Benthic foraminifera analysis of two sediments cores from the northern Cape Basin in the eastern South Atlantic Ocean

Two late Quaternary sediment cores from the northern Cape Basin in the eastern South Atlantic Ocean were analyzed for their benthic foraminiferal content and benthic stable carbon isotope composition. The locations of the cores were selected such that both of them presently are bathed by North Atlantic Deep Water (NADW) and past changes in deep water circulation should be recorded simultaneously at both locations. However, the areas are different in terms of primary production. One core was recovered from the nutrient-depleted Walvis Ridge area, whereas the other one is from the continental slope just below the coastal upwelling mixing area where present day organic matter fluxes are shown to be moderately high. Recent data served as the basis for the interpretation of the late Quaternary faunal fluctuations and the paleoceanographic reconstruction.
During the last 450,000 years, NADW flux into the eastern South Atlantic Ocean has been restricted to interglacial periods, with the strongest dominance of a NADW-driven deep water circulation during interglacial stages 1, 9 and 11. At the continental margin, high productivity faunas and very low epibenthic d13C values indicate enhanced fluxes of organic matter during glacial periods. This can be attributed to a glacial increase and lateral extension of coastal upwelling. The long term glacial-interglacial paleoproductivity cycles are superimposed by high-frequency variations with a period of about 23,000 yr. Enhanced productivity in surface waters above the Walvis Ridge, far from the coast, is indicated during glacial stages 8, 10 and 12. During these periods, cold, nutrient-rich filaments from the mixing area were probably driven as far as to the southeastern flank of the Walvis Ridge.

本研究对采自南大西洋东部开普盆地北部的两根晚第四纪（late Quaternary）沉积岩芯，开展了底栖有孔虫（benthic foraminifera）组分与底栖稳定碳同位素组成分析。岩芯站位的选取遵循以下原则：两处岩芯现今均处于北大西洋深层水（North Atlantic Deep Water, NADW）的影响范围之内，且两地可同步记录深层水环流的历史演化。但两处区域的初级生产（primary production）特征存在显著差异：其中一根岩芯取自营养盐匮乏的沃尔维斯海岭（Walvis Ridge）区域，另一根则取自沿岸上升流混合区（coastal upwelling mixing area）下方的大陆坡，该区域现今的有机质通量（organic matter fluxes）已被证实处于中等偏高水平。本研究以最新观测数据为基础，对晚第四纪动物群波动展开解读并完成古海洋学重建（paleoceanographic reconstruction）。

在过去45万年中，北大西洋深层水向南大西洋东部的输入仅局限于间冰期（interglacial period）阶段，其中在1、9、11号间冰期，北大西洋深层水主导的深层水环流表现最强。在大陆边缘（continental margin）区域，冰期（glacial period）时出现的高生产力动物群与极低的表生底栖生物稳定碳同位素（δ¹³C）值，指示当时有机质通量显著升高。这一现象可归因于冰期时沿岸上升流的增强与横向扩张。长期的冰期-间冰期古生产力（paleoproductivity）旋回之上，还叠加了周期约2.3万年的高频波动信号。研究显示，在远离海岸的沃尔维斯海岭上方表层水域，8、10、12号冰期阶段的生产力水平显著升高。在这些冰期阶段，源自混合区的冷性营养盐富集丝状流（filament），可能被输送至沃尔维斯海岭的东南侧翼。