Formation and preservation of authigenic pyrite in the methane dominated environment
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Dr. Lan-Feng Fan and Prof. Saulwood Lin from the Institute of Oceanography, explore authigenic pyrite formation and preservation in the methane dominated Environment. This study, published in Deep Sea Research Part I (08/2018), finds approximately 50% of sulfate were preserved in sediments as pyrite, which is 10 fold higher than that in normal marine environments. Time require for the pyrite formation is about 4400 years, based on diffusion model calculation of barium sulfate precipitation.
Sulfate reduction could go through dissimilatory sulfate reduction and anaerobic methane oxidation couple with sulfate reduction (AOM-SR) with pyrite the end product. While AOM-SR is an important process in oxidizing methane and limiting methane entering the ocean, there is limited information available regarding pyrite formation and preservation under methane dominated environment. The purpose of this study is to report pyrite formation and preservation at a methane dominated environment, the YuanAn Ridge, where methane seeps have been observed, and to evaluate how would that differ from typical anoxic environment. Pore water methane, sulfate, dissolved sulfide, barium, and sediment pyrite, barium/Al ratio and organic carbon in sediments were analyzed from sediments collected by piston cores on board the R/V Ocean Researcher I (OR-I) from the study environment.
The results showed methane flux is controlling pyrite formation in this methane dominated environment. Pyrite concentration is linearly correlated with methane flux with exceptions to shallower sulfate methane transition zone (SMTZ) sites where methane could have vent directly to the overlying water and contribute less to the pyrite formation. The more methane entering the SMTZ, the more pyrite formed and preserved in the sulfate methane transition zone sediments. Authigenic pyrite from dissimilatory sulfate reduction is a small fraction of the pyrite found in the methane dominant and low in organic carbon environment, with majority of pyrite derived from AOM-SR. Large spatial variations on rate of sulfate reduction, pyrite and methane concentrations were observed in the studied area sediments. Depth of sulfate methane transition zone varied between 1 and 14 m and is a log function of methane flux. Pore water sulfate profiles displayed three different types, linear, concave up and down, indicating methane flux have varied in time.
Pyrite burial efficiency is high, approximately 50% of sulfate entering the SMTZ were preserved in sediments as pyrite. This efficiency of sulfate reduction through AOM-SR is much higher than pyrite formation from dissimilatory sulfate reduction in normal marine sediments. The AOM-SR and pyrite formation occurred at depth within the SMTZ favor a higher degree of pyrite preservation. Time require for the pyrite formation is about 4400 years in the YAR sediments, based on diffusion model calculation of barium sulfate precipitation.
Extended reading
- Fan, L., S. Lin, C.-W. Hsu, I.-T. Tseng, and K. M. Huang. Formation and preservation of authigenic pyrite in the methane dominated environment” Deep-Sea Research Part I, 2018(8), 138, 60–71.
- Egger, M., N. Riedinger, J. M. Mogollon, and B. B. Jorgensen. Nat. Geosci., 2018, 11, 421-425.
Fig. 7. Conceptual diagram (A) of pyrite accumulation in a methane dominated environment. (B) a model diagram showing position of maximum concentrations of barium sulfate, pyrite and vertical profiles of pore water dissolved sulfate, barium and methane based on data from Station N6. Station N6, 24, 26 and 21 referring to data in text. Black arrow bar: relative magnitude of methane flux; White arrow bar in dark shaded area: relative magnitude of pyrite accumulation. Dot shaded area: Barite buildup zone. Dark shaded area: pyrite accumulation zone. Gray shaded area: lower boundary of the sulfate methane transition zone. Bubbles: methane gas seep into water column on top of the YAR. (from Fan et al., DSR I, 2018)