Evidence of climate change I the SW Chad basin from the geochemistry of unsaturated zone moisture and groundwater.
by
I.B. Goni
Coauthors: W.M. Edmunds

Conservative solutes and isotopic content of unsaturated zone moisture and groundwater can store information that could give an idea of past recharge and climatic event of an area. In the SW Chad basin, 480 mm average rainfall for the past 30 years and 8 years average Cl in rainfall of 1.77 mg/l have been used as input data to estimate recharge and climatic history of the area. The unsaturated zone Cl profiles in this region provides a record of recharge and climatic events of the past 80 years. Fluctuations in Cl concentrations in these profiles were modeled against annual rainfall records of Maiduguri (1915-2000). The best fit is however achieved using a lower average Cl (0.65 mg/l) than that obtained from field results (1.77 mg/l). Another profile (the MF profile) obtained close to lake Chad is characterised by fine sand at the top 3 m and clayey deposits at the lower section, probably representing the former bed of lake Chad. The profile shows very high Cl concentration at the lower section (below 3 m), with concentration up to 7000 mg/l at certain depth. This high Cl concentration is likely the pore waters of the former lake sediments. Enriched deuterium of -12‰ as against -30‰ for the upper section and high solute concentrations support evaporation as the likely control. Oscillation of solutes and deuterium values in this section of the profile indicates cycles of flooding of lake Chad that is probably related to changes in the past climate.

Geochemical evidence from the deep confined aquifers' water indicates that they were recharged some 20-30 ka BP. Oxygen-18 data indicate that the period is wetter than present with depleted 18O. Results from noble gases analysis show that the waters in these confined aquifers were recharged during climates that are at least 6oC cooler than present. The effective absent of groundwater of post 20 ka age ties with the evidence that the subsequent period (Kanemian) was an arid phase. Although waters from the wetter interlude during Holocene could not be accounted for, they probably have flown to the mega Chad and/or into the Pays Bas depressions in Chad.

Date received: June 22, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahr-37.