Chlorine isotopic constraint on contrastive genesis of representative coastal and inland shallow brine in China

• Shallow brine in coastal zone and inland basin were contrasted based on Cl isotope.
• Brine in coastal zone experienced evaporation formerly and salt dissolution later.
• Brine in inland basin experienced salt dissolution formerly and evaporation later.
• δ37Cl and δ81Br of shallow brine were reverse in both coastal zone and inland basin.

There are great scientific interests to reveal the shallow brine genesis in different geologic environments and also practical significance for resource production and environmental protection, which have not yet gained much attention, however. This study took shallow Quaternary brine in the coastal plain of Laizhou Bay and salt lake brine/intercrystalline brine in arid inland Qaidam Basin in China as two kinds of representative shallow brines, and comparatively analyzed the Cl stable isotopic signatures, coupled with hydrochemistry and water isotopes, aiming at finding out the salinity origin of shallow brines in the coast and inland basin, and further discussing Cl stable isotopic behavior in actual hydrogeological systems. The results indicated that shallow brine in the coastal zone originated from evaporation and precipitation formerly, and halite dissolution by tidewater or rainwater later, as a result of which, most of shallow brine exhibited enriched Cl isotopic signatures. As a contrast in arid inland basin, superficial brine originated from the dissolution of precipitated salt minerals by river water formerly, and intensive evaporation with subsequent precipitation in basin center later, for which all the brines finally exhibited the depleted Cl isotopic signatures. In addition, diffusion and ion filtration in the coast zone, and inflow of deep oilfield water in inland basin, were proposed as potential processes contributing to respective Cl isotopic signatures. In the coast, 37Cl are enriched and 81Br are depleted in most of shallow brines, while all the superficial brine (saline water) samples in arid inland basin exhibit negative δ37Cl and positive δ81Br values. Precipitation and dissolution of salt minerals were believed to contribute primarily to the contrary isotopic signatures, which would not change the Br isotopic compositions a lot for the solution in which the original Br concentration has been relatively high, which is not the case for Cl isotope, however.