Labelling of precipitation by stable isotopes ( 18O, 2H) over the Jos Plateau and the surrounding plains (north-central Nigeria)

Abstract

Stable isotopes oxygen-18 and deuterium were studied in precipitation at five meteorological stations on the Jos Plateau and the surrounding plains. Rainwater is progressively depleted in heavy isotopes from the beginning of the rainy season towards the peak of the season, with the most depleted values being recorded in the August rains. Isotopic variation with altitude is very evident between the stations on the plateau and that on the plain. The isotope-altitude gradient is probably concealed by the mass-effect of the Jos Plateau. A Local Meteoric Line (LML) of the type ϖD=7.8 ϖ 18O+10.6 was obtained with all the available isotopic data. When only the data for the rain for the months of June through August are used, a relation ϖD=8.4 ϖ 18O+14, close to the Global Meteoric Water line but with a higher deuterium excess (d) is obtained. This suggests a contribution of continental vapour mass to precipitation over the region. On the basis of the stable isotope relationships, three categories of recharging rainwater are defined, which may have implications in groundwater tracing: 1. i) precipitation of the early and perhaps late rainy season, enriched in heavy isotopes 2. ii) precipitation of the advanced stage of the rainy season, marked by non-evaporated rains (slope 8, d > 10) 3. iii) precipitation of the peak of the rainy season (August, September), depleted in heavy isotopes.