Collaborative Research: Improving the Late Cretaceous-Eocene geomagnetic polarity time scale by integrating the global magnetic anomaly record and astrochronology

每隔几万到几百万年，地球磁场就会发生逆转，磁北和磁南就会交换位置。地磁极性时标 (GPTS) 记录了这些反转的日期。从 20 世纪 60 年代开始，GPTS 的后续版本均以全球海洋磁异常模式为基础，该模式保存了由在大洋中脊扩张轴处形成的磁化火山岩记录的过去磁场反转的档案。最近，天文年代学为 GPTS 提供了独立的信息来源。天文年代学利用地球轨道和自转轴方向的天文周期驱动的沉积周期，磁场反转的时间可以通过沉积物序列来估计，这些沉积物记录了沉积在海底时的磁场方向。该项目的目标是建立一个 GPTS，将所有来源的信息结合起来：磁异常、天文年代学和放射性同位素测年。由此产生的 GPTS 精度的提高对于确定板块运动速率和沉积物序列中记录的过去气候变化的时间至关重要。为了扩大其影响力，该项目将支持一名学者参加为期两年的非学位项目，在该项目中，来自代表性不足群体的大学毕业生参与一项活跃的研究项目，以支持他们申请研究生院。时间尺度构建的科学也将在 K-12 教育工作者和公开讲座中介绍，该项目中开发的软件例程将在 Astrochron 中提供，Astrochron 是一个被学生和研究人员广泛使用的开源软件包。项目的总体目标是将 GPTS 构建从“赢家通吃”策略（时间尺度基于单一“最佳”信息源）转变为严格集成不同信息源，利用相关错误的独立性。将应用贝叶斯公式和蒙特卡罗抽样方法来组合所有数据，根据每条信息的不确定性对其进行权衡，并将不确定性传播到最终的 GPTS。该项目将重点关注晚白垩世-始新世（约 84-3300 万年前，或 Ma），这是 GPTS 仍在变化的时间间隔。这一时期包含约 50 Ma 的极端温暖气候时期，随后出现降温趋势，导致南极冰盖的形成。此外，一些全球构造事件发生在50-45Ma左右，当时印度洋海底扩张速度因印度与欧亚大陆碰撞而降低；与此同时，南大西洋和北太平洋的传播速度加快，恰逢夏威夷-皇帝海山链的弯曲。改进的时间尺度对于增进我们对新生代气候变化的理解并探索其与全球构造可能的相关性至关重要。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Every few tens of thousands to millions of years Earth’s magnetic field reverses, with magnetic north and south swapping places. The geomagnetic polarity time scale (GPTS) dates these reversals. Starting in the 1960s, successive versions of the GPTS were based on the global pattern of oceanic magnetic anomalies, which preserve an archive of past field reversals recorded by magnetized volcanic rocks that formed at the spreading axes of mid-ocean ridges. More recently, astrochronology has provided an independent source of information on the GPTS. Astrochronology uses sedimentary cycles driven by astronomical cycles in Earth’s orbit and spin axis orientation, and the timing of magnetic field reversals can be estimated in sequences of sediments that recorded the field orientation when they were deposited at the bottom of the sea. The objective of this project is to build a GPTS that combines information from all sources: magnetic anomalies, astrochronology, and radioisotopic dating. The improved accuracy of the resulting GPTS will be crucial to determine rates of plate motion and the timing of past climatic changes recorded in sediment sequences. To broaden its impact, the project will support a scholar in a two-year non-degree program where college graduates from underrepresented groups participate in an active research project to support their applications to graduate school. The science of time scale construction will also be presented to K-12 educators and in public lectures, and the software routines developed in the project will be made available in Astrochron, an open source package that is widely used by students and researchers.The overarching project goal is to transform GPTS construction from a “winner-take-all” strategy, where the time scale is based on a single “best” source of information, to a rigorous integration of different sources of information that exploits the independence of the associated errors. A Bayesian formulation and Monte Carlo sampling methods will be applied to combine all the data, weighing each piece of information on the basis of its uncertainty and propagating uncertainties to the final GPTS. The project will focus on the Late Cretaceous-Eocene (~84-33 million years ago, or Ma), a time interval where the GPTS is still in flux. This period contains a time of extreme warm climate around 50 Ma, which was followed by a cooling trend that resulted in the formation of the Antarctic ice sheet. Moreover, several global tectonic events took place around 50-45 Ma, when seafloor spreading rates decreased in the Indian Ocean as India collided with Eurasia; simultaneously, spreading became faster in the southern Atlantic and northern Pacific, coinciding with a bend in the Hawaii-Emperor seamount chain. An improved time scale is critical to advance our understanding of Cenozoic climatic changes and to explore their possible correlation to global tectonics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.