GROUNDWATER RECHARGE AND PALAEOCLIMATE IN THE SIRTE AND KUFRA BASINS, LIBYA

Stable and radio-isotope results (C, H, O) for groundwaters from the Sirte and northern Kufra basins are used to determine the recharge history during the Holocene and late Pleistocene. Radiocarbon ages have been corrected on the basis of their stable carbon isotope ratios and on environmental samples from the areas, and two groups may be recognised: (1) low 14C activity groundwaters (13000-34000 yr. BP) with δ 13C-5.6 to -11.7‰; and (2) higher 14C activity groundwaters (5000-7800 yr. BP) enriched in 13C up to δ 13C = -3.2‰. There is a general correlation of age with depth. A well defined freshwater (< 50 mg/l Cl-) channel can be traced within the aquifer for some 130 km through the region, which is considered to represent recharge from a former wadi. This water with an age of ± 7800 yr. BP is chemically and isotopically distinct from the regional groundwaters and provides direct evidence of a significant recharge event during the Holocene. The stable isotope (O and H) composition of groundwater from the Kufra and Sirte basins are all related by an evaporative line with slope δ D = 4.5δ 13O - 35 with an intercept on the meteoric line of -11‰. This suggests a recharge source continuing into the Holocene from air masses, analogous to current heavy monsoon rain derived from south of the Sahara. The spatial and temporal distribution of groundwaters in relation to the evaporative line suggests a progressive change in character of the recharge which is controlled by a shift towards strongly convective rainfall during the Holocene. The direct hydrogeological and geochemical evidence supports climatic models proposed by several workers in which discrete humid episodes during the Holocene are inferred. © 1979.