Behavior of Carbonate Associated Sulfate During Authigenic Carbonates Precipitation at Cold Seeps

The carbonate associated sulfate (CAS) has been widely used in investigating geochemistry of ancient seawater sulfate. The reliability of CAS as a proxy of contemporaneous seawater sulfate has been examined in multiple cases and the results have been somewhat assuring in most cases involving open ocean deposits. Many geological carbonate deposits, however, were the product of early diagenesis, the CAS behavior in them, especially among different carbonate mineral phases that are sensitive to microbial activity and pore-water chemistry have not been examined. Distinct mineral phases are occurring among modern cold-seep carbonates in the Gulf of Mexico, which provides us a unique opportunity to examine the relationship between mineral formation condition and the CAS within. We found that the CAS concentration in different minerals varies widely without a clear pattern. The δ³⁴S_CAS and δ¹⁸O_CAS also vary considerably, ranging from 21.9% to 56.2% (V-CDT) and from 10.5% to 24.8% (V-SMOW), respectively. On δ³⁴S_CAS versus δ¹⁸O_CAS plots, both aragonite and calcite show linear trends that project down toward those of open seawater sulfate. The trends suggest that sulfate has been isotopically modified to various degrees in pore fluids before being incorporated into carbonate lattice. The much narrower δ³⁴S_CAS and δ¹⁸O_CAS ranges for aragonite than for calcite suggests a much "pickier" condition for aragonite formation during early diagenesis, which is consistent with the fact that aragonite is more prone to precipitate in high sulfate concentration environments. Our results suggest that concentration and isotopic composition of CAS in seep carbonates may be controlled by the supply of pore-water sulfate during carbonate precipitation. The reliability of CAS in carbonate of early diagenetic origin as a proxy of contemporaneous seawater sulfate is therefore questioned.