Collaborative Research: Reversal frequency versus ancient field strength: Clues from the Jurassic and Cretaceous

我们对地球磁场强度随地质年代变化的认识已经取得了很大进展，但是仍然存在一些相当基本和影响深远的问题。一个关键的问题是磁场强度与其在一个被称为逆转的过程中改变符号的趋势之间的关系。个别地磁倒转的记录清楚地表明，它们与低磁场强度有关。这一观察结果导致了平均场强与反转率之间的假设关系，其中反转少的时期（如白垩纪正常超时）平均场强较高，而反转快的时期（如侏罗纪安静带）平均场强较低。虽然现有资料为侏罗纪（中生代偶极子低）存在一段较长时期的平均场强减弱提供了一些支持，但从极快速逆转状态中恢复过来显然并不伴随着场强的恢复；低磁场强度显然持续到白垩纪，在反转率恢复“正常”很久之后。问题是，MDL假设所依据的几乎所有数据都是基于亚美尼亚在20世纪80年代发表的记录不佳的数据。现代古强度实验所得的高质量数据与侏罗纪和白垩纪的直接辐射测定数据相结合，对于确定中生代地磁场是否减弱以及减弱了多长时间至关重要。pi将与亚美尼亚科学家合作，并将在亚美尼亚收集样本。ISE为旅行提供共同资金。这笔资金将使研究小组能够对亚美尼亚关键的烤触点进行重新采样，新的数据将有助于确定中生代偶极低终止的年龄和性质。此外，来自亚美尼亚弧层序的高质量年龄数据，其中有海相沉积物与可测定的熔岩流互层，可以直接校准侏罗纪生物地层时间尺度，这是一个罕见的校准时期。在已知的外加磁场条件下，在实验室中逐步用残余热磁化（TRM）取代原始的残余热磁化（TRM），对亚美尼亚暴露的中生代弧序列进行完整的古强度实验。这些数据，连同对化学变化和非理想行为的检查，为确定古代磁场强度提供了基础。成功的样品将使用39Ar/40Ar方法进行放射性定年。样品还将进行一系列岩石磁性测试，以确定样品是否适合进行古强度实验。

Much progress has been made in our understanding of the variation of Earth's magnetic field strength through geological time, but some rather fundamental and far-reaching questions remain. One key question bears on the relationship between magnetic field strength and its tendency to change sign in a process known as reversing. Records of individual reversals clearly show that they are associated with low magnetic field strength. This observation lead to the hypothesized relationship of average field strength with reversal rate in which periods with few reversals (e.g., the Cretaceous Normal Superchron) would have higher average field strengths and periods with rapid reversals (e.g., the Jurassic Quiet Zone) would have lower average field strengths. While existing data provide some support for the existence of a long period of reduced average field strength in the Jurassic (the Mesozoic Dipole Low, or MDL), the recovery from the state of extremely rapid reversals apparently was not accompanied by a recovery in field strength; the low field strengths apparently persisted well into the Cretaceous, long after reversal rates returned to 'normal'. The problem is that nearly all of the data on which the MDL hypothesis is based are based on poorly documented data from Armenia, published in the 1980s. High quality data derived from modern paleointensity experiments combined with direct radiometric dates from the Jurassic and Cretaceous are essential to resolve whether and for how long the geomagnetic field was depressed during the Mesozoic. The PIs will be collaborating with Armenian scientists, and will collect samples in Armenia. ISE is providing co-funding for travel. Funding from this grant will allow the team to resample the key Armenian baked contacts and new data will help constrain the age and nature of the termination of the Mesozoic Dipole Low. Additionally, high quality age data from the Armenian arc sequences in which there are marine sediments interbedded with datable lava flows may allow direct calibration of the Jurassic biostratigraphic time scale, a period in which such calibration is rare. Samples will be collected from the Mesozoic arc sequences exposed in Armenia and samples will be subjected to complete paleointensity experiments whereby the original thermal remanent magnetization (TRM) is progressively replaced by a TRM induced in the laboratory under known conditions of the applied magnetic field. Such data, along with checks for chemical alteration and non-ideal behavior provide the basis for determining the ancient field strength. Successful samples will be radiometrically dated using the 39Ar/40Ar method. Samples will also be subjected to a host of rock magnetic tests designed to characterize the suitability of the samples for paleointensity experimentation.