Microfossil cyclicity in Late Albian sediments of Kirchrode I borehole

We studied the biological response to orbital forcing in marine Upper Albian sediments recovered from the 245 m-long Kirchrode I borehole in the Lower Saxony basin in northwestern Germany. Results from quantitative analysis of planktonic and benthic foraminifera, of calcareous nannofossils, and radiolaria were used for this study. Spectral analysis in the depth domain indicates for the high sedimentation rate part of the Upper Albian dominant periods with wavelengths of 10±13 m, 5±6 m, and 2±3 m, which we interpret to represent the biological response to orbital forcing in the Milankovitch frequency bands eccentricity, obliquity, and precession, respectively. In addition, a low amplitude 40±50 m cycle was found, which would represent the long-term eccentricity variation of roughly 400 ka. Microfossil cyclicity does not change significantly within the whole core indicating sedimentation rates of 11±12 cm/ka on an average, with variations between 3.5 and 13 ka.
Microfossils show greater variability in their abundance changes than the physical and chemical parameters and also greater power in the higher-frequency bands (obliquity and precession). While most of the planktonic foraminifer species studied are dominated by variations in the obliquity, most benthic foraminifer species show an additional strong influence of precession. These differences in the cyclicity of the abundance changes are interpreted as reflecting a stronger influence of low latitude water in the deep waters of the Late Albian Lower Saxony basin than in the shallow waters. This basin was part of a wide, 'Boreal' epicontinental sea, which was connected to the Tethys to the south via the Polish strait and via the Paris basin, and which was connected with the North Atlantic and Arctic to the north. In analogy to results from analysis of data from the Late Neogene, strong effects of precession interpreted as being more characteristic for changes/influences triggered in the low latitudes and those of obliquity to be more characteristic for influences from the high latitudes.
The presence of a relatively strong eccentricity cycle, not only in the compound parameters, but also in the abundance changes of single species during the Late Albian means that there must have been a non-linear response to orbital forcing and internal feedbacks.

本研究针对德国西北部下萨克森盆地245米长的Kirchrode I钻孔（Kirchrode I borehole）所采集的上阿尔必阶（Upper Albian）海相沉积物，开展了轨道强迫（orbital forcing）的生物响应研究。本研究采用了浮游有孔虫（planktonic foraminifera）、底栖有孔虫（benthic foraminifera）、钙质超微化石（calcareous nannofossils）以及放射虫（radiolaria）的定量分析结果。深度域频谱分析（spectral analysis in the depth domain）结果显示，上阿尔必阶沉积速率较高的层段存在主导周期，其波长分别为10±13 m、5±6 m与2±3 m；我们将其分别解释为轨道强迫在米兰科维奇频段（Milankovitch frequency bands）下的偏心率（eccentricity）、黄赤交角（obliquity）与岁差（precession）生物响应。此外，本次研究还发现了一个振幅较弱的40±50 m周期，其对应约400 ka的长期偏心率变化。全岩心的微体化石（microfossil）旋回未出现显著变化，据此推算平均沉积速率为11±12 cm/ka，其波动区间为3.5~13 cm/ka。
相较于物理与化学参数，微体化石的丰度变化具有更强的变异性，且在高频频段（黄赤交角与岁差）中具有更高的谱功率。多数被研究的浮游有孔虫物种的丰度变化以黄赤交角驱动为主，而多数底栖有孔虫物种的丰度变化则额外受到岁差的强烈影响。上述丰度变化旋回的差异，被解释为晚阿尔必期（Late Albian）下萨克森盆地深水环境受到低纬度水体的影响强于浅水环境。该盆地曾属于广阔的“北方”陆表海（Boreal epicontinental sea），其南侧经波兰海峡与巴黎盆地与特提斯洋（Tethys）相连，北侧则与北大西洋及北冰洋相通。结合晚新近纪（Late Neogene）的相关研究结果，岁差的显著影响通常被认为是低纬度触发的变化或影响的典型特征，而黄赤交角的影响则更多体现为高纬度作用的特征。
晚阿尔必期不仅在复合参数中，且在单个物种的丰度变化中均存在较强的偏心率周期，这表明当时的生态系统对轨道强迫存在非线性响应，并伴有内部反馈机制。