Fate of rising methane bubbles in stratified waters: How much methane reaches the atmosphere?

Abstract

[1] There is growing concern about the transfer of methane originating from water bodies to the atmosphere. Methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport. This work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere. The bubble model predicts the evolving bubble size, gas composition, and rise distance and is suitable for almost all aquatic environments. The model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep, oxic, saline water with excellent results. Methane bubbles from within the hydrate stability zone (typically below $500 m water depth in the ocean) are believed to form an outer hydrate rim. To explain the subsequent slow dissolution, a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone. The calibrated model explains the impressively tall flares (>1300 m) observed in the hydrate stability zone of the Black Sea. This study suggests that only a small amount of methane reaches the surface at active seep sites in the Black Sea, and this only from very shallow water areas (<100 m). Clearly, the Black Sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere, although substantial amounts of methane may reach the surface in shallow lakes and reservoirs.

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

“…During dives with the submersible JAGO in October 2004 (as part of the EC-funded METROL project) gas bubbles at -92 m seep site were collected directly at the seafloor. The initial gas composition of the bubbles was almost pure methane (80 to 90 %) of presumed microbial origin as indicated by the isotopic composition (-62 to -68 13 C‰ PDB) (McGinnis et al, 2006). Peckmann et al (2001) assumes that the methane originates from organic-rich lacustrine sediments deposited during the Black Sea's fresh-water phase.…”

“…4.1.A.). The region is wellknown for abundant gas seeps, carbonate buildups and shallow gas (Polikarpov et al, 1989;Polikarpov et al, 1992;Egorov et al, 1998;Luth et al, 1999;Peckmann et al, 2001;Thiel et al, 2001;Amouroux et al, 2002;Michaelis et al, 2002;Kruglyakova et al, 2004;Kutas et al, 2004;Popescu et al, 2004;Pape et al, 2005;Pimenov and Ivanova, 2005;Reitner et al, 2005;Schmale et al, 2005;Treude et al, 2005;McGinnis et al, 2006;Naudts et al, 2006). During the 58 th (May-June 2003) and 60 th (MayJune 2004) cruise of R.V.…”

“…Recent studies have shown that methane transfer from marine and lacustrine seeps to the atmosphere is only effective when methane is transported by bubbles released in relatively shallow water (< 100 m water depth) (Leifer and Patro, 2002;MacDonald et al, 2002;Schmale et al, 2005;McGinnis et al, 2006). In most cases, even if a bubble reaches the surface with a significant size, most of the methane is dissolved into the water column and replaced by other stripped gases, particularly oxygen (in oxic conditions) and nitrogen.…”

“…In most cases, even if a bubble reaches the surface with a significant size, most of the methane is dissolved into the water column and replaced by other stripped gases, particularly oxygen (in oxic conditions) and nitrogen. Therefore, only a massive release of large amounts of bubbles may create a bubble plume and enable significant volumes of methane to be transferred to the atmosphere from deeperwater seeps (> 100 m water depth) (Judd, 2004;McGinnis et al, 2006;Judd and Hovland, 2007).…”

Integral models for bubble, droplet, and multiphase plume dynamics in stratification and crossflow

“…The centerline dilution, plume trajectory, the locations of intrusions, and the pathways of rising particles are common metrics these models need to predict. Since the Deepwater Horizon accident, we have been developing a modeling system for subsea oil and gas plumes based on the discrete particle model [1,2] within an integral plume model framework [3][4][5] that synthesizes modeling approaches in single-and multiphase plumes across the literature [6][7][8]. In this paper, we present this general modeling system for multiphase plumes in stratification and crossflow, its validation to laboratory and field experiments, and we discuss its predictions for canonical test cases of accidental oil-well blowouts in deep water to illustrate the complex role that thermodynamics and mass transfer often plays in multiphase plumes.…”