Cyclostratigraphic calibration of the Famennian stage (Late Devonian, Illinois Basin, USA)

The Late Devonian biosphere was affected by two of the most severe biodiversity crises in Earth's history, the Kellwasser and Hangenberg events near the Frasnian–Famennian (F–F) and the Devonian–Carboniferous (D–C) boundaries, respectively. Current hypotheses for the causes of the Late Devonian extinctions are focused on climate changes and associated ocean anoxia. Testing these hypotheses has been impeded by a lack of sufficient temporal resolution in paleobiological, tectonic and climate proxy records. While there have been recent advances in astronomical calibration that have improved the accuracy of the Frasnian time scale and part of the Famennian, the time duration of the entire Famennian Stage remains poorly constrained. During the Late Devonian, a complete Late Frasnian–Early Carboniferous succession of deep-shelf deposits accumulated in the epieric sea in Illinois Basin of the central North-American mid-continent. A record of this sequence is captured in three overlapping cores (H-30, Sullivan and H-32). The H-30 core section spans the F–F boundary; the Sullivan section spans almost all of the Famennian and the H-32 section sampled spans the interval of the Upper Famennian and the D–C boundary. To have the best chance of capturing Milankovitch cycles, 2000 rock samples were collected at minimum 5-cm-interval across the entire sequence. Magnetic susceptibility (MS) was measured on each sample and the preservation of climatic information into the MS signal was verified through geochemical analyses and low-temperature magnetic susceptibility acquisition. To estimate the duration of the Famennian Stage, we applied multiple spectral techniques and tuned the MS signal using the highly stable 405 kyr cycle for Sullivan and the obliquity cycle for the H-30 and H-32 cores. Based on the correlation between the cores we constructed a Famennian floating astronomical time scale, which indicates a duration of 13.5 ± 0.5 myr. An uncertainty of 0.5 myr was estimated for the uncertainties arising from the errors in the stratigraphic position of the F–F and D–C boundaries, and the 405 kyr cycle counting. Interpolated from the high-resolution U–Pb radiometric ages available for the Devonian–Carboniferous boundary we recalibrated the Frasnian–Famennian boundary numerical age to 372.4 ± 0.9 Ma.晚泥盆世生物圈受到地球历史上两次最严重的生物多样性危机冲击，分别是弗拉斯期-法门期（Frasnian–Famennian, F–F）界线附近的凯尔瓦斯事件（Kellwasser event），以及泥盆纪-石炭纪（Devonian–Carboniferous, D–C）界线附近的亨根贝格事件（Hangenberg event）。当前针对晚泥盆世生物灭绝事件成因的主流假说聚焦于气候变化及其伴生的海洋缺氧现象，而相关检验工作长期受制于古生物学、构造学及气候代用指标记录的时间分辨率不足。尽管近年天文定标研究取得进展，提升了弗拉斯阶时间标尺以及法门阶部分时段的定年精度，但整个法门阶的持续时限仍未得到精准限定。晚泥盆世时期，北美中部大陆中西部的伊利诺伊盆地陆表海（epicontinental sea）中沉积了一套完整的晚弗拉斯期-早石炭世陆棚深沉积序列，该序列的地层记录保存在三个重叠钻探岩芯（H-30、Sullivan与H-32）中：其中H-30岩芯剖面覆盖F–F界线；Sullivan岩芯剖面几乎涵盖整个法门阶；H-32岩芯的采样段则覆盖上法门阶与D–C界线区间。为最大化捕获米兰科维奇旋回（Milankovitch cycles）的信号，研究团队在整个沉积序列中以至少5厘米的间隔采集了2000件岩石样品。对每件样品开展了磁化率（magnetic susceptibility, MS）测试，并通过地球化学分析与低温磁化率测定，验证了磁化率信号中气候信息的保真度。为估算法门阶的持续时限，研究团队应用了多种频谱分析技术，并以Sullivan岩芯的405千年（kiloyear, kyr）高稳定旋回、H-30与H-32岩芯的斜度旋回对磁化率信号进行轨道调谐。基于各岩芯间的地层对比，研究人员构建了法门阶浮动天文时间标尺，结果显示法门阶的持续时长为13.5 ± 0.5百万年（million years, myr）。针对F–F与D–C界线的地层位置误差以及405 kyr旋回计数误差带来的不确定性，研究估算得到0.5 myr的误差范围。通过对泥盆纪-石炭纪界线已有的高分辨率铀-铅（U–Pb）放射性同位素测年数据进行插值，研究人员将弗拉斯期-法门期界线的数值年龄重新校准为372.4 ± 0.9百万年前（mega-annum, Ma）。