The stratigraphic depth in the Cismon APTICORE borehole and the age of the average age model sampled by the Monte Carlo tuning algorithm

Orbital tuning, the process of fitting sedimentary cycles to orbital periodicities, can estimate with high resolution the timing and duration of key events in the geological record. We formulate here orbital tuning as the inverse problem of finding the variation in sedimentation rate that matches sediment cycles with orbital periodicities. Instead of obtaining a single best estimate, we apply a Bayesian formulation and define a probability distribution of sedimentation rate variations that result in powerful orbital periodicities. By sampling this distribution with a Monte Carlo method, we quantify the uncertainty in the inferred sedimentation rates due to uncertainties in the tuning periods and to components of the sedimentary signal that are unrelated to orbital cycles. The method is applied to the chronology of a 30 m interval in the Cismon APTICORE borehole (Southern Alps, Italy) that includes the early Aptian oceanic anoxic event 1a (Selli Level), estimated to last 1.11 ± 0.11 Ma (95% interval). The d13C record shows a sudden negative shift of about -1 per mill at the base of the Selli Level (22-47 ka) followed by a recovery to preshift values in approx. 240 ka, consistent with a scenario where light carbon is quickly added and then flushed out of the oceanatmosphere system.轨道调谐（Orbital tuning）是将沉积旋回与轨道周期相匹配的研究过程，可实现地质记录中关键事件发生时间与持续时长的高分辨率估算。本文将轨道调谐建模为一类反问题：即寻找可使沉积旋回契合轨道周期的沉积速率变化量。不同于仅获取单一最优估算结果的传统方法，我们采用贝叶斯（Bayesian）建模框架，定义了能够产生显著轨道周期信号的沉积速率变化的概率分布。通过蒙特卡洛（Monte Carlo）方法对该概率分布进行采样，我们量化了两类不确定性导致的推断沉积速率误差：一是调谐周期本身的不确定性，二是沉积信号中与轨道周期无关的组分带来的偏差。本方法被应用于意大利南阿尔卑斯山Cismon APTICORE钻孔中一段30米岩心的年代学研究，该岩心段包含早阿普第期海洋缺氧事件1a（Selli阶，Selli Level），据估算该事件的持续时长为1.11±0.11百万年（Ma，百万年），对应95%置信区间。d13C记录显示，在Selli阶底部（22~47千年（ka，千年））处出现了约-1‰的突然负偏移，随后在约240千年的时间内逐步恢复至偏移前的数值，这与轻碳快速注入后从海-气系统中被冲刷移除的理论情景相符。