Sabkha dolomite as an archive for the magnesium isotope composition of seawater

Recent studies have uncovered the potential of Mg isotopes (δ²⁶Mg) for studying past ocean chemistry, but records of such data are still scarce. Dolomite has been suggested as a promising archive for δ²⁶Mg of seawater. However, its enigmatic formation mechanism and the difficulty in precipitating dolomite in the laboratory at surface temperatures decrease confidence in the interpretation of δ²⁶Mg values from the rock record. To evaluate factors determining the δ²⁶Mg of dolomite, we studied pore water and sediment from Dohat Faishakh Sabkha, Qatar— one of the rare environments where dolomite is currently forming. The δ²⁶Mg values of the dolomite (−2.56‰ to −1.46‰) are lower than that of seawater (−0.83‰), whereas δ26Mg values of pore water (−0.71‰ to −0.14‰) are higher. The isotope fractionation accompanying dolomite formation is generally in accordance with an empirical fractionation from the literature, extrapolated to the sabkha’s temperature (−1.84‰ to −1.51‰). The results suggest that evaporated seawater is the sole source of Mg, and isotopically light dolomite is the major sink, so that the δ²⁶Mg of the dolomite-forming pore water is equal to or greater than that of seawater. Thus, provided that the lowest δ²⁶Mg value among several dolomite samples is used, and the formation temperature is known, similar sabkha-type dolomites can be utilized as an archive for δ²⁶Mg values of ancient seawater.