T172 World of Lakes

Monday, 28 October 2013
1:00 PM - 5:00 PM
Room 501

THE LINK BETWEEN LITTLE ICE AGE RAINFALL AND MODERN FRESHWATER WETLANDS IN THE EAST AFRICAN RIFT VALLEY

Paper number: T172-9
Program: Technical Sessions
Time: 2:55 PM – 3:10 PM

Authors

Abstract

Groundwater discharge (GWD) zones, i.e. springs, seeps and wetlands are a critical natural resource for humans and animals in arid regions of the East Africa Rift System (EARS). Seasonal rains and high evapotranspiration (ET) yield ephemeral rivers and saline-alkaline lakes, thus wetlands sustained by groundwater flow may be the only dependable sources of freshwater. The GWD zones also focus biological diversity supporting a wide range of flora and fauna. We summarize over a decade of paleoenvironmental and paleoclimatological research in the EARS from 4 groundwater-fed wetlands in locations spanning ~1ºN to 3ºS. Evidence is derived from microfossils (pollen, testate amoeba, and diatoms), clay mineralogy and organic carbon. Our studies demonstrate that wetlands are a valuable, but often overlooked paleoclimate archive as their initiation and persistence record the longevity of potable water in an otherwise arid region.

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.