Behavior of Mg isotopes during precipitation of methane-derived carbonate: Evidence from tubular seep carbonates from the South China Sea

The magnesium (Mg) isotopic composition of marine carbonate has been used as a proxy to constrain the geochemical composition of seawater and to trace the global Mg cycle. Sulfate-driven anaerobic oxidation of methane (SD-AOM) is a prominent process that facilitates the formation of marine authigenic carbonate. Yet the understanding of the impact of SD-AOM on the Mg isotope composition of authigenic carbonate under natural conditions is limited. Here we present the analysis of two tubular methane-seep carbonates from the South China Sea. The tubular carbonates reveal a trend of decreasing delta C-13 values and increasing carbonate contents from the periphery (i.e., the outer surface of the carbonate) to the inner portion (i.e., close to the inner surface of the carbonate pipes). The tubular carbonates are interpreted as former fluid conduits, indicating intense and lasting seepage that apparently increased with time. The observed variability in composition probably results from changes in the intensity of SD-AOM and correlating carbonate precipitation rates. Trends of enrichment of middle rare earth elements (MREEs), molybdenum (Mo), and uranium (U) from the periphery inward indicate more reducing formation conditions toward the inner surface of the former conduit. Along the same transects, delta Mg-26 values decrease from periphery to inner portion in one tubular carbonate (-2.63 parts per thousand to -3.24 parts per thousand), while delta Mg-26 values are close to constant for the second tubular carbonate (-3.42 parts per thousand to -3.37 parts per thousand); such patterns highlight the complexity of Mg isotope fractionation during seep carbonate formation. A positive correlation between delta Mg-26 and delta C-13 values and a negative correlation between delta Mg-26 and Mg/Ca molar ratios suggests that enhanced SD-AOM facilitates the incorporation of light Mg isotopes into the carbonate lattice. However, such assumption contradicts with the fact that higher carbonate precipitation rates reduce Mg isotope fractionation due to incomplete dehydration of Mg ions. It is hypothesized that hydrogen sulfide produced by SD-AOM may enlarge the extent of fractionation by weakening the hydration of Mg. If this interpretation is correct, the effect of sulfide on the fractionation of Mg isotopes can even offset or exceed the effect of precipitation rate. Our study indicates that SD-AOM has an effect on the Mg isotope fractionation during carbonate precipitation. Similar effects may have occurred in geological periods with more widespread carbonate authigenesis. A better understanding of the factors affecting the Mg isotopic fractionation during marine carbonate authigenesis is needed to efficiently use this archive of paleoenvironments and element cycling.

Automated summary

This study investigates the impact of sulfate-driven anaerobic oxidation of methane (SD-AOM) on the magnesium isotopic composition of marine authigenic carbonate. The analysis of tubular methane-seep carbonates from the South China Sea reveals complex variations in magnesium isotope composition, influenced by factors such as seepage intensity and carbonate precipitation rates. The findings suggest that SD-AOM plays a role in magnesium isotope fractionation during carbonate precipitation, highlighting the need for a better understanding of this process for interpreting paleoenvironmental records.