Magnesium isotope compositions of seep carbonates fingerprint dolomite formation pathways

Dolomite is a common yet poorly understood constituent of modern seep carbonates. To decipher the fundamental controls governing its formation, we investigate authigenic carbonates from Dongsha and Shenhu areas of the South China Sea using magnesium isotopes (δ²⁶Mg) combined with mineralogy, rare earth elements, δ¹³C, and δ¹⁸O. Dongsha samples mainly comprise Mg calcite precipitating under dynamic redox conditions and exhibit broad δ²⁶Mg variability (−3.77 ‰ to −1.46 ‰). The δ²⁶Mg values are positively correlated with δ¹³C values and negatively correlated with δ¹⁸O, Ce anomalies, and Mg/Ca ratios, reflecting kinetic Mg isotope fractionation during rapid incorporation of seawater-derived Mg enhanced by intense sulfate-driven anaerobic oxidation of methane (SD-AOM). In contrast, Shenhu samples are dominated by ordered dolomite formed under persistently reducing conditions. The low δ²⁶Mg values with narrow range (−3.60 ‰ ∼ −3.16 ‰) are close to high-Mg calcite containing weakly ordered dolomite, implying a deeper, closed-system porewater Mg source and near-equilibrium precipitation during ordering. The contrasting δ²⁶Mg signatures coupled with a paragenetic sequence of Mg calcite and dolomite support a two-step formation pathway for seep dolomite: (1) precipitation of initial Mg calcite precursors, followed by (2) cation ordering development. Sustained SD-AOM activity in locally restricted environments maintains sulfide-rich, supersaturation conditions that favoring ordered dolomite formation at seeps.