EVIDENCE FROM DOLOMITE MG ISOTOPES REVEAL MG CYCLE IN PALEOPROTEROZOIC OCEAN

As one of the main components of seawater, magnesium isotopes have a relatively stable and uniform distribution in modern oceans. Negative isotopic fractionation of Mg occurred during the formation of dolomite in seawater, causing Mg isotopes in dolomite generally lighter than the isotopic value of seawater, except for evaporites. In this study, we collected Mg isotope values of massive dolomite of different geological ages and tested Mg isotope values in Proterozoic dolomite, and found that the Mg isotope values in dolomite tend to be lighter since Mesoproterozoic. After considering the influence of temperature, water mobility and sedimentation rate during diagenesis, we propose that this continuous negative drift indicates the shift of magnesium isotopes in seawater. Mg in the ocean mainly sources from river input, and is mainly consumed by dolomite precipitation and hydrothermal activity. In the modern ocean, mass and isotopic balance between Mg source and sink has been achieved. Assuming that the intensity of hydrothermal activity kept stable, comparing the Mg cycle of Proterozoic oceans with modern oceans, we propose that the negative drift of Mg isotope in seawater is caused by the changes in the same direction of Mg input by rivers. In the modern terrestrial weathering system, the amount of Mg from silicate and carbonate in river input is basically equivalent, while Mg from silicate is heavier. Therefore, the heavier continental margin input of the Proterozoic earth may reflect the difference of weathering intensity between different types of rocks, and further reflect the change of the proportion of silicate rocks and carbonate rocks on the continental surface of the Proterozoic earth. This proportion may profoundly affect the early Earth's climate system, and then influence the survival and evolution of life.