Holocene Paleomagnetic Secular Variation (Directions and Intensity) from Southast Alaska

阿拉斯加东南部的全新世古地磁长期变化（方向和强度）

• 批准号: 0711584
• 负责人: Joseph Stoner
• 金额: $ 32.5万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-08-01 至 2010-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0711584&HistoricalAwards=false
• 关键词: Holocene; Paleomagnetic; Secular; Variation; Directions

Observations of the historical geomagnetic field and its secular variations from a wide variety of sources reveal transient and persistent geomagnetic features. Persistent features include the subdued secular variation in the Pacific relative to the Atlantic hemisphere, and regions of concentrated geomagnetic flux over Canada and Siberia at ~60° N latitude. Transient historical features include movement of the North Magnetic Pole and a growing region of reversed magnetic flux at the core/mantle boundary below the South Atlantic. Along with the present dipole intensity decay rate of 5% per century (five times the free decay rate), these observations have led some researchers to speculate that the geomagnetic field may be heading toward a reversal or an excursion. The question of whether the recently accelerated dipole decay is likely to continue is important to society because the magnetic field shields the earth from cosmic ray bombardment, with practical implications for telecommunications, human health, and global ecosystems. A recent proposition that the present rate of dipole decay is driven by the South Atlantic anomaly and is only a few hundred years old suggests that long-term changes could result from a series of transient centennial scale features, which are difficult to resolve in the short historical record. Understanding the interplay between transient and persistent geomagnetic features over longer time intervals in the paleomagnetic record is, therefore, likely to play a significant role in deciphering the dynamics of the geodynamo, as well as having implications for mechanisms of long-term production of cosmogenic isotopes in the atmosphere, which have been used to infer solar variability and its impact on past climate changes. Recent paleomagnetic observations are now beginning to document transient features as abrupt sub-millennial and even sub-centennial paleomagnetic field behaviors. Paleomagnetic secular variation (PSV) records that record abrupt features are so-far limited to the Atlantic hemisphere (eastern North America, Iceland/Greenland, Europe/Middle East). Whether this reflects asymmetry of the field or a lack of available data is presently unclear. This project will test the spatial scale of abrupt geomagnetic behavior and beginning to assess PSV of the Pacific, while providing high quality data for the next generation of spherical harmonic models, through a detailed study of sediments from the Southeast Alaska continental margin. This opportunity arises because of recently collected new jumbo piston cores of extraordinarily high sedimentation rate (meters to 10's of meters per thousand years) with favorable magnetic properties, on cruse EW0408 of R/V Ewing. Paleomagnetic secular variation will be studied on eight out of 27 available jumbo piston cores using u-channel and discrete sample methods. Biogenic components suitable for radiocarbon dating are present, although severely diluted by the terrigenous sediment. Detailed chronologies based on radiocarbon will be developed using new techniques of precise accelerator 14C dating of ultra-small samples, developed at the Keck AMS Laboratory at UC Irvine. This study will, therefore, fill a significant data gap, test hypotheses on magnetic field variability and employing new methods for both high-resolution paleomagnetic measurements and fine-scale chronologic constraints that will substantially advance understanding of the dynamics of Earth's magnetic field. The data generated will contribute to differentiating the timescales of geomagnetic versus solar control on terrestrial cosmic ray flux and cosmogenic isotope production. This is significant for understanding both climatic and solar variability, and important to society because the magnetic field shields the earth from cosmic ray bombardment, with practical implications for telecommunications, human health, and global ecosystems. Regional PSV and relative paleointensity master curves will be developed and used as stratigraphic templates for a variety of marine and terrestrial paleoclimatic studies. Data will be contributed to databases and used to supplement ongoing spherical harmonic studies of the field from a region with little data. Radiocarbon results will contribute to understanding of changing reservoir ages in the North Pacific region, of interest to improving chronologies of past climate changes, and to understanding variations in the oceans carbon cycle. This project will support the training of both undergraduate honors students and graduate students.

从各种各样的来源对历史地磁场及其长期变化的观测揭示了短暂和持久的地磁特征。持续的特征包括太平洋相对于大西洋半球的较弱的长期变化，以及在北纬~60°的加拿大和西伯利亚地区的地磁通量集中。短暂的历史特征包括北磁极的运动和南大西洋以下地核/地幔边界的反向磁通量增长区域。再加上目前偶极子强度衰减率为每世纪5%（是自由衰减率的5倍），这些观测结果使一些研究人员推测地磁场可能正在走向反转或偏移。最近加速的偶极子衰变是否可能继续下去的问题对社会很重要，因为磁场保护地球免受宇宙射线的轰击，对电信、人类健康和全球生态系统具有实际意义。最近的一项主张认为，目前的偶极子衰变速率是由南大西洋异常引起的，而且只有几百年的历史，这表明长期的变化可能是由一系列短暂的百年尺度特征引起的，而这些特征很难在短期的历史记录中得到解决。因此，了解古地磁记录中较长时间间隔的瞬变地磁特征和持久地磁特征之间的相互作用，可能在破译地球动力学方面发挥重要作用，并对大气中宇宙成因同位素的长期产生机制产生影响，这些同位素已被用于推断太阳变率及其对过去气候变化的影响。最近的古地磁观测现在开始记录瞬态特征，如亚千年甚至亚百年的古磁场行为。古地磁长期变化（PSV）记录的突变特征迄今仅限于大西洋半球（北美东部、冰岛/格陵兰岛、欧洲/中东）。目前尚不清楚这是否反映了磁场的不对称性或缺乏可用的数据。该项目将测试突变地磁行为的空间尺度，并开始评估太平洋的PSV，同时通过对阿拉斯加东南部大陆边缘沉积物的详细研究，为下一代球面调和模型提供高质量的数据。这个机会的出现是因为最近在R/V Ewing的EW0408号cruuse上收集了新的巨型活塞芯，其沉降速率非常高（每千年一米到十米），具有良好的磁性。利用u型通道和离散采样方法，对27个大型活塞岩心中的8个进行古地磁长期变化研究。虽然被陆源沉积物严重稀释，但仍存在适合放射性碳测年的生物成分。基于放射性碳的详细年表将使用由加州大学欧文分校Keck AMS实验室开发的对超小型样品进行精确加速器14C定年的新技术来开发。因此，这项研究将填补一个重要的数据空白，测试磁场变异性的假设，并采用高分辨率古地磁测量和精细尺度年代学约束的新方法，这将大大促进对地球磁场动力学的理解。所产生的数据将有助于区分地磁与太阳对地球宇宙射线通量和宇宙成因同位素生产控制的时间尺度。这对理解气候和太阳的变化具有重要意义，对社会也很重要，因为磁场保护地球免受宇宙射线的轰击，对电信、人类健康和全球生态系统具有实际意义。开发区域PSV和相对古强度主曲线，并将其作为各种海相和陆相古气候研究的地层模板。数据将提供给数据库，并用于补充数据很少的地区正在进行的实地球谐研究。放射性碳的结果将有助于了解北太平洋地区储层年龄的变化，有助于改进过去气候变化的年表，并有助于了解海洋碳循环的变化。该项目将同时支持本科生和研究生的培养。