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

由于缺乏高质量的观测资料，对地球动力学过程的更全面了解受到了重大限制。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认识来自于对全新世材料的古地磁和考古地磁研究。相比之下，来自更新世的高分辨率古地磁长期变化（PSV）研究要少得多。没有考古资料；熔岩流是不连续的，很难确定年代。与全新世PSV记录（50 cm/kyr）相当的沉积速率的更新世沉积层序很少得到恢复。因此，长期的PSV和偏移场行为，这完全是一个全新世之前的现象，很少被仔细研究到充分理解它们的动力学所需要的程度。为了减轻这种观测数据的差距，这在南半球尤其普遍，正在对ODP第202阶段智利边缘沉积物的晚更新世地磁场行为进行u通道古地磁研究。1233号（南纬41度，西经74度，水深838米）、1234号（南纬36度，西经73度，水深1015米）和1235号（南纬36度，西经73度，水深489米）恢复了重复的沉积物序列，积累速率约为50至200 cm/kyr。对这些层序全新世部分的古地磁研究表明，这些沉积物提供了可靠的、高质量的地磁行为古地磁档案。在Leg 202期间进行的船上研究表明，从全新世底部到大约70 ka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。随着这些沉积物的高堆积速率（高达5年/厘米）和优异的磁性，高分辨率的独立放射性碳年代学和基于千年气候的地层学已经发展起来。这三个地点的PSV记录基于u通道和离散样本古地磁测量，提供了一个机会来重建最完整和最准确的晚更新世定向PSV记录，相对古强度，以及从南半球，也许是世界上任何地方恢复的相关偏移。这些结果将以前所未有的时间分辨率和独立的时间控制描述地磁场强度、法向（PSV）和极端事件（漂移）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的Laschamp偏移（约41 ka）记录，并将更好地理解其与正常长期变化、古强度和偏移场行为的关系。对所有晚更新世PSV记录进行区域和全球的相互比较，以评估数据质量和磁场变化的时空模式，从而促进对这一最近的地磁极端事件（Laschamp偏移）的低阶球谐分析。需要这种观测来检验地磁场变异性的时间尺度概念、偏移的起源和性质、它们与发电机行为的关系以及它们对地球系统的重要性。

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.