Geomagnetic field behaviour and geodynamo characterisation at the end of the Cretaceous Normal Superchron

地磁场是由地球流体外核中一个自我维持的发电机产生的，它是由一个以地球为中心的轴向偶极子近似模拟的，但在百年到千年的时间尺度上经历波动——被称为古长期变化——并且能够随机地逆转其极性。在地质时间尺度上，地磁场也可以令人惊讶地产生稳定极性的长周期（bb0 - 20myr），最近的一个是白垩纪正常超时（CNS, 84-121 Ma）。地球发电机的“超时”和“反转”状态之间的转变是如何发生的仍然是一个有争议的问题，逐渐的转变是地幔对地核施加的热边界条件变化的潜在标志，而突然的转变将更符合地球发电机的自发非线性转变。为了更好地描述这种转变的渐进或突然性质，我们建议在这个项目中研究60-100 Ma的地磁场行为和地球动力状态。为此，我们将首先获得东苏拉威西蛇绿岩（ESO，位位在低古纬度~90 Ma）和马达加斯加洪水玄武岩（MFB，位位在中古纬度84 ~ 92 Ma）熔岩流序列的新的古地磁方向和强度。与鄂霍次克-楚科奇火山带（OCVB，位于高古纬度84-90 Ma）的观测结果相比，这些新的观测结果将使我们能够更好地约束CNS结束前的古磁场纬向剖面。为了在从CNS到地球发电机“反转”状态的转变过程中对地磁场行为进行更全面和动态的描述，我们将编制适合于60-100 Ma区间的场建模的高质量古磁记录。这将使我们能够在过渡前后的一个或两个精心选择的时间窗口内调整由一个巨大的高斯过程产生的统计场模型的参数。为了推断地球深部动力学的潜在变化，最后将得到的参数与各种边界条件下的数值发电机模拟结果进行比较。

The geomagnetic field, generated by a self-sustaining dynamo in the Earth’s fluid outer core, is modelled in first approximation by a geocentric axial dipole, yet experiencing fluctuations on centennial-to-millennial timescales—termed palaeosecular variation—and being able to stochastically reverse its polarity. Over geological timescales, the geomagnetic field can surprisingly also produce prolonged (> 20 Myr) periods with stable polarity, the most recent one being the Cretaceous Normal Superchron (CNS, 84–121 Ma). How the transition between ‘superchron’ and ‘reversing’ states of the geodynamo occurs remains a controversial issue, a gradual transition being the potential signature of changes in the thermal boundary conditions imposed by the mantle on the core whereas an abrupt transition would be more consistent with a spontaneous non-linear transition of the geodynamo. To better characterise the gradual or abrupt nature of the transition, we propose to investigate in this project the geomagnetic field behaviour and the geodynamo state at 60–100 Ma. To this end, we will first acquire new palaeomagnetic directions and intensities on sequences of lava flows from the East Sulawesi Ophiolite (ESO, emplaced ~90 Ma at low paleolatitude) and the Madagascan Flood Basalts (MFB, emplaced 84–92 Ma at mid palaeolatitude). Compared to previous results from the Okhotsk-Chukotka Volcanic Belt (OCVB, emplaced 84–90 Ma at high palaeolatitude), these new observations will allow us to better constrain the latitudinal profile of the palaeomagnetic field just before the end of the CNS. In order to produce a more global and dynamical description of the geomagnetic field behaviour across the transition from the CNS to a ‘reversing’ state of the geodynamo, we will then compile high-quality palaeomagnetic records suitable for field modelling over the 60–100 Ma interval. This will allow us to adjust the parameters of statistical field models produced by a giant gaussian process over one or two well-chosen time windows before and after the transition. With the view of inferring potential changes in the Earth’s deep dynamics, the obtained parameters will finally be compared with the outcomes of numerical dynamo simulations submitted to various boundary conditions.