Effects of sulphate reduction and geogenic CO2 incorporation on the determination of 14C groundwater ages – a case study of the Palaeogene groundwater system in north-eastern Syria

Abstract

Groundwater from the Palaeogene aquifer system in north-eastern Syria has been studied using chemical and isotopic methods to determine the effects of carbonate dissolution, sulphate reduction and geogenic CO2 incorporation in the dilution of 14C activity, and later to correct the conventional 14C water ages. The reason for this non-classical approach of correction is because the groundwater in this karstified and Nummulitic carbonate aquifer occurs under confined and partly confined conditions, and is located in an area very close to deep faults and fractures. Furthermore, the interconnection with the Upper Cretaceous formations, which commonly contain gypsum and hydrocarbons, can facilitate the processes of sulphate reduction and geogenic CO2 incorporation, which should not be excluded. The dilution factor related to carbonate dissolution was estimated to be about 0.60–0.75. The dilution factor associated with sulphate reduction, which only depends on H2S content, was rather low (about 0.95). However, as a result of the local tectonic setting in this area, the influence of geogenic CO2 incorporation was clearly high. The dilution factor associated with this effect ranges between 0.24–0.64. Consequently, the corrected 14C ages are considerably reduced compared with those determined by classical models. Accordingly, the groundwater in the study area can be divided into three main groups: (1) fresh, shallow and cold water of less than 1 ka age; (2) brackish, deep and thermal water of rather old age (10.9–12.3 ka B.P.); and (3) an admixed groundwater of intermediate quality and age (1.9–6.7 ka B.P.).