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

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，因为导致其融化的所有过程都没有得到很好的理解。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。在目前对未来几十年海平面上升的估计中，最重要的不确定性是南极冰盖的潜在贡献。增加我们对大冰原如何对自然因素对气候变化作出反应的认识的一种方法是检查地质历史。地球历史上的自然全球变暖（和变冷）事件提供了我们可以用来更好地理解我们今天无法直接观察到的过程、相互作用和反应的例子。其中一个时期，大约在300万年前（被称为上新世），是大气中二氧化碳含量最后一次像今天这样高，因此，这是一个可以更好地研究冰盖对气候变暖的反应的时期。具体来说，这个项目感兴趣的是了解南极洲附近的洋流在过去的气候事件中是如何运输热量和储存碳的。过去冰盖与海洋相互作用的历史记录在沉积物中，这些沉积物一层一层地沉积在南极洲近海的深海中。2018年1月至2月，一支由科学家和船员组成的团队乘坐科学海洋钻探船JOIDES Resolution前往南极洲西部近海的罗斯海，以恢复这些沉积物档案。这个项目的重点是那次探险的沉积物岩心，它捕捉到了相对温暖的上新世时期，以及随后向过去200万年中典型的较冷气候的过渡。研究人员将通过多种互补测量来分析沉积物，包括：粒度、组成、有机物化学、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完成，然后将其整合到冰盖与海洋相互作用的统一记录中。最终，这些结果将用于改进南极冰盖如何应对未来气候变化的模型预测。第二部分：技术说明：来自南极冰盖（AIS）边缘的地质记录表明，冰盖的振荡响应于日照的轨道变化（即~400、100、41和20 kyr），它似乎对调节年平均日照的特定频率（即41 kyr倾角）更为敏感，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，轨道强迫与南极底水（AABW）产生之间的关系是无限制的。因此，在了解日照变化如何影响冰缘和南大洋条件，从而直接影响全球海洋通风方面存在知识缺口。研究人员假设，从上新世到更新世，日照驱动的变化直接影响了AABW的产生和向南大洋的输出。例如，上新世温暖时期的倾角放大可能导致由于AIS程度降低而导致稠密水从陆架产生和输出增加，这反过来又导致了更大的AABW流出。为了确定AABW的产生与轨道状态的关系，他们计划从罗斯海大陆隆起的希拉里峡谷大堤的单一连续记录中重建两者，希拉里峡谷是AABW流出的主要渠道。为了验证他们的假设，他们将分析IODP站点U1524的沉积物（2018年在国际海洋发现计划远征374期间回收），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级联流出和AABW产量的代表。他们将利用形态、粒度和河床厚度作为输入参数，通过浊度流特性的数值模拟来估计下坡通量。(2)他们将利用粒度数据、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)他们将利用大块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响应轨道强迫而变化。所有这些数据将与沉积学记录相结合，以反卷积其沉积或由AABW流出引起的再动员的有机质生产，作为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中，以确定在上新世-更新世不同气候条件下AABW流出与轨道强迫情景之间的关系。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.