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

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 1a，简称OAE1a，120 Ma）是白垩纪中期温室地球环境中一段地质尺度上极为短暂的时段，其典型特征包括海洋沉积物中有机碳累积量升高、生物群落发生突发性更替，以及指示全球碳循环发生显著扰动的碳同位素异常漂移。然而，这类剧烈环境变化的持续时长不足100万年（<10^6 a），而现有年代学数据的时间分辨率通常仅为10^6 a量级，这导致我们对OAE1a的认知存在关键空白。本研究对三处分布广泛的沉积剖面开展了高分辨率调查，分别为意大利Cismon APTICORE岩心、墨西哥东北部Santa Rosa Canyon峡谷剖面，以及北大西洋伊比利亚陆缘外的深海钻探计划（Deep Sea Drilling Project，简称DSDP）398号站位；这些剖面涵盖了记录有压缩型与中等延展型OAE1a地层的多种沉积环境。本研究的核心目标为：为上述三处剖面建立轨道年代学框架，并构建适用于OAE1a的统一高分辨率时间标尺。对各剖面的高频测量数据开展频谱分析后，均检出具有统计学显著性的沉积旋回：其中APTICORE岩心的碳酸钙含量采样间距约为5~10 kyr；Santa Rosa Canyon剖面的样品测量了磁化率（magnetic susceptibility，简称MS）与非滞后剩磁（anhysteretic remanent magnetization，简称ARM）；DSDP 398号站位的样品则测量了磁化率、非滞后剩磁，以及非滞后剩磁与等温剩磁（isothermal remanent magnetization，简称IRM）的比值。这些旋回的周期比值与调制模式与白垩纪中期的轨道旋回高度相似，表明轨道尺度的气候波动可能对沉积过程起到了调控作用。基于轨道调控的沉积旋回，我们可以估算OAE1a各全球可识别独特阶段的持续时长。尽管OAE1a的总持续时长约为1.0~1.3 Myr，但其以负碳同位素漂移为代表的初始扰动过程极为迅速，仅持续约27~44 kyr。这一估算结果可为约束OAE1a的触发机制提供重要依据。