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

缺乏高质量的观测对更全面地了解地球发电机过程造成了很大的限制。最近的历史长期变异(HSV)研究表征了短期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的最完整的了解来自于对全新世物质的古地磁和古地磁研究。相比之下，从更新世开始的高分辨率古地磁长期变化(PSV)研究要罕见得多。没有考古资料；熔岩流不连续，很难确定日期。而与全新世PSV记录(50 cm/Kyr)相当的更新世沉积序列很少被恢复。因此，长期的PSV和偏移场行为，这完全是一种全新世前的现象，很少被仔细研究到充分了解它们的动力学所需的程度。为了缩小这一在南半球特别普遍的观测数据差距，正在对ODP202航次期间取心的智利边缘沉积物的晚更新世地磁行为进行U通道古地磁研究。1233站(S 41°0，西经74°26，水深838m)、1234站(S 36°13，西经73°40，水深1015m)和1235站(S 36°9，西经73°33，水深489m)恢复了重复沉积序列，累积速率约为50~200 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和高质量的地磁行为古地磁档案。202航次期间进行的船上研究表明，从全新世底部到大约70ka的晚更新世沉积以前所未有的时间分辨率记录了PSV、相对古强度(RPI)和漂移记录。除了与这些沉积物相关的高积累速率(高达5年/厘米)和优异的磁性外，还开发了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。这三个地点基于U-通道和离散样本古地磁测量的PSV记录提供了一个机会，可以重建最完整和最古老的晚更新世方向PSV、相对古强度和相关的漂移记录，这些记录曾从南半球，也许是世界任何地方恢复。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、法线方向(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恢复的最完整的拉尚漂移记录(约41ka)，并将更好地了解其与正常的长期变化、古强度和漂移场行为的关系。正在对所有晚更新世PSV记录进行区域和全球内部比较，以评估数据质量和场变异性的空间/时间模式，以促进对这一最近的地磁极端事件(拉尚漂移)进行低次球面调和分析。需要进行这种观测，以检验地磁场可变性的时间尺度概念、漂移的起源和性质、它们与发电机行为的关系以及对地球系统的重要性。

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.