Pliocene-Quaternary upwelling in the Southeastern Atlantic may reflect changes in water mass production

Abstract

Key words: Upwelling; Pliocene; Quaternary; Atlantic; water masses; opaline silica; organic carbon. The sediments recovered at Deep Sea Drilling Project Sites 362 and 532 on Walvis Ridge Abutment Plateau and at Site 530 in the southeastern Angola Basin record long-term changes in the rates of upwelling. Deposition of opaline silica and organic carbon increased from latest Miocene to latest Pliocene then declined to present. The sediments display light-dark cycles. The dark cycles contain more terrigenous material and represent glacials. During the Late Miocene the productivity maxima were characteristic of glacial maxima in the Antarctic. Since the beginning ofthe Pliocene productivity maxima have occurred during interglacials. The most likely causes of these changes are: 1) desiccation and reflooding of the Mediterranean. The desiccation drew the ITCZ to its most northerly position. After reflooding the Mediterranean had a positive fresh-water balance until about 2.5 Ma, when it changed to its present negative balance and lagoonal circulation. The period during which productivity increased along the southwest African margin corresponds to the time when the Mediterranean had a positive fresh-water balance and estuarine circulation. During this time the Mediterranean supplied no intermediate water to the North Atlantic. The decline in productivity off southwest Africa corresponds to the time when lagoonal circulation developed in the Mediterranean and, as at present, its outflow forms a major intermediate water mass. During glacials the more dilute saline Mediterranean outflow resulted in the expansion of nutrient-poor North Atlantic Intermediate Water (NAIW) at a higher level in the ocean. The NAIW replaced AAIW in the South Atlantic during glacials. Upwelling along Southwest Africa may have increased as a result of increased wind stress, but the upwelled water was NAIW, and did not result in increased productivity. 2) growth of the Antarctic and Northern Hemisphere ice caps. During the Late Miocene growth of the Antarctic ice cap forced northward migration of the subtropical highs and Intertropical Convergence Zone (ITCZ). These changes in atmospheric circulation may have initiated productive upwelling over the Walvis Abutment Plateau. As Northern Hemisphere glaciation was initiated, the Earth changed from a unipolar to a bipolar glaciated state. This forced southward migration of the ITCZ and an increase in the intensity of the southeast trade winds. 3) closing of the Central American Straits. The resulting salinization of the North Atlantic forced increased production of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). The production of NADW resulted in nutrient export from the North Atlantic and development of the contrast between nutrient-rich southern and nutrient-poor northern intermediate and deep water masses. The combination of all these changes is probably responsible for the observed pattern of change in productivity. Hay and Brock's (1992) explanation of lessened productivity during glacials being due to upwelling of nutrient-poor NAIW rather than AAIW remains a viable hypothesis.