Model Uncertainties
Here, we discuss the impacts of hydrate concentration, heat flow and climate model
uncertainties to our results.
Hydrate saturation in the pore space is one of the least certain parameters in our
models. For our default model we used a hydrate saturation of the pore space of 5%,
which lies between the 6-13%, estimated for the same area from P and S wave
velocities in water depths of ~1285-1500 m [Chabert et al., 2011; Westbrook et al.,
2008] and less than 5%, estimated in water depths of 480-866 m [Chabert et al.,
2011]. Thatcher et al. [2013] showed that for our intrinsic permeability value of 10-13
m2, the time for methane to reach the seabed is independent of the hydrate saturation
for values between 5-30% (their Figure 6b). Figure S2a supports the results of
Thatcher et al. [2013], but shows that for a hydrate concentration of 2.5% there is a
delay of ~50 yr, due to the limited amount of methane from dissociated hydrate to
reach the irreducible gas saturation. The maximum rate of seabed methane outflow
and the time period over which it is emitted increases with increasing hydrate
saturation. With the 20% model there is still significant methane in the system by
2300 yr (Figure S2a). For hydrate saturations of the pore space within 2.5-20%, the
maximum rate of seabed methane outflow is between 29-120 mol yr-1 m-2.
Importantly, because the period of gas emission increases with increasing hydrate
concentration, the magnitude of total gas emission from the area increases, because
gas emission is active simultaneously from a greater range of depths and, therefore,
from a larger area.
In our modeling approach, we decided to impose a constant heat flow instead of a
constant geothermal gradient, because the gradient changes with the phase (hydrate,
water or gas) occupying the pore space. For a given thermal conductivity, our heat
flows were estimated by an iterative process of varying them until our present-day
seabed distribution of gas and hydrate (for a 100% methane hydrate) matched current
seismic data that image BSR depth in water depths of more than 580 m and the depth
of the upper limit of gas-related reflectors in shallower waters. We were more
confident about the range of possible thermal conductivities in our study area than
about heat flow values, based on the type of sediments, published measurements
(Table S2) and recent measurements on the top few meters of sediment in the plume
area [1.8-2.1 W m-1K-1, T. Feseker, personal communication], and hence we iterated
over the heat flow. If the real thermal conductivity of the sediment is
over/underestimated, the heat flow will be over/underestimated by the same relative
amount, and so, the temperature profile with depth will remain the same as that which
gives a present-day seabed distribution of gas and hydrate consistent with the seismic
observations. In our models, uncertainties in heat flow arising from uncertainties in
thermal conductivity (within a range of ±30%) do not significantly affect the time for
methane to reach the seabed (Figure S2b). However, they do influence the maximum
rate of methane outflow, ranging from 46-74 mol yr-1 m-2, and the time for methane
from dissociation at the base of the GHSZ to contribute to the methane outflow from
dissociation in the upper part, which produces the second significant increase in
methane outflow (Figure S2b).
The model (or structural) uncertainty was captured by using two different climate
models, HadGEM2 and CCSM4, and the scenario-related uncertainty was examined
by using the two most extreme scenarios, RCPs 8.5 and 2.6, which represent high and
low greenhouse emissions, respectively, to cover the full range of possible future
scenarios. The temperatures predicted by these global climate models were offset to
make the temperature at 2005 yr the same as the mean temperature for the period
1975-2005 given by CTD measurements in our study area (supporting information
Temperature Series Construction). By this means, the bias in the temperatures given
by the global climate models was normalised to our study location. The parametric
uncertainty of climate models, which is an active area of research in its own right, is
beyond the scope of this work. A detailed discussion on climate model uncertainty is
given by Hawkins and Sutton, [2009].
Figure S2: Rate of flow of methane from the seabed at 420 m water depth (mwd),
using HadGEM2 climate model and climate-forcing scenario RCP 8.5. (a) Results for
hydrate saturations (HS) of 2.5, 5,10, and 20% of pore space. All other parameters are
identical to those for the default model (supporting information, Table S3). (b) Results
for thermal conductivity (TC) of 0.98 W m-1K-1 and (HF) heat flow of 54 mW m-2, TC
of 1.4 W m-1K-1 and HF of 54 mW m-2, and TC of 1.82 W m-1K-1 and HF of 100 mW
m-2. All other parameters are identical to those for the default model.
Table S2: Thermal gradient (TH), thermal conductivity (TC) and heat flow (HF)
values west of Svalbard.
Water
Depth
[m]
>2200
>900
850
840
820
813
800
700
688
600-350
560
~400
~250
Thermal
Gradient
[°C km-1]
115
85-122
70
113
73
119
108
97
53-61
44
Thermal
Conductivity
[W m-1K-1]
1.2
1.0-1.2
1.2
1.16
1.09
1.16
1.4
1.4
1.6
1.4
1.14
Heat
Flow
[mW m-2]
102.5
85-122
102.5
131
80
138
88
85
102.5
77
110
Reference
Observations
Vanneste et al., [2005]
Crane et al., [1991]
Vanneste et al., [2005]
Crane et al., [1991]
Eldholm et al., [1999]
Crane et al., [1991]
This study
This study
Crane et al., [1991]
This study
Crane et al., [1991]
Sarkar et al., [2012]
Rajan et al., [2012]
BSR-derived TG
TG, TC, and HF data in the top ~5 m
BSR-derived TG
TG, TC, and HF data in the top ~5 m
TG and TC data in the top ~5 m
TG, TC, and HF data in the top ~5 m
TC and HF constrained by data
TC and HF constrained by data
TG, TC, and HF data in the top ~5 m
TC and HF constrained by data
TG, TC, and HF data in the top ~5 m
Inferred TG from BSR at other water depths
Inferred TG from BSR at other water depths
References
Chabert, A., T. A. Minshull, G. K. Westbrook, C. Berndt, K. E. Thatcher, and S.
Sarkar (2011), Characterization of a stratigraphically constrained gas hydrate system
along the western continental margin of Svalbard from ocean bottom seismometer
data, J. Geophys. Res. Solid Earth, 116, B12102, doi: 10.1029/2011JB008211.
Crane, K., E. Sundvor, R. Buck, and F. Martinez (1991), Rifting in the northern
Norwegian-Greenland Sea: Thermal tests of asymmetric spreading, Journal of
Geophysical Research: Solid Earth, 96(B9), 14529-14550, doi: 10.1029/91jb01231.
Eldholm, O., E. Sundvor, P. R. Vogt, B. O. Hjelstuen, K. Crane, A. K. Nilsen, and T.
P. Gladczenko (1999), SW Barents Sea continental margin heat flow and Håkon
Mosby Mud Volcano, Geo-Marine Letters, 19(1-2), 29-37, doi:
10.1007/s003670050090.
Hawkins, E., and R. Sutton (2009), The Potential to Narrow Uncertainty in Regional
Climate Predictions, Bulletin of the American Meteorological Society, 90(8), 10951107, doi: 10.1175/2009bams2607.1.
Rajan, A., J. r. Mienert, and S. Bünz (2012), Acoustic evidence for a gas migration
and release system in Arctic glaciated continental margins offshore NW-Svalbard,
Marine and Petroleum Geology, 32(1), 36-49, doi: 10.1016/j.marpetgeo.2011.12.008
Sarkar, S., C. Berndt, T. A. Minshull, G. K. Westbrook, D. Klaeschen, D. G. Masson,
A. Chabert, and K. E. Thatcher (2012), Seismic evidence for shallow gas-escape
features associated with a retreating gas hydrate zone offshore west Svalbard, Journal
of Geophysical Research: Solid Earth, 117(B9), doi: 10.1029/2011JB009126.
Thatcher, K.E, G.K. Westbrook, S. Sarkar, and T.A. Minshull (2013), Methane
release from warming-induced hydrate dissociation in the West Svalbard continental
margin: Timing, rates, and geological controls, J. Geophys. Res. Solid Earth, 118, doi:
10.1029/2012JB009605.
Vanneste, M., S. p. Guidard, and J. r. Mienert (2005), Bottom-simulating reflections
and geothermal gradients across the western Svalbard margin, Terra Nova, 17(6),
510-516, doi: 10.1111/j.1365-3121.2005.00643.x.
Westbrook, G. K., et al. (2008), Estimation of gas hydrate concentration from multicomponent seismic data at sites on the continental margins of NW Svalbard and the
Storegga region of Norway, Mar. Pet. Geol., 25(8), 744-758, doi:
10.1016/j.marpetgeo.2008.02.003.