Collaborative Research: Pleistocene Geomagnetic Field Variability from the High-Latitude Southern Hemisphere

合作研究：南半球高纬度更新世地磁场变化

• 批准号: 0635348
• 负责人: Steven Lund
• 金额: $ 16.5万
• 依托单位: University of Southern California
• 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: 2007-01-01 至 2009-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0635348&HistoricalAwards=false
• 关键词: Collaborative; Research; Pleistocene; Geomagnetic; Field

A significant limitation toward a more complete understanding of the geodynamo process results from the scarcity of high quality observations. Recent historic secular variation (HSV) studies have characterized the global pattern of short-term HSV and have related its variability to the core dynamo process. Paleomagnetic studies make it clear, however, that the Earth's magnetic field has undergone a much wider range of spatial and temporal variations than have been seen in historic times. From an observational perspective, our most complete understanding of paleomagnetic secular variation has come from paleo-and archeomagnetic studies of Holocene materials. In contrast, high-resolution paleomagnetic secular variation (PSV) studies from the Pleistocene are much rarer. Archeological materials are absent; lava flows are discontinuous and difficult to date. And Pleistocene sediment sequences with accumulation rates comparable to Holocene PSV records ( 50 cm/kyr) have only rarely been recovered. Therefore, long-term PSV and excursional field behavior, which is exclusively a pre-Holocene phenomenon, are rarely scrutinized to the extent required for a full understanding of their dynamics. To alleviate this observational data gap, that is especially prevalent for the southern hemisphere, a u-channel paleomagnetic study of Late-Pleistocene geomagnetic field behavior from Chilean margin sediments cored during ODP Leg 202 is being undertaken. Sites 1233 (41 degrees 0 S, 74 degrees 26W, water depth 838 m), 1234 (36 degrees 13 S, 73 degrees 40W, water depth 1015 m), and 1235 (36 degrees 9 S, 73 degrees 33 W, water depth 489 m) recovered replicate sediment sequences with accumulation rates ranging from approximately 50 to 200 cm/kyr. Paleomagnetic studies on the Holocene part of these sequences demonstrate that these sediments provide a reliable and high quality paleomagnetic archive of geomagnetic behavior. Shipboard studies undertaken during Leg 202 show that the late-Pleistocene sediments, from the base of the Holocene to approximately 70 ka, record PSV, relative paleointensity (RPI) and excursional records at unprecedented temporal resolution. Along with the high accumulation rates (up to 5 yrs/cm) and excellent magnetic properties associated with these sediments, high-resolution independent radiocarbon chronologies and millennial climate based stratigraphies have been developed. PSV records from these three sites based on u-channel and discrete sample paleomagnetic measurements provide an opportunity to reconstruct the most complete and best dated record of late-Pleistocene directional PSV, relative paleointensity, and associated excursions ever recovered from the Southern Hemisphere and, perhaps, anywhere in the world. These results will describe the interrelationships between geomagnetic field intensity, normal directions (PSV), and extreme events (excursions) with unprecedented temporal resolution and independent chronologic control. Particularly, these sediments preserve the most completely resolved record of the Laschamp Excursion (ca 41 ka) ever recovered and will provide a better understanding of its relationship to normal secular variation, paleointensity and excursional field behavior. A regional and global inter-comparison of all late Pleistocene PSV records to assess data quality and space/time patterns of field variability is being undertaken to facilitate a low-order spherical harmonic analysis of this most (Laschamp Excursion) recent geomagnetic extreme event. Such observations are required to test concepts of timescales of geomagnetic field variability, the origin and nature of excursions, their relationship to dynamo behavior and there importance to the Earth system.

由于缺乏高质量的观测结果，对地球发电机过程的更全面理解受到了重大限制。最近的历史长期变异 (HSV) 研究描述了短期 HSV 的全球模式，并将其变异性与核心发电机过程联系起来。然而，古地磁研究清楚地表明，地球磁场经历了比历史时期更广泛的空间和时间变化。从观测的角度来看，我们对古地磁长期变化最完整的理解来自于全新世材料的古地磁和古地磁研究。相比之下，更新世的高分辨率古地磁长期变化（PSV）研究则要少得多。缺乏考古材料；熔岩流是不连续的，很难确定年代。更新世沉积物序列的积累率与全新世 PSV 记录 (50 cm/kyr) 相当，但很少被恢复。因此，长期 PSV 和偏移场行为（这完全是全新世前的现象）很少受到充分了解其动力学所需的程度的审查。为了缩小这种在南半球尤其普遍的观测数据差距，正在对 ODP 第 202 段期间取芯的智利边缘沉积物进行晚更新世地磁场行为的 u 通道古地磁研究。站点 1233（南纬 41 度，西经 74 度 26，水深 838 m）、1234（南纬 36 度 13，西经 73 度 40，水深 1015 m）和 1235（南纬 36 度 9，西经 73 度 33，水深） 489 m）恢复的重复沉积物序列，积累率范围约为 50 至 200厘米/克尔。对这些序列的全新世部分的古地磁研究表明，这些沉积物提供了可靠且高质量的地磁行为古磁档案。第 202 段期间进行的船上研究表明，从全新世底部到大约 70ka 的晚更新世沉积物，以前所未有的时间分辨率记录了 PSV、相对古强度 (RPI) 和偏移记录。除了与这些沉积物相关的高积累率（高达 5 年/厘米）和优异的磁性之外，还开发了高分辨率独立放射性碳年代学和基于千年气候的地层学。这三个地点基于 u 通道和离散样本古地磁测量的 PSV 记录提供了重建晚更新世定向 PSV、相对古强度以及从南半球恢复的相关偏移的最完整和最好日期记录的机会，也许，在世界任何地方。这些结果将以前所未有的时间分辨率和独立的时间控制来描述地磁场强度、法向（PSV）和极端事件（偏移）之间的相互关系。特别是，这些沉积物保存了迄今为止最完整解析的 Laschamp Excursion（约 41ka）记录，并将提供对其与正常长期变化、古强度和偏移场行为的关系的更好理解。正在对所有更新世晚期 PSV 记录进行区域和全球相互比较，以评估数据质量和场变化的空间/时间模式，以促进对这一最近（拉尚偏移）最近的地磁极端事件进行低阶球谐分析。需要进行此类观测来测试地磁场变化的时间尺度概念、偏移的起源和性质、它们与发电机行为的关系以及对地球系统的重要性。