Clay mineralogy of sediment cores from the South-East Atlantic off Namibia

Independent proxies were assessed in two Late Quaternary sediment cores from the eastern South Atlantic to compare deep-water changes during the last 400 kyr. Two cores were recovered from beneath North Atlantic Deep Water (NADW) at approximately 3 000 m depth. Late Quaternary presence of NADW is indicated by the Cibicidoides wuellerstorfi assemblage on the Walvis Ridge (Core GeoB 1214) and the Bulimina alazanensis assemblage on the Namibian continental slope (Core GeoB 1710). The propagation of NADW is exclusively observed during interglacials, with maximum factor loadings in Stages 1, 5, 7, 9 and 11. These maxima are consistent with peaks in kaolinite/chlorite ratios and maxima of poorly crystalline smectite in the clay-mineral record. Kaolinite and poorly crystalline smectite are products of intense chemical weathering. They are injected into the NADW at low latitudes, north of the study area, and advected south. Chlorite, which is stable under cold weathering regimes, is a characteristic mineral of water masses of southern origin. During glacial stages, it is advected north with Southern Component Water (SCW). Above the NADW/SCW depths, kaolinite/chlorite ratios vary only slightly without a significant glacial-interglacial pattern, as measured in a core (GeoB 1712) from 1 000 m deep on the same profile of the Namibian continental slope off Walvis Bay.

本研究针对南大西洋东部的两根晚第四纪沉积岩芯开展独立代用指标分析，以对比过去40万年以来的深水环流变化。
两根岩芯采自水深约3000米的北大西洋深层水（North Atlantic Deep Water, NADW）影响区域。晚第四纪时期北大西洋深层水的存在，可通过沃尔维斯海岭GeoB 1214岩芯中的武氏卷包虫（Cibicidoides wuellerstorfi）组合，以及纳米比亚大陆坡GeoB 1710岩芯中的阿拉萨那靴形虫（Bulimina alazanensis）组合予以指示。
北大西洋深层水的扩张仅见于间冰期，在第1、5、7、9、11阶段达到因子载荷峰值。
该峰值与黏土矿物记录中的高岭石/绿石比值峰值以及弱结晶蒙脱石的含量峰值相一致。
高岭石与弱结晶蒙脱石均为强烈化学风化的产物，它们在研究区以北的低纬度区域注入北大西洋深层水，并向南平流输送。
绿石在冷风化环境中性质稳定，是南源水团的特征矿物；冰期阶段，其随南源水团（Southern Component Water, SCW）向北平流输送。
在纳米比亚大陆坡沃尔维斯湾附近同一剖面、水深1000米的GeoB 1712岩芯中，北大西洋深层水/南源水团深度之上的高岭石/绿石比值仅存在小幅波动，未呈现显著的冰期-间冰期变化模式。