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

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.

Automated summary

Recent studies have found that dolomite could potentially be used as an archive for studying ancient seawater Mg isotope values, but uncertainties remain around the factors influencing dolomite Mg isotopes. Research in a rare environment in Qatar suggests that pore water involved in dolomite formation may have higher delta Mg-26 values than seawater.