Magnetic susceptibility (MS) measurements on the Sancerre (France) drill-core

The Toarcian Oceanic Anoxic Event (T-OAE) of the early Jurassic period involves one of the largest perturbations of the carbon cycle in the past 250 Ma, recorded by a pronounced negative carbon-isotope excursion (CIE). Numerous studies have focused on potential causes of the T-OAE and CIE, but are hampered by an uncertain timescale. Here we present high-resolution (~2 kyr) magnetic susceptibility (MS) measurements from the marine marls of the Sancerre-Couy drill-core, southern Paris Basin, spanning the entire Toarcian Stage. The MS variations document a rich series of sub-Milankovitch to Milankovitch frequencies (precession, obliquity and eccentricity) with the periodic g2-g5 (405 kyr) and quasi-periodic g4-g3 (~2.4 Myr Cenozoic mean periodicity) eccentricity terms being the most prominent. The MS-related g4-g3 variation reflects third-order eustatic sequences, and constrains the sequence stratigraphic framework of the Toarcian Stage. In addition, MS variations reveal a modulation of g2-g5 by g4-g3 eccentricity related cycles, suggesting that sea-level change was the main control on the deposition of the Toarcian Sancerre marls, in tune with the astro-climatic frequencies. The stable 405 kyr cyclicity constrains a minimum duration of the Toarcian Stage to ~8.3 Myr, and the well documented CIE, associated with the T-OAE, to ~300 to 500 kyr. The 405 kyr MS timescale calibrates the periodicity of the prominent high-frequency d13C cycles that occur in the decreasing part of the CIE to 30 to 34 kyr, consistent with the Toarcian obliquity period predicted for an Earth experiencing sustained tidal dissipation.

早侏罗世托阿尔阶海洋缺氧事件（Toarcian Oceanic Anoxic Event, T-OAE）是过去2.5亿年间规模最大的碳循环扰动事件之一，其地球化学记录为显著的负碳同位素偏移（carbon-isotope excursion, CIE）。此前诸多研究围绕T-OAE与CIE的潜在成因展开，但受限于不确定的年代标尺。本研究基于巴黎盆地南部桑瑟尔-库伊（Sancerre-Couy）钻孔的海相泥灰岩岩芯，获取了覆盖整个托阿尔阶的高分辨率（~2千年）磁化率（magnetic susceptibility, MS）数据。磁化率变化记录了丰富的亚米兰科维奇至米兰科维奇周期信号，涵盖岁差、黄赤交角与偏心率周期，其中以405千年周期的g2-g5项以及约2.4百万年（新生代平均周期）的准周期g4-g3偏心率项最为突出。与磁化率相关的g4-g3变化反映了三级海平面变化层序，约束了托阿尔阶的层序地层格架。此外，磁化率变化揭示g2-g5周期受g4-g3偏心率相关旋回的调制，表明海平面变化是托阿尔阶海相泥灰岩沉积的主控因素，与天文气候周期相契合。稳定的405千年周期将托阿尔阶的最小持续时长约束至约8.3百万年，而与T-OAE伴生的已被充分记录的CIE持续时长则被限定为约300至500千年。基于405千年磁化率年代标尺，本研究将CIE下降段显著的高频δ¹³C周期的周期范围校准为30至34千年，这与地球经历持续潮汐耗散时预测的托阿尔阶黄赤交角周期一致。