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.

早侏罗世托阿尔期海洋缺氧事件（Toarcian Oceanic Anoxic Event，T-OAE）是过去2.5亿年间碳循环遭受的最显著扰动之一，其标志性特征为显著的负向碳同位素偏移（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，研究进展受阻。本文基于巴黎盆地南部桑瑟尔-库伊（Sancerre-Couy）钻探岩心的海相泥灰岩，开展了高分辨率（~2千年）的磁化率（magnetic susceptibility，MS）测量，采样层位完整覆盖整个托阿尔阶。磁化率变化记录了丰富的亚米兰科维奇至米兰科维奇尺度旋回频率，涵盖岁差、黄赤交角与偏心率三类周期，其中以周期为405千年的g2-g5项，以及近似周期为2.4百万年（新生代平均周期）的g4-g3偏心率项最为显著。与磁化率相关的g4-g3旋回反映了三级海平面变化层序，并为托阿尔阶构建了约束性的层序地层格架。此外，磁化率变化揭示出g2-g5旋回受到g4-g3偏心率周期的调制，表明海平面变化是控制托阿尔期桑瑟尔海相泥灰岩沉积的主导因素，这与天文气候旋回的演化规律相符。稳定的405千年旋回将托阿尔阶的最小持续时限约束为约8.3百万年，而与T-OAE伴生、已有充分记录的CIE持续时限则被限定为约30万至50万年。基于405千年磁化率年代标尺，我们将CIE下降段显著的高频δ13C旋回周期校准为30至34千年，这一结果与地球经历持续潮汐耗散时预测的托阿尔期黄赤交角周期完全一致。