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

对各种来源的历史地磁场及其长期变化的观测揭示了地磁的瞬变和持久特征。持续的特征包括太平洋相对于大西洋半球的长期变化减弱，以及加拿大和西伯利亚上空约60°N纬度的地磁集中区域。瞬变的历史特征包括北极的运动和南大西洋下方核/地幔边界的一个不断增长的反向磁通量区域。加上目前偶极强度衰减率为每世纪5%(是自由衰减率的5倍)，这些观测结果让一些研究人员推测，地磁场可能正朝着反转或漂移的方向发展。最近加速的偶极衰变是否可能继续下去的问题对社会来说很重要，因为磁场保护地球免受宇宙射线的轰击，这对电信、人类健康和全球生态系统具有实际影响。最近提出的一个命题是，目前的偶极子衰减率是由南大西洋异常驱动的，而且只有几百年的历史，这表明长期变化可能是一系列短暂的百年尺度特征的结果，这些特征很难在短暂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，可能会在破译地球发电机的动力学方面发挥重要作用，并可能对大气中宇宙成因同位素的长期产生机制产生影响，这些同位素已被用来推断太阳的可变性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变特征，如突然的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、欧洲/中东)。这是否反映了这一领域的不对称性，还是缺乏可用的数据，目前尚不清楚。该项目将测试突然的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大陆边缘沉积物的详细研究，为下一代球谐模型提供高质量的数据。这一机会的出现是因为最近在R/V尤文的EW0408型铁芯上收集到了具有良好磁性能的新的巨型活塞芯，其沉积速率极高(米到10‘S每千年)。将使用U形通道和离散采样方法对27个可用巨型活塞岩芯中的8个进行古地磁长期变化研究。存在适合于放射性碳测年的生物成分，尽管被陆源沉积物严重稀释。基于放射性碳的详细年表将使用加州大学欧文分校凯克AMS实验室开发的超小样本的精确加速器14C测年新技术来开发。因此，这项研究将填补一个重大的数据空白，测试关于磁场可变性的假设，并采用新的方法进行高分辨率的古地磁测量和精细的年代学约束，这将大大促进对地球磁场动力学的理解。产生的数据将有助于区分地磁和太阳对地球宇宙线通量和宇宙成因同位素产生的控制的时间尺度。这对于理解气候和太阳变化都很重要，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影响。将开发区域PSV和相对古强度主曲线，并用作各种海洋和陆地古气候研究的地层模板。数据将被提供给数据库，并用于补充一个数据很少的地区正在进行的球面调和研究。放射性碳的结果将有助于理解北太平洋地区储集层年龄的变化，有助于改进过去气候变化的年表，并有助于理解海洋碳循环的变化。该项目将支持本科生和研究生的培养。

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.