Rainfall geochemistry in the Sahel region of northern Nigeria

Chemical data and stable isotope (delta O-18, delta H-2) results are presen ted for monsoon rains for several years during the 1990s from northern Nige ria in the Sahel region of Africa. The isotopic data from Garin Alkali (Nig eria) are related by a line delta H-2 = 6.33 delta O-18 + 9.9 with a weight ed mean value of - 3.6 parts per thousand for delta O-18. The heaviest rain s have the lightest isotopic compositions, the lighter rains' enrichment is as a result of convection. The mean 1992 concentrations of Cl in rain rang ed from 1.3 to 2.8 mg l(-1) for the two stations in Nigeria. The early rain s have higher Cl than the later events although Cl accumulations are in gen eral directly related to rainfall amount. The Br/Cl ratios of all rains are enriched above marine values, which may in part be attributed to a prefere ntial concentration of Br in smaller size particles, although more likely, is related to release from the biomass as the air masses pass over vegetate d areas. The high Br/Cl ratios rule out dust from halite sources during the monsoon. The ratios of Na and CI are similar to those in sea water, althou gh all other elements (especially Ca, SO4, NO3 and K) are enriched relative to marine aerosols and indicate continental sources. The element ratios (C a/SO4); K/Mg; K/Na) are remarkably similar to those in ash leachates from t ropical vegetation and this is proposed as the main solute source in the pr esent day monsoon rains, reinforcing the evidence of Br/Cl ratios. The chem ical results show the considerable terrestrial influence and are in line wi th isotopic evidence, which demonstrates considerable modification by conve ctive circulation and continental influence as the monsoon air masses track northwards over the Sahel. (C) 2001 NERC. Published by Elsevier Science Lt d. All rights reserved.