Continuous Geomagnetic Field Records During Snowball Earth: Critical Tests of Cryogenian Uniformitarianism

雪球地球期间的连续地磁场记录：低温均变论的关键测试

• 批准号: 0739105
• 负责人: Joseph Kirschvink
• 金额: $ 20.3万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-01-01 至 2011-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0739105&HistoricalAwards=false
• 关键词: Continuous; Geomagnetic; Field; Records; During

Neoproterozoic time is simply weird compared to younger intervals of Earth History, as sediments of this age contain evidence for unusual or extraordinary climatic, biogeochemical, and geological events. Of particular geophysical interest are the late Neoproterozoic stratigraphic intervals on all continents that show firm lithologic evidence of glaciers marching over low-latitude carbonate platforms, a situation that clearly has not happened in the past 500 million years. This "Snowball Earth" climate catastrophe relies heavily on paleomagnetic studies of late Neoproterozoic glacial sediments in South Australia, which have shown repeatedly that low-paleolatitude magnetization wraps around purported syn-sedimentary folds in a ~10 m thick, rhythmic-laminated "tidal" deposit, reverses polarity frequently in thicker, multi-environment glacial deposits, and pre-dates tectonic folding. Recent paleomagnetic investigations of isotopically-anomalous "cap" carbonates, deposited during final Snowball deglaciation, have documented multiple reversals, suggesting surprisingly frequent geomagnetic field changes and/or unexpectedly long deglacial timescales. Either way, something about the Neoproterozoic Earth system and its response to climate change forcing seems to have been fundamentally different from our understanding of modern global change. We are producing two possibly continuous geomagnetic field records from critical intervals of South Australia's Snowball Earth sedimentary succession to test the veracity of the existing, surprising paleomagnetic records from those units. In a unit known as "Elatina rhythmites," millimeter-scale sand and silt couplets are considered to represent semi-diurnal tides. A positive "syn-sedimentary fold" test on Elatina rhythmites establishes an apparently depositional age of magnetization, so millimeter-scale magnetostratigraphy through ~10 m of rhythmites should recover a continuous ~60-year geomagnetic field record suitable for answering one or more of the following questions: 1) Are Elatina rhythmite laminations genuinely semi-diurnal? Any signal resembling geomagnetic secular variation through the rhythmite section would seem to preclude this possibility and argue for a longer, perhaps annual, lamination periodicity. 2) Is the famous, low-paleolatitude paleomagnetic result for Elatina rhythmites genuinely syn-depositional in origin? Detailed sedimentologic and paleomagnetic investigation of purported "syn-sedimentary folds" would address remote alternative hypotheses of diagenetic origin for rhythmite magnetization. 3) Was the geomagnetic field during Snowball glaciation anomalously weak? Such a scenario could explain unusually frequent geomagnetic reversals in Elatina glacial deposits and in the overlying, deglacial cap carbonate. If answers to 1) and 2) proved magnetization to be depositional on a short timescale, then fabric analysis of Elatina rhythmites could prove that part of the anomalous signal derives from inefficient magnetization processes, possibly linked to a weak geomagnetic field. Finally, we are also producing a ~50,000 - 500,000-year, nearly continuous geomagnetic field record from Nuccaleena cap dolostone, the deglacial carbonate postdating Elatina glacial deposits. The structure of an expected geomagnetic secular variation signal in this unit should also address whether or not the strength and character of Earth's Neoproterozoic geomagnetic field was similar to that of the modern Era. The mystery of Snowball Earth, and the "magic" of paleomagnetism merit wider public communication. We are collaborating with Australian colleagues to produce an international, traveling museum exhibit exhibiting some of these exceptionally interesting -- and visually stunning -- rocks. We are also advising a National Geographic TV documentary focused on the Earth history story of Snowball Earth and on the scientific method by which geologists and paleomagnetists read the ancient rock record.

与地球历史上更年轻的时期相比，新元古代的时间简直是奇怪的，因为这个时代的沉积物包含了不寻常或特别的气候、生物地球化学和地质事件的证据。在地球物理学中，特别值得关注的是新元古代晚期各大洲的地层间隔，这些地层间隔显示出确凿的岩性证据，表明冰川在低纬度碳酸盐台地上移动，这种情况在过去的5亿年里显然没有发生过。这种“雪球地球”气候灾难在很大程度上依赖于对南澳大利亚新元古代晚期冰川沉积物的古地磁研究，这些研究反复表明，在约10米厚的节奏层状“潮汐”沉积物中，低古纬度磁化包裹着所谓的同沉积褶皱，在较厚的多环境冰川沉积物中，极性频繁反转，并且早于构造褶皱。最近的古地磁研究表明，在雪球消融期间沉积的同位素异常“帽”碳酸盐岩记录了多次逆转，表明地磁场变化异常频繁和/或消融时间异常长。不管怎样，新元古代地球系统及其对气候变化强迫的反应似乎与我们对现代全球变化的理解有着根本的不同。我们正在从南澳大利亚雪球地球沉积演替的关键间隔中提取两个可能连续的地磁场记录，以测试这些单元现有的令人惊讶的古地磁记录的准确性。在一个被称为“Elatina rhythmites”的单位中，毫米尺度的沙子和淤泥对联被认为代表了半日潮。对Elatina节律岩进行正面的“同沉积褶皱”试验，建立了一个明显的磁化沉积时代，因此，毫米尺度的磁地层学通过~10米的节律岩应该恢复一个连续的~60年的地磁场记录，适合回答以下一个或多个问题：1)Elatina节律岩片层真的是半昼夜的吗？任何类似于地磁长期变化的信号通过节奏剖面似乎都排除了这种可能性，并主张一个更长的，也许是每年一次的层压周期。2) Elatina节律岩著名的低古纬度古地磁结果真的是同沉积的吗？对所谓的“同沉积褶皱”进行详细的沉积学和古地磁研究，将解决有关节律岩磁化成因的遥远替代假说。3)雪球冰期地磁场是否异常弱？这种情况可以解释在埃拉蒂纳冰川沉积物和上覆的脱冰帽碳酸盐中异常频繁的地磁反转。如果第1)和第2)的答案证明磁化是在短时间内沉积的，那么对Elatina节律虫的结构分析可以证明部分异常信号来自低效的磁化过程，可能与弱地磁场有关。最后，我们还得到了一个约5万- 50万年的、几乎连续的地磁场记录，这些记录来自于nuccaleene盖层白云岩和Elatina冰川沉积后的去冰期碳酸盐。该单元中预期的地磁长期变化信号的结构也应解决地球新元古代地磁场的强度和特征是否与现代相似的问题。雪球地球之谜和古地磁的“魔力”值得更广泛的公众传播。我们正在与澳大利亚的同事合作，制作一个国际性的、巡回的博物馆展览，展示这些非常有趣的、视觉上令人惊叹的岩石。我们还为国家地理频道的一部纪录片提供建议，该纪录片聚焦于雪球地球的地球历史故事，以及地质学家和古地磁学家解读古代岩石记录的科学方法。