The Middle Miocene Antarctic Climate Transition: Investigating Magnitude, Phasing, and Processes Involving Cryosphere Expansion and Global Cooling

该奖项由极地项目办公室的南极地质和地球物理项目提供，支持一个研究中新世气候转变的项目，该项目记录在南塔斯曼隆起的海洋钻探项目岩心中。理解导致气候变化的机制是地球科学的一个重大挑战。中新世中期（~13-15 Ma（百万年前））的地质记录描述了与南极冰冻圈扩张和北半球缺乏明显冰盖同时发生的显著的全球海洋/大气冷却。这表明南极气候和冰冻圈演化在晚新生代气候发展中起着核心作用。考虑到传统代用物（如d18O）的复杂性，关于中新世中期冰量和温度变化的幅度、阶段和过程存在根本性问题。该项目的主要目标是更好地了解：1)全球变冷和南极冰冻圈在这一区间扩张的幅度和阶段；2)当时迫使南极气候/冰冻圈演化的过程；3)在轨道周期（约40万年、10万年和40万年），不断扩大和日益稳定的南极冰冻圈对气候反馈的影响。最近发展的地球化学古地温指标（Mg/Ca）应该是中中新世d18O温度和冰体积信号的反褶积。来自ODP站点1171中新世有孔虫的新数据表明，Mg/Ca将产生评估冰体积和温度变化所必需的古温度。该项目将整合高分辨率（2- 6kyr（千年））浮游和底栖有孔虫Mg/Ca， d18O， d13C和动物记录来实现这些目标。这种新颖的多代理方法将检验中新世中期南极冰冻圈演化和全球变冷的假设。进一步的研究将涉及：1)强迫中新世中期气候变化的过程，以及2)在这种变化中气候反馈的动态影响。研究将：1)评估西南太平洋/南大洋水文对这一气候变化的响应；2)利用~2-6 kyr的年龄分辨率记录，评估冰冻圈扩张可能涉及的过程/反馈，以区分40和100 kyr的轨道周期；3)通过中新世中期气候转变，在轨道时间尺度上表征气候谱的演变。为了实现上述目标，将对ODP Leg 189期间在南塔斯曼隆起（STR）钻探的一对（ODP Sites 1170和1171）中中新世（~16.8 ~ 12 Ma）异常高质量的海相CaCO3层序进行高分辨率（~2-6 kyr）多代理地球化学和动物调查。STR序列提供了一个难得的机会来研究中中新世全球气候变化在一个敏感的海洋界面在南方高纬度地区。中新世中期，STR位于副热带太平洋环流与南极环极流汇合处附近。STR标志着东澳大利亚流的南部范围，这是一个敏感的西部边界流，为南部高纬度地区提供热量和水分。1170和1171地点CaCO3保存良好，深海沉积速率较高（~6 cm/kyr），回收率达80%。这项研究的主要知识价值在于，它将提供一个前所未有的机会，在轨道时间尺度上研究影响中新世中期南极冰冻圈扩张和全球冷却的幅度、阶段和过程。本研究将集中于新生代气候演化的观点，包括影响地球系统的过程和反馈。南极洲发生的剧烈变化可能与观测到的热带太平洋变率（ENSO）有关，这突出表明有必要进一步了解在较长（数万年至数百万年）和较短（十年至千年）时间尺度上影响南极冰冻圈发展、范围和稳定性的反馈。这项研究的影响是广泛的，因为它解决了与南极冰冻圈演变及其对气候重组的响应有关的重大问题，这对社会至关重要。更广泛的影响是将在研究过程中完成的学生培训。

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate the Miocene climate transition as recorded in Ocean Drilling Program cores from the South Tasman Rise. Understanding mechanisms that force climate change is a major challenge in Earth Sciences. Middle Miocene (~13-15 Ma (million years ago)) geologic records describe significant global ocean/atmosphere cooling coincident with Antarctic cryosphere expansion and a lack of appreciable Northern Hemisphere ice sheets. This suggests that Antarctic climate and cryosphere evolution played a central role in late Cenozoic climate development. Given the complexity of traditional proxies (e.g. d18O), fundamental questions exist concerning the magnitude, phasing, and processes driving mid-Miocene ice volume and temperature change. The primary objectives of this project are to better understand: 1) magnitude and phasing of global cooling and Antarctic cryosphere expansion across this interval; 2) processes that forced Antarctic climate/cryosphere evolution at this time; and 3) effects of an expanding and increasingly stable Antarctic cryosphere on climate feedbacks at orbital periods (about 40, 100, and 400 thousand years). Recently developed geochemical paleotemperature proxies (Mg/Ca) should deconvolve mid-Miocene d18O temperature and ice volume signals. New data from ODP Site 1171 Miocene foraminifers suggest that Mg/Ca will yield paleotemperatures necessary to evaluate ice volume and temperature changes. This project will integrate high-resolution (2-6 kyr (thousand year)) planktonic and benthic foraminifer Mg/Ca, d18O, d13C, and faunal records to address these objectives. This novel multi-proxy approach will test hypotheses regarding mid-Miocene Antarctic cryosphere evolution and global cooling. Further inquiry will address: 1) processes forcing the mid-Miocene climate shift, and 2) dynamic influences of climate feedbacks across this shift. The research will: i) assess the southwest Pacific/Southern Ocean hydrographic response to this climate shift, ii) evaluate possible processes/feedbacks involved in cryospheric expansion using records with age resolution of ~2-6 kyr to distinguish 40 and 100 kyr orbital periodicity, and iii) characterize the evolution of the climate spectrum at orbital time-scales through the mid-Miocene climate transition. To achieve the proposed objectives, high-resolution (~2-6 kyr) multi-proxy geochemical and faunal investigations will be conducted on a pair (ODP Sites 1170 and 1171) of unusually high quality marine CaCO3 sequences of mid-Miocene age (~16.8 to 12 Ma) drilled on the South Tasman Rise (STR) during ODP Leg 189. STR sequences provide a rare opportunity to study the mid-Miocene global climate shift at a sensitive oceanographic interface in the high southern latitudes. In the mid Miocene, the STR was situated proximal to the confluence of subtropical Pacific circulation and the Antarctic Circumpolar Current. The STR marks the southern extent of the East Australian Current, a sensitive western boundary current supplying heat and moisture to high southern latitudes. Sites 1170 and 1171 are marked by superb-quality CaCO3 preservation, relatively high deep-sea sedimentation rates (~6 cm/kyr), and 80% recovery. The primary intellectual merit of this research is that it will provide an unprecedented opportunity to examine the magnitude, phasing, and processes influencing middle Miocene Antarctic cryosphere expansion and global cooling at orbital timescales. This research will focus the view of Cenozoic climate evolution, including processes and feedbacks influencing Earth's systems across a major climate transition. Dramatic changes occurring in Antarctica, which may be linked to observed tropical Pacific variability (ENSO), underscore the need to improve understanding of feedbacks influencing the development, extent, and stability of the Antarctic cryosphere on both long (tens of thousands to millions of years) and short (decadal to millennial) timescales. The impact of this research is broad, as it addresses major issues related to the evolution of the Antarctic cryosphere and its response to climate reorganizations that is of vital interest to society. Further broader impacts are the student training that will be accomplished during the course of the research.