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.

整个固体地球在大约5.65亿年前旋转了90度，这一事件引发了地球上生命的爆发（当时大多数现存的动物门和纲首次出现），这是地球科学中最有争议的假设之一。最终，这一观点的准确性取决于过去的地磁场在古代岩石样本中记录得有多好（这是古地磁学学科的范围）。 PI开发了一种从地质样品中获取古地磁数据的方法-称为硅酸盐单晶方法-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团队得出了另一种假设。这些数据并没有记录整个固体地球的旋转，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，两极经常逆转。该团队将通过对加拿大南部三个地区样本的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决定地球是否能屏蔽太阳风的一个主要因素，这项研究还将有助于在地球历史的这一关键时期限制高能太阳粒子对大气和生物圈的潜在影响。收集到的数据也将有助于测试过去大陆板块的速度是否高于均变论的假设。这项工作将支持研究生和本科生，他们将通过课堂培训、实地研究和实验室培训获得宝贵的技能。在埃迪卡拉纪到寒武纪早期（大约635-530 Ma），整个固体地球旋转了90度，这引发了寒武纪早期生命的爆发，这是非常有争议的。这不是一个被称为惯性交换真极移（IITPW）的事件在理论上是否可能的问题，而是它是否发生过的问题。最终，事件的准确性依赖于古地磁数据的保真度。PI团队最近使用硅酸盐单晶古地磁分析来研究这个谜，这种方法允许隔离单畴磁性载体，这些载体是最好的磁场记录器，能够在十亿年的时间尺度上保留剩磁。在他们对Sept-Iles（大约565 Ma）侵入（加拿大）的研究中，他们没有发现IITPW的支持。相反，他们发现了地磁场在入侵冷却期间逆转的证据。初步分析进一步表明异常低的古强度。这些观测结果为另一种假设提供了基础：埃迪卡拉纪到寒武纪早期的地磁场异常微弱，并且经常逆转。该团队将研究加拿大南部的三个地区，以验证他们的假设。这项工作也将广泛限制生物进化的边界条件，在关键埃迪卡拉纪早寒武纪的间隔，并将对核心，地幔和地表环境的影响。具体来说，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.如果磁层顶的间隔距离减小，则允许高能太阳粒子更大的穿透，以及iii.如果大陆板块的速度高于均变论的假设。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的培训获得宝贵的技能。他们将结合联合收割机实地考察，课堂学习和暑期课程，给学生一个全面的研究经验。