著者

本山 功

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.121, no.3, pp.478-492, 2012-06-25 (Released:2012-07-09)

参考文献数

50

被引用文献数

2 1

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The Benguela Current is an eastern boundary current in the South Atlantic subtropical gyre, associated with strong coastal upwelling off Namibia, which plays a major role in heat transport from the Indian Ocean to the Atlantic Ocean through the Agulhas Current and in the global carbon cycle through high biologic productivity. Seven sedimentary cores (Sites 1081–1087) recovered during the Ocean Drilling Program Leg 175 cruise from continental slopes consist of upper Cenozoic continuous siliceous and calcareous hemipelagic sequences, which allowed high-resolution paleoceanographic analyses over the upwelling region from 20°S to 30°S. Onboard and post-cruise scientific research has revealed and discussed the history of the oceanic system and the relation to global climate changes as follows. (1) Calcareous sequences (Sites 1085 and 1087) in the southern part of the region record sedimentary inprints that can be correlated with the Miocene global carbonate crash events and latest Miocene to early Pliocene biogenic bloom with the first signal of wind-driven upwelling at 11.2 Ma. (2) Off Namibia diatom concentrations dramatically increased after 3.1 Ma and reached a maximum spanning from 2.6 to 2.0 Ma, which was called the Matuyama Diatom Maximum associated with a moderate increase in organic matter accumulation and lowering of sea-surface temperature. This elevated bioproductivity and cooling occurred in response to changes in water circulation caused by gateway closures and enhanced bipolar glaciation. During the last 2 million years, the decreasing trend of diatom deposition coincided with an overall increase of coastal upwelling intensity. (3) The Walvis Opal Paradox is another prominent feature observed in orbitally controlled climate cycles in the upwelling system during the Quaternary. It is characterized by a decrease of diatom/opal deposition, which coincided with increased upwelling during glacial periods and vice versa during interglacials. Its possible causes include waning of North Atlantic Deep Water production during glacials. Despite these great advances in the reconstruction of the evolution of the Benguela Current upwelling system, causal links to global climate and regional events in other oceans are less well understood. To evaluate the interplay between opal/organic carbon deposition in the upwelling system and a series of climatic, tectonic, oceanographic, and biologic events in the world ocean, a better understanding of sedimentary processes on shelves and slopes in terms of glacio-eustatic sea level changes, improvement of paleoproductivity reconstructions, and reevaluation of dissolution of siliceous microfossil shells is needed.