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Abstract
Although gas hydrates often occur in seismically active regions, the role of earthquakes as triggers of hydrocarbon seepage through gas hydrate-bearing sediments has been only superficially addressed. The Makran continental margin offshore Pakistan hosts hydrocarbon-laden sediments and gas hy- drates and is prone to vigorous seismicity. The area was visited in the frame of RV METEOR expedition M 74/3 in 2007 (Bohrmann et al. 2008).
Here we present geochemical evidence for a substantial increase in upward gas flux inducing methane emission into the water column and gas hydrate formation in the sediment, a phenomenon which occurred within a few de- cades of the strongest earthquake ever reported for the entire Arabian Sea. We propose a causal relation and present reflection seismic data supporting our hypothesis that co-seismic ground shaking induced mechanical fracturing of gas hydrate-bearing sediments creating pathways for free gas to migrate from a shallow reservoir within the gas hydrate stability zone into the water column.
Our findings lead to conclude that hydrocarbon seepage triggered by earth- quakes might play a role for carbon budgets at other seismically active conti- nental margins. The newly identified process presented here can help interpret data from similar sites.
Find the corresponding paper here:
Fischer D, Mogollón JM, Strasser M, Pape T, Bohrmann G, Fekete N, Spiess V, Kasten S (2013) Subduction zone earthquake as potential trigger of submarine
hydrocarbon seepage. NATURE GEOSCIENCE 6(8) 647-651 62°40'E 62°50'E
24°20'N24°15'N24°10'N
2000 m
3500 m Water depth
~0.5–1 m
a
Gas bubbles
b
c
Sediment fissure
Earthquake epicentre 1945
Deformation front
Proto-deformation front Hydrate Site
Non-Hydrate Site Seismic line
0 5 10 km
T [yr after 1945]
040 50
6270 90
1501000 measured
0 5 10SO42-15[mM]20 25 30
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Depth [m]
1.5 1.0 0.5 0.0
Depth [m]
20 22 SO2442- [mM]26 28 30
y = -0.74x+20.98 R2 = 0.89
b a
c
T [yr after 1945]
02 5
62 measured
0 5 10SO42-15[mM]20 25 30
1.5 1.0 0.5 0.0
Depth [m]
Non-Hydrate-site Hydrate-site
Depth-integrated AOM rates:
0.01 mol m-2 yr-1 (for T=0) 0.45 mol m-2 yr-1 (for T=62)
Depth-integrated AOM rates:
0.01 mol m-2 yr-1 (for T=0) 3.5 mol m-2 yr-1 (for T=62)
Estimation of the pre-event SMT depth
Pre-event (T=0)
Pre-event (T=0) Steady state
Steady state (T= 5 yr)
1500 2000 2500 3000 3500 4000 4500
380039003850Two-way traveltime [ms]
Offset [m]
S N
Hydrate-site ~123 m to the W of seismic line
S N
Reflection amplitude
Interference between seafloor and gas polarity reflector
Hydrate-site ~123 m to the W of seismic line
Acoustic Turbidity
285029252887.5
1500 2000 2500 3000 3500 4000 4500
380039003850Two-way traveltime [ms]
Offset [m]
(1)
(2)
(3)
Approx. depth below sea level [m]
250 m
~37.5 m VE=6.7
a
b
Seafloor
Elevated gas reflectors Initial depth of gas reflector Gas migration pathways a
b
200 300 400 500
Ba [mg·kg-1]
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
3 6 9 12 15
Ba/Al [(g·g-1)·1,000]
Ba(diagenetic)
Ba(total) Ba/Al
0 10SO42- [mM]20 30
0 25 50 75 100
Ba2+ [µM]
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Depth [m]
SO42- Ba2+
SMT
200 300 400 500
Ba [mg·kg-1]
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
3 6 9 12 15
Ba/Al [(g·g-1)·1,000]
Ba(diagenetic)
Ba(total) Ba/Al
0 10 20 30
SO42-, CH4 /3 [mM]
0 25 50 75 100
Ba2+ [µM]
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Depth [m]
SO42- Ba2+
CH4 SMT
Gas hydrates
Current ba- rite formation
Current ba- rite formation Non-Hydrate-site
Hydrate-site
"Kink type"
sulphate profile
"Concave-up type"
sulphate profile
Gas hydrates
45-91 yr
38-78 yr Earthquake: 62 yr
Formation time of barite enrichments Transport/reaction modeling of pore water profiles Seismic evidence for free gas mobilization
M
W8.1 in 1945
Study site/earthquake epicenter ROV images/ bathymetry
Formation times of authigenic barite enrichments were calculated based on diffusive fluxes of dissol- ved Ba
2+into the precipitation zones of both cores.
They encompass the time that has elapsed bet-
ween the earthquake in 1945 and sampling in 2007 and suggest causal relation.
Transport/reaction modeling of pore water profiles
enabled us to simulate the evolution of the measured sul- fate profiles over time. Best fit to measured profiles when assuming:
a) the injection of free methane gas to shallow depths of 5.9 mbsf. at the Non-Hydrate Site during or shortly after the earthquake and
b) a pre-event depth of the SMT of 21 m. At the Hydrate
Site steady-state conditions are already reached within 5 yr after the event (bubble irrigation).
Conclusions
• Three independent geochemical and seismic indicators suggest substantial increase in CH
4flux a few decades
before sampling
• Mechanical fracturing of gas hydrate- rich sediments through seismic ground shaking
• Earthquakes can trigger the release of hydrocarbons from the seafloor
EARTHQUAKE-INDUCED METHANE MIGRATION THROUGH THE GAS HYDRATE STABILITY ZONE IN THE SUBDUCTION REGIME OFFSHORE PAKISTAN
David Fischer
1,2, José M. Mogollón
1,3, Michael Strasser
4, Thomas Pape
2,
Gerhard Bohrmann
2, Noemi Fekete
2, Volkhard Spiess
2and Sabine Kasten
1,21: Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany 2: Marum - Center for Marine Environmental Sciences, Leobener Strasse, 28359 Bremen, Germany
3: Utrecht University, Faculty of Geosciences, P.O. Box 80125, 3508 TC Utrecht, The Netherlands 4: Swiss Federal Institute of Technology Zürich; Sonneggstrasse 5, 8092 Zürich, Switzerland
Acknowledgements:
We thank captain and crew of RV METEOR for excellent support and cooperation at sea. This work has been supported through the DFG Research Centre/Cluster of Excellence `The Ocean in the Earth System' (MARUM) with additional funding by the Helmholtz Association (Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven)
Cited references:
Bohrmann G and cruise participants (2008) Report and preliminary results of R/V Meteor cruise M74/3, Fujairah-Male, 30 October-28 November, 2007. Cold seeps of the Makran subduction zone (Continental margin off Pakistan); Berichte, Fachbereich 5, Universität Bremen, edited by: Bohrmann, G., and Ohling, G.., Bremen, 161 pp.
Pendse C G (1945) The Mekran earthquake of the 28th November 1945. India Meteorological Department Scientific Notes 10, 141-146 Cover page of the first scientific
description of the 1945 earthqua- ke (Pendse 1945).
The global context of the study area. The nort- hern Arabian Sea hosts the Makran subduction zone forming an accretionary prism with a se- diment thickness of up to 7 km. The study site, Nascent Ridge (3165 m water depth), is the
youngest tectonic feature of the local structural framework. The epicenter of the MW 8.1 eart- quake (Pendse 1945) was 15 km to the W of the study sites.
Contact: David Fischer david.fischer@awi.de 0049-471-48312389
