Oceanic methane layers: the hydrocarbon seep bubble deposition hypothesis

Abstract

Bubble plumes from hydrocarbon seeps drive upwelling flows in the water column that can disappear if the bubbles dissolve. This may lead to formation of a layer enriched in gases and substances transported by the bubbles, a process we term bubble deposition. A review of observed dissolved methane layers in the North Sea showed their existence in an area of active seeping pockmarks at a height of ∼ 20–30 m above the sea bed, well below the thermocline. To test the bubble deposition hypothesis, rising seep bubbles were simulated numerically. The model predicted a dissolution depth consistent with the observed methane layer for ∼ 2700‐µm‐radius bubbles. The model also predicted that bubbles smaller than 3400 µm dissolved subsurface, decreasing to 2000 µm for a 10‐cm s−1 upwelling flow. We speculate that this layer may be attractive to marine organisms. Although North Sea seeps are not oily, this mechanism also applies to oily bubbles from hydrocarbon seeps or a leaking undersea gas/oil pipeline. Thus bubble deposition can create a subsurface oil layer which rises far slower than either the bubble stream or droplets entrained in the stream.

Effects of Salinity on Surface Lifetime of Large Individual Bubbles

“…Deep-water seeps form large bubbles too [33]. Our result in Figure 5 implies that salinity stratification of the water column would not affect the rise velocity and gas exchange of seep bubbles rising from ocean depth.…”

First evidence of widespread active methane seepage in the Southern Ocean, off the sub-Antarctic island of South Georgia

“…The rapidity of this process strongly depends on the bubble size, the rising velocity, as well as the composition and conditions of the surrounding medium and the presence of upwelling flows 59 . Several studies have demonstrated that methane escapes the 320 bubbles well before final bubble dissolution 11,56,60,61 .…”

Compositional changes in natural gas bubble plumes: observations from the Coal Oil Point marine hydrocarbon seep field

“…Bubbles from seeps that are unable to saturate the plume water continue to lose CH 4 throughout the water column. Given enough time (sufficient depth), the bubbles may even dissolve completely (e.g., Leifer and Judd 2002).…”