Time series of terrigenous source elements (Al, K, Ti, Zr) from core GeoB4901-8 recovered from the deep-sea fan of the Niger River record variations in riverine sediment discharge over the past 245,000 yr. Although the flux rates of all the elements depend on physical erosion, which is mainly controlled by the extent of vegetation coverage in central Africa, element/Al ratios reflect conditions for chemical weathering in the river basin. Maximum sediment input to the ocean occurs during cold and arid periods, when precipitation intensity and associated freshwater runoff are reduced. High carbonate contents during the same periods indicate that the sediment supply has a positive effect on river-induced marine productivity. In general, variations in the terrestrial signals contain a strong precessional component in tune with changes in low-latitude solar radiation. However, the terrestrial signal lags the insolation signal by several thousand years. K/Al, Ti/Al, and Zr/Al records reveal that African monsoonal precipitation depends on high-latitude forcing. We attribute the shift between insolation cycle and river discharge to the frequently reported nonlinear response of African climate to primary orbital configurations, which may be caused by a complex interaction of the secondary control parameters, such as surface albedo and/or thermohaline circulation.

The olfactory system is a unique part of the central nervous system since it retains neuronal turnover and regenerative capacities in adulthood. Thus it provides an ideal model to study plasticity of membrane moities involved in cell-cell interactions. One structure particularly involved in cell-cell interaction is the tight junction, which establishes polarization of epithelial cells and creates diffusion barriers to paracellular passages. ZO-1 is a phosphoprotein peripherally associated with tight junctions. We have studied expression of ZO-1 protein in the developing and adult olfactory system of the mouse in order to get information about the localization and developmental expression of this tight junction component. ZO-1 expression has also been determined in cell cultures of olfactory bulbs.ZO-1 was present in the olfactory placode prior to formation of tight junctions. ZO-1 was localized in the developing and mature olfactory epithelium at heterotypic contacts between supporting cells and olfactory neurons as well as at homotypic contacts between both these cell types. Confocal microscopy showed quantitative differences in the ZO-1 expression among different olfactory dendrites. In the olfactory nerves ZO-1 immunolabeling was detectable between olfactory ensheathing cells. From the seventh postnatal day ZO-1 immunolabeling was detected at the mitral cell layer of the bulb on cells tentatively identified as oligodendrocytes. Myelinated tracts of the bulb were ZO-1 negative. Cell cultures of olfactory bulbs showed ZO-1 immunoreaction, mostly localized on glial fibrillary acidic protein (GFAP)-positive cells. Our results provide further evidence that ZO-1 serves functions unrelated to the tight junction complex and indicate molecular heterogeneity of these cell-cell contacts. (C) 1994 Wiley-Liss, Inc.