Release of multiple bubbles from cohesive sediments

Abstract

[1] Methane is a strong greenhouse gas, and marine and wetland sediments constitute significant sources to the atmosphere. This flux is dominated by the release of bubbles, and quantitative prediction of this bubble flux has been elusive because of the lack of a mechanistic model. Our previous work has shown that sediments behave as elastic fracturing solids during bubble growth and rise. We now further argue that bubbles can open previously formed, partially annealed, rise tracts (fractures) and that this mechanism can account for the observed preferential release at low tides in marine settings. When this mechanical model is applied to data from Cape Lookout Bight, NC (USA), the results indicate that methanogenic bubbles released at this site do indeed follow previously formed rise tracts and that the calculated release rates are entirely consistent with the rise of multiple bubbles on tidal time scales. Our model forms a basis for making predictions of future bubble fluxes from warming sediments under the influence of climate change. Citation:

Methane bubble growth in fine-grained muddy aquatic sediment: Insight from modeling

“…Therefore, to agree with the representative equivalent radii of the bubbles measured by Wilkens and Richardson (1998), Anderson et al (1998), Abegg and Anderson (1997), the critical SIF must be notably smaller than 200 Pa m 3/2 suggested in the literature and used in this study. In addition, in nature when the growth of bubbles occurs in sediments with a net of open or temporally closed ducts of low SIF (Chanton et al, 1989;Martens and Klump, 1980;Algar et al, 2011a), the bubbles will grow and rise at significantly lower terminal sizes than those obtained in our simulations.…”

“…The observed preferential bubbles release under the low tides or water level (Chanton et al, 1989;Martens and Klump, 1980;Algar et al, 2011a) points onto the long-time fractures existence. Permeability evaluation of the fractured muddy sediment still remains scarce.…”

Size Does Matter: Importance of Large Bubbles and Small-Scale Hot Spots for Methane Transport

“…Of course, sediment properties such as cohesiveness also play a role in bubble release, but perhaps in a more unexpected manner as existing fractures may allow for an easier bubble transit through the sediments. 35 Ultimately, the increase in WMD with depth that we observed in Lake Wohlen falls within the range of increasing bubble volume with increasing pressure, which in turn likely impacted the observed depth-flux relationship or lack thereof in Lake Wohlen. Since the SMD takes into account the disproportionate volume contribution from larger bubbles, Figure S1 in the Supporting Information for true depths).…”

The physics of bubbles in surficial, soft, cohesive sediments

“…If pressure changes have little effect on initial bubbles, this is not true of subsequent bubbles. Algar et al (2011b) use this latter concept to model bubble release at the CBL site and find that the release rates and the bubble size distribution are well predicted e see Figure 16. Scandella et al (2011) further advance that drops in hydrostatic pressure will cause pre-existing bubble conduits to re-open.…”