Global Records of the Iceland Basin Geomagnetic Excursion

地球的磁场在许多不同的时间尺度上有所不同。在几年到几个世纪的短时间尺度上，磁极在地理极周围漂移，这一过程被称为长期变化。因此，磁极很少在地理极点，这就是为什么磁罗盘在用于导航之前要根据这种变化进行调整。如果磁极的位置在大约10,000年的较长时间尺度上平均，则平均位置与地理极的位置重合。在数百万年的很长的时间尺度上，地球的磁场是已知的反转其极性，指南针将指向南方而不是北方。这些反转并不是定期发生的，不同极点的持续时间差异很大。我们目前正处于一个“正常的极性”时期(即指南针指向北方)，上一次磁场逆转是在大约780,000年前。地磁漂移发生在长期变化和磁场反转之间的时间尺度上。在地磁漂移期间，磁极偏离地理极很远(&gt；45 dgerees)，但又回到原来的位置。这与完全反转不同，全反转是指磁极在落在相反的地理极点之前，偏离地理极很远的地方。有人提出，在过去的78万年里(即自上一次反转以来)，有多达12次这样的漂移，由于它们是全球性事件，它们可以用来对比世界各地的不同沉积序列。这些漂移的标志也是地球磁场总体强度的下降。这个磁场保护我们不受入射太阳辐射的影响，我们最近表明，由于场强降低导致入射太阳辐射增加，大气中放射性核素的产生速度增加是偏移的标志。这种放射性核素保存在沉积物和冰芯中，可能提供了一种将气候变化的冰芯记录与海洋沉积物中识别的气候变化模式相关联的方法。由于缺乏对地磁漂移的良好记录，人们对地磁漂移的原因、漂移持续多长时间以及它们对关联不同气候事件的作用有多大的了解一直受到阻碍。它们通常在来自沉积物的磁信号中被识别出来，在那里，地球磁场在磁性矿物融入沉积物期间对齐，或者当磁性矿物在岩石形成期间生长时。因为它们被认为是短期的，所以我们需要快速积累的沉积物。根据对来自大西洋的两个沉积物岩心的分析，我们最近发现，其中一次被称为冰岛盆地事件的漂移发生在大约18.6万年前，持续了大约7000年。其他作者也提出了同一活动的持续时间越来越短的建议。我们的建议旨在研究分布在大西洋和太平洋、分布在一定纬度范围内的其他五个岩心的磁信号。这将使我们能够评估该信号的全球性质。我们还将携带我们的地球化学分析，使我们能够计算事件的持续时间。最近的研究表明，与反转相关的磁不稳定性在赤道似乎只持续了2000年，而在地理两极大约持续了10000年。我们将能够测试地磁漂移是否也是如此。最后，对这些事件持续时间的了解将使气候科学家了解到，短途旅行可以提供他们与气候记录的关联。短持续时间将提供非常精确的相关性，但更难识别，而长持续时间应该更容易找到和识别，但提供的相关性精度要低得多。

The Earth's magnetic field varies on a number of different timescales. On short timescales of years to centuries the magnetic poles drift around the geographic poles, in a process known as secular variation. Hence, the magnetic pole is rarely at the geographic pole, and this why magnetic compasses are adjusted for this variation before they can be used for navigation. If the location of the magnetic poles is averaged over longer timescales of about 10,000 years the mean positions coincide with those of the geographic poles. On very long time scales of millions of years the Earth's magnetic field is known to reverse its polarity, and compasses would point to the south rather than to the north. These reversals do not occur regularly and the duration of the different polarities varies enormously. We are currently in a period of 'normal polarity' (i.e. compasses point to the north) and the last reversal of the field was about 780,000 years ago. Geomagnetic excursions occur on timescales between secular variation and field reversals. During a geomagnetic excursion the magnetic poles wander far away (>45 dgerees) from the geographic poles, but return to their previous location. This distinguishes them from full reversals where the magnetic pole wanders far away from the geographic pole, before settling at the opposite geographic pole. There have been suggestions that there have been up to 12 such excursions in the last 780,000 years (i.e. since the last reversal), and because they are global events, they can be used to correlate different sedimentary sequences worldwide. The excursions are also marked by a decrease in the overall intensity of the Earth's magnetic field. This field shields us from incoming solar radiation, and we have recently shown that excursions are marked by an increase in the rate of production of radionuclides in the atmosphere, due to the increased incoming solar radiation that results from the reduction in field strength. Such radionuclides are preserved in sediments and in ice cores, and potentially offer a way of correlating ice-core records of climate change with patterns of climate change recognised in marine sediments. An understanding of the causes of geomagnetic excursions, how long they last, and how useful they can be for correlating different climate events has been hampered by a lack of good records of these events. They are typically recognised in the magnetic signal from sediments, where the Earth's magnetic field aligns magnetic minerals during their incorporation into the sediment, or when magnetic minerals grow during formation of the rocks. Because they are thought to be of short duration we need sediments that accumulate at a rapid rate. We have recently shown that one of these excursions known as the Iceland Basin Event, which took place about 186,000 years ago, lasted for about 7000 years, based on analyses of two sediment cores from the Atlantic. Other authors have proposed shorter and longer durations for the same event. Our proposal aims to study the magnetic signal for five other cores, scattered through the Atlantic and Pacific oceans, and spread over a range of latitudes. This will enable us to assess the global nature of the signal. We will also be carrying our geochemical analyses that will enable us to calculate the duration of the event. Recent work has shown that the magnetic instabilities associated with reversals appear to only last 2,000 years at the equator, and about 10,000 years at the geographic poles. We will be able to test if the same holds true for geomagnetic excursions. Finally, a knowledge of the duration of these events will inform climate scientists as to the resolution that excursions can offer their correlations of climate records. Short durations will offer very precise correlation, but are harder to identify, whereas long durations should be easier to find and identify but offer correlation at a much lower precision.