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Early diagenetic dolomitization in methane seep carbonates constrained by coupled Mo and Mg isotopes
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冯
DOI：10.1016/j.gca.2026.07.015.png)
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Abstract
En 中文
Marine carbonates are widely regarded as excellent archives for recording the geochemical evolution of seawater. However, the long-standing “dolomite problem” has hindered the reconstruction of formation mechanisms and depositional settings of the widespread occurrences of dolomite throughout Earth’s history. Cold seeps, where sulfate-driven anaerobic oxidation of methane (SD-AOM) promotes the formation of dolomite, are key to deciphering low-temperature dolomitization processes. Here, we investigate late Miocene to early Pleistocene seep dolomite from Chiahsien, Taiwan, together with modern seep dolomite from the Gulf of Mexico and South China Sea, using a coupled molybdenum (Mo) and magnesium (Mg) isotope approach to constrain the dolomitization environment and processes within seep systems. The wide range of authigenic Mo isotope compositions (δ98Moauth: 0.29 to 2.94‰), correlating with Fe/Al ratios, tracks changes of methane flux spanning from diffusion-dominated (enrichment of heavy Mo) to oxide-shuttle-dominated (enrichment of light Mo) regimes. This contrasts with the homogeneous δ26Mg values of seep dolomite (−2.72 ± 0.21‰), reflecting equilibration with seawater during dolomitization in a shallow, porewater environment with high replenishment of seawater Mg. The Chiahsien seep dolomite further displays higher crystallographic ordering expressed as I(015)/I(110), generally lower Sr/(Mg + Ca) ratios, and more euhedral crystals than modern seep dolomite. Taken together, our new observations suggest that precursor carbonate minerals (aragonite, calcite) were dolomitized during early diagenesis at shallow depth. The lack of Rayleigh fractionation effects of Mg isotope distinguishes seep dolomite from dolomite formed in more restricted, deeper settings, such as environments dominated by organic sulfate reduction and methanogenesis. This study demonstrates that combined Mo–Mg isotope systematics can effectively decipher the redox history and the process of dolomitization. Furthermore, it suggests that seep dolomite, which tends to form in open, seawater-buffered porewater environments, is a promising archive for reconstructing the Mg isotope composition of ancient seawater.
Keywords:
Dolomitization
Mg isotopes
Mo isotopes
Seep carbonate
Early diagenesis
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