231Pa/230Th-Verhältnisse im Auftriebsgebiet vor Südwest-Afrika

Abstract

English abstract provided by author:

Th-, U-isotopes and 231Pa were measured in four cores from the Cape- and Angola Basin. The cores in the Cape Basin are located in the upwelling area off Namibia, the core in the Angola Basin in the open ocean. Over the past ca. 70 ka, the open ocean core shows no temporal variation in both the 230Th-normalized particle flux and the terrigeneous input. In contrast, the cores in the margin area display an increase in terrigeneous input during glacial times with a simultaneous decrease in particle flux. The depth of the redoxcline, which is derived from the very high uranium contents, shows significant variations in the Glacial and the Holocene. Comparison of the depth of the redoxcline with the flux of organic carbon reveals that observed minima of the oxygen penetration depth are caused by a low oxygen content in the bottom water. The measured 231Pa/230Th-ratios indicate, that during certain intervals of the glacial ocean circulation the production of NADW was temporarily strongly reduced. These intervals correlate with the northward shift of the Antarctic Polar Frontal System. From the mass balance of the Holocene distribution of 231Pa it is concluded that 231Pa is transported from the open ocean into the high productivity areas. The use of the 231Pa/230Th-ratio as a tracer for ocean circulation without a careful mass balance should thus be met with caution