Controls on subsurface methane fluxes and shallow gas formation in Baltic Sea sediment (Aarhus Bay, Denmark)

Abstract

Shallow gas accumulates in coastal marine sediments when the burial rate of reactive organic matter beneath the sulfate zone is sufficiently high and the methanogenic zone is sufficiently deep. We investigated the controls on methane production, methane accumulation, and gas bubble formation along a 400 m seismo-acoustic transect across a sharp transition from gas-free into gas-bearing sediment in Aarhus Bay (Denmark). Twelve gravity cores were taken, in which the pore water was analyzed for inorganic solutes while rates of organic carbon mineralization were measured experimentally by 35 SO 4 2radiotracer method. The thickness of organic-rich Holocene mud increased from 5 to 10 m along the transect concomitant with a shallowing of the depth of the sulfate-methane transition from >4 m to 2.5 m. In spite of drastic differences in the distribution of methane and sulfate in the sediment along the transect, there were only small differences in total mineralization, and methanogenesis was only equivalent to about 1 % of sulfate reduction. Shallow gas appeared where the mud thickness exceeded 8-9 m. Rates of methanogenesis increased along the transect as did the upward diffusive flux of methane. Interestingly, the increase in the sedimentation rate and Holocene mud thickness had only a modest direct effect on methanogenesis rates in deep sediments. This increase in methane flux, however, triggered a shallowing of the sulfate-methane transition which resulted in a large increase in methanogenesis at the top of the methanogenic zone. Thus, our results demonstrate a positive feedback mechanism that causes a strong enhancement of methanogenesis and explains the apparently abrupt appearance of gas when a threshold thickness of organic-rich mud is exceeded.

Microbial community assembly and evolution in subseafloor sediment

“…1A and SI Appendix, Table S1). Although the depth distribution of sediment age and microbial carbon mineralization rates show little variation across stations in Aarhus Bay (19), the depth of the SMT zone varies between stations due to differences in the thickness of the Holocene mud layer forming the marine sediment (19). Across the sampled sediment depths (0-7 m) the microbial cell density decreased from 10 9 to 10 7 cells·g −1 wet weight (Fig.…”

“…Differences in the number of reads mapped on SAGs do explain some of the diversity differences observed, but genuine differences among lineages remain (P = 0.016). Our generation time estimate is based on empirically determined parameters (Material and Methods); of these, cellular carbon content varies twofold (27) and sediment age by 2% (4), whereas rates of carbon mineralization were modeled from experimental data with a SD of 5% (19,28). Finally, cellular growth yields likely vary by a factor of 2-3, according to pure culture studies and subsurface sediment modeling (3,6,8).…”

“…The cores were subsampled for DNA extraction by a protocol, which removes extracellular DNA (30), and for whole-cell extraction for single-cell sorting. The biogeochemistry of the studied cores is reported elsewhere (19).…”

“…Sulfate reduction rates therefore represent the metabolic activity of the entire anaerobic microbial food chain in this zone (6,8). In marine sediments, including Aarhus Bay, sulfate reduction rates decrease with sediment depth according to a power law function that can be extrapolated below the SMT zone to reflect carbon mineralization rates in the methanogenic zone (i.e., by methanogenesis) (19,32). We therefore used such a power law function determined for Aarhus Bay sediments (19) to calculate organic carbon oxidation rates.…”

“…In marine sediments, including Aarhus Bay, sulfate reduction rates decrease with sediment depth according to a power law function that can be extrapolated below the SMT zone to reflect carbon mineralization rates in the methanogenic zone (i.e., by methanogenesis) (19,32). We therefore used such a power law function determined for Aarhus Bay sediments (19) to calculate organic carbon oxidation rates. Cell-specific carbon oxidation rates were calculated from total cell abundances and were used to estimate biomass turnover of cells assuming a growth yield of 8% (8) and a cellular carbon content of 21.5 fg (27).…”