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

合作研究：反转频率与古代场强：来自侏罗纪和白垩纪的线索

• 批准号: 1520698
• 负责人: Anthony Koppers
• 金额: $ 7.74万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-08-15 至 2021-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1520698&HistoricalAwards=false
• 关键词: Collaborative; Research; Reversal; frequency; versus

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.

我们对地球磁场强度随地质时间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影响深远的问题。 一个关键问题涉及磁场强度与其在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。 这一观察结果得出了平均场强与反转率的假设关系，其中反转较少的时期（例如，白垩纪正常超纪元）将具有较高的平均场强，而反转较快的时期（例如，侏罗纪安静区）将具有较低的平均场强。平均场强。虽然现有数据为侏罗纪（中生代低偶极子，或 MDL）存在长期平均场强降低提供了一些支持，但从极快反转状态的恢复显然并没有伴随着场强的恢复;低场强显然一直持续到白垩纪，在逆转率恢复到“正常”很久之后。 问题在于，MDL 假设所依据的几乎所有数据都基于亚美尼亚 20 世纪 80 年代发表的记录不足的数据。 来自现代古强度实验的高质量数据与侏罗纪和白垩纪的直接辐射数据相结合，对于确定中生代地磁场是否减弱以及持续多久至关重要。 PI 将与亚美尼亚科学家合作，并在亚美尼亚收集样本。 ISE 正在为旅行提供共同资助。这笔赠款的资金将使团队能够对关键的亚美尼亚接触点进行重新采样，新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，来自亚美尼亚岛弧序列的高质量年龄数据，其中存在海洋沉积物与可测量数据的熔岩流互层，可以允许直接校准侏罗纪生物地层时间尺度，在这个时期，这种校准是罕见的。 样品将从亚美尼亚暴露的中生代弧层序中采集，样品将接受完整的古强度实验，原始热剩磁 (TRM) 逐渐被实验室在已知的外加磁场条件下感应的 TRM 所取代。 这些数据以及对化学变化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 39Ar/40Ar 方法进行放射性测年。 样品还将接受一系列岩石磁性测试，旨在表征样品是否适合古强度实验。