TEMPORAL AND SPATIAL VARIATION OF SPRING RECORDS (TUFA): CALCIUM-LIMITED SYSTEMS IN VOLCANIC TERRAINS

Springs are important components of the hydrologic system in continental settings, especially in semi-arid climates like East Africa. Long-term wet-dry cycles in the low latitudes are thought to be astronomically controlled, i.e. Milankovitch precession cycles (19-23 ka) (Kutzbach, 1981). Precipitation (P) varies up to 50% with changes in solar insolation that fluctuates 8-10 % over a precession cycle (Ruddiman, 2000). Stronger insolation drives stronger summer monsoon maxima increasing P. Consequently during wetter periods the regional groundwater reservoirs enlarge; the water table rises and springs increase in number and in size compared to drier periods.

The records of springs (tufas) vary in both time and space and do not always coincide with periods of higher rainfalls. In extensional rift settings, they are commonly associated with faults (acting as conduits for groundwater) and thus the episodic occurrence of tufa is often interpreted as “tectonic”. It is not clear whether these deposits are a record of climate change (e.g., increased spring discharge linked to a wetter climate) or if they record a tectonic event (e.g. a phase of faulting that induced a change in CO2 flux or enhanced permeability). The calcium needed for carbonate deposits may well be the major limiting factor for tufa and travertine formation.

Alkaline volcanic rocks are a source of calcium in the East African Rift. Calcium sequestered in soil as cements and calcrete during dry portions of the precession cycle is partly dissolved and flushed by rising groundwater during wetter portions when weathering intensity increases, and is supplemented by increased Ca2+ and HCO3- produced by greater silicate hydolysis. Mineral records of springs will form only as long as the calcium supply lasts and thus spring deposits should not be interpreted directly as a climatic signal. Examples of tufas from several East African rift basins (Olduvai, Magadi, and Bogoria) that formed at the beginning of precession cycles (globally recognized Marine Isotopes Stages) are likely reflecting the limited supply of calcium in these volcanic terrains, as well as climate change.