Cyclostratigraphy, orbital tuning and inferred productivity for the type Kimmeridge Clay (Late Jurassic), Southern England

Three independently measured variables (magnetic susceptibility, photoelectric factor and total gamma-ray) obtained from throughout the type Kimmeridge Clay Fm in Dorset (Southern England) were used to identify regular metre-scale, sedimentary cycles. Spectral analysis demonstrates that for long stratigraphical intervals the cycles are expressed as large-amplitude cycles of 1.87–4.05 m wavelength and smaller-amplitude cycles of around half that wavelength. These cycles are interpreted to record orbital obliquity and precession, respectively. The much larger amplitude of the inferred obliquity cycles compared with the precession cycles may indicate a high-latitude climatic forcing transferred to lower latitudes via sea-level variations. Orbital tuning indicates that the Early Kimmeridgian (<em>sensu anglico</em>) lasted at least 3.6 Ma (95 longer-wavelength cycles) and the Late Kimmeridgian at least 3.9 Ma (103 longer-wavelength cycles). The first detailed productivity estimates for the Kimmeridge Clay Fm, on a cycle-by-cycle calculation, indicate that average productivity of the type Kimmeridge Clay (220 g m⁻² a⁻¹) was less than the average productivity on modern continental shelves. The high average organic carbon content of the type Kimmeridge Clay (3.8% total organic carbon) cannot be attributed to high average productivity. However, the average organic carbon content is consistent with low siliciclastic mineral dilution of organic matter and/or elevated preservation linked to reduced bottom-water oxygenation.

研究采用采自英格兰南部多塞特郡金默里奇黏土组（Kimmeridge Clay Formation，地层单位“组”缩写为Fm）全域的三项独立测量变量——磁化率（magnetic susceptibility）、光电因子（photoelectric factor）与总伽马射线（total gamma-ray），识别出规则的米级沉积旋回。频谱分析结果显示，在较长地层间隔中，此类旋回可分为两类：波长1.87~4.05米的大振幅旋回，以及波长约为其一半的小振幅旋回。研究认为这两类旋回分别对应轨道倾角（orbital obliquity）与岁差（precession）信号。相较于岁差旋回，推断得到的倾角旋回振幅显著更大，这表明高纬度气候强迫可通过海平面变化传递至低纬度区域。轨道调谐结果表明，按英文狭义定义（sensu anglico）的早基默里吉亚期持续时长至少为3.6百万年（对应95个长波长旋回），晚基默里吉亚期则至少为3.9百万年（对应103个长波长旋回）。本研究首次针对金默里奇黏土组开展逐旋回的详细古生产力估算，结果显示该组典型剖面的平均古生产力为220克每平方米每年（g·m⁻²·a⁻¹），低于现代大陆架的平均古生产力水平。金默里奇黏土典型剖面的平均总有机碳（total organic carbon）含量高达3.8%，但这一高含量无法用较高的平均古生产力来解释。不过，其平均有机碳含量与两种情形相符：一是有机质被硅质碎屑矿物（siliciclastic mineral）的稀释程度较低，二是与底层水含氧量降低相关的有机质保存效率提升。