Micropaleontological investigation of sediment core Kronsmoor

Benthic foraminiferal and calcareous nannofossil assemblages, as well as stable isotope data from the Campanian/Maastrichtian boundary interval (~71.4 to ~70.7 Ma) of the Kronsmoor section (North German Basin), were investigated in order to characterize changes in surface-water productivity and oxygen content at the seafloor and their link to climatic and paleoceanographic changes. A nutrient index based on calcareous nannofossils is derived for the high-latitude, epicontinental North German Basin, reflecting changes in surface-water productivity. Oxygen isotopes of well-preserved planktic foraminiferal specimens of Heterohelix globulosa reflect warmer surface-water temperatures in the lower part of the studied succession and a cooling of up to 2°C (0.5 per mil) in the upper part (after 71.1 Ma). For the lower and warmer part of the investigated succession, benthic foraminiferal assemblages and the calcareous nannofossils indicate well-oxygenated bottom waters and low-surface water productivity. In contrast, the upper part of the succession is characterized by cooler conditions, lower oxygen content at the seafloor and increasing surface-water productivity. It is proposed that the cooling phase starting at 71.1 Ma was accompanied by increasing surface-water mixing caused by westerly winds. As a consequence of mixing, nutrients were advected from sub-surface waters into the mixed layer, resulting in increased surface-water productivity and enhanced organic matter flux to the seafloor. We hypothesize that global sea-level fall during the earliest Maastrichtian (~71.3 Ma), indicated by decreasing carbon isotope values, may have led to a weaker water mass exchange through narrower gateways between the Boreal Realm and the open North Atlantic and Tethys oceans. Both the weaker water mass exchange and enhanced surface-water productivity may have led to slightly less ventilated bottom waters of the upper part of the studied section. Our micro-paleontological and stable isotopic approach indicates short-term (<100 kyr) changes in oxygen consumption at the seafloor and surface-water productivity across the homogeneous Boreal White Chalk succession of the North German Basin.本研究针对北德盆地（North German Basin）克龙斯莫尔剖面（Kronsmoor section）坎潘期/马斯特里赫特期（Campanian/Maastrichtian）界线间隔（约71.4 Ma至约70.7 Ma）中的底栖有孔虫（benthic foraminiferal）与钙质超微化石（calcareous nannofossil）组合，以及稳定同位素数据（stable isotope data）展开分析，旨在阐明表层水生产力与海底氧含量的变化特征，及其与气候和古海洋学（paleoceanographic）变化的内在联系。本研究基于钙质超微化石构建了适用于高纬度陆表海北德盆地的营养盐指数，用以反映表层水生产力的变化。对保存完好的异旋球虫（Heterohelix globulosa）浮游有孔虫（planktic foraminiferal）标本的氧同位素分析显示，研究层序下部的表层海水温度偏高，而上部（71.1 Ma之后）出现了最高达2℃（0.5‰）的降温过程。在研究层序下部的暖期环境中，底栖有孔虫组合与钙质超微化石均指示底层水体含氧量充足，且表层水生产力较低。与之相反，层序上部的环境特征为温度偏低、海底氧含量下降，且表层水生产力升高。本研究提出，始于71.1 Ma的降温阶段伴随西风驱动的表层水混合作用增强。混合作用将次表层水体中的营养盐输送至表层混合层，进而提升了表层水生产力，并增强了有机质向海底的沉降通量。我们假设，以碳同位素值降低为指示的最早马斯特里赫特期（约71.3 Ma）全球海平面下降，可能导致北极域（Boreal Realm）与开阔北大西洋、特提斯洋（Tethys oceans）之间的通道变窄，进而削弱了不同水团间的交换。水团交换减弱与表层水生产力提升共同作用，可能使得研究剖面上部的底层水体通风程度略有降低。本项微古生物学（micro-paleontological）与稳定同位素研究方法，揭示了北德盆地均质北陆白垩层序中，海底氧消耗与表层水生产力的短期（<100 kyr）变化。