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

Abstract 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 δ 11 B 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 δ 11 B, B, Mg, Sr and Ca. Using our new data as well as published pH values, total dissolved solids (TDS), element concentrations, and δ 11 B values of brines in western China, we reached the following conclusions. (1) During evaporation process, salinity increases, the δ 11 B values of brines in salt lakes increase, and the pK B (−logK B ) and pH values simultaneously decrease. (2) The large heterogeneity in the δ 11 B values of the carbonates (−2.74‰ to +7.64‰) from core ISL1A is mainly due to the δ 11 B 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 δ 11 B values of these carbonates with other records from core ISL1A suggests that the δ 11 B values of the carbonates in the arid Qaidam Basin can be used as a new proxy for the salinity of paleo-lake water.