Intensity of methane seepage reflected by relative enrichment of heavy magnesium isotopes in authigenic carbonates: A case study from the South China Sea

Abstract

Changing conditions including steep redox gradients result in different Ca-Mg-carbonates forming at methane seeps as a consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM). Since Mg2+ is one of the main cations in Ca-Mg-carbonates, Mg isotope composition may reflect the mode of mineral authigenesis at seeps. Seep carbonates were collected from the Shenhu area and the Southwest (SW) Taiwan basin in the South China Sea. The carbonate phases in Shenhu samples are dolomite accompanied by accessory Mg-calcite, while SW Taiwan samples consist of Mg-calcite and dolomite. Correspondingly, Shenhu carbonates show systematically higher Mg/Ca ratios. Low δ13C values of seep carbonates confirm their derivation from the oxidation of methane, δ18O values reflect formation in equilibrium with coeval seawater. The δ26Mg value of a reference sample of biodetrital carbonate (−4.28‰) is lower than those of seep carbonates (−3.25 to −2.95‰). Since only little variability of δ26Mg values of pore waters is expected based on previous work, differences in the δ26Mg values of seep carbonates were apparently caused by changing degrees of isotopic fractionation during precipitation. Trends between δ26Mg values and Mg/Ca ratios and between δ26Mg and δ13C values suggest that Mg isotope fractionation was controlled by a kinetic mechanism affecting the incorporation of Mg2+ ions into the carbonate lattice in the course of SD-AOM. By consuming sulfate and by producing sulfide, SD-AOM reduces the energy differences for the dehydration of ions of light and heavy Mg isotopes, lowering isotope fractionation. Additionally, the two trends have been found to be steeper for Shenhu samples, suggesting more pronounced SD-AOM in the Shenhu area. Our study indicates that Mg isotope composition of methane-derived carbonates is affected by the process that drives carbonate precipitation – SD-AOM. Future work is required to confirm the utility of Mg isotopes as a new proxy for this major biogeochemical process.