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

摘要

Recent studies have uncovered the potential of Mg isotopes (delta Mg-26) for studying past ocean chemistry, but records of such data are still scarce. Dolomite has been suggested as a promising archive for delta Mg-26 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 delta Mg-26 values from the rock record. To evaluate factors determining the delta Mg-26 of dolomite, we studied pore water and sediment from Dohat Faishakh Sabkha, Qatar-one of the rare environments where dolomite is currently forming. The delta Mg-2(6) values of the dolomite (-2.56 parts per thousand to -11.46 parts per thousand) are lower than that of seawater (-0.83 parts per thousand), whereas delta Mg-26 values of pore water (-0.71 parts per thousand to -0.14 parts per thousand) 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 parts per thousand. to -1.51 parts per thousand). The results suggest that evaporated seawater is the sole source of Mg, and isotopically light dolomite is the major sink, so that the delta Mg-2(6) of the dolomite-forming pore water is equal to or greater than that of seawater. Thus, provided that the lowest delta Mg-2(6) value among several dolomite samples is used, and the formation temperature is known, similar sabkha-type dolomites can be utilized as an archive for delta Mg-2(6) values of ancient seawater.