Geomagnetic field characteristics in the Paleozoic, based on three very extensive extrusive sequences

基于三个非常广泛的喷出层序的古生代地磁场特征

• 批准号: 0909288
• 负责人: Rob Van der Voo
• 金额: $ 21.5万
• 依托单位: Regents of the University of Michigan - Ann Arbor
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-06-15 至 2013-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0909288&HistoricalAwards=false
• 关键词: Geomagnetic; field; characteristics; Paleozoic; based

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The Earth's magnetic field is known to change annually in strength, orientation and in its location of the magnetic poles, which move on average some 3000 feet per month. These changes are readily observed by comparing the directions in which a compass needle points from year to year. Geographic north and the Earth's rotation axis will stay more or less in place, as magnetic north moves considerably. Thus, the compass-needle direction is typically different from the true ("geographical") north-pointing direction, and is called the geomagnetic declination. Records of the magnetic field from 19th-century logs of navigational parameters and from magnetic observatories have revealed, for instance, a shift of some 1500 km in the location of the magnetic northpole in the last 150 years. This shift may be seen in declination changes in the continental United States up to a degree every five or six years. Such changes are called secular variation, derived from one of the meanings of the latin word seculum as "century". For older times, it turns out that ancient lava flows have recorded the direction of the magnetic field that existed when they solidified and cooled. Thus, rocks formed in the last five million years have recorded the geomagnetic field and reveal secular variation. However, this variation is only sparsely documented for the preceding 250 million years of Earth history, whereas before that time, in the Paleozoic and the Precambrian, secular variation characteristics have not been determined in any reliable and robust ways. The is remedying that situation by characterizing the secular variation in three unusually thick lava sequences in central Asia, with ages of about 500, 370 and 270 million years. The project is also assessing whether an evolutionary trend may exist in the magnitude of the geomagnetic field's secular variation as the Earth's solid inner core has grown steadily at the expense of the liquid outer core where most of the Earth's magnetic field is generated. Because the geomagnetic field generation in the outer core is influenced by the thickness of the outer core layer, secular variation studies may, in turn, reveal geodynamo characteristics. A more technical evaluation of the total geomagnetic field observable at the surface of the Earth describes its structure as constituting a main ("dipole") field plus higher-order fields (e.g., quadrupole, octupole etc.). During the last 550 million years the field consisted predominantly of a dipole field, but secular variation implies that on short time-scales higher-order fields exist and may wax or wane at any moment in time. A well-known application of records of the magnetic field has been to determine the ancient latitudes of continents; the method is called paleomagnetism. In order for a given magnetic direction to be interpreted as originating at a given ancient latitude, one must assume that the averaged magnetic field was purely that of a dipole. This assumption is testable for limited geological times, and here too this project will significantly extend the knowledge about the characteristics of the magnetic field. Lastly, the magnetic field is known to reverse its polarity at irregular intervals, in a process whereby the (magnetic) northpole becomes southpole and vice versa, in an otherwise unchanging, steadily rotating Earth. In a few geological periods, very long intervals of constant polarity are known, and it has been speculated that in these intervals secular variation may be significantly reduced. The proposed research is testing this idea for one of these long polarity intervals ending at about 265 million years ago. In addition to the research goals of this project, the award is providing support for the education and training of a graduate and undergraduate student at the University of Michigan.

该奖项是根据2009年的《美国回收与再投资法》（公法111-5）资助的。众所周知，地球的磁场每年都在强度，定向以及磁极的位置发生变化，该磁极平均每月移动约3000英尺。 通过比较指南针逐年指向指示的方向，可以很容易地观察到这些变化。随着磁性北部的移动，地理北部和地球旋转轴将或多或少地停留在适当的位置。因此，指南针的方向通常与真实（“地理”）北点方向不同，被称为地磁偏斜。 从19世纪的导航参数日志和磁性观测站的记录显示，例如，在过去150年中，磁性北波尔的位置大约有1500 km的变化。在美国大陆的偏差变化中，可能每五年或六年都可以看到这种转变。这种变化称为世俗变化，源自拉丁语sculum的含义之一，称为“世纪”。 在较旧时，事实证明，古老的熔岩流已经记录了固化和冷却时存在的磁场方向。因此，在过去的五百万年中形成的岩石记录了地​​磁领域并揭示了世俗的变化。但是，这种变化仅在前2.5亿年的地球历史上稀少，而在此之前，在古生代和前寒武纪和前寒武纪，世俗的变异特征尚未以任何可靠且可靠的方式确定。这种情况是通过表征中亚三个异常厚的熔岩序列的世俗变化，年龄约为500、370和2.7亿年，从而解决了这种情况。 该项目还在评估地磁场的世俗变化的大小可能存在进化趋势，因为地球的固体内核已经以液体外核的代价稳定增长，而地球大部分磁场则是液体外核的。由于外核中的地磁田间产生受外核层厚度的影响，因此世俗的变异研究可能会揭示Geodynamo的特征。 对地球表面可观察到的总地磁场的更技术评估将其结构描述为构成一个主要（“偶极”）磁场，以及高阶田地（例如，四极杆，八振长等）。 在过去的5.5亿年中，该领域主要由偶极场组成，但是世俗的变化意味着在短时间尺度上存在高阶场，并且在任何时候都可能蜡或减弱。 磁场记录的一种众所周知的应用是确定大陆的古代纬度。该方法称为古磁性。为了使给定的磁方向解释为起源于给定的古代纬度，必须假设平均磁场纯粹是偶极子的磁场。对于有限的地质时代，该假设是可以测试的，此项目也将显着扩展有关磁场特征的知识。最后，已知磁场在不规则的间隔中扭转了其极性，在该过程中，（磁性）北极变为南柱，反之亦然，在原本不变的，稳步旋转的地球中。在几个地质时期，已知的恒定极性间隔很长，并且已经推测，在这些间隔中，世俗变异可能会大大减少。拟议的研究是在大约2.65亿年前的这些长极性间隔之一测试了这一想法。除了该项目的研究目标外，该奖项还为密歇根大学的研究生和本科生的教育和培训提供了支持。