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.

地球磁场对于导航仍然有用，但也许更重要的是，今天它形成了一个针对宇宙辐射的防护罩，否则会导致臭氧层进一步消耗，而臭氧层可以保护我们免受紫外线辐射并干扰至关重要的卫星通信为社会。场越强，屏蔽越大。地球磁场已经严重减弱，极性反转数百次，并且在许多其他场合试图这样做，这是科学的最大谜团之一。 地球科学家可以通过多种方式增进我们对磁场的理解，人们普遍认为磁场是由在地球液体外核中旋转的热铁水流产生的，从而产生磁发电机。 其中包括：（1）创建和研究由熔融钠或其他（危险）导电金属制成的模拟发电机，（2）对产生极性反转和场偏移的发电机过程进行超级计算机建模，以及（3）收集和解释观察结果记录地球过去磁场的天然岩石。 这项研究属于后一类，我们建议大大扩展观测范围，以了解大约 77 万年前发生的地球磁场最后一次反转。 这笔赠款将允许一个科学家合作小组使用钾-氩放射性同位素衰变时钟获得新的年龄测定，以及有关该场强度和方向的新信息，如在火山爆发期间喷发的熔岩流序列中所记录的那样。地球磁场最后一次完全极性反转。 我们计划收集的信息反过来将提供来自真实地球过去的基本数据，这些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。地球地磁场的反转和偏移在沉积岩、火山岩和极地冰中形成了全球标记地平线。 这些地磁场不稳定性的精确年代学对于量化气候变化、地表过程和生物进化的时间和速率至关重要。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。 过去 20 年来，沉积物和熔岩流中的岩石磁性测量技术、应用于沉积物岩心的天文年代学方法以及使用 40Ar/39Ar 钾氩测年方法对熔岩流进行放射性同位素测年的进展，已产生了许多成果。从松山反转年代到布鲁尼正常年代的最近极性转变的古地磁记录。 鉴于准确和精确的年龄对于Matuyama-Brunhes反转的重要性，我们的目标是：（1）大大扩大与该反转时间有关的熔岩流40Ar/39Ar年龄集，同时提高精度和这些年龄测定的准确性，(2) 检验沉积岩芯中记录的 773,000 年前的天文年代年龄与实际年龄之间的差异这一假设熔岩流的 40Ar/39Ar 年龄反映了熔岩流中非常有限的 40Ar/39Ar 数据的准确性，并且 (3) 确定记录与毛伊岛和智利的 MB 逆转及其前兆相关的方向变化的熔岩古强度。 该项目是日本、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中对本科生和研究生进行最先进的放射性同位素地质年代学和古地磁方法的培训。 华盛顿大学将为研究生和本科生举办地质年代学夏季短期课程。 此外，PI 将与威斯康星大学地质博物馆的工作人员合作，创建一个展览，突出地质学家如何计时，包括使用 40Ar/39Ar 测年和天文年代学。