The nature of the Ediacaran to early Cambrian geomagnetic field

埃迪卡拉纪至早寒武世地磁场的性质

• 批准号: 1520681
• 负责人: John Tarduno
• 金额: $ 29.55万
• 依托单位: University of Rochester
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-08-01 至 2018-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1520681&HistoricalAwards=false
• 关键词: nature; Ediacaran; early; Cambrian; geomagnetic

The suggestion that the entire solid Earth rotated by 90 degrees, approximately 565 million years ago, and that this event sparked the explosion of life on the planet (when most existing animal phyla and classes first appeared), is one of the most controversial hypotheses in the geosciences. Ultimately, the veracity of this idea rests on how well the past geomagnetic field is recorded in ancient rock samples (the purview of the discipline of paleomagnetism). The PIs have developed a method for obtaining paleomagnetic data from geologic samples - known as the single silicate crystal approach - having higher fidelity than standard procedures. Preliminary data applying this method leads the team to an alternative hypothesis. Rather than recording a rotation of the entire solid Earth, the data reflect a geomagnetic field 565 million years ago that was in an unusual state: it was weak, and the poles reversed frequently. The team will test their hypothesis through a single silicate crystal paleomagnetic study of samples from three areas of southern Canada. Because the strength of the geomagnetic field is a principal factor defining shielding of the planet from the solar wind, this study will also help constrain the potential influence of energetic solar particles on the atmosphere and biosphere during this critical time in Earth history. The data collected will also be useful for testing if continental plate velocities were higher than uniformitarian assumptions, in the past. The work will support graduate students and undergraduates, who will acquire valuable skills though training in the classroom, field studies and in the laboratory. The suggestion that the entire solid Earth rotated by 90 degrees during Ediacaran to early Cambrian times (approximately 635-530 Ma), and that this sparked the early Cambrian explosion of life, is highly contentious. This is not a question of whether such an event, known as inertial interchange true polar wander (IITPW), is theoretically possible, but rather whether it occurred. Ultimately, the veracity of the event relies on the fidelity of paleomagnetic data. The PI team has recently examined this enigma using single silicate crystal paleomagnetic analysis, a method that allows the isolation of single-domain magnetic carriers that are the best field recorders, capable of preserving remanences on billion-year time scales. In their study of the Sept-Iles (approximately 565 Ma) intrusion (Canada), they have not found support for IITPW. Instead, they find evidence that the geomagnetic field was reversing during cooling of the intrusion. Preliminary analyses further suggest unusually low paleointensities. These observations provide the basis for an alterative hypothesis: the Ediacaran to early Cambrian geomagnetic field was unusually weak, and reversed frequently. The team will study three areas in southern Canada to test their hypothesis. This work will also broadly constrain the boundary conditions for biotic evolution during the key Ediacaran to early Cambrian interval, and will have implications for the core, mantle and surface environment. Specifically,the investigators hope to constrain i. whether the geodynamo was in an unusual state (perhaps associated with inner core growth), ii. if the magnetopause standoff distance was reduced, allowing greater penetration of energetic solar particles, and iii. if continental plate velocities were higher than uniformitarian assumptions. The work will support graduate students and undergraduates, who will acquire valuable skills though training in field studies in southern Canada and in the laboratory. They will combine field excursions, classroom studies and summer programs to give students a comprehensive research experience.

地球科学中最具争议性的假说之一是，整个固体地球大约在5.65亿年前自转了90度，这一事件引发了地球上生命的爆炸(当时大多数现存的动物门和纲第一次出现)。归根结底，这一观点的准确性取决于古代岩石样本(古地磁学学科的范畴)对过去的地磁场的记录情况。PIS开发了一种从地质样品中获取古地磁数据的方法--称为单硅酸盐晶体方法--比标准程序具有更高的保真度。应用这种方法的初步数据使研究小组得出了另一种假设。这些数据没有记录整个固体地球的自转，而是反映了5.65亿年前的地磁场处于一种不寻常的状态：它很弱，两极经常颠倒。该团队将通过对加拿大南部三个地区的样品进行单一硅酸盐晶体古地磁研究来验证他们的假设。由于地磁场的强度是确定地球不受太阳风影响的主要因素，这项研究还将有助于限制高能太阳粒子在地球历史上这一关键时刻对大气层和生物圈的潜在影响。收集的数据也将有助于测试大陆板块的速度是否高于过去的统一论假设。这项工作将支持研究生和本科生，他们将通过课堂、实地研究和实验室培训获得宝贵的技能。关于整个固体地球在埃迪卡拉纪到早寒武世(大约635-530 Ma)期间旋转了90度，并引发了早寒武世生命爆发的说法，是非常有争议的。这不是一个被称为惯性互换真极移(IITPW)的事件在理论上是否可能的问题，而是它是否发生的问题。最终，这一事件的准确性取决于古地磁数据的保真度。PI团队最近使用单硅酸盐晶体古地磁分析研究了这个谜团，这种方法允许隔离作为最佳现场记录器的单域磁性载体，能够在数十亿年的时间尺度上保留剩磁。在他们对加拿大9月-ILES(约565 Ma)侵入体的研究中，他们没有发现对IITPW的支持。相反，他们发现了在侵入体冷却期间地磁场发生逆转的证据。初步分析进一步表明，古强度异常低。这些观察结果为另一种假设提供了基础：埃迪卡拉纪到早寒武世的地磁场异常微弱，而且经常发生逆转。该团队将对加拿大南部的三个地区进行研究，以验证他们的假设。这项工作还将广泛地限制关键埃迪卡拉纪至早寒武世期间生物演化的边界条件，并将对核、地幔和地表环境产生影响。具体地说，研究人员希望限制I。地球发电机是否处于不寻常的状态(可能与内核生长有关)，II。如果磁层顶的对峙距离减小，允许更多的高能太阳粒子穿透，以及III。如果大陆板块的速度高于统一论的假设。这项工作将支持研究生和本科生，他们将通过在加拿大南部和实验室进行实地研究培训获得宝贵的技能。他们将结合实地考察、课堂学习和暑期项目，为学生提供全面的研究体验。