Paleomegnetic and geochemistry of sediments from the early Aptian Oceanic Anoxic Event@en

The early Aptian Oceanic Anoxic Event (OAE1a, 120 Ma) represents a geologically brief time interval in the mid-Cretaceous greenhouse world that is characterized by increased organic carbon accumulation in marine sediments, sudden biotic changes, and abrupt carbon-isotope excursions indicative of significant perturbations to global carbon cycling. The brevity of these drastic environmental changes (< 10**6 year) and the typically 10**6 year temporal resolution of the available chronologies, however, represent a critical gap in our knowledge of OAE1a. We have conducted a high-resolution investigation of three widely distributed sections, including the Cismon APTICORE in Italy, Santa Rosa Canyon in northeastern Mexico, and Deep Sea Drilling Project (DSDP) Site 398 off the Iberian margin in the North Atlantic Ocean, which represent a range of depositional environments where condensed and moderately expanded OAE1a intervals are recorded. The objectives of this study are to establish orbital chronologies for these sections and to construct a common, high-resolution timescale for OAE1a. Spectral analyses of the closely-spaced (corresponding to ~5 to 10 kyr) measurements of calcium carbonate content of the APTICORE, magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) of the Santa Rosa samples, and MS, ARM and ARM/IRM, where IRM is isothermal remanent magnetization, of Site 398 samples reveal statistically significant cycles. These cycles exhibit periodicity ratios and modulation patterns similar to those of the mid-Cretaceous orbital cycles, suggesting that orbital variations may have modulated depositional processes. Orbital control allows us to estimate the duration of unique, globally identifiable stages of OAE1a. Although OAE1a had a duration of ~1.0 to 1.3 Myr, the initial perturbation represented by the negative carbon-isotope excursion was rapid, lasting for ~27-44 kyr. This estimate could serve as a basis for constraining triggering mechanisms for OAE1a.早阿普第期大洋缺氧事件（Oceanic Anoxic Event，OAE1a，120 Ma）是白垩纪中期温室地球背景下一段地质时限极短的时期，其标志性特征包括海洋沉积物中有机碳堆积量增加、生物群发生突变，以及指示全球碳循环发生显著扰动的碳同位素偏移（carbon-isotope excursions）。然而，这些剧烈环境变化的短暂性（<10^6年），以及现有年代学框架通常仅具备10^6年的时间分辨率，成为我们认识OAE1a的关键知识缺口。本研究对三处分布广泛的剖面开展了高分辨率研究，包括意大利的奇蒙APTICORE钻孔、墨西哥东北部的圣罗莎峡谷，以及北大西洋伊比利亚缘外的深海钻探计划（Deep Sea Drilling Project，DSDP）398号站位，这些剖面覆盖了记录有凝缩型与适度延展型OAE1a地层段的各类沉积环境。本研究的目标是为这些剖面建立轨道年代学，并构建适用于OAE1a的统一高分辨率时间标尺。对间距紧密（对应约5~10千年）的测试数据开展频谱分析后发现：APTICORE钻孔的碳酸钙含量、圣罗莎峡谷样品的磁化率（magnetic susceptibility，MS）与非滞后剩磁（anhysteretic remanent magnetization，ARM），以及398号站位样品的磁化率、非滞后剩磁以及ARM/等温剩磁（isothermal remanent magnetization，IRM，其中IRM为等温剩磁），均揭示出具有统计学显著性的周期信号。这些周期的周期比值与调制模式与白垩纪中期轨道周期特征相似，表明轨道尺度的气候变化可能调控了沉积过程。轨道调控作用使我们得以估算OAE1a各可全球识别的独特阶段的持续时长。尽管OAE1a的总持续时长约为1.0~1.3百万年，但以负碳同位素偏移为代表的初始扰动过程极为迅速，仅持续约27~44千年。这一估算结果可作为约束OAE1a触发机制的研究基础。