1.
Introduction
Methane is an environment-altering molecule, function as a fuel for human activities, a carbon source for
benthic, water-column, and wetland ecosystems (Levin,
2005
; Valentine et al.,
2001
), and a greenhouse gas
in the atmosphere (Sundquist & Visser,
2003
). Assessing methane's impact on contemporary and future
climate, ecosystems, and ocean chemistry requires tracking and quantifying methane migration through
the natural environment.
In marine systems, methane has a complex role and history. Below the seafloor, methane is primarily pro
-
duced either by microbes that consume buried organic matter in shallow sediments or by thermal break
-
down of organic carbon at greater depths (Reeburgh,
2007
; Whiticar,
1999
). Once produced, methane may
be dissolved in sediment pore waters, exist in the vapor phase between sediment grains or in fractures, or
combine with water to form solid gas hydrate under certain pressure and temperature conditions (e.g., Rup
-
pel & Waite,
2020
). Methane that finds a pathway for upward migration through the sedimentary section is
mostly consumed by anaerobic oxidation of methane (AOM) before reaching the seafloor (Reeburgh,
1976
).
However, rapid advection of methane through sediments can effectively overwhelm the AOM sink (Joye
et al.,
2004
; Orcutt et al.,
2005
), allowing methane to be emitted from seafloor seeps as bubbles.
Abstract
Methane released from seafloor seeps contributes to a number of benthic, water column,
and atmospheric processes. At seafloor seeps within the methane hydrate stability zone, crystalline gas
hydrate shells can form on methane bubbles while the bubbles are still in contact with the seafloor or
as the bubbles begin ascending through the water column. These shells reduce methane dissolution
rates, allowing hydrate-coated bubbles to deliver methane to shallower depths in the water column than
hydrate-free bubbles. Here, we analyze seafloor videos from six deepwater seep sites associated with a
diverse range of bubble-release processes involving hydrate formation. Bubbles that grow rapidly are
often hydrate-free when released from the seafloor. As bubble growth slows and seafloor residence time
increases, a hydrate coating can form on the bubble's gas-water interface, fully coating most bubbles
within
∼
10 s of the onset of hydrate formation at the seafloor. This finding agrees with water-column
observations that most bubbles become hydrate-coated after their initial
∼
150 cm of rise, which takes
about 10 s. Whether a bubble is coated or not at the seafloor affects how much methane a bubble contains
and how quickly that methane dissolves during the bubble's rise through the water column. A simplified
model shows that, after rising 150 cm above the seafloor, a bubble that grew a hydrate shell before
releasing from the seafloor will have
∼
5% more methane than a bubble of initial equal volume that did not
grow a hydrate shell after it traveled to the same height.
Plain Language Summary
Methane is the primary component of natural gas. Processes
that affect the formation, consumption, and redistribution of methane in natural settings have significant
environmental consequences, such as ocean acidification and greenhouse warming. In water deeper than
500–600 m, methane bubbles can acquire shells of gas hydrate, a crystalline solid made of methane and
water molecules. Here, we study how the process of hydrate shell formation alters the fate of methane that
naturally bubbles up from the seafloor. We find that bubbles taking longer than
∼
10 s to release at seafloor
that is within the local methane hydrate stability often grow hydrate shells before rising into the water
column. As bubbles ascend through the water column, the hydrate shells slow down bubble dissolution
and gas exchange, allowing more methane to be delivered to shallower water depths.
FU ET AL.
© 2021. The Authors.
This is an open access article under
the terms of the
Creative Commons
Attribution
License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.
Hydrate Formation on Marine Seep Bubbles and the
Implications for Water Column Methane Dissolution
X. Fu
1
, W. F. Waite
2
, and C. D. Ruppel
2
1
Department of Mechanical and Civil Engineering, California Institute of Technology, Pasadena, CA, USA,
2
U. S.
Geological Survey, Woods Hole, MA, USA
Key Points:
•
In deepwater marine environments,
some methane bubbles emerging
from the seafloor acquire gas hydrate
shells before they release
•
Gas hydrate shells form when bubble
release rates from seeps within the
hydrate stability zone are slower
than
∼
10 s per bubble
•
Within 1.5 m of rise, most bubbles
grow shells, with initially shelled
bubbles having
∼
5% more gas than
initially clean bubbles
Supporting Information:
Supporting Information may be found
in the online version of this article.
Correspondence to:
X. Fu,
rubyfu@caltech.edu
Citation:
Fu, X., Waite, W. F., & Ruppel, C. D.
(2021). Hydrate formation on marine
seep bubbles and the implications for
water column methane dissolution.
Journal of Geophysical Research:
Oceans
,
126
, e2021JC017363.
https://
doi.org/10.1029/2021JC017363
Received 15 MAR 2021
Accepted 11 AUG 2021
10.1029/2021JC017363
RESEARCH ARTICLE
1 of 27
Journal of Geophysical Research: Oceans
FU ET AL.
10.1029/2021JC017363
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To maintain its pressure equilibrium with the surrounding water, a gas bubble will expand as it rises, de
-
pressurizing as the surrounding water pressure decreases with decreasing depth. Leifer and Patro (
2002
)
note, however, that bubbles also seek to equilibrate chemically with the surrounding waters, and that a
rapid net diffusion of gas out of the bubble can cause the bubble to shrink rather than expand as it rises.
Because oceanic dissolved methane concentrations are typically low relative to their solubility or equilib
-
rium concentrations (Ruppel & Kessler,
2017
), methane diffuses out of bubbles so rapidly that methane
bubbles are observed to shrink as they rise (Rehder et al.,
2002
,
2009
). The diffusive loss of methane from
bubbles is rapid enough that, once released at the seafloor, typical bubbles are completely stripped of their
methane during the first
∼
100 m of the bubble's ascent (McGinnis et al.,
2006
). The dissolved, bubble-de
-
rived methane is consumed in the deep ocean (Leonte et al.,
2018
) during microbially mediated aerobic
oxidation (Valentine et al.,
2001
) that leaves a byproduct of carbon dioxide, which in turn may contribute to
ocean acidification (Garcia-Tigreros et al.,
2021
; Garcia-Tigreros & Kessler,
2018
; Reeburgh,
2007
). Analysis
of dissolved methane in the near-surface ocean in deepwater areas confirms that very little can be traced
to seafloor emissions (Leonte et al.,
2020
; Sparrow et al.,
2018
). Instead, most methane in the near-surface
ocean proximal to seafloor seeps appears to be produced
in situ
by plankton (Leonte et al.,
2020
; Pohlman
et al.,
2017
), and enhanced sea-air methane flux traceable to seafloor methane emissions has not been found
in deepwater seep provinces surveyed to date (e.g., Myhre et al.,
2016
).
This article focuses on the fate of methane bubbles emitted from the seafloor at locations where the local
pressure and temperature conditions are within the hydrate stability zone. It has long been postulated that
such bubbles may form enclosing shells of gas hydrate that could reduce the diffusion of methane into the
surrounding water during the bubble's ascent, thereby allowing the methane to reach shallower depths in
the water column before dissolution occurs. This hypothesis was further supported by field experiments
(Rehder et al.,
2002
) and later theorized into models (McGinnis et al.,
2006
), which suggest that a hydrate
shell can slow the rate of transport of methane out of a bubble by 80%. Thus, establishing how and when
a bubble might grow a hydrate shell has significant implications for methane transport through the water
column. With the increasing number of remotely operated vehicle (ROV) explorations of seafloor methane
seeps, substantial direct observational data have accumulated to document the rate and pattern of hydrate
formation around methane bubbles at and near the seafloor. Here, we combine theoretical constraints with
the quantitative analyses of seafloor videos collected at deepwater seeps on North American continental
margins to determine the characteristics of bubbles that are most likely to acquire hydrate coatings and to
assess the impact of hydrate shells on the preservation of methane within bubbles.
2.
Background
The amount of methane emitted from the seafloor into the overlying ocean is estimated to lie in the broad
range of 16–3,200 Tg yr
−1
based on mass balance considerations that take into account the contemporary
concentration of methane in the oceans and aerobic methane oxidation in the water column (Ruppel & Kes
-
sler,
2017
). There are few reliable bottom-up estimates for global methane emissions into the ocean, mostly
because the number and distribution of deepwater methane seeps and their flux are so poorly constrained
(Phrampus et al.,
2020
; Skarke et al.,
2014
). Hovland et al. (
1993
) estimate 8–65 Tg methane (CH
4
) per year
emitted from seeps on global continental shelves. That study, however, focused on a setting where water
depths are too shallow and water temperatures too high for gas bubbles to acquire the hydrate shells that
are the focus of this article. Kvenvolden et al. (
2001
) gave an estimate of 20 Tg CH
4
yr
−1
emissions into the
water column from seafloor seeps at a time when widespread seafloor seepage had been described mostly in
petroleum basins (Bernard et al.,
1976
; Cranston et al.,
1994
). Recent discoveries made using modern acous
-
tic and mapping techniques suggest that seafloor methane seeps are a common occurrence along deepwater
global continental margins (Boetius & Wenzhöfer,
2013
; Johnson et al.,
2015
; Riedel et al.,
2018
; Ruppel &
Kessler,
2017
; Skarke et al.,
2014
; Westbrook et al.,
2009
) and are likely to number in the tens of thousands
worldwide (Boetius & Wenzhöfer,
2013
; Phrampus et al.,
2020
).
As noted above, most vapor phase methane released from the seafloor as bubbles at water depths exceed
-
ing
∼
100 m (McGinnis et al.,
2006
) diffuses into the water column and is consumed by microbial aerobic
oxidizers, a process that depletes both the dissolved methane and the water column's oxygen and leaves
behind carbon dioxide (Heintz et al.,
2012
; Valentine et al.,
2001
). Microbial communities respond rapidly
Journal of Geophysical Research: Oceans
FU ET AL.
10.1029/2021JC017363
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to changes in dissolved methane concentrations (Redmond & Valentine,
2012
), even consuming nearly all
of the methane released during the Deepwater Horizon incident (Kessler et al.,
2011
).
The water depth at which microbial oxidation is most active strongly depends on the availability of dis
-
solved methane, which is controlled by three parameters: (a) the mass transfer rate across the surface of
ascending methane bubbles, which describes how rapidly methane dissolves into the water; (b) the rise
velocity of the bubble, which influences the vertical scale of methane dissolution into the water column
and potentially the atmosphere; and (c) the methane mass fraction (or partial pressure) in a bubble, which
controls how much methane is available at the bubble’s surface for dissolution (e.g., Leifer & Patro,
2002
;
Li & Huang,
2016
; McGinnis et al.,
2006
; Topham,
1984a
; Wang et al.,
2020
; Yapa et al.,
2010
). All three
parameters change when a gas hydrate shell grows around the methane bubble (see Sections
5.1
and
5.2
).
Where seafloor pressure-temperature conditions are within the hydrate stability zone, hydrate shells may
grow around emitted bubbles either while the bubbles are still attached to the seafloor or soon after they
are released from the seafloor. Understanding the processes related to bubble release at seafloor seeps, the
growth of hydrate shells, and the evolution of bubbles in the water column is essential for quantifying how
methane released from the seafloor becomes distributed throughout the water column.
In the deep ocean, gas hydrate formation is often limited by methane availability, even when the
P
-
T
con
-
ditions are well within the gas hydrate stability field (Ruppel & Waite,
2020
). As reviewed by Ruppel and
Kessler (
2017
), the ocean as a whole is greatly undersaturated in methane, and a bubble exposed to water
depleted in methane begins dissolving via methane diffusion across the bubble-water interface even under
favorable hydrate-forming conditions (Chen et al.,
2014
; Maini & Bishnoi,
1981
; Rehder et al.,
2002
,
2009
;
Warzinski et al.,
2014
). With time, however, methane dissolution from the bubble increases the dissolved
methane content in a thin boundary layer of water around the gas-water interface and can promote hydrate
formation (Boewer et al.,
2012
) on the bubble's surface. At the gas-water interface, localized competition
between the rates of methane diffusion and hydrate nucleation determines whether a hydrate layer can
eventually form. When the driving force for hydrate nucleation increases, such as in deeper and colder
waters that are far within the hydrate stability field, hydrate nucleation rates can outpace the rate of meth
-
ane diffusion, and macroscopic hydrate formation can occur even in methane-poor environments (Chen
et al.,
2014
; Maini & Bishnoi,
1981
).
The formation of gas hydrate on emitted gas bubbles is favored where ocean waters have higher concen
-
trations of methane, even if the concentrations are still significantly below methane saturation. Elevated
dissolved methane concentrations have been measured near the seafloor above continental slope methane
seeps in many settings, including the United States Atlantic, Cascadia, Gulf of Mexico (GOM), and Svalbard
margins (Garcia-Tigreros et al.,
2021
; Graves et al.,
2015
; Lapham et al.,
2013
; Thomsen et al.,
2012
). Close
to seafloor seeps, dissolved methane concentrations can be two to seven orders of magnitude (Lapham
et al.,
2013
; Thomsen et al.,
2012
) larger than the background value for deep ocean waters (
∼
2.5–3.5 nM;
Rehder et al.,
1999
).
Water column observations and laboratory measurements (Maini & Bishnoi,
1981
; Wang et al.,
2016
; War
-
zinski et al.,
2014
) have described the rapid formation of hydrate shells on rising bubbles, but less is known
about when and how such coatings initiate and grow on bubbles emitted from natural seeps. Existing bub
-
ble rise models (Topham,
1984b
) and the family of models (Li & Huang,
2016
; McGinnis et al.,
2006
; Wang
et al.,
2020
; Yapa et al.,
2010
) based on the experimental results of Rehder et al. (
2002
,
2009
), assume that
bubbles released at the seafloor are initially hydrate-free and grow hydrate shells after some elapsed time.
McGinnis et al. (
2006
) parameterize this transition by assuming the hydrate shell only forms once the bub
-
ble diameter shrinks to 3.5 mm during the bubble’s ascent. With this assumption, McGinnis et al. (
2006
)
conclude that a Black Sea bubble plume (flare) detectable at 1,300 m above the seafloor would require
bubbles to be 20 mm in diameter when emitted. Not only do bubbles of this size tend to break into smaller
bubbles (McGinnis et al.,
2006
), but Black Sea bubble sizes are mostly in the range of 1.4–18.2 mm diameter
(Egorov et al.,
2003
), with an average emission diameter of 6 mm (McGinnis et al.,
2006
). If the bubbles are
instead initially assumed to be hydrate-coated when released from the seafloor, the bubble diameters only
need to average 9 mm to create the 1,300 m-tall Black Sea flare (McGinnis et al.,
2006
).
Journal of Geophysical Research: Oceans
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In this study, we analyze seafloor videos from six natural deepwater seep sites on North American marine
margins to assess the impact of hydrate shell formation on bubble release characteristics. From these videos,
we classify bubbles into two general categories: (a) hydrate-free, or “clean” bubbles with fully mobile surfac
-
es that tend to be shiny and clear; (b) hydrate-coated bubbles, for which the hydrate coating is characterized
by gray or silvery, semi-rigid, or rigid surfaces. These distinctions are not always obvious, and dedicated
bubble-seep research endeavors would improve on the observations made here by utilizing dual high-speed
cameras (Wang & Socolofsky,
2015
; Wang et al.,
2016
), and reference grids for size analysis (Padilla & We
-
ber,
2021
; Rehder et al.,
2002
).
We examine the role of local lithology, bubble release rates, and extant gas hydrate in controlling the size,
shape, and nature of the hydrate coating for bubbles emitted at the seafloor. We apply a simplified model
to determine the relationship between the methane content of a bubble and the fraction of the bubble that
is hydrate-coated after 150 cm of rise in the water column. In the absence of any site-specific gas or water
chemistry information, our simplified model assumes all bubbles are oil-free and formed purely from meth
-
ane, though we do indicate how model results would vary in the presence of oil or a mixed gas with higher
hydrocarbons. Our objective in this study is to couple visual observations with modeling results to show
how a range of observed initial bubble conditions and methane emission processes contribute to methane
dissolution in the water column over deepwater methane seeps emitting bubbles within the gas hydrate
stability zone.
3.
Geologic Setting
We use videos recorded during ROV dives at six actively bubbling seafloor methane seeps (Figure
1a
; Ta
-
ble
1
) to constrain seep lithology, the characteristics of bubble emissions (e.g., rate and bubble size), and the
extent of hydrate coating on bubbles before they are released from the seafloor and, when possible,
∼
150 cm
into their water column ascent. All of the seeps are located at water depths and associated with bottom
water temperatures that place them within the methane hydrate stability field (Figure
1b
; Table
2
). The six
seep sites are located on North American marine margins, including the active margin offshore Vancouver
Island and on passive margins south of New England and in the salt tectonic province of the northern GOM
petroleum basin. Despite the range of tectonic settings, probable gas sources, and seafloor conditions rep
-
resented by these seeps, the observed bubble emission and hydrate formation processes reveal fundamental
physical and chemical insights that apply regardless of seep setting. Here, we review the key characteristics
and video-based observations about the six sites.
3.1.
Cascadia Margin
The Barkley Canyon seep (Figure
1a
, panel 1) lies on the northern part of the Cascadia active margin in the
Northeast Pacific Ocean. Since 2015, over a thousand previously unknown bubble plumes have been dis
-
covered along this margin from offshore Vancouver Island to northern California at water depths of tens of
meters to more than 2,500 mbsl (e.g., Embley et al.,
2016
; Johnson et al.,
2015
; Merle & Embley,
2016
; Merle
et al.,
2021
; Riedel et al.,
2018
). The seep we focus on here is at the NEPTUNE cabled observatory node re
-
ferred to as “Barkley Canyon Axis” near the “Barkley Canyon Hydrates” node that is frequently monitored
by ROVs (e.g., Juniper et al.,
2013
) and a remotely operated deep-sea crawler (Chatzievangelou et al.,
2020
;
Doya et al.,
2017
; Purser et al.,
2013
). For this study, we use a 2012 ROV Ropos video described by Seabrook
et al. (
2019
) that captures active methane seepage at 985 mbsl (Table
1
), which is well within the stability
field for methane hydrate (Table
2
). Seep emission dynamics at this location have previously been described
by Thomsen et al. (
2012
). In the Ropos video from Seabrook et al. (
2019
), the seep is producing mostly
hydrate-free, clear-surfaced, “clean” gas bubbles that rapidly release from small orifices (also referred to as
“emitters”) in sediment that appears relatively uniform and fine-grained in the video (Text
S1c
). Though
the video does not contain images of hydrate outcrops or bacterial mats, two slightly gray, hydrate-coated
bubbles that do not release from the seafloor are visible. Additionally, gas bubbles collect under the crab
on the right side of Figure
2a
and begin forming hydrate. Enough gas eventually accumulates to overcome
the crab's mass, lifting and flipping the crab over (Figure
2b
). The hydrate detaches and begins ascending
through the water column as the crab somersaults to the seafloor (Figure
2c
).
Journal of Geophysical Research: Oceans
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3.2.
Northern Gulf of Mexico
Four of the seafloor seeps that we analyze are located in the northern GOM salt tectonics province (Fig
-
ure
1a
, panel 2), a passive margin setting that hosts one of the world's most productive marine petroleum
basins. Seafloor morphology of this area is controlled by salt diapirism and faulting, with salt withdrawal
basins developing between the buried salt bodies (Worrall & Snelson,
1989
). Especially at basin boundaries,
faults form to accommodate salt ascent through the sediment and serve as the loci for seafloor leakage of
fluids (gas, petroleum, and brines). Gas hydrate systems have been recognized in the northern GOM from
Figure 1.
(a) Large map shows stars at the locations of North American margin seeps highlighted in this article. Inset 1: Cascadia margin with the location of
the Barkley Canyon site and seeps (red) that have been mapped at greater than 500 mbsl (nominal top of gas hydrate stability; Ruppel & Waite,
2020
) by Riedel
et al. (
2018
), Embley et al. (
2016
), Merle and Embley (
2016
), and Merle et al. (
2021
). Inset 2: Northern Gulf of Mexico showing the location of GB648, Confetti,
Sleeping Dragon, and Horn Dome. Seeps indicated in red are georeferenced from those shown for the continental slope from Fisher et al. (
2015
). From west
to east, the white outlines indicate the Garden Banks, Green Canyon, and Mississippi Canyon protraction areas. Bathymetry from Kramer and Shedd (
2017
).
Inset 3: United States Atlantic margin showing the location of New England Seep 2 and seeps (red) deeper than 550 mbsl (nominal top of gas hydrate stability)
from Skarke et al. (
2014
). (b) Gas hydrate stability curve is shown in green for Structure I methane-only hydrate calculated for 3.5% weight percent NaCl in
water using the fit to the Sloan and Koh (
2007
) equation as given in Table
1
of Ruppel and Waite (
2020
). The corresponding water depth and bottom water
temperatures at each of the seep sites discussed here are indicated by the black dots. The degree of supercooling given in Table
2
is the difference between
observed temperatures and the corresponding phase boundary temperature at that depth.
Journal of Geophysical Research: Oceans
FU ET AL.
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the seafloor to great depths within the sediments and at locations ranging from cold seeps to mud volca
-
noes to conformably deposited sediments and associated fractures in the salt withdrawal basins (Boswell
et al.,
2012
; Brooks et al.,
1984
; Cook et al.,
2008
; Daigle et al.,
2018
; Hutchinson et al.,
2011
; Roberts,
2001
).
Gas mixtures emitted at northern GOM seeps may contain hydrocarbons other than methane (e.g., Brooks
et al.,
1984
), but we analyze the bubble dynamics assuming pure methane, which is the dominant gas
at some seeps (Sassen et al.,
2003
) and which simplifies the analyses and comparisons undertaken here.
Northern GOM sites are identified by their protraction area and lease block designator.
The seep at Garden Banks 648 (GB648) was explored by
ROV Deep Discoverer
in 2014 (Lobecker et al.,
2019
).
The site is characterized by active bubble emissions set in an area with authigenic carbonate outcrops and
bare seafloor sediment. Hydrate has accumulated beneath a carbonate ledge
∼
10 m away from the area
shown in Figure
3
(see, e.g., Figure
2
; Text
S2a
and also Ruppel & Kessler,
2017
). We consider a seafloor seep
site that is covered in bacterial mats and fine-grained sediment adjacent to snails and a carbonate outcrop
(Figure
3a
). Bacterial mats are interpreted as being associated with higher seafloor methane fluxes (Lev
-
in,
2005
; Lloyd et al.,
2010
), and the bubbles (Figure
3b
) originate from small orifices within the bacterial
mats and surrounding fine-grained sediment. Even within the small area studied here (Figure
3b
), bubbles
show diversity in size, shape and fractional hydrate coverage.
Horn Dome (HD) (Figures
1
and
4
, inset), located in Mississippi Canyon lease block 36 (MC36), is at the
northwest edge of a salt-cored bathymetric high in a location where seafloor exploration has revealed sev
-
eral hydrate outcrops (Ruppel & Amon,
2017
). The video used for our analysis (Text
S3b
) was collected by
the ROV
Deep Discoverer
(Kennedy et al.,
2019
) and excerpted by Ruppel and Waite (
2020
) in their analysis
Site name
Latitude
Longitude
Reference
Sleeping Dragon (MC118)
28°51.1421′N
88°29.5109′W
Wang and Socolofsky (
2015
) Wang et al. (
2016
)
Garden Banks 648 (GB648)
27°20.3465′N
92°21.6182′W
Lobecker et al. (
2019
)
Barkley Canyon
48°19.0057′N
126°3.0098′W
Seabrook et al. (
2019
)
Horn Dome (MC36)
28°57.9380′N
88°11.6970′W
Kennedy et al. (
2019
)
Confetti (GC600)
27°22.1954′N
90°34.2624′W
Wang et al. (
2016
)
New England Seep 2
39°52.2625′N
69°17.1560′W
Shank et al. (
2014
)
Note
. Lease block designations are given in parentheses for the four sites in the Gulf of Mexico. Barkley Canyon is located offshore Vancouver Island and New
England Seep two is offshore Massachusetts (Figure
1
). Links to site and video information are provided in Text
S1–S6
.
Table 1
Study Site Locations and References for the Original Seafloor Videos
Site
Water
depth (m)
Seafloor temperature
(°C)
CH
4
hydrate
subcooling (°C)
CH
4
gas density
(kg/m
3
)
Water viscosity
(μPa s)
Diffusion
coefficient
(cm
2
/s)
Sleeping Dragon
a
(MC118)
890
5.3
5.3
76
1,490
1.84 × 10
−5
GB648
965
5.3
5.9
84
1,489
1.84 × 10
−5
Barkley Canyon
b
985
3.6
7.8
87
1,570
1.82 × 10
−5
Horn Dome (MC36)
1,034
4.2
7.6
93
1,540
1.82 × 10
−5
Confetti
a
(GC600)
1,190
4.4
8.6
108
1,528
1.83 × 10
−5
New England Seep 2
1,418
4.2
10.2
132
1,534
1.82 × 10
−5
Note
. Subcooling below the local methane hydrate stability temperature is calculated using the stability curve formulation from Tishchenko et al. (
2005
).
Methane gas density is calculated for the seafloor pressure and temperature conditions according to the methane equation of state from Sychev et al. (
1987
).
Water viscosity is from Lemmon et al. (
2016
). The diffusion coefficient is calculated based on a quadratic fit in temperature to data from Akgerman and
Gainer (
1972
).
a
Depth and temperature data from Wang et al. (
2016
).
b
Depth and temperature data from Seabrook et al. (
2019
).
Table 2
Site Characterization
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FU ET AL.
10.1029/2021JC017363
7 of 27
of seafloor hydrate phenomena. The video grab (Figure
4
) shows a hydrate outcrop draped with a thin
layer of fine-grained sediment. Exposed hydrate has an orange tint. Though we do not have any direct
sampling from this site, previous work has shown that oil staining (Beaudoin et al.,
2014
; Kennicutt,
2017
;
Sassen et al.,
2004
) or perhaps bacterial crusting (Kennicutt,
2017
) can give hydrate an orange coloring. In
addition to the eelpouts, the outcrop hosts ice worms (Ruppel & Amon,
2017
) feeding directly on the ex
-
posed hydrates and likely contributing to the formation of burrows and cavities in the hydrate itself (Fisher
et al.,
2000
). Whereas the GB648 emitters are orifices in the fine-grained sediment, HD emitters (Figure
4b
)
appear directly connected to the hydrate and primarily protrude from underneath exposed hydrate. Emitter
HD5 is an extreme example in which the visible hydrate shell is part of an accumulating gas pocket under
-
neath the hydrate ledge. HD also offers a chance to peer inside certain hydrate-coated methane emitters.
HD4 is the primary example of a preexisting hydrate tube from which bubbles are released at the top. The
tube is transparent enough that rising menisci are visible, a phenomenon that suggests the simultaneous
flow of gas and water in the subsurface environment that feeds this seep.
The Sleeping Dragon seep, which is located west of Whiting Dome in MC118, has orange-tinted hydrate
covered in some places by a thin layer of fine-grained sediment (Figure
5
). Similar to the HD site, the orange
color could be due to oil impurities or bacterial crusting. According to the descriptions of Wang and Socolof
-
sky (
2015
) and Wang et al. (
2016
) for the ROV
Hercules
dives in 2014, Sleeping Dragon also has hydrate tube
vents. Bubbles produced by the line of individual vents made of hydrate (see
hydrate tube vents
in Figure
8
)
are described by Wang and Socolofsky (
2015
) as being primarily free of hydrate or oil films.
Figure 2.
Inset: Map of known seeps (red) from Riedel et al. (
2018
) and the focus seep site (star) in Barkley Canyon,
offshore Vancouver Island. (a) Seep orifices identified prior to the crab flip (see text) are indicated by circled numbers.
As the tanner crabs (
Chionoecetes tanneri
) feed, seep gas collects beneath the crab on the right, eventually lifting the
crab off the seafloor. (b) As the crab begins to flip, white hydrate flakes detach from the crab’s underbelly. (c) After the
crab lands to the right of its takeoff position, two additional emitter sites could be tracked. Note that emitter 12 (blue
circle) is the same in panels (a) and (c). The rising bubbles were shiny and clear, from which we interpret they did
not have hydrate coatings. Two bubbles that appeared to have gray, rigid hydrate shells and never released from the
seafloor are indicated in panels (a) and (c). For scale, the flipping crab’s carapace is
∼
7 cm across based on the ROV’s
laser spacing (visible in the video but located outside the area imaged here). Imagery is modified from bottom water
video archived by Ocean Networks Canada and from the supplement to Seabrook et al. (
2019
). See Text
S1
for video
descriptions and links (Text
S1c
contains the specific clip referred here).
1
3
4
5
6
10
11
7
2
8
12
9
14
13
12
a
b
c
Non-releasing,
hydrate-coated bubbles
Dislodged hydrate formed
under crab
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The Confetti seep site (Figures
1a
and
6
), which is located in Green Can
-
yon (GC) 600, lies along a steep scarp bounding a salt withdrawal basin
∼
30 km NNW of where Green Canyon cuts into the Sigsbee Escarpment.
The site was visited by ROV
Hercules
in 2014 (Wang et al.,
2016
). Frac
-
tured authigenic carbonate is exposed at the seafloor, and bubbles emerge
primarily in a single, rapid stream from a seafloor crack. No hydrate is vis
-
ible, although the site is well within the hydrate stability field (Table
2
).
Unlike the small bubbles released at the other sites in this study, the
Confetti seep produces relatively larger bubbles that Wang et al. (
2016
)
observe breaking up into 1–4 mm radii bubbles within the first meter of
rise. These reduced size bubbles are typical of those at the other sites in
our study.
3.3.
United States Atlantic Margin
The final seep site examined in this study lies at
∼
1,400 mbsl on the heav
-
ily eroded continental slope of the New England passive margin. New
England Seep 2 (Figure
7
) is located on a ridge between unnamed can
-
yons east of Shallop Canyon (McVeigh et al.,
2018
; Skarke et al.,
2014
).
Similar to the Barkley Canyon site, New England Seep 2 was emitting pri
-
marily hydrate-free bubbles from fine-grained sediment and had a high
concentration of red crabs,
Chaceon quinquedens
(Quattrini et al.,
2015
)
during exploration with ROV
Deep Discoverer
in 2013. The seep site is
surrounded by white bacterial mats that may also contain gas hydrate
(Shank et al.,
2014
), and the bubble emissions are slightly slower than
at Barkley Canyon. One of the slower emitters (NE1, Figure
7b
) produc
-
es tubular-shaped bubbles similar to those from emitter GB3 (Figure
3
),
each with a clear, shiny hemispherical top and an extended lower portion
that we interpret to be developing a hydrate coating as it becomes gray
and opaque (see Text
S6b
for video link and also the supplementary vide
-
os in Skarke et al.,
2014
).
4.
Seafloor Observations
Seafloor bubble release is characterized here according to bubble release
time, hydrate coating morphology and seafloor lithology. Bubble release
time is defined as the time it takes for a bubble to grow prior to being released from the seafloor. Ob
-
servations suggest that bubble release time correlates with the extent and rate of hydrate film growth on
the bubble surface prior to the moment of release. The hydrate coating itself also affects the final bubble
shape, and hence its rise velocity (Bigalke et al.,
2010
) and methane loss rate (McGinnis et al.,
2006
; Reh
-
der et al.,
2002
,
2009
). The seafloor lithology controls the sediment's pore size distribution, which in turn
influences the bubble size (Leifer & Culling,
2010
) and hence its rise velocity (Clift et al.,
1978
) and initial
methane content. Understanding these bubble release characteristics provides a foundation for assessing
the extent to which gas hydrate formation may slow down the dissolution of methane into the water column
once a bubble leaves the seafloor.
4.1.
Bubble Release Rate
Figure
8
summarizes the results of analyzing the visual observations of bubble release within the hydrate
stability zone at the selected study sites. The timing of our video analysis, calculated by counting individual
video image frames, is based on the reported frame rate (generally 30 fps), and is discussed in Table
S11
description. A key finding is that bubbles tend to accumulate more extensive hydrate coatings the longer
they remain attached to the seafloor (slower release rate). The average release time for different types of
hydrate coatings is site specific and reflects a localized competition between the rate at which the gas-water
Figure 3.
Inset: Location of Garden Banks seep (GB648) at the edge of
a salt withdrawal basin in the northern Gulf of Mexico. (a) Overview
image estimated to be
∼
3 m across (Text
S2b
). The seep is within the white
bacterial mat on a mound draped in light brown, fine-grained sediment.
Upper left inset shows rigid-walled bubbles imaged
∼
2 m above the
seep (Text
S2d
). Bubble analysis area is highlighted in yellow. (b) Close-
up: Emitter identifiers are shown for orifices with measurable bubble
release rates (average release times shown in Figure
8
). This site has a
combination of the shiny, clear bubble surfaces associated with hydrate-
free bubble surfaces (e.g., GB2, GB5, and the tops of GB6-9) and the gray
or silvery surfaces associated with hydrate (e.g., GB1, GB3, and GB4, the
bottom portions of GB6-9 and the circled bubbles). The two hydrate-
crusted bubbles circled in yellow did not release during the
∼
5-min this
site was imaged (Text
S2b
and
S2c
).
b
GB1
Eelpout
(
Zoarcidea
)
GB2
GB3
GB4
GB5
GB9
GB7
GB8
~1 cm
GB6
Sea snails
(
Cantrainea
)
Rigid bubbles
above seep
Fig. 3b
a
Bacterial mat
s
(
Beggiatoa
)
Non-ellipsoidal,
hydrate-coated
bubbles
Carbonate
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interface expands and the rate at which hydrate grows on that expand
-
ing surface. This competition depends on the local methane solubility
and dissolved-phase methane concentration. In general, release times for
clean bubbles are less than 5 s, while forming a complete hydrate coating
requires release times
∼
10 s or longer.
When gas flow rate is sufficiently high, the rate of bubble surface creation
and freshening outpaces the rate of hydrate shell growth (Fu et al.,
2020
).
At the Barkley Canyon site, bubble release rates are high enough that
no hydrate appears to form on the growing bubbles prior to seafloor re
-
lease. As the crab-flip video demonstrates (Seabrook et al.,
2019
), how
-
ever, hydrate forms readily on gas bubbles that are collected and trapped
beneath the crab (Figure
2b
). Similar effects have been observed on Blake
Ridge, where the local methane saturation beneath a carbonate over
-
hang is sufficiently high to promote hydrate formation and accumulation
from an underlying gas bubble source (Ruppel & Waite,
2020
; Van Dover
et al.,
2003
).
At the HD tube-vent emitter HD4 (Figure
4b
), the bubble surface grows
only slightly faster than the hydrate formation rate, as illustrated in Fig
-
ure
9
. During bubble growth, internal gas circulation (Clift et al.,
1978
;
Pinczewski,
1981
) helps shift growing hydrate films away from the emerg
-
ing meniscus toward the base of the bubble (Figure
9
, upper panel). Such
balance between gas flow rate and hydrate growth rate, however, is del
-
icate and can be easily disrupted by changes in gas flux (Fu et al.,
2020
).
As discussed in Section
4.3
, when the growing bubble is cut off from the
tube’s gas source, the bubble stops growing and the surface becomes fully
hydrate-coated in less than 0.3 ± 0.03 s (Figure
9
, lower panel).
It is important to note that the release time for the HD tube-vent emitter
(HD4) is shorter than that of the neighboring hydrate-free bubble emitter
(Figure
4b
, emitter HD1). Some aspects of HD4’s gas or water chemistry
may differ from that of HD1, accelerating hydrate growth at HD4. This
observation is a reminder that the relationship between release time and
hydrate formation (Figure
8
) is also extremely sensitive to localized hy
-
drate formation conditions (methane concentration, surfactants, etc.). In
general, however, bubbles with release times below 2 s rarely have hy
-
drate coatings for the sites considered here.
Despite some variabilities, Figure
8
implies that after
∼
5–10 s bubbles attached to the seafloor generally
become hydrate coated. We hypothesize that this timescale also serves as a good estimate for the total sea
-
water exposure time for a bubble to become hydrate coated. The total seawater exposure time includes both
the time a bubble spends attached to the seafloor (seafloor residence time) and the time of its subsequent
ascent through the water column.
To elaborate on this hypothesis, we note that an initially clean bubble could become hydrate-coated during
its subsequent rise. When an initially clean bubble releases from the seafloor, drag from the surrounding
water on the bubble surface maintains meniscus motion from the top toward the bottom of the rising bub
-
ble (McGinnis et al.,
2006
; Warzinski et al.,
2014
), just as observed for seafloor-grown bubbles (Figure
9a
).
Though this can slow hydrate growth, a hydrate film nonetheless begins growing almost immediately
(McGinnis et al.,
2006
; Rehder et al.,
2009
). Within the first 100–200 cm of rise within the water column,
many of the bubbles visible at GB648 appear to be gray and rigid and are probably coated in hydrate (Fig
-
ure
3
, inset). Wang et al. (
2016
) indicate that all bubbles observed 150 cm above the Sleeping Dragon and
Confetti seeps are definitely hydrate-coated. As discussed in Text
S9
, for the observed bubble diameters
(3–5 mm for HD, 2–6 mm for Sleeping Dragon, and 2–10 mm for Confetti), rise velocities range from 15 to
22 cm/s, suggesting the bubbles would take
∼
7–10 s to rise 150 cm. This calculated time is consistent with
Figure 4.
Inset: Location of the Horn Dome (HD) seep (MC36) northwest
of the bathymetric high associated with the buried salt diapir in the
northern Gulf of Mexico. (a) Overview: Bubbles form on the surface of an
orange-tinted hydrate outcrop that is thinly draped with gray, fine-grained
sediment. Bubble analysis area is highlighted in the yellow box. (b) Close-
up: Emitter identifiers for the HD seep study area (average release times
shown in Figure
8
). Emitter HD5 is a gas-filled hydrate shell trapped under
the hydrate ledge. Hydrate-coated bubbles not indicated by yellow arrows
did not release during the 5-min video (Text
S4b
). Ice worms (Ruppel &
Amon,
2017
) identifiable by their thin, white antennae, can be seen in
depressions and beneath the overhang.
Exposed
hydrate
10-cm laser spacing
Fig. 4b
Eelpouts
(
Zoarcidea
)
a
HD2
HD1
HD3
HD4
(T
ube-vent emitter)
HD5
Ice worms
(
Hesiocaeca
methanicola
)
1 cm
b
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the seafloor observations shown in Figure
8
, where only a single one of
the nine bubbles with release times longer than
∼
10 s is hydrate free.
Thus, for the sites described here, our analysis suggests that bubble sur
-
faces are generally hydrate-coated within 10 s of exposure to seawater,
regardless of whether the bubble is initially coated at the seafloor. This
10 s includes both the seafloor residence time and subsequent bubble
rise. For instance, a bubble releasing after only 2 s would be anticipated
to release as a clean, hydrate-free bubble (Figure
8
), but become hydrate
coated within the first 8 s of rise.
4.2.
Hydrate Coating Morphology During Bubble Growth
Figure
8
indicates not only that bubble release time correlates with the ex
-
tent of hydrate coverage on a bubble, but also where and how thickly that
hydrate forms. Extensive hydrate growth can significantly alter both the
shape and methane content of a bubble, two characteristics that in turn
influence where the bubble's methane will dissolve in the water column.
In general, hydrate growth begins at the base of the bubble, where the
gas has been in contact with seawater the longest. As the release time in-
creases, gas hydrate can grow farther up the sides of the bubble until only
a small cap of hydrate-free bubble surface remains (Figure
8
, right pan
-
el, chimney-growth bubble). Hydrate can completely cover slow-grow
-
ing bubbles (Figure
8
, right panel, film-coated bubbles), forming thicker,
more rigid shells over time (Figure
8
, right panel, shell-coated bubbles).
Pronounced hydrate shell growth significantly alters the shape of sea
-
floor bubbles. Figure
10
shows three stages of a chimney-growth bub
-
ble. This chimney-like bubble growth shape occurs because the hydrate
growth directs bubble expansion toward the surface of least resistance
(Fu et al.,
2018
,
2020
), which is generally the top of the bubble. The cou
-
pled effect of hydrate formation and bubble expansion also creates minor
bends in the final bubble shape (Figure
10
here, and also Figure
4
in Fu
et al.,
2018
). These rigid, non-spherical, large aspect ratio (width/height)
shapes have been observed to yield reduced rise velocity in hydrate-coat
-
ed bubbles (Bigalke et al.,
2010
).
Numerous fully coated bubbles are observed (e.g., Figures
3
and
4b
)
when the bubble release time becomes sufficiently long. Though direct
thickness measurements are not possible, we assume that the apparent
elasticity of the coating correlates with the shell thickness. Thus, a shell
that is more easily stretched is assumed to be thinner than a more rigid
one. HD emitter HD2 (Figure
4b
) produces thinly coated bubbles, which
act like inflating balloons (Figure
11a
). These pliable, thin-shelled bub
-
bles have 38 s average release times. HD emitter HD3 produces more rigid
bubbles that grow via cracking, stepwise expansion, and rapid crack heal
-
ing (Figure
11b
), with an average release time of 33 s. In comparison, two
rigid, opaque, potentially thicker-shelled bubbles from GB648 (emitters
GB1 and GB4, Figure
3b
) each release only once during the video. Their
minimum possible release times are 41 and
∼
170 s, respectively. HD has
several hydrate-coated bubbles that do not release during the 5-min video
(e.g., the two larger tubes to the left of HD1in Figure
4b
). These bubbles
appear to have thicker shells than those that do release.
An important consequence of these diverse hydrate-coating process
-
es is the emission of irregularly shaped bubbles into the water column
Figure 5.
Inset: Location of the Sleeping Dragon (MC118) site near
Whiting Dome in the northern Gulf of Mexico. (a) Overview: Orange-
tinted hydrate outcrop covered with thin sediment. Broken clam shells
(
Calyptogena
) are piled on the left side of the mound. (b) Close-up of the
yellow box from (a), showing ice worms on the hydrate surface. A line
of hydrate vent tubes, similar to those at Horn Dome (HD) emitter HD4
(Figure
4b
) produce bubbles rapidly (
∼
0.1 s per bubble or faster). Vent
tubes and bubbles are more easily distinguished in the original video
(Wang & Socolofsky,
2015
, also Text
S4b
), from which this imagery is
extracted.
a
b
Fig. 5b
Clam Shells
(
Calyptogena
Ponderosa
)
Exposed hydrate
Ice worms
(Hesiocaeca methanicola)
5 cm
Rapidly released,
hydrate-free
bubble
s
Bubble emission tube
s
Figure 6.
Confetti seep site (GC600; Figure
1a
). Inset: Map showing the
seep location (star) at the edge of a salt withdrawal basin in the northern
Gulf of Mexico. Main image: A stream of relatively large bubbles is emitted
from a seafloor crack within carbonate-dominated rock. Wang et al. (
2016
)
note these bubbles break up within the first meter of rise. After breakup,
1–4 mm bubble radii were measured by Wang et al. (
2016
). The image is
∼
1 m across. Imagery is modified from video in the supplement to Wang
et al. (
2016
), see also Text
S5b
.
Crack
Rapid-release
bubble stream
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(Figures
10
and
11
, right-hand panels). As noted in Section
4.1
, many of the bubbles observed 150–200 cm
above the seafloor at GB648 are hydrate coated, rigid, and irregularly shaped. Wang et al. (
2016
) makes the
same observation above the Sleeping Dragon and Confetti sites (see Figure
6
in Wang et al.,
2016
). This
suggests that, at seep sites where hydrate coating occurs, bubble plumes likely consist of a combination of
irregularly shaped and rigid shelled bubbles as well as clean bubbles. The hydrodynamic interactions within
these shelled-bubble swarms may differ significantly from that of hydrate-free bubbles and influence plume
dissolution dynamics.
4.3.
Lithology and Bubble Release Controls on Bubble Size
Lithologic control on the size of bubbles released at the seafloor has been examined by Leifer and Cull
-
ing (
2010
), who classify seep lithologies according to the sediment grain size relative to the bubble size. For
fine-grained lithologies in which bubble radii are much larger than sediment grain radii (e.g., Barkley Can
-
yon and GB648), Leifer and Culling (
2010
) note that gas can become temporarily trapped and accumulate
below the seafloor before being released in a pulsed mode capable of lifting and mobilizing the sediment
grains. The Leifer and Culling (
2010
) study utilizes non-cohesive grains without pre-existing flow paths,
however. Though sediment motion in response to bubble release can be seen in the video (e.g., at NE7 in
Figure
7b
), the generally steady, non-pulsing bubble releases at Barkley Canyon, GB648, and New England
Seep 2 suggest the fine-grained sediments at these sites are sufficiently cohesive and the seeps are mature
enough to have developed a subsurface network of unobstructed gas pathways and emission points. Such a
near-surface network is likely connected to a larger, dendritic subsurface conduit system, as imaged in 3D
beneath the Blake Ridge seep field (Hornbach et al.,
2007
).
For unobstructed emission at the seafloor-water-column interface, a capillary tube is a good analog for the
shape of an emitter’s pore structure (Leifer & Culling,
2010
). Here we assume that seafloor bubbles are re
-
leased from a capillary tube-like structure with a circular orifice. The typical bubble size should then reflect
a balance between gas buoyancy and gas/water surface tension at the orifice. For hydrate-coated bubbles,
the tensile strength of hydrate coating can enhance the gas/water surface tension, potentially allowing larg
-
er bubbles to form. Here we test this hypothesis by calculating the theoretical bubble size under hydrate-en
-
hanced surface tension and compare our calculations to field observations.
Assuming a circular orifice, Davidson and Amick (
1956
) and Oguz and Prosperetti (
1993
) demonstrate that
the maximum volume of a clean bubble,
V
max
, at the moment of release should be determined by the bal
-
ance between the bubble’s buoyancy and the surface tension at the orifice:
Figure 7.
Inset: location of New England Seep two on the continental slope south of Massachusetts. (a) Overview
image showing bubble release primarily within the tan patch of bare seafloor surrounded by white bacterial mats.
Bubble analysis area is highlighted in yellow. (b) Emitter identifiers for the New England Seep 2 (NE) seep study area.
Yellow circles indicate bubbles that remain fixed at the seafloor during the entire
∼
2 min of video for this site. The two
yellow circles mark bubbles that remain fixed to the seafloor during the video. These stationary bubbles appear less
reflective than the neighboring bubbles from emitters 5, 8, and 9, suggesting they may be coated by thin hydrate shells.
The video can be accessed in Text
S6b
.
NE1
NE2
NE3
NE4
NE6
NE5
NE10
NE9
NE8
NE7
1 cm
b
Fig. 2b
Bacterial mats
(
Beggiatoa
), potentially
with surficial gas hydrate
a
10 cm
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(1)
where
r
orifice
is the emission orifice radius (m),
σ
denotes the bubble's sur
-
face tension (taken to be 0.061 N/m for methane at the water depths in
this study (Sachs & Meyn,
1995
)),
ρ
w
represents the seawater density of
1,030 kg/m
3
(Fofonoff,
1985
),
ρ
g
is the methane gas density (Table
2
), and
g
gives the gravitational acceleration of 9.807 m/s
2
. Davidson and Am
-
ick (
1956
) note that bubbles can grow up to 2
V
max
for the low flow rates
observed in this study (Text
S7
). By analogy to Equation
1
, the maximum
volume,
V
max, hydrate
of a bubble held to the orifice by the tensile strength,
τ
, of a hydrate film of thickness,
δ
, is given by:
(2)
Jung and Santamarina (
2011
) measured the tensile strength for methane
hydrate to be 0.2 MPa. The initial film thickness of methane hydrate form
-
ing on a gas/water interface is
∼
10 μm (Li et al.,
2017
; Yin et al.,
2018
). To
obtain a conservative volume estimate of
V
max, hydrate
bubble size, we will
assume only a 1 μm shell thickness.
V
r
g
wg
max
.
.
.
,
2
orifice
V
r
g
wg
max,
..
.
.
.
hydrate
orifice
2
Figure 8.
Relationship between bubble release time and type of bubble released. Release times for the clean bubbles at Sleeping Dragon could not be
precisely determined but are estimated to be 0.1 s or less (not plotted). Example snapshots from Horn Dome and GB648 are shown on the right (yellow scale
bars = 2 mm), along with schematic diagrams of gas (blue), hydrate (gray), and ground (brown) to illustrate various bubble types and their growth processes.
In these schematic diagrams, solid gray lines represent pre-existing hydrate that remains unchanged from bubble to bubble; dashed gray lines represent hydrate
films that can grow along with the bubble, progressively coating the gas/water interface. Longer release times generally correlate with more hydrate coverage of
the meniscus and thicker hydrate walls/shells, but the precise release-time transitions between different shell morphologies will depend on the local solubility
and dissolved-methane concentration conditions. The tube release schematic illustrates a potential internal fluid meniscus-based mechanism for controlling
bubble release (see also Figure
13
). NE seeps corresponds to New England Seep 2, offshore Massachusetts.
Figure 9.
Bubble growth rate relative to hydrate film growth rate for the
Horn Dome (HD) tube-vent emitter (HD4, Figure
4b
). (a) Time series
showing active bubble growth producing a gas/water meniscus (outlined
in yellow). Internal gas circulation (red arrows at time 0 s; Clift et al.,
1978
;
Pinczewski,
1981
), helps shift the growing hydrate film (gray regions
surrounding yellow outline) away from the active meniscus growth. (b)
When flow is shut off to this emitter, the gas/water meniscus (outlined in
yellow) is rapidly coated in hydrate (<0.3 s).