Boron isotope geochemistry of a brine-carbonate system in the Qaidam Basin, western China

The boron (B) isotope-pH proxy for marine carbonates has been widely used to reconstruct past ocean pH values. Unlike seawater, the apparent dissociation constant of boric acid (K_B) and the δ¹¹B values of brines from the terrestrial salt lakes in western China vary. However, relevant studies concerning the B isotope geochemistry of brine-carbonate systems in terrestrial salt lakes are limited. In this study, thirty-nine clastic samples from a sediment core (ISL1A) from the Qarhan Salt Lake in the Qaidam Basin were collected. These carbonates were analyzed for δ¹¹B, B, Mg, Sr and Ca. Using our new data as well as published pH values, total dissolved solids (TDS), element concentrations, and δ¹¹B values of brines in western China, we reached the following conclusions. (1) During evaporation process, salinity increases, the δ¹¹B values of brines in salt lakes increase, and the pK_B (-logK_B) and pH values simultaneously decrease. (2) The large heterogeneity in the δ¹¹B values of the carbonates (-2.74‰ to +7.64‰) from core ISL1A is mainly due to the δ¹¹B values of the brines, rather than pK_B and pH. (3) B isotopic fractionation with a small α_{carbonate-brine} of 0.997 in a brine-carbonate system conforms to the Rayleigh fractionation model. (4) Our comparison of the δ¹¹B values of these carbonates with other records from core ISL1A suggests that the δ¹¹B values of the carbonates in the arid Qaidam Basin can be used as a new proxy for the salinity of paleo-lake water.