THE LINK BETWEEN LITTLE ICE AGE RAINFALL AND MODERN FRESHWATER WETLANDS IN THE EAST AFRICAN RIFT VALLEY
The GWD wetlands differ in size from meters to kilometers and reflect varied lake margin environments (deltaic, alluvial fan and fault-related) and different hydrologic settings (gravity-fed and artesian). Multiple sediment cores were collected at each site. All the sites exhibit similar basal stratigraphies, with inorganic sediments overlain by organic rich wetland deposits. AMS radiocarbon dates of this contact zone indicate that wetland inception at each site occurred within a narrow window of recent geologic time known as the Little Ice Age (1400 to 1850 A.D.; LIA). It appears that the LIA was characterized by a time of higher precipitation and/or lower ET producing a positive hydrologic balance within this tropical region. A proposed physical model is that the rainfall trapped on topographic highs (fault blocks, volcanoes) moves slowly, i.e. m/yr, in the subsurface into adjacent low areas. Although the LIA rainwater is still nourishing modern GWD areas, the resource is not expected to be permanent. Our data differ significantly from studies undertaken to the west in Uganda and the Congo, where a negative hydrologic balance occurred during the LIA. Whereas, the atmospheric dynamics causing the disparity are not yet understood, interactions between the Intertropical Convergence Zone and the Congo Air Boundary are likely involved.