CONTRAST ON CHLORINE STABLE ISOTOPE GEOCHEMISTRY OF COASTAL AND INLAND SHALLOW BRINE

In recent years, pore water with high salinity in shallow stratum has been payed much attention due to its attribute of salt mine resource. In China, shallow Quaternary brine in the coastal plain of Laizhou Bay and salt lake brine / intercrystalline brine in inland arid Qaidam Basin are two kinds of representative shallow brines, which are exploited to produce salt. Chlorine is a perfect representative of salinity in natural waters, the geochemistry and stable isotope of which can provide valuable insight into origin and evolution of salt and coexisting brine.

The d³⁷Cl values and related hydrochemistry were determined for shallow brines within a depth of 80 m in the south coast of Laizhou Bay, salt lake brines / intercrystalline brines in Qaidam Basin, and other waters which may have hydrologic connection with them in these two areas. The results indicate that d³⁷Cl values of shallow brines in coastal zone range from - 0.33‰ to +0.34‰, but most of them (>70%) are enriched relative to SMOC. On the contrast, salt lake brines / intercrystalline brines in inland basin exhibit a d³⁷Cl variation from -0.62‰ to -0.10‰, which are all negative. The Cl isotopic compositions of shallow brines close to SMOC in the south coast of Laizhou Bay, in conjunction with other geochemical parameters, imply that they originated from evaporated seawater. While, the relative enrichment of ³⁷Cl in these waters could be attributed to redissolution of salt precipitation, which was previously formed by surficial seawater evaporation, in tidewater or meteoric water, making more ³⁷Cl release into solution preferentially. In addition, horizontal diffusion during brine intrusion to inland and vertical ion filtration when the groundwater in lower aquifer moves towards upper aquifer might contribute to the enrichment of ³⁷Cl. The negative d³⁷Cl values in salt lake brines / intercrystalline brines can be explained by halite precipitation resulting from intensive evaporation, making more ³⁷Cl enriched in solid phase, which can be supported by positive d³⁷Cl values in coexisting halite and positive correlation between d³⁷Cl and γNa/γCl in brines. In addition, Cl stable isotopic characteristics of salt lake brines / intercrystalline brines could be influenced by recharge from deep oilfield water through fracture, the d³⁷Cl values of which are all negative.