GROUNDWATER DISCHARGE DEPOSITS (GWD) AS A RECORD OF CLIMATE CHANGE IN ARID TROPICAL CONTINENTAL SETTINGS

Groundwater discharge deposits (GWD) are an important sedimentological component of the paleohydrologic system in continental settings. GWD may record the timing and magnitude of past changes in regional moisture balance in endorheic basins and thus are an additional, or alternative source, of paleohydrologic and paleoclimatic information traditionally derived only from lake records. East African Rift System (EARS) basins are mostly hydrologically closed and are thus responsive to shifts in climate, and specifically to changes in the hydrologic budget (P-ET). Milankovitch precession cycles (19-23 ka) lead to wet-dry cycles at low latitudes. Precipitation (P) varies with changes (8-10 % over a cycle) in solar insolation. Stronger isolation drives stronger summer monsoon maxima increasing P. Transpiration, ET (~ 2500-3000 mm/yr) does not vary much. Consequently during wetter periods the regional groundwater reservoirs enlarge, the water table rises and ground water-fed systems increase in number and in size compared to drier periods. Lake levels fluctuate in response to change in hydrologic budget and GWD appear to respond similarly. The records of GWD are easily identifiable: they are composed of peat and green clay deposits that contain eolian-transported mineral matter, macrophyte plant remains (e.g. roots, stems) pollen, phytoliths, diatoms, root casts, charcoal, carbonate and manganese-rich nodules.

Aquifers are recharged from areas of higher rainfall (P). Meteoric water then moves under hydraulic head to lower elevations exiting as groundwater seeps or springs at the valley bottom or along faults. In arid, tropical regions, GWD are distinct from surface run-off wetlands (river floodplains, deltas, and lacustrine marshes) as these systems are significantly affected by the relative difference of precipitation (P) vs. evapotranspiration (ET), whereas aquifer-fed systems are shielded from evaporation and are direct function of groundwater recharge (P). I present recent findings from three basins in the EARS (Baringo, Bogoria and Solai) that contain dated wetland records that can be linked to the global climate record and thus support the idea that GWD are records of climate and are a viable option to lake records as paleoclimatic indicators in arid, low-latitude (tropical) continental settings.