Mid-Pliocene upwelling and nutrient (Si, N) conditions in the Benguela upwelling system and their relationship to the Southern Ocean paleoceanography

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The closure of the Panama seaway for surface water exchange between about 4 and 2.6 Ma BP led to a profound reorganisation of the ocean thermohaline circulation, the advanced growth of continental ice-sheets, and probably to a more vigorous atmospheric circulation. Concomitant with these processes an extreme increase in diatom productivity and a dominance of Antarctic diatom species in the Benguela upwelling system occurred. This phenomenon is known as the Matuyama Diatom Maximum (MDM) based on results of ODP Leg 175 drill sites off Namibia, and was inferred to be a result of intensification of coastal upwelling and larger entrainment of subAntarctic mode waters. On the other hand, alkenone SST reconstructions and the δ15N signal of organic matter from these sites strongly imply that during the MDM still well stratified, warm surface waters prevailed and nutrient supply was fostered by local nitrogen fixation. This argues against a strong intensification of coastal upwelling conditions, which would have led to surface ocean cooling and subsurface nutrient supply. To resolve this controversy it is proposed to compare detailed proxy records for changes in sea surface temperatures, plankton productivity, as well as for surface ocean stratification or upwelling intensity with records documenting past changes of silicon and nitrogen supply to the Benguela system. This will allow a better understanding of the major factors leading to very high siliceous plankton productivity in eastern boundary current settings under mean states of warmer and more stratified surface water conditions than those prevailing today. The Pliocene conditions may serve as an analogue for the development of coastal upwelling settings under future global warming conditions.