Microbial Sulfate Reduction at Cold Seeps Based on Analysis of Carbonate Associated Sulfate

Abstract:

Microbial sulfate reduction and coupled anaerobic oxidation of methane (AOM) are the dominant biogeochemical processes occurring at cold seeps in marine settings. These processes not only support the growth of chemosynthetic communities but also promote the precipitation of authigenic carbonates. However, investigations of microbial sulfate reduction have been conducted only using porewaters or seep-related barites. The fact is that many seeps are either inactive or do not precipitate any barite minerals. Thus, little is known about the microbial sulfate reduction at these seep environments. The occurrence of authigenic carbonate has been documented at almost all cold seep sites, which provide a unique opportunity to investigate the microbial sulfate reduction using such carbonate. The presentation is focused on the concentrations and isotopic signatures of carbonate associated sulfate (CAS). The aim of the project is to determine the role of sulfate and sulfate reduction during carbonate precipitation at cold seeps. The CAS concentrations are 67-537 ppm in high-Mg calcite, 51-181 ppm in low-Mg calcite, and 116-565 in aragonite. The δ³⁴S_CAS and δ¹⁸O_CAS also vary considerably, ranging from 21.9‰ to 56.2‰ (V-CDT) and from 10.1‰ 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. 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.

Acknowledgment: This study was partially supported by NSFC (41373085).