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，地球的磁场就会反转，南北磁场互换位置。地磁极性时间尺度(GPTS)显示了这些倒转的日期。从1960年代开始，GPTS的连续版本以全球海洋磁异常模式为基础，保存了大洋中脊扩张轴线上形成的磁化火山岩记录的过去磁场反转的档案。最近，天体年代学提供了关于GPTS的独立信息来源。天体年代学使用由地球轨道和自旋轴方向的天文周期驱动的沉积周期，可以通过记录沉积在海底时的磁场方向的沉积物序列来估计磁场反转的时间。这个项目的目标是建立一个综合了来自所有来源的信息的GPTS：磁异常、天体年代学和放射性同位素测年。由此产生的GPTS的精确度的提高将对确定板块运动速度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，该项目将支持一名学者参加一个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将参与一个积极的研究项目，以支持他们申请研究生院。时间尺度构建的科学还将被介绍给K-12教育工作者和公开讲座，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。总体项目目标是将GPTS的构建从基于单一的“最佳”信息源的“赢家通吃”策略转变为利用相关错误的独立性的不同信息源的严格集成。将采用贝叶斯公式和蒙特卡罗抽样方法合并所有数据，根据信息的不确定度对每一条信息进行加权，并将不确定度传递到最终的GPTS。该项目将专注于晚白垩世-始新世(约84-3300万年前，或Ma)，这是一个GPTS仍在变化的时间段。这段时间包含了50 Ma左右的极端温暖气候，随后出现了降温趋势，导致了南极冰盖的形成。此外，在50-45 Ma左右发生了几次全球构造事件，当时随着印度与欧亚大陆的碰撞，印度洋的海底扩张速度下降；同时，南大西洋和北太平洋的扩张速度变得更快，恰逢夏威夷-帝王海山链的弯曲。改进的时间尺度对于促进我们对新生代气候变化的理解和探索它们与全球构造的可能相关性至关重要。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.