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

该奖项由极地方案办公室南极地质和地球物理方案提供，支持一个研究塔斯曼隆起南部大洋钻探方案岩芯记录的中新世气候转变的项目。了解迫使气候变化的机制是地球科学领域的一大挑战。中新世中期(约13-15 Ma(百万年前))的地质记录描述了与南极冰冻圈扩张相一致的显著的全球海洋/大气冷却和明显的北半球冰盖的缺乏。这表明南极气候和冰冻圈演化在晚新生代气候发展中发挥了核心作用。考虑到传统指标(例如d18O)的复杂性，存在着关于中中新世冰量和温度变化的规模、相位和过程的基本问题。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段时间内膨胀的大小和阶段；2)此时迫使南极气候/冰冻圈演化的过程；3)不断扩大和日益稳定的南极冰冻圈对轨道周期(约40、100和40万年)气候反馈的影响。新近发展起来的地球化学古温指标(Mg/Ca)应能解卷出中中新世d18O的温度和冰量信号。来自ODP1171站中新世有孔虫的新数据表明，镁/钙将产生评估冰量和温度变化所需的古温度。该项目将综合高分辨率(2-6 KYR(千年))浮游和底栖有孔虫的mg/Ca、d18O、d13C和动物群记录，以实现这些目标。这一新的多代理方法将检验关于中新世中期南极冰冻圈演化和全球变冷的假说。进一步的研究将涉及：1)迫使中新世中期气候变化的过程，以及2)气候反馈对这种变化的动态影响。这项研究将：i)评估西南太平洋/南大洋对这种气候变化的水文响应；ii)利用年龄分辨率为~2-6kyr的记录，评估可能涉及冰冻层扩张的过程/反馈，以区分40kyr和100kyr的轨道周期；以及iii)描述中新世中期气候过渡期间气候谱在轨道时间尺度上的演变。为了实现拟议的目标，将对ODP189航次期间在南塔斯曼隆起(STR)钻探的一对(ODP1170和1171站)异常高质量的中中新世(~16.8 Ma至12 Ma)海相CaCO3序列进行高分辨率(~2-6KYR)多代理地球化学和动物群调查。STR序列为在南部高纬度敏感海洋界面研究中中新世全球气候变化提供了难得的机会。中新世中期，STR位于副热带太平洋环流和南极绕极流的交汇处附近。STR标志着东澳大利亚流的南部范围，这是一种敏感的西部边界流，为南部高纬度地区提供热量和水分。1170和1171号遗址的特点是高质量的碳酸钙保存，相对较高的深海沉积速率(~6厘米/KYR)和80%的回收率。这项研究的主要学术价值是，它将提供一个前所未有的机会来研究影响中新世中期南极冰冻圈扩张和轨道时间尺度上的全球冷却的大小、阶段和过程。这项研究将聚焦于新生代气候演化的观点，包括在重大气候过渡期间影响地球系统的过程和反馈。南极洲发生的巨大变化可能与观测到的热带太平洋变率(ENSO)有关，这突出表明需要更好地了解在长(数万至数百万年)和短(十至千年)时间尺度上影响南极冰冻圈的发展、范围和稳定性的反馈。这项研究的影响是广泛的，因为它涉及与南极冰冻圈的演变及其对社会至关重要的气候重组的反应有关的重大问题。更广泛的影响是在研究过程中将完成的学生培训。

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.