Miocene–Pliocene stepwise intensification of the Benguela upwelling over the Walvis Ridge off Namibia

Abstract

Abstract. Upwelling is a significant part of the ocean circulation controlling largely the transport of cold waters to the surface and therefore influences ocean productivity and global climate. The Benguela Upwelling System (BUS) is one of the major upwelling areas in the world. Previous reconstructions of the BUS mainly focused on the onset and intensification in southern and central parts, but changes of the northern part have been rarely investigated in detail. Using the organic-walled dinoflagellate cyst record of ODP Site 1081 from the Late Miocene to the Pliocene we reconstruct and discuss the upwelling history on the Walvis Ridge with a special focus on the movement of the Angola–Benguela Front (ABF). We show that during the Late Miocene the Angola Current flowed southwards over the Walvis Ridge more frequently than today because the ABF was probably located further south as a result of a weaker meridional temperature gradient. A possible strengthening of the meridional gradient during the latest Miocene to early Pliocene in combination with uplift of south-western Africa intensified the upwelling along the coast and increased the upwelling's filaments over the Walvis Ridge. An intermediate period from 6.2 to 5.5 Ma is shown by the dominance of Habibacysta tectata, cysts of a cool-tolerant dinoflagellate known from the northern Atlantic, indicating changing oceanic conditions contemporaneous with the Messinian Salinity Crisis. From 4.4 Ma on, the upwelling signal got stronger again and waters were well-mixed and nutrient-rich. Also effects of Cunene River discharge into the South Atlantic are recorded since 4.4 Ma. Our results show a northward migration of the ABF and the initial stepwise intensification of the BUS.