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

我们对地质时期地球磁场强度变化的理解已经取得了很大进展，但仍然存在一些相当基本和深远的问题。 一个关键的问题是磁场强度和它在一个被称为反转的过程中改变符号的趋势之间的关系。 个别反转的记录清楚地表明，它们与低磁场强度有关。 这一观察导致了平均场强与反转率的假设关系，其中具有很少反转的周期（例如，白垩纪正超时）将具有更高的平均场强和具有快速反转的周期（例如，侏罗纪安静区）将具有较低的平均场强。虽然现有的数据提供了一些支持，存在一个长期的平均场强降低在侏罗纪（中生代偶极子低，或MDL），从极其快速的反转状态的恢复显然是不伴随着恢复的场强;低场强显然持续到白垩纪，反转率恢复到“正常”后很长一段时间。 问题在于，几乎所有的MDL假设所依据的数据都是基于亚美尼亚在20世纪80年代发表的记录不足的数据。 高质量的数据来自现代古强度实验结合直接辐射日期从侏罗纪和白垩纪是必不可少的，以解决是否和多久的地磁场在中生代被压抑。 研究所将与亚美尼亚科学家合作，并将在亚美尼亚收集样本。伊势正在为旅行提供共同资金。这笔赠款的资金将使该团队能够重新采样关键的亚美尼亚烘烤接触，新的数据将有助于限制中生代偶极子低压终止的年龄和性质。 此外，高质量的年龄数据，从亚美尼亚弧序列中有海洋沉积物夹层与可定年的熔岩流可能允许直接校准的侏罗纪生物地层时间尺度，在此期间，这种校准是罕见的。 将从亚美尼亚暴露的中生代弧序列中采集样品，并对样品进行完整的古强度实验，在实验室已知的外加磁场条件下，用感应的TRM逐步取代原来的热反射磁化。 这些数据，沿着对化学蚀变和非理想行为的检查，为确定古代场强提供了基础。 成功的样品将使用39 Ar/40 Ar方法进行放射性测年。 样品还将接受一系列岩石磁性测试，旨在表征样品对古强度实验的适用性。

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.