Groundwater flow system as an archive of palaeotemperature: Noble gas, radiocarbon, stable isotope and geochemical study in the Pannonian Basin, Hungary

Research highlights: → NGTs show a 9.1 deg. C difference between the Last Glacial Maximum and the Holocene in Hungary. → The 9.1 deg. C difference led to different C chemistry and 14C0 in the recharging waters. → The elevated crustal He at the end of the flow path is attributed to presence of old formation water. → Correction of 14C ages requires a more detailed knowledge about the hydrology. - Abstract: To establish the increase in temperature and the time span of the transition between the Late Glacial Maximum (LGM) and the Holocene, the noble gas content, 18O, 2H, 13C δ values, 3H and 14C activity and chemistry were studied in a groundwater flow system in Quaternary sediments in Hungary. The study area is a sub-basin of the Pannonian Basin, where the C isotope ratios are not influenced by carbonate reactions along the flow path, because the only water-rock interaction is ion exchange. The δ18O and δ2H values indicate a cold infiltration period, followed by warming, and, finally, warm temperature conditions. The noble gas data show that the average infiltration temperature was 3.3 deg. C in the cold, 12.9 deg. C in the warm, and intermediate in the transitional stage. Using the noble gas temperatures, geochemical batch modelling was performed to simulate the chemical processes. Based on the geochemical model, δ13C and 14C0 (initial radiocarbon activity) in the recharging water were calculated. Transport modelling was used to simulate the distribution of chemical components, δ18O, δ2H and 14C0, along the flow path. It was found that the main processes determining the chemical composition of the groundwater were dissolution/precipitation of calcite and dolomite during infiltration near the surface, and ion exchange along the flow path. In the recharge area the δ13C and 14C0 were controlled by dissolution and precipitation of carbonate minerals, C speciation, and fractionation processes. All these processes were influenced by the recharge temperature. NGTs calculated from the dissolved noble gas concentrations showed an average of 3.3 deg. C for cold, and 12.9 deg. C for warm infiltration, i.e. for the LGM and for the Holocene. The temperature difference was thus 9.1 ± 0.8 deg. C, which is one of the largest degree of warming detected by noble gases so far. The alkalinity indicates that carbonate reactions were unimportant along the flow path. Owing to the temperature dependence of the equilibrium constants, temperature conditions during infiltration have to be taken into consideration in radiocarbon age calculation. Dispersive transport along the flow path modified the chemical and isotopic composition of infiltrated water. The contribution of the old pore water, which was free of the 14C isotope, resulted in uncertainties in radiocarbon age determination. It was concluded that determination of the radiocarbon age or mean residence time requires detailed knowledge of the hydraulic conditions of groundwater.