COLLABORATIVE RESEARCH: Orbital-scale Variability of the West Antarctic Ice Sheet and the Formation of Bottom Water in the Ross Sea during the Pliocene-Pleistocene

合作研究：上新世-更新世期间南极西部冰盖的轨道尺度变化和罗斯海底层水的形成

• 批准号: 2000992
• 负责人: Brian Romans
• 金额: $ 20.35万
• 依托单位: Virginia Polytechnic Institute and State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2000992&HistoricalAwards=false
• 关键词: COLLABORATIVE; RESEARCH; Orbital; scale; Variability

Part I: Non-technical description: Predicting how polar ice sheets will respond to future global warming is difficult because all the processes that contribute to their melting are not well understood. This is important because the more ice on land that melts, the higher sea levels will rise. The most significant uncertainty in current estimates of sea-level rise in the coming decades is the potential contribution from the Antarctic Ice Sheet. One way to increase our knowledge about how large ice sheets respond to climate change in response to natural factors is to examine the geologic past. Natural global warming (and cooling) events in Earth’s history provide examples that we can use to better understand processes, interactions, and responses we can’t directly observe today. One such time period, approximately three million years ago (known as the Pliocene), was the last time atmospheric carbon dioxide levels were as high as they are today and, therefore, represents a time period to study to better understand the ice sheet response to a warming climate. Specifically, this project is interested in understanding how ocean currents near Antarctica, which transport heat and store carbon, behaved during these past climate events. The history of past ice sheet-ocean interactions are recorded in sediments that were deposited, layer upon layer, in the deep sea offshore Antarctica. In January-February 2018, a team of scientists and crew set sail to the Ross Sea, offshore west Antarctica, on the scientific ocean drilling vessel JOIDES Resolution to recover such sediment archives. This project focuses on a sediment core from that expedition, which captures the relatively warm Pliocene time interval, as well as the subsequent transition into cooler climates typical of the past two million years. The researchers will analyze the sediment with multiple complementary measurements, including: grain size, composition, chemistry of organic matter, physical structures, microfossil type and abundance, and more. These analyses will be done by the research team, including several students, at their respective laboratories and will then integrated into a unified record of ice sheet-ocean interactions. Ultimately, the results will be used to improve modeled projections of how the Antarctic Ice Sheet could respond to future climate change. Part II: Technical description: Geological records from the Antarctic Ice Sheet (AIS) margin demonstrate that the ice sheet oscillated in response to orbital variations in insolation (i.e., ~400, 100, 41, and 20 kyr), and it appears to be more sensitive to specific frequencies that regulate mean annual insolation (i.e., 41-kyr obliquity), particularly when the ice sheet extends into marine environments and is impacted by ocean circulation. However, the relationship between orbital forcing and the production of Antarctic Bottom Water (AABW) is unconstrained. Thus, a knowledge gap exists in understanding how changing insolation impacts ice marginal and Southern Ocean conditions that directly influence ventilation of the global ocean. The researchers hypothesize that insolation-driven changes directly affected the production and export of AABW to the Southern Ocean from the Pliocene through the Pleistocene. For example, obliquity amplification during the warmer Pliocene may have led to enhanced production and export of dense waters from the shelf due to reduced AIS extent, which, in turn, led to greater AABW outflow. To determine the relationship of AABW production to orbital regime, they plan to reconstruct both from a single, continuous record from the levee of Hillary Canyon, a major conduit of AABW outflow, on the Ross Sea continental rise. To test their hypothesis, they will analyze sediment from IODP Site U1524 (recovered in 2018 during International Ocean Discovery Program Expedition 374) and focus on three data sets. (1) They will use the occurrence, frequency, and character of mm-scale turbidite beds as a proxy of dense-shelf-water cascading outflow and AABW production. They will estimate the down-slope flux via numerical modeling of turbidity current properties using morphology, grain size, and bed thickness as input parameters. (2) They will use grain-size data, physical properties, XRF core scanning, CT imaging, and hyperspectral imaging to guide lithofacies analysis to infer processes occurring during glacial, deglacial, and interglacial periods. Statistical techniques and optimization methods will be applied to test for astronomical forcing of sedimentary packages in order to provide a cyclostratigraphic framework and interpret the orbital-forcing regime. (3) They will use bulk sedimentary carbon and nitrogen abundance and isotope data to determine how relative contributions of terrigenous and marine organic matter change in response to orbital forcing. All of these data will be integrated with sedimentological records to deconvolve organic matter production from its deposition or remobilization due to AABW outflow as a function of the oscillating extent of the AIS. These data sets will be integrated into a unified chronostratigraphy to determine the relationship between AABW outflow and orbital-forcing scenarios under the varying climate regimes of the Plio-Pleistocene.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.

第一部分：非技术描述：预测极地冰盖将如何应对未来的全球变暖是困难的，因为导致它们融化的所有过程都没有被很好地理解。这一点很重要，因为陆地上融化的冰越多，海平面就会上升得越高。目前对未来几十年海平面上升的估计中最大的不确定性是南极冰盖的潜在贡献。要增加我们对大冰盖如何响应气候变化对自然因素的响应的知识，一种方法是研究地质过去。地球历史上的自然全球变暖(和变冷)事件提供了一些例子，我们可以用这些例子来更好地理解我们今天无法直接观察到的过程、相互作用和反应。大约300万年前(称为上新世)就是这样一个时期，这是大气二氧化碳水平最后一次达到今天的水平，因此，这是一个需要研究的时期，以更好地了解冰盖对气候变暖的反应。具体地说，这个项目感兴趣的是了解南极洲附近的洋流在这些过去的气候事件中表现如何，这些洋流输送热量和储存碳。过去冰盖与海洋相互作用的历史被记录在沉积物中，这些沉积物层层堆积在南极洲近海的深海中。2018年1月至2月，一组科学家和船员乘坐科学海洋钻探船JOIDES Resolve前往南极洲西部近海的罗斯海，以找回这些沉积物档案。该项目的重点是那次探险的沉积物岩芯，它捕捉到了上新世相对温暖的时间间隔，以及随后向过去200万年典型的凉爽气候的过渡。研究人员将通过多项互补测量来分析沉积物，包括：颗粒大小、成分、有机质化学、物理结构、微化石类型和丰度等。这些分析将由包括几名学生在内的研究小组在各自的实验室进行，然后将整合成冰盖与海洋相互作用的统一记录。最终，这些结果将被用于改进南极冰盖如何应对未来气候变化的建模预测。第二部分：技术描述：南极冰盖(AIS)边缘的地质记录表明，冰盖对太阳辐射的轨道变化(即~400、100、41和20KYR)作出了响应，而且它似乎对调节年平均日照角度(即41 KYR倾角)的特定频率更加敏感，特别是当冰盖延伸到海洋环境并受到海洋环流的影响时。然而，轨道强迫与南极底层水(AABW)的产生之间的关系是不受限制的。因此，在理解日照变化如何影响直接影响全球海洋通风的冰缘和南大洋状况方面存在知识鸿沟。研究人员推测，日照驱动的变化直接影响了AABW的生产和从上新世到更新世向南大洋的出口。例如，在较温暖的上新世，倾角放大可能由于AIS范围的减少而导致陆架稠密水的生产和出口增加，这反过来又导致更多的AABW外流。为了确定AABW产量与轨道状况的关系，他们计划从罗斯海大陆隆起的Hillary Canyon堤坝上重建两者的连续记录。Hillary Canyon是AABW外流的主要渠道。为了验证他们的假设，他们将分析IODP站点U1524(2018年国际海洋发现计划374期间发现的)的沉积物，并专注于三个数据集。(1)他们将利用毫米级浊积层的产状、频率和特征作为密集陆架水梯级外流和AABW生产的替代。他们将使用形态、颗粒大小和床层厚度作为输入参数，通过浊流特性的数值模拟来估计下坡通量。(2)他们将使用粒度数据、物理性质、XRF岩心扫描、CT成像和高光谱成像来指导岩相分析，以推断冰期、去冰期和间冰期发生的过程。统计技术和最优化方法将用于测试沉积包的天文强迫，以提供旋回地层框架和解释轨道强迫制度。(3)他们将使用大量沉积碳、氮丰度和同位素数据来确定陆源和海洋有机质的相对贡献如何随着轨道强迫的变化而变化。所有这些数据都将与沉积学记录相结合，以消除由于AABW外流导致的有机质沉积或再活化产生的有机质，这是AIS振荡程度的函数。这些数据集将被整合到一个统一的年代地层学中，以确定在上更新世不同气候制度下AABW外流和轨道强迫情景之间的关系。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。