Pierre Henry, Tiphaine Zitter, Fabienne Fornacciari, Louis Geli Collge de France, Chaire de Godynami

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Cold seeps along the Main Marmara Fault context
and perspectives for monitoring.

• Pierre Henry, Tiphaine Zitter, Fabienne
  Fornacciari, Louis Geli Collège de France,
  Chaire de Géodynamique, Trocadéro, Europôle de
  lArbois BP80, 13545 Aix-en-Provence, France
  Ifremer, BP 70, 29280 Plouzané, France
• Data sources Marmara1 (Suroît) cruise
  bathymetry and HR seismics
• Marmacore, MarmaraVT (Marion-Dufresne) coring
• Marmarascarps (Atalante and Victor ROV)
  observation, microbathymetry, THR.
• Content Cold seep distribution along the active
  fault(s)
• Hydrogeological systems importance of the
  lacustrine past
• Understanding the evolution with time
• Response to strain exemples from subductions
• Marnaut cruise in 2007
• ESONET observatory project

http//www.cdf.u-3mrs.fr/henry/marmara/index.html
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Marmara Sea

•  Following the Kocaeli earthquake, many gas
  seepages have been observed on the sea bed. The
  most prominent of them was one about one km NW of
  Topçular where significant gas bubbles have been
  observed on the sea surface.  
•  Gas plumes are observed on the echo-sounder.
  They are sometimes very thick and diverted from
  vertical by currents. 
•  It is well-known that the subaquaeous
  sedimentary units in the Izmit Bay are charged
  with gas and the Holocene postransgressional
  marine deposits act as a cap layer. The
  gas-charged sediments are generally placed in the
  central parts of the basins where the gulf
  becomes wider. 
• From Bedri Alpar, Turkish Journal of Marine
  Sciences, 5 (3), 1999, "Underwater signatures of
  the Kocaeli earthquake,(August 17th 1999)",
  111-130. http//www.geocities.com/CapeCanaveral/St
  ation/8361/Quake99Turkey.html

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Cold seeps a consequence of methane
(hydrocarbon) release in seawater and/or
oxidation at a shallow depth in the sediment

• Examples from Marmara Sea - Marmarascarps Cruise

Black Patches
Bacterial Mats
Carbonate crusts
Bubble emission
Brackish water emission
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Cold seep distribution in Marmara Sea

• Mostly based on ROV observations during
  Marmarascarps cruise
• All known seeps are on strike-slip fault traces
• Most, but not all fault segments explored have
  cold seeps (black patches bacterial mats)
• A few sites display highly focused flow
• Brackish water expulsion
• Methane free gas migration in the sediment

ROV visual observation track
Black patches
Authigenic carbonates
Active chimneys
Bacterial mats
Fault
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Marion Dufresne Cores (MARMARA VT, Mai 2004)

• Pore fluid sampling
• Determine past Marmara water composition
• Composition of expelled fluids, flow pathways and
  flow rates
• Tectonics and sedimentology
• Connexion Aegean-Marmara Sea-Black Sea

http//www.cdf.u-3mrs.fr/henry/marmara/index.html
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Central basin gas plume in sediment

• Geophysical evidence for free gas (3.5 kHz chirp)
• No geochemical evidence for pore water advection
  (MD04-2742)
• Carbonates and bacterial mats at fault outcrop

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Tekirdag basin chimneys Brackish water outflow

• Active chimneys aligned on fault rupture (ROV
  micro bathymetry)
• Near outlet of canyon

Zone of disturbed echo gas charged sand layers
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Tekirdag basin chimneys kilometer scale flow
cell
N
S
1 km
marine
lacustrine
thick sand layers

• Thick sand layers (gt 1m) in lacustrine section
  contain low salinity pore fluid
• Stronger gradient at MD04-2740 than at MD04-2737
  is evidence for lateral flow toward the fault
  zone

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Smaller scale flow cells influence on
sulfate-methane reaction

• Transient methane migration following earthquake
  1063 A.D. (Halbach et al., 2004)
• Model is too circumstantial and does not explain
  chloride profiles
• Steady state convection driven by salinity
  difference (Henry et al. 1992, 1996)
• Minimum permeability required is very high for
  marine clay-rich mud (10-14 m2)
• Episodic bubble expulsion drives seawater entry
  (Tryon et al., 2002)
• Bubble emission maintains high permeability
  conduits

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A relationship between seawater intrusion and
methane oxidation
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Factors influencing localized pore fluid expulsion

• Driving forces
• Buoyancy from salinity contrast
• Compaction
• 2-phase flow gas generation, migration and
  expulsion
• Conduits
• Coarse turbidites gt lateral flow toward fault
  zone
• Erosion gt open fault conduit at seafloor
• Gas bubble migration paths (gas buoyancy and
  capillary forces)
• Canyon outlets into the deep basins appear as
  favorable sites
• Most active sites probably correspond to local
  flow cells
• basin scale(s), H 1 - 5 km, Z gt 50 m
• small scale, Z 3-10 m, H lt 200 m
• What could be the relationships with the
  earthquake cycle ?
• Gas expulsion and seawater intrusion (and
  landslides Kuscu et al., 2005) as a consequence
  of ground shaking.
• Conduit permeability expected to decrease with
  time due to authigenic carbonate formation.
• Deeper source, variable in time and space, cannot
  be excluded.
• Local flow cells may respond to strain and stress
  variations.

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Cold seep response to strain
OBS flow meter deployments

• Correlated fluid pulsing and seismic tremors in
  the Costa Rica subduction zone (Brown et al.,
  2005)

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Marnaut - Atalante cruise with manned submersible
Nautile - 2007

• Initial objectives (2004)
• Tectonic context of cold seeps
• Fluid sampling
• Biogeochemistry of authigenic carbonates
• Additional objectives
• Gas distribution in the sediment (deep-tow CHIRP
  - availability issue)
• Canyons and slope instability
• Hydrogeology flow-meters (SCRIPPS, M. Tryon),
  piezometers (Ifremer)
• Rare gasses (Helium isotopes)
• Initiate studies of temporal variability
• Strain - fluid flow coupling
• Flow meters, OBSs (4), Piezometers
• Time series of chemical and isotopic composition
  (SCRIPPS (D. Hilton) - CRPG (B.Marty, S.
  Bourlange))
• Osmotic pumps with copper tubes
• Extend the experiment from 1 month (cruise
  duration) to 1 year.
• Cruise proposal in January 2007 to use an Ifremer
  ship for instrument recovery and additional work
  (High resolution bathymetry, and sounding using
  AUV or deep-tow).
• Alternate solutions in Turkey for instrument
  recovery.

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ESONET observatory sites (N. Cagatay)
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Scientific hypotheses to be tested with permanent
observatory

• Strike-slip motion is localized on a main crustal
  shear zone, microseismically active, but locked.
• Faults observed on the seafloor are locked and
  accomodate slip only during earthquakes.
• Sediment input to the deep basins is episodic
  (earthquakes, floods, landslides).
• Fluid expulsion varies in time and space
  throughout the seismic cycle.
• Formation of carbonate crusts and chimneys is
  fast (100 yrs - what is the evolution since 1912
  ?).
• Fluid expulsion at the seafloor has an impact on
  the water column and may contribute to the
  injection of methane into the atmosphère.