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 (sensu anglico) 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.

通过对英格兰南部多塞特地区基默里奇粘土组（Kimmeridge Clay Fm）全层段独立测量的三个变量（磁化率、光电因子和总伽马射线）进行分析，识别出规律性的米尺度沉积周期。频谱分析表明，在较长的地层间隔中，周期性表现为波长为1.87至4.05米的较大振幅周期以及约为该波长一半的小振幅周期。这些周期被认为分别记录了轨道倾斜和进动。与进动周期相比，推断出的轨道倾斜周期幅值显著较大，这或许暗示了高纬度气候强迫通过海平面变化传递至低纬度地区。轨道调谐显示，早期基默里奇阶（按英文名称理解）至少持续了3.6百万年（95个较长波长的周期），而晚期基默里奇阶至少持续了3.9百万年（103个较长波长的周期）。针对基默里奇粘土组，基于周期性计算的首次详细生产力估计表明，典型基默里奇粘土的平均生产力（220克每平方米每年）低于现代大陆架的平均生产力。然而，典型基默里奇粘土较高的平均有机碳含量（3.8%总有机碳）并不能归因于其高平均生产力。相反，平均有机碳含量与有机物质的低硅质沉积矿物稀释和/或与底部水体氧气减少相关的较高保存度相一致。