龄年表

• 批准号: 1542976
• 负责人: Gregory Balco
• 金额: $ 5.06万
• 依托单位: Berkeley Geochronology Center
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-05-01 至 2021-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1542976&HistoricalAwards=false
• 关键词: COLLABORATIVE; RESEARCH; Resolving; Ambiguous; Exposure

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.

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 25,000 年前的最后一个冰河时代的高峰期，海平面比现在低 120 米（400 英尺），因为现在属于海洋的水已成为北美、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，陆地冰的融化和消退导致海平面上升。在这个项目中，我们的目标是加深对南极冰盖大小变化如何影响这一过程的理解。实现这一目标的总体策略包括（i）访问南极洲目前未被冰雪覆盖的地区； (ii) 寻找地质证据，特别是岩石表面和沉积物，表明这些地区过去被较厚的冰覆盖； (iii) 确定这些地质表面和沉积物的年龄。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中的微量元素，这些微量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之前访问南极洲时收集的岩石样本，可以确定冰盖过去膨胀和收缩的时间。该项目预期的主要科学成果是：（i）加深对南极冰盖变化如何导致过去全球海平面上升的了解； (ii) 提高对现代观测到的南极冰盖长期变化的理解。 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对本科生和研究生进行培训，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杜兰大学教授，新奥尔良这座城市目前正受到海平面变化的影响。该项目将使用在南极洲多个地点收集的现有样品中原位产生的宇宙成因碳 14 进行测量，以解决现有最新报告中的主要模糊之处。 盛冰期呈现冰盖重建。该项目很重要，因为准确重建冰盖变化对于限制过去海平面变化的重建至关重要。虽然碳 14 最常被用作地质计时仪，它是在高层大气中产生并掺入有机材料中，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内产生。在后一种情况下，其浓度与表面暴露的持续时间成正比，并且对原位产生的碳14的测量可用于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。尽管碳14是可用于此目的的几种痕量放射性核素之一，但它在其中的独特之处在于其半衰期相对于冰期-间冰期变化的时间尺度较短。因此，在许多冰期-间冰期循环期间，极地地区的岩石表面反复被冰盖变化覆盖和暴露，碳14测量特别适合准确测定最近一次冰盖前进和后退的时间。 我们的目标是利用这一特性来提高我们对南极冰盖在许多关键地点的变化的理解，在这些地点，其他表面暴露测年方法已经产生了模糊的结果。从地理上看，这些集中在南极洲的威德尔海海湾，该地区的南极大陆的几何形状可能允许冰期-间冰期的冰量发生巨大变化，但现有的冰盖变化地质记录特别模糊。 此外，在已经存在独立约束的冰消年表的情况下进行原位碳 14 测量，有可能使我们能够确定冰盖前进的最后时期以及最近的后退时期。