Geochemical and stable isotope composition of Middle Campanian marl-limestone rhythmites of the Lehrte West Syncline near Hannover (Lower Saxony, northern Germany)

A cyclic marl–limestone succession of Middle–Late Campanian age has been investigated with respect to a Milankovitch-controlled origin of geochemical data. In general, the major element geochemistry of the marl–limestone rhythmites can be explained by a simple two-component mixing model with the end-members calcium carbonate and 'average shale'-like material. Carbonate content varies from 55 to 90%. Non-carbonate components are clay minerals (illite, smectite) and biogenic silica from sponge spicules, as well as authigenically formed zeolites (strontian heulandite) and quartz. The redox potential suggests oxidizing conditions throughout the section. Trace element and stable isotopic data as well as SEM investigations show that the carbonate mud is mostly composed of low-magnesium calcitic tests of planktic coccolithophorids and calcareous dinoflagellate cysts (calcispheres). Diagenetic overprint results in a decrease of 2% d18O and an increase in Mn of up to 250 ppm. However, the sediment seems to preserve most of its high Sr content compared to the primary low-magnesium calcite of co-occurring belemnite rostra. The periodicity of geochemical cycles is dominated by 413 ka and weak signals between 51 and 22.5 ka, attributable to orbital forcing. Accumulation rates within these cycles vary between 40 and 50 m/Ma. The resulting cyclic sedimentary sequence is the product of (a) changes in primary production of low-magnesium calcitic biogenic material in surface waters within the long eccentricity and the precession, demonstrated by the CaCO3 content and the Mg/Al, Mn/Al and Sr/Al ratios, and (b) fluctuations in climate and continental weathering, which changed the quality of supplied clay minerals (the illite/smectite ratio), demonstrated by the K/Al ratio. High carbonate productivity correlates with smectite-favouring weathering (semi-arid conditions, conspicuously dry and moist seasonal changes in warmer climates). Ti as the proxy indicator for the detrital terrigenous influx, as well as Rb, Si, Zr and Na, shows only low frequency signals, indicating nearly constant rates of supply throughout the more or less pure pelagic carbonate deposition of the long-lasting third-order Middle–Upper Campanian sedimentary cycle.

本次研究针对中-晚坎潘期（Middle–Late Campanian）的旋回性泥灰岩-石灰岩序列，开展了地球化学数据的米兰科维奇旋回成因分析。总体而言，该泥灰岩-石灰岩纹层的主量元素地球化学特征，可通过以碳酸钙与“平均页岩”端元构成的二元混合模型进行合理解释。其碳酸盐含量介于55%~90%之间，非碳酸盐组分包括黏土矿物（伊利石、蒙脱石）、海绵骨针成因的生物硅，以及自生形成的锶型辉沸石（strontian heulandite）与石英。氧化还原电位数据显示，整个剖面均处于氧化环境。微量元素、稳定同位素数据以及扫描电子显微镜（SEM）观测结果表明，该碳酸盐泥主要由浮游颗石藻与钙质鞭毛藻孢囊（钙球）的低镁钙质介壳构成。成岩叠加改造导致δ¹⁸O值降低2%，锰（Mn）含量最高可达250 ppm。但与同期产出的箭石鞘低镁方解石相比，该沉积物仍保留了大部分原始高锶（Sr）含量。地球化学旋回的周期以413 ka为主，同时伴随51~22.5 ka的弱信号，该特征可归因于轨道强迫（orbital forcing）作用。旋回内的沉积速率介于40~50 m/Ma之间。该旋回沉积序列的形成受控于两类过程：(a) 表层水体中低镁钙质生物成因物质的初级生产力变化，该变化受长偏心率与岁差旋回调控，可通过碳酸钙含量以及Mg/Al、Mn/Al、Sr/Al比值予以体现；(b) 气候与大陆风化的波动，该波动改变了供给黏土矿物的组分特征（伊利石/蒙脱石比值），可通过K/Al比值进行指示。高碳酸盐生产力与蒙脱石型风化作用（半干旱环境、温暖气候下显著的干湿季节变化）呈现正相关关系。钛（Ti）作为陆源碎屑输入的代用指标，辅以铷（Rb）、硅（Si）、锆（Zr）与钠（Na），仅显示出低频信号，表明在持续时间较长的三级中-晚坎潘期远洋碳酸盐沉积旋回中，陆源供给速率基本保持稳定。