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

新元古代时期与更年轻的地球历史时期相比简直是奇怪的，因为这个时代的沉积物包含了不寻常或不寻常的气候、生物地球化学和地质事件的证据。特别具有地球物理意义的是所有大陆的新元古代晚期地层段，它们显示出确凿的岩性证据，表明冰川在低纬碳酸盐台地上移动，这显然是过去5亿年来从未发生过的情况。这场“雪球地球”气候灾难在很大程度上依赖于对南澳大利亚州新元古代晚期冰川沉积物的古地磁研究，这些研究一再表明，低纬度磁化作用包裹在约10米厚的韵律叠层“潮汐”沉积褶皱周围，在较厚的多环境冰川沉积中经常反转极性，并在构造褶皱之前发生。最近对Snowball末期冰川消融期间沉积的同位素异常“帽状”碳酸盐的古地磁研究记录了多次逆转，这表明地球磁场变化出人意料地频繁，和/或脱冰时间尺度出人意料地长。无论如何，关于新元古代地球系统及其对气候变化压力的反应，似乎与我们对现代全球变化的理解有根本不同。我们正在从南澳大利亚州雪球地球沉积序列的关键区间产生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。在一种被称为“Elatina韵律”的单位中，毫米尺度的沙子和淤泥对子被认为代表半日潮。Elatina韵律岩的正“同沉积褶皱”测试建立了一个明显的磁化沉积年龄，因此通过~10m韵律岩的毫米尺度磁性地层学应该可以恢复一个连续的~60年的地磁场记录，适合回答以下一个或多个问题：1)Elatina韵律岩纹层真的是半日活动的吗？任何通过节律区段的类似地磁长期变化的信号似乎都排除了这种可能性，并主张更长的、也许是每年一次的分层周期。2)Elatina韵律岩著名的低古纬度古地磁结果是否真的是同沉积成因？对所谓的“同沉积褶皱”进行详细的沉积学和古地磁研究，将解决韵律岩磁化的成岩起源的遥远的替代假说。3)雪球冰期的地磁场异常弱吗？这样的情景可以解释Elatina冰川沉积物和上覆的脱冰盖碳酸盐中异常频繁的地磁反转。如果1)和2)的答案证明磁化作用在短时间尺度上是沉积的，那么对Elatina韵律岩的组构分析可以证明，异常信号的一部分来自无效的磁化过程，可能与微弱的地磁场有关。最后，我们还从Nuccaleena帽白云岩中产生了大约50,000-500,000年的几乎连续的地磁场记录，Nuccaleena帽白云岩是Elatina冰川沉积之后的脱冰期碳酸盐。该单元中预期的地磁长期变化信号的结构还应考虑地球新元古代地磁场的强度和特征是否与现代类似。雪球地球的奥秘和古地磁学的“魔力”值得更广泛的公众交流。我们正在与澳大利亚同事合作，制作一个国际性的巡回博物馆展览，展示其中一些非常有趣的--视觉上令人惊叹的--岩石。我们还为国家地理电视台的一部纪录片提供建议，该纪录片专注于雪球地球的地球历史故事，以及地质学家和古地磁学家阅读古代岩石记录的科学方法。

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.