Collaborative Research: An EARTHTIME Chronology for the Matuyama-Brunhes Geomagnetic Field Reversal

合作研究：松山-布伦赫斯地磁场反转的地球时间年表

• 批准号: 1250446
• 负责人: Bradley Singer
• 金额: $ 39.97万
• 依托单位: University of Wisconsin-Madison
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-03-01 至 2017-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1250446&HistoricalAwards=false
• 关键词: Collaborative; Research; EARTHTIME; Chronology; Matuyama

The Earth?s magnetic field is still useful for navigation, but perhaps more importantly today it forms a protective shield against cosmic radiation that otherwise would lead to further depletion of the ozone layer that protects us from UV radiation and interfere with the satellite communications so essential for society. The stronger the field, the greater the shielding. That the Earth?s magnetic field has weakened profoundly, reversed polarity hundreds of times, and attempted to do so on many other occasions, is one of the great enigmas of science. There are a variety of ways by which geoscientists advance our understanding of the magnetic field, which is widely thought to be generated by the flow of hot molten iron that swirls around in Earth?s liquid outer core to generate a magnetic dynamo. These include: (1) creation and study of analog dynamos made of molten sodium or other (dangerous) electrically conducting metals, (2) supercomputer modeling of dynamo processes that produce polarity reversals and field excursions, and (3) amassing and interpreting observations from natural rocks that record the magnetic fields of Earth?s past. This study falls into the latter category, and we have proposed to greatly expand the set of observations that bear on understanding the last reversal of Earth?s magnetic field that took place about 770 thousand years ago. This grant will permit a collaborative team of scientists to acquire new age determinations using the potassium-argon radio-isotope decay clock, as well as new information on the intensity and direction of the field, as recorded in sequences of lava flows that erupted during the last full polarity reversal of Earth?s magnetic field. The information we plan to collect will, in turn, provide fundamental data from the real Earth?s past that can help to inform and validate approaches that rely on analog or supercomputer simulations of this process. Reversals and excursions of Earth?s geomagnetic field create global marker horizons in sedimentary and volcanic rocks and polar ice. An accurate and precise chronology of these geomagnetic field instabilities is fundamental to quantifying the timing and rates of climate change, surface processes, and biological evolution. Moreover, knowing the structural and temporal evolution of reversals and excursions is essential to understanding the geodynamic process by which reversals occur. Advances in rock magnetic measurement techniques in both sediments and lava flows, astrochronologic methods applied to sediment cores, and radio-isotopic dating of lava flows during the past two decades, using the 40Ar/39Ar variant of potassium-argon dating, has produced a number of paleomagnetic records of the most recent polarity transition from the Matuyama reversed chron to the Brunhes normal chron. Given the importance of an accurate and precise age for the the Matuyama-Brunhes reversal, our objectives are to: (1) Greatly enlarge the set of 40Ar/39Ar ages from lava flows that bear on the timing of this reversal while improving the precision and accuracy of these age determinations, (2) Test the hypothesis that the discrepancy between the astrochronologic age of 773,000 years before present recorded in sediment cores and the 40Ar/39Ar age of lava flows reflects the accuracy of the very limited 40Ar/39Ar data available from the lava flows, and (3) Determine the paleointensity of lavas that record directional changes associated with the MB reversal and its precursor on Maui and in Chile. This project is an international collaboration between scientists in Japan, the Netherlands, Denmark, and the USA. We will train undergraduate and graduate students in both state-of-the-art radio-isotopic geochronology and paleomagnetic methods in the labs at UW Madison and UC Santa Cruz, respectively. A geochronology summer short course will be held at UW for graduate and undergraduate students. Moreover, The PI will work with staff in the UW Geology museum to create a display that highlights how geologists tell time, including the use of 40Ar/39Ar dating and astrochronology.

地球吗?地球的磁场对导航仍然有用，但也许今天更重要的是，它形成了一个防止宇宙辐射的保护层，否则将导致臭氧层的进一步消耗，臭氧层保护我们免受紫外线辐射，并干扰对社会至关重要的卫星通信。磁场越强，屏蔽作用就越强。那是地球吗？地球的磁场已经大大减弱，极性逆转了数百次，并在许多其他场合试图这样做，这是科学上最大的谜团之一。地球科学家通过多种方式加深了我们对磁场的理解，人们普遍认为磁场是由在地球上旋转的热铁水流产生的。S液体外核产生磁发电机。这些包括：(1)创建和研究由熔融钠或其他（危险的）导电金属制成的模拟发电机；(2)对产生极性反转和磁场偏移的发电机过程进行超级计算机建模；(3)收集和解释记录地球磁场的天然岩石的观测结果。年代的过去。这项研究属于后一类，我们建议大大扩展对理解地球最后一次逆转的观测集。大约77万年前发生的美国磁场。这笔拨款将允许一个科学家合作小组利用钾-氩放射性同位素衰变时钟获得新的年龄测定，以及关于磁场强度和方向的新信息，这些信息记录在地球上一次完全极性逆转期间爆发的熔岩流序列中。S磁场。我们计划收集的信息将反过来提供真实地球的基本数据。可以帮助告知和验证依赖于该过程的模拟或超级计算机模拟的方法。地球的倒转和漂移？美国的地磁场在沉积岩、火山岩和极地冰中形成了全球标志层。对这些地磁场不稳定性的准确和精确的年表是量化气候变化、地表过程和生物进化的时间和速率的基础。此外，了解反转和偏移的结构和时间演化对于理解反转发生的地球动力学过程至关重要。在过去的二十年中，沉积物和熔岩流的岩石磁测量技术、沉积物岩心的天体年代学方法以及熔岩流的放射性同位素定年（使用40Ar/39Ar的钾氩定年法）的进步，已经产生了大量的古地磁记录，记录了最近从Matuyama逆时到Brunhes正时的极性转变。考虑到准确和精确的Matuyama-Brunhes反转年龄的重要性，我们的目标是：(1)大大扩大了与这一逆转时间有关的熔岩流40Ar/39Ar年龄集，同时提高了这些年龄测定的精度和准确性；(2)检验了沉积物岩心中记录的距今773万年前的天体年代学年龄与熔岩流40Ar/39Ar年龄之间的差异，这反映了熔岩流中非常有限的40Ar/39Ar数据的准确性；(3)确定毛伊岛和智利的熔岩古强度，这些熔岩记录了与MB反转及其前体相关的方向变化。这个项目是日本、荷兰、丹麦和美国科学家之间的国际合作。我们将分别在威斯康星大学麦迪逊分校和加州大学圣克鲁斯分校的实验室培养最先进的放射性同位素地质年代学和古地磁方法的本科生和研究生。地质年代学夏季短期课程将在华盛顿大学为研究生和本科生开设。此外，PI将与威斯康星大学地质博物馆的工作人员合作，创建一个突出地质学家如何判断时间的展览，包括使用40Ar/39Ar测年和天体年代学。