Reconstruction of Seawater Mg Isotopes and Its Implications for the "dolomite Problem": Insights from Late Ediacaran Massive Dolomite Deposition

The “dolomite problem” is a long-standing enigma in geology. The seawater chemistry and the abundance of marine dolomite during the Late Ediacaran were significantly different from those of the modern ocean. Reconstructing the Mg isotopic composition and chemical conditions of Late Ediacaran seawater and comparing them with modern seawater may provide important insights into solving the “dolomite problem”. This study systematically analyzes the C-O-Sr-Mg isotopes, petrography, and trace element characteristics of Late Ediacaran massive dolomite from the Upper Yangtze Platform, South China. It is found that the Late Ediacaran dolomite effectively preserves primary sedimentary signatures, with its δ²⁶Mg values remaining unaltered by calcite mixing, terrigenous detritus, or diagenetic alteration. The Late Ediacaran massive dolomite is interpreted as large-scale, syn-depositional dolomite induced by microbial activity, serving as a valid archive for reconstructing the Mg isotopic composition of Late Ediacaran seawater. The reconstructed δ²⁶Mg values of Late Ediacaran seawater range from -0.45‰ to 0.14‰, which are higher than that of modern seawater (-0.83‰). Further modeling confirms that intense silicate weathering, oceanic anoxia, and prolific anaerobic microbes created seawater chemical conditions characterized by high Mg/Ca ratios and low SO₄²⁻ concentrations. These conditions were crucial for the widespread syn-depositional massive dolomite formation during the Late Ediacaran and, by extension, the Precambrian. In contrast, weaker silicate weathering, ocean oxygenation, and the decline of anaerobic microbes have resulted in modern oceans having biological-seawater chemical conditions—namely, low anaerobic microbial activity and high SO₄²⁻ concentrations—that are unfavorable for large-scale, syn-depositional massive dolomite formation. This study not only provides crucial geological evidence for understanding paleo-seawater chemistry but also offers new insights into unraveling the “dolomite problem”.