Origin of the δ18O and δ2H composition of meteoric waters in Ethiopia

Understanding the origin of the delta O-18 and delta H-2 content of meteoric waters is the initial step in using these isotopes in water resources, hydrological and hydro-climatic investigations. Specifically, isotopes of water are proven tools in a) constraining rate and mechanism of groundwater recharge, b) tracing movement of groundwaters, c) reconstructing past climate (rainfall amount, humidity and temperature) at various time scales, d) quantifying water flux across boundaries (e.g. evaporation rates, mixing), and e) complementing regional and global climate models. The delta O-18 and delta H-2 values in rainfall waters of Ethiopia have long been recognised as a 'regional anomaly'. Regardless of the station's high altitude (2360 m asl) and the region's low mean annual temperature (16 degrees C), two conditions that would otherwise lead to relative isotope depletion, the rains show the highest O-18 composition with no sign of effect of evaporative enrichment (as revealed by d-excess). This enrichment is also reflected in other local meteoric waters (shallow groundwaters, lakes, ambient vapour and rivers). Here, the delta O-18 and delta H-2 of 600 lake water samples, 3000 groundwater samples, and rainfall isotope data base from IAEA/WMO stations at Addis Ababa have been used to show the linkage between rainfall derivation processes and isotope signals. The major sources of moisture in Ethiopia are recycled moisture from continental sources for western and northern Ethiopia, and direct moisture from the Southern Indian Ocean for eastern lowlands of Ethiopia. Southern Ethiopia is influenced by South Indian Ocean moisture, which is characterised by relatively depleted O-18 and H-2. Spatial variation (altitude and latitude) in O-18 and H-2 is weak, with altitude effect accounting for 0.1 parts per thousand/100 m depletion in delta O-18. Detailed investigation of the relation between isotope pattern and isotope effects (amount, seasonality, temperature, etc.) shows that these effects are not pronounced. The -4.5 parts per thousand delta O-18 shift recorded in deeper groundwaters and other paleoclimate achieves (from the current wt. mean rainfall value of -0.5 parts per thousand) can only be explained by a shift in source of moisture, such as the northward penetration of the South Indian Ocean moisture into the Ethiopian highlands, or changes in evaporation conditions at the source. (C) 2011 Elsevier Ltd and INQUA. All rights reserved.