A Mid-Brunhes Climatic Event: Long-Term Changes in Global Atmosphere and Ocean Circulation
ON A TIME SCALE OF 1 10(6) years, climatic development in the Late Cenozoic is primarily related to plate movements affecting the geometry of the continents and the ocean basins. On a much shorter term, 1 10(4) to 1 10(5) years, many climatic records indicate a dominance of glacial and interglacial fluctuations that are relatable to orbital cycles. In this report we present evidence of a global climatic change 4.0 10(5) to 3.0 10(5) years ago on a time scale of 1 10(5) to 1 10(6) years which is superimposed on the glacial and interglacial cycles. Unlike other Late Cenozoic climatic variations reported so far, the change shows opposite trends in the Northern and Southern hemispheres.
Results from multidisciplinary studies of piston cores from the Zaire (Congo) deepsea fan of the eastern Angola Basin (location 1 in Fig. 1) and the Canary Basin (location 2) show long-term environmental changes occurring in the mid-Brunhes (1, 2).
Angola Basin. In the Zaire deep-sea fan this change is reflected by a distinct break in sediment accumulation rates and mineralogical composition. During glacial periods, terrigenous accumulation rates were high (1, 3) and the Zaire River transported mainly high-crystalline smectites toward the ocean floor (4). These are signals of aridification in tropical Africa and desertification of the Congo and Angola coastal zone (1, 3, 4). During interglacials, the tropical rain forest expanded and the Zaire River supplied less sediment (1, 3). In these periods the clay-mineral associations were dominated by kaolinite and low-crystalline smectites, which are weathering products from igneous rocks in the tropical rain forest (4).
From 4.0 10(5) to 3.5 10(5) years ago there was a decrease in the rates of terrigenious accumulation (1) and in the supply of high-crystalline smectites (4) (Fig. 2). The decreasing accumulation was recorded in all six piston cores that contain sediments of this age. This fan-wide event is recognizable either by the decrease in accumulation rates and the disappearance of silt and mica from the sediments (Fig. 2) or by the extinction of a large-scale turbidite sequence (1). Decreases in the accumulation rate of terrigenous matter and of high-crystalline smectite reflect a long-term change from arid to more humid conditions in equatorial Africa.
Canary Basin. In all cores from the Canary Basin reaching back to over 3 10(5) years ago, a change in planktonic foraminiferal composition is observed. Between 3.7 10(5) and 3.1 10(5) years ago, faunal assemblages changed and tropical and subtropical forms such as Pulleniatina spp. and Globorotalia menardii/tumida became absent in the sediments in the central part of the basin (30| to 33|N, 24| to 25|W) (Fig. 3) (5). At present these species have their northern limit at about 28|N (6). More to the west, however, near 30|W, tropical and subtropical foraminifera do persist after 3.0 10(5) years ago (7). Apparently, in the central and eastern part of the basin a southward migration of the interglacial subtropical front took place over a...
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