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Anatomy of a Plume (Cold Seep)

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The interplay between seep structure, tides, hydrate formation and temperatures results in seep emissions that are complex and variability. This subsequently results in complex plume variability.

Illustrative figure adapted from Sultan, N. et al, Nat. Commun. 2020, 11 (1), 5087.

Seep Structure

Seep Structure

Gas seeping from the seafloor typically originates from a reservoir and is concentrated by fractures, which direct the gas through specific pathways. This seepage may collect salt, forming brine or mud, potentially leading to the creation of a mud volcano. Linear fractures can channel gas to emerge in a more focused and concentrated manner, resulting in pronounced vents with vigorous bubble emissions. Conversely, a network of interconnected fractures may disperse gas more broadly, causing widespread miniseepage.


At the seafloor, compacted mud or areas with substantial hydrate deposits may restrict seepage flow to a few primary vents. In contrast, a porous, sandy seafloor may facilitate more widespread seepage.

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Bubble Composition

An illustration of the changing bubble composition with respect to depth.  An example acoustic profile is added to illustrate how bubble quantity may change with depth.

Bubble Composition

Rising bubbles typically composed of hydrocarbons, carbon dioxide, and hydrogen sulfide dissolve rapidly into water due to a pronounced concentration gradient. The dissolution rate is influenced by factors such as hydrostatic pressure, Henry's law, the formation of hydrate and oil films, as well as bubble shape and size. Concurrently, dissolved atmospheric gases will diffuse into the bubbles from the water.

 

Consequently, bubbles reaching heights of 70-80 m above seafloor no longer contain significant amounts of hydrocarbons and will approximate the composition of atmospheric air. These bubbles will expand as they rise, while hydrostatic pressure will force continued bubble dissolution, making it uncertain whether they ultimately reach the surface.

 

McGinnis, D. F.; Greinert, J.; Artemov, Y.; Beaubien, S. E.; Wüest, A. Journal of Geophysical Research: Oceans 2006, 111 (C9).

Wind-driven Ekman transport causes net transport of water to flow 90 degrees from the wind direction. At depths of about 100 m current direction is opposite of wind direction.

Illustration credit to Danielle Hall at NOAA, Currents, Waves, and Tides.

Complex Currents

Complex Currents

 

Upward transport caused by rising bubbles will distribute plume creation across the water column, though it is typically limited to a range of less than 50 meters due to the finite availability of seep gas as it dissolves into the water. Oceanographic currents will then determine the shape and dilution of a seep’s plume, generally moving along an isopycnal plane, which may cause the plume to extend horizontally over some distance. Additionally, the complex interaction between tide-, wind-, and thermohaline-driven currents can result in a significantly variable plume structure. A tidal current may drive a plume in opposite directions throughout the day, while wind-driven currents may drive a plume in different directions at various depths (see Ekman transport).

Vast bacterial mats in the area of a cold seep consume methane inhibiting its release.

Credit: oceanexplorer.noaa.gov

Benthic Community

Benthic Community

Bacterial mats form near a vent, utilizing methane and hydrogen sulfide as energy sources. These mats can extend over large areas if there is significant secondary seepage through a porous substrate. Such areas can be highly visible. The bacteria facilitate the precipitation of carbonate rock, creating a surface that supports benthic communities. Well-established vents have the potential to sustain diverse ecosystems and intricate food chains, which may attract apex predators. The sonar signatures of these biological communities are often highly pronounced. 

NOAA has an excellent educational portal for cold seeps:

https://oceanexplorer.noaa.gov/edu/themes/cold-seeps/welcome.html

Air-Sea Interface

Sheen and decomposed crude oil on the ocean's surface near a vent

Air-Sea Interface

 

Wave action facilitates the movement of dissolved gases at the ocean's surface toward equilibrium with the atmosphere. Due to ultraviolet oxidation, atmospheric hydrocarbon concentrations remain very low, thereby driving hydrocarbon concentrations at the air-sea interface to extremely low levels.

Frequently, the composition of seep emissions include crude oil components that are highly insoluble in water and possess very low vapor pressure. These components form a sheen on the ocean’s surface, which can be detected by satellite or through direct observation.

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Plume structure can be influenced by various venting attributes. Illustration adapted from Etiope, G. Natural Gas Seepage: The Earth’s Hydrocarbon Degassing, 2015. 

Plume
Structure

Plume Structure
 

The structure of a plume can vary significantly due to various currents and may be further affected by differing seep attributes. A primary vent at a seep may exhibit rapid ebullition of bubbles, resulting in a plume with a pronounced vertical structure. Additionally, a primary vent may be accompanied by substantial miniseepage within the surrounding area, producing plume characteristics with extensive ‘ground cover’. Finally, a seep may release high-density fluids, such as brine or mud-slurry, causing the plume source to extend laterally across the seafloor as it follows the high-density fluid's flow.

An lively gas seep documented by NOAA.  Gas seepage rate can vary dramatically between vents, this results in large variations in plume size.

Seep Emissions

Seep Emissions

A seep's source reservoir will determine the gas composition, which may consist mainly of methane or carbon dioxide. Hydrogen sulfide and various hydrocarbons are also likely to be present. Gas from the source reservoir may undergo alterations during migration due to dilution from secondary sources or decomposition. Hydrocarbon composition can vary widely, ranging from >99.99% pure methane to a mixture containing a few percent of 'wet' natural gases (ethane, propane, butane) or the bubble may be primarily composed of the liquid phase components of crude oil. 


The dissolution of gas from a bubble into seawater can be slowed by the presence of liquid phase hydrocarbons behaving like a surfactant or the formation of solid-phase methane hydrate films.  The presence of these films will extending the transport of hydrocarbons further vertically.


Solomon, E. A., et al. Nature Geosci 2009, 2 (8), 561–565.

Plume from a North Sea sea cold-seep characterized by underwater mass spectrometry.

Credit: Gentz et al, 2016

https://core.ac.uk/download/pdf/42907548.pdf

Plume

Plume

Plume structure generally includes a vertical component due to the vertical migration of bubbles, a horizontal component along isopycnal planes, and sometimes a seafloor component associated with miniseepage, brine, or mud-slurry features. The plume's extent is limited by advective dilution, methanotrophic bacterial decomposition, and atmospheric release. Plume extent can vary significantly, but typically spans a few hundred meters for an average cold seep. Intense plumes (e.g. from well-head blowouts) can extend many kilometers, and carbon dioxide signatures (from the oxidation of the hydrocarbon plume) may extend significantly further still.

Typical plume extent has been well documented by Gentz et al, 2016 using underwater mass spectrometry.

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