GROUNDWATER-FED WETANDS AS AN ARCHIVE OF MILANKOVITCH AND MILLENNIAL-SCALE CLIMATE VARIABILITY IN ARID EAST AFRICA

Springs and groundwater-fed wetlands are common in modern arid East Africa and are an integral part of hydrologic system in drylands. Although small and localized, the presence of wetlands and their changes over time appear to reflect rainfall fluctuations on both long (Milankovitch precession cycles (19-23 ka), intermediate (millennial-scales ~1500-1800 yrs) and short (50-100 yrs) decadal timescales. In the tropics, precipitation (P) varies with changes in insolation which fluctuates <8-10 % over a precession cycle. Stronger summer insolation drives a stronger summer monsoon response increasing annual rainfall. P is the main variable as mean high annual temperatures and evapo-transpiration, ET (~ 2500 mm/yr) vary little in the tropics. Groundwater discharge zones are fed from reservoirs that are less vulnerable to the vagaries of annual P fluctuations than is surface runoff. Consequently, during sustained wetter periods, regional groundwater reservoirs enlarge, the water table rises and springs and wetlands increase in number and in size compared to drier periods. The cause of the millennial and decadal climate fluctuations is not known, but may be due to changes in solar activity (sun spots) that are then amplified within the atmospheric-oceanic system producing more or less moisture. Biological proxy records (pollen, testate amoeba, ostracodes, and diatoms) indicate that wetlands are archives for long-, intermediate-, and short-term climate changes, although, local geology (lithology and structure), regional topography, and distance to recharge areas may affect the specific wetland response to a specific change of climate. Wetlands commonly form during wetter phases and shrink during dry phases of Milankovitch cycles. Examples of millennial-scale responses are: (1) development of a new swamp during a wet period (LIA) with sufficient permanent water with stable water chemistry to support diatoms, (2) expansion of swamp forest (Syzygium) and reduction of Typha vegetation during dry periods, and (3) increase in abundance and diversity of freshwater ostracode species during wet periods. Possible decadal-scale responses are revealed by records of testate amoeba and changes in proportions of Cyperaceae (wetter) and Typha (drier) pollen that appear to coincide with sun spot minima (Spörer and Maunder).