ARTICLE IN PRESS
Marine and Petroleum Geology 25 (2008) 457–472
www.elsevier.com/locate/marpetgeo
Complex plumbing systems in the near subsurface: Geometries of
authigenic carbonates from Dolgovskoy Mound (Black Sea)
constrained by analogue experiments
Adriano Mazzinia,, Michael K. Ivanovb, Anders Nermoena, André Bahrc,
Gerhard Bohrmannc, Henrik Svensena, Sverre Plankea,d
a
Physics of Geological Processes, University of Oslo, P.O. Box 1048, 0364 Oslo, Norway
b
Moscow State University, Vorobjevy Gory, Moscow 119992, Russia
c
Research Centre Ocean Margins, University of Bremen, Post Box 330 440, D-28334 Bremen, Germany
d
Volcanic Basin Petroleum Research, Oslo Research Park, 0349 Oslo, Norway
Received 28 April 2007; received in revised form 20 September 2007; accepted 3 October 2007
Abstract
Targeted sampling on the Dolgovskoy Mound (northern Shatsky Ridge) revealed the presence of spectacular laterally extensive and
differently shaped authigenic carbonates. The sampling stations were selected based on sidescan sonar and profiler images that show
patchy backscatter and irregular and discontinuous reflections in the near subsurface. The interpretation of acoustic data from the top
part of the mound supports the seafloor observations and the sampling that revealed the presence of a complex subsurface plumbing
system characterized by carbonates and gas. The crusts sampled consist of carbonate cemented layered hemipelagic sedimentary Unit 1
associated with several centimetres thick microbial mats. Three different carbonate morphologies were observed: (a) tabular slabs,
(b) subsurface cavernous carbonates consisting of void chambers up to 20 cm3 in size and (c) chimney and tubular conduits vertically
oriented or forming a subhorizontal network in the subsurface. The methanogenic origin of the carbonates is established based on visual
observations of fluids seepage structures, 13C depletion of the carbonates (d13C varying between 36.7% and 27.4%), and by thin
carbonate layers present within the thick microbial mats. Laboratory experiments with a Hele–Shaw cell were conducted in order to
simulate the gas seepage through contrasting grain size media present on the seafloor. Combined petrography, visual observations and
sandbox simulations allowed a characterization of the dynamics and the structures of the plumbing system in the near subsurface. Based
on sample observations and the experiments, three observed morphologies of authigenic carbonates are interpreted, respectively, as (a)
Darcian porous flow through the finely laminated clayey/coccolith-rich layers, (b) gas accumulation chambers at sites where significant
fluid escape was impeded by thicker clayey layers forming the laminated Unit1 and (c) focussed vertical fluid venting and subhorizontal
migration of overpressured fluids released from (b). The Hele–Shaw cell experiments represent a promising tool for investigating shallow
fluid flow pathways in marine systems.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: Black Sea; Shatsky Ridge; Dolgovskoy Mound; Anaerobic methane oxidation; Authigenic carbonate; Hele–Shaw cell modelling
1. Introduction
The Black Sea mud volcanoes, diapirs and cold seeps
have been a target of investigation of numerous marine
expeditions with the aim of investigating the mechanisms
of hydrocarbon-rich fluids seepage (Ivanov et al., 1989;
Corresponding author.
E-mail address: adriano.mazzini@fys.uio.no (A. Mazzini).
0264-8172/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.marpetgeo.2007.10.002
Ginsburg et al., 1990; Ivanov et al., 1992; Limonov et al.,
1994; Woodside et al., 1997; Ivanov et al., 1998; Bohrmann
and Schenck, 2002; Ergun et al., 2002; Kenyon et al., 2002).
Differently shaped authigenic carbonates (slabs, chimneys,
irregular, blocky) have been observed and sampled from
various sites of the Black Sea Basin often associated with
microbial colonies (Woodside et al., 1997; Peckmann et al.,
2001; Thiel et al., 2001; Michaelis et al., 2002; Mazzini
et al., 2004). The precipitation of these carbonates is
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considered to be the result of coupled sulphate reduction
and anaerobic methane oxidation (AOM) operated by
consortia of archaea and bacteria (Ritger et al., 1987;
Valentine and Reeburgh, 2000; Michaelis et al., 2002;
Boetius and Suess, 2004).
The authigenic carbonates collected worldwide have a
broad variety in shape and mineralogy. This variety is
interpreted to be related with a combination of numerous
factors; the four main ones include the varying rate and
composition of the seeping fluids, the type of sediment
where fluid seepage occurs and the type of biota thriving at
the seepage sites (Mazzini, 2004). Several authors tried to
model the various carbonate shapes based mainly on
morphological and petrotraphy studies (e.g. Kulm and
Suess, 1990; Michaelis et al., 2002; Diaz-del-Rio et al.,
2003; Mazzini et al., 2004, 2006). However a modelling approach trying to combine these disciplines with
analogue laboratory modelling has not been investigated in
detail yet.
In 2005 a joint venture of Training Through Research
and METRO Research Programmes (TTR15 cruise),
visited the Shatsky Ridge focussing on the study of
Dolgovskoy Mound (DM) situated along the northwestern Shatsky Ridge (Fig. 1(A)).
This paper reports the data and the observations
collected from the DM during the TTR15 cruise
(Akhmetzhanov et al., 2007). Petrography and geochemical
studies on a unique authigenic carbonate collection were
complemented by laboratory modelling studies representing a novel approach for seepage studies. The described
multidisciplinary approach, including sandbox laboratory
simulations, aims to give new insights into the marine
subsurface plumbing system.
2. Geological setting
Despite the numerous cruises exploring the Black Sea,
little documentation is available from the Shatsky Ridge
region that was broadly explored mainly by Russian
scientists during the last decade (e.g. Grinko et al., 2004;
Afanasenkov et al., 2005a).
The Shatsky Ridge is located between the Tuapse
Trough and the Eastern Black Sea Basin. The ridge has
no bathymetric expression observable on the seafloor
due to a thick cover of Cenozoic sediments. The ridge
overlies continental crust (Starostenko et al., 2004) and
appeared as a positive structure on the seafloor during
the Cretaceous separating the deep water basins of the
Great Caucasus from the new ‘‘young’’ Eastern Black
Sea Basin (Nikishin et al., 2003). Since no deep drilling
has ever been conducted in the Eastern Black Sea region,
most interpretations are based on the geology of the
surrounding coast areas and seismic investigations. Recent
seismic investigations (2001–2004) revealed structures,
suggesting that a significant portion of the ridge consists
of Late Jurassic carbonate build-ups (Afanasenkov et al.,
2005a, b).
More detailed bathymetric studies were conducted in
1996 by the RV Gelendzik completing a Simrad EM-12S
multibeam mosaic along the Shatsky Ridge and the Tuapse
Trough (Andreev, 2005). This mosaic combined with the
previously acquired seismic data allowed the identification
of several new structures (possible mud volcanoes) in these
two areas.
3. Sedimentary units and authigenic carbonates from the
Black Sea
A common pattern of three distinct modern sedimentary
units characterize most of the deep water areas of the Black
Sea (Andrusov, 1890; Degens and Ross, 1972; Ross and
Degens, 1974; Shimkus et al., 1975). Below the seafloor, the
typical geological sequence includes three main units:
(a) Unit 1 consists of thin alternating coccolith ooze
(mostly Emiliania huxleyi) and clayey sediment laminae.
The thickness of the clayey laminae can occasionally reach
several centimetres. At seepage locations, Unit 1 (and
sometimes Unit 2) can be interbedded by exotic centimetre
thick structureless and light coloured clayey layers. The
origin of these so called ‘‘Degens layers’’ has been discussed
for long since their first discovery and several hypotheses
have been suggested (Degens et al., 1978). Degens et al.
(1978) initially proposed a turbidite origin of these layers,
Calvert et al. (1987) linked them to mudflow events,
however the most likely hypothesis is the one described by
Kempe et al. (2001) that suggested the localized expulsion
of fluidized and overpressured clayey sediments on the
seafloor. (b) Unit 2 consists of thin laminations of sapropel;
(c) Unit 3, interpreted to be lacustrine facies, consists of
laminations of terrigenous (silty–clayey) sediment, commonly containing hydrotroilite layers. These units reach
their maximum thickness in the central part of the
Black Sea; Units 1 and 2 are on average 30 cm thick, while
Unit 3 can reach several metres in thickness (Ross and
Degens, 1974).
Likewise for the sedimentary units, a broad pattern of
authigenic carbonates has been described by Mazzini et al.
(2004). The typification includes: Type U1 (consisting of
layered slabs of carbonate cemented clayey and coccolith
ooze laminae from Unit 1); Type U2 (consisting of
layered slabs of carbonate cemented sapropelic Unit 2),
Type U3 (layered carbonate cemented Unit 3), Type MSa
(structureless micrite-cemented clay) usually forming
laterally extensive pavements at the seafloor or within
Unit 1. This type of slabs possibly represents the carbonate
cementation of the described ‘‘Degens layers.’’ Carbonate
types U1 and MSa were sampled from the study area
described in this paper.
4. Methods and experimental setup
The marine expedition was conducted with the RV
Professor Logachev equipped with the long range sidescan
sonar OKEAN, single channel seismic system, MAK-1 M
35°0'E
30'
36°0'E
30'
37°0'E
30'
38°0'E
30'
36° 40.5'E
39°0'E
36° 41'E
36° 41.5'E
36° 42'E
44° 1.5'N
30'
A
20
45°0'N
B
30'
10
MAK
58BS
45°0'N
50
100
200
12
8
00 00 400
1400
600
30'
BS347G
100
0
30'
1600
1800
Tu
aps
eT
rou
gh
2000
BS348G
BS349Gr
44°0'N
Dolgovskoy
44° 1'N
Sh
ats
ky
Rid
ge
30'
30'
30'
36°0'E
30'
37°0'E
30'
38°0'E
30'
39°0'E
500
C
500
1970 m
SE
acoustic shadows
NW
2020 m
2070 m
20:30
20:00
19:00
18:00
17:00
459
Fig. 1. (A) Bathymetric map around the study area with Shatsky Ridge and Dolgovskoy Mound (red dot), inset of Black Sea map; (B) detail from MAK58BS sidescan sonar line (perimeter framed in
image C) through Dolgovskoy Mound with sampling stations and (C) sub bottom profiler and image of deep towed MAK58BS sidescan sonar line through Dolgovskoy Mound.
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A. Mazzini et al. / Marine and Petroleum Geology 25 (2008) 457–472
44°0'N
BS346Gr
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460
deep towed high resolution 100 kHz sidescan sonar system
and 5 kHz subbottom profiler, with a TV remote controlled
grab, and a 1.5 tonnes gravity coring device that sampled
the targeted locations. The samples collected were described and selected for further analyses.
Thin sections of carbonate samples were studied using
both optical and electronic microscopes. X-ray diffraction
(XRD) analyses were performed on bulk carbonate
samples to determine the dominant carbonate phase.
Carbon and oxygen isotopic analyses were completed on
bulk carbonate samples at the Institute for Energy
Technology at Kjeller, Norway. Carbonate cements were
ground and digested with a 0.1 ml 100% H3PO4 solution
for 2 h at 30.0 1C in a vacuumed environment. The released
CO2 was transferred to a Finnigan MAT DeltaXP isotope
ratio mass spectrometer (IRMS), for determination of d13C
and d18O. Results are reported in % relative to the VPDB
standard (Table 1). The precision for d13C is 70.1% and
70.2% for d18O.
Sandbox experiments on granular media (Gidaspow,
1994; Jaeger et al., 1996) were conducted at the Physics of
Geological Processes Laboratory (University of Oslo) on a
quasi two dimensional Hele–Shaw cell (e.g. Woolsey et al.,
1975; Tanaka and Toyokumi, 1991; Nichols et al., 1994)
consisting of two 600 600 12 mm3 glass plates 8 mm
distant from each other. Several set-ups and tests have been
collected before selecting the ideal parameters where the
subsurface deformation in fluid flushed discontinuous
media are best observable. The cell was filled with an
18 cm thick interval of spherical Beijer glass beads
(ø ¼ 420–840 mm) interbedded by a 0.3 mm thick layer of
devolite (china clay) in the upper part. Air was used as the
analogue fluid to induce overpressure in the media. Air was
flushed in the system through an inlet with inner diameter
of 3.8 mm, placed 6.3 cm into the cell in order to prevent
preferential flow along the bottom and edges of the cell. Air
flux and pressure data (i.e. monotonic increase of air flow
and pressure drops across the sedimentary media) were
collected in real time by an Omega pressure sensor and
Omega FV-135 flow meter. The pressure sensors were
placed by the air inlet and at the top of the bed. Pictures
were taken during the experiment using a high resolution
digital camera at a rate of 10 images per second.
Experiments were performed in a two dimensional set-up
and the quantitative measurements of the pressure and
flow velocity are thus not directly applicable to a three
dimensional setting. The walls of the Hele–Shaw cell set
definite limits on the deformation of the top layer.
Furthermore there exists friction between the walls and
the flowing grains that does not occur in natural systems
(Mourgues and Cobbold, 2003). Nevertheless, adding a
third dimension does not introduce any new physical
effects in the processes of interest. The main processes of
interests (i.e. Darcy flow in porous media and deformation)
and their scaling are independent of the number of spatial
dimensions (Gidaspow, 1994). Therefore two dimensional
experiments capture the same physical processes as in a
three dimensional setting. Further experiments were done
in order to constrain the parameters controlling the
deformation of the media. This was achieved using Beijer
glass beads as single media and varying its thickness h
(Fig. 8(A)) from the major permeability contrast (i.e. here
represented by the interface between the inlet and the glass
beads) to the surface. In this way the deformation observed
on the contact between the inlet and the Beijer glass beads
follows the same principles of the deformation occurring
between the glass beads and the impermeable clayey layer
as the abrupt permeability contrast (from higher to lower
Table 1
Summary of sampling points described and main petrographic, morphological and geochemical characteristicsa
Sample number
Structure
d13C%
(VPDB)
d18O%
(VPDB)
XRD analyses
%a
Comments
BS346–2.1
Vertical chimney
–
–
Py ¼ 100
BS346–2.2
Vertical chimney
–
–
Py ¼ 100
BS346–2.3
Cavernous
27.4
0.8
CC ¼ 100
BS346–2.4
Slab
36.7
0.2
CC ¼ 100
BS346–2.5
Cavernous
36.4
0.4
CC ¼ 100
BS346–2.6
Horizontal tube
33.5
0.1
Py ¼ 100
BS346–2.7
Slab
33.5
0.0
MgCC ¼ 37,
CC ¼ 37,
Qz ¼ 26
Black iron rich mineral (pyrite)
from central part of seepage pipes
Black iron rich mineral (pyrite)
from central part of seepage pipes
White calcite minerals associated
with bacterial mats
Carbonate cement from
laminated slab Type U1
White calcite minerals close to
pyrite deposits
Carbonate crystals (isotopes
analyses) close to black iron-rich
mineral (XRD analyses) from
tubular shaped precipitate
White slab Type MSa capping
Type U1 crust
Pv ¼ pyrite, MgCC ¼ Mg calcite, CC ¼ calcite, Qz ¼ quartz.
a
Average estimates.
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permeability) generates a zone of overpressured fluids. This
set-up allowed us to acquire a significant amount of data
simply by changing the thickness h of the single granular
media.
5. Results
5.1. Acoustic survey: the structure of the mound
A sidescan sonar survey was conducted in order to select
the primary targets for further sampling. Sidescan sonar
line MAK58BS was acquired in a NW–SE direction along
the Shatsky ridge (Fig. 1(C)).
The acoustic image shows that the mound has a
subcircular shape with a diameter of approximately
900 m and a height of more than 70 m (Fig. 1(B) and
(C)). The mound is highly asymmetric and shows at least
three sharp positive features on its top part. The northwestern (and probably southern) slope is steep and slump
deposits (possibly representing mud breccia) are observed
461
at its foot. The south-eastern slope is gentler and is
interrupted by a sharp step in its middle part. A moat
frames the foot of this slope. Backscatter record appears
patchy throughout the structure, revealing areas with
stronger reflection where carbonate crusts and/or coarser
sediment deposits (e.g. mud breccia) are inferred. Strong
backscatter is not observed in the central part of the
structure. The record of weak and medium backscattering
signal between time marks 18:31–18:37 is associated with
partial recording error when the fish was rapidly pulled up
along the steep slope close to the top of the structure.
Both sidescan sonar and subbottom profiler records are
inconsistent with other profiles from the deep parts of the
Black Sea. Usually sonar images are characterized by weak
grey monotonous backscatter, and profiler displays flat
relief and relatively deep penetration of the acoustic signal
through the stratified Quaternary deposits. The images
recorded around the DM show the opposite. The sonar
image shows a complex backscatter consisting of patches of
different size and shape while the profiler reveals irregular
5 cm
d
l
d
l
d
l
500 m
200
Fig. 2. Slab shaped carbonates. (A) Seafloor image showing laterally extensive carbonate slabs; image view approximately 1.2 m; (B) hand specimen of
finely laminated and carbonate cemented sedimentary Unit 1; note some small degassing vesicles on the upper surface filled by microbial colonies
(arrowed); (C) thin section image perpendicular to lamination showing layering of alternated coccolith-rich and clay-rich layers; (D) detail of area framed
in image (C) showing alternations of darker pyrite-organic-calcite-rich layers (d) and lighter coloured clay-rich layers (l).
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A. Mazzini et al. / Marine and Petroleum Geology 25 (2008) 457–472
8 cm
5 cm
2 cm
500 μm
Fig. 3. Cavernous shaped carbonate. (A) Seafloor image showing irregular morphology; image view approximately 1.2 m; (B) large block of carbonate
cemented sedimentary Unit 1, the internal part of the block is void and coated by thick mats of microbial colonies; the upper part if the block is
characterized by up to 1 cm sized degassing vesicles and several centimetres sized ‘‘chimney like’’ features that pierce the hollow block; the deformed
carbonate cemented laminae are visible throughout the block; on the central part of the carbonate sample are visible pipes horizontally oriented and tubes
that form an intricate network in the subsurface; (C) details from area framed in image (B) showing some of the pipes horizontally oriented; (D) example
of cavernous hand specimen entirely coated with thick pinkish microbial mat in the internal part; (E) details of microbial mat with authigenic carbonate
layers precipitating inside (arrowed); (F) thin section image of sparitic authigenic carbonate precipitating within the microbial mat.
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morphology with numerous positive shapes a few meters in
height and with no layering of sedimentary units.
Numerous subcircular depressions up to tens of meters in
size were also observed through the sonar record. Some of
these features resemble the pockmarks observed in other
settings. Areas with very strong backscatter are present
between time mark 16:20–16:40 and 17:10–17:20 starboard
side. This type of backscatter is not associated with sharp
relief and is interpreted as (a) lithological heterogeneity due
to deposits from the distal part of the Kerchensky Fan or
(b) associated with localized hydrocarbon-rich fluids
seepage (e.g. authigenic carbonate). However, in case of
the section from 19:20 to 20:20 we can observe clear
topographic image with strong backscatter towards the
sidescan and accompanied by acoustic shadow. The
apparent subcircular form of this feature might be
interpreted as the existence of a mud volcano with crater
walls (?). A similar kind of buried mud volcanoes or large
collapse structures have been observed previously in the
463
Sorokin Through (Ivanov et al., 1998) and Mediterranean
Ridge (Ivanov et al., 1996). An alternative interpretation
suggests that these features could represent the remains of
the channel-levee system recorded in SE part of this line.
The acoustic reflectivity of the bottom deposits are
changing significantly along the subbottom profiler record,
suggesting strong lithological heterogeneity.
5.2. Seafloor and carbonate observations
TV line profiles around the top part of DM were selected
based on the acquired sidescan sonar images. A preliminary report of the seafloor sampling and observations with a
discussion of the results was initially given by Mazzini et al.
(2007). The TV record revealed the presence of numerous
escarpments (up to 1–2 m high), differently shaped
carbonate deposits and localized microbial mats. The three
gravity core sampling stations retrieved ordinary hemipelagic sequences, showing distinct carbonate cemented
Fig. 4. ‘‘Chimney-like’’ carbonates. (A) Image showing chimney-like carbonates rising from the seafloor (framed on dashed line); image view
approximately 1.2 m; (B) hand specimen showing sections of chimneys entirely cemented by framboidal pyrite; (C) details of internal part of the chimney
(framed in image B) entirely consisting of black framboidal pyrite, the reddish colour represents the oxidation that appeared the day after the sampling and
(D) thin section image of pyrite framboid aggregates.
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layers (Type U1 and Type MS) and sandy-silty intervals.
Spectacular samples were recovered from the topmost part
of DM (station BS346Gr, 1965 m water depth) were the
sidescan sonar record had high backscatter and where the
TV images showed an irregular seafloor. Using a TV
remote controlled grab, a large collection of authigenic
carbonates (up to 60 cm wide) was retrieved onboard. The
recovery revealed that these carbonates have a complex
geometry in the subsurface. Analyses and observations
(Table 1) distinguished three main shapes of carbonate
features: (a) slab, (b) cavernous and (c) chimney-like and
tubular.
The slab type carbonates appear laterally extensive on
the seafloor (Fig. 2(A)) and mainly consists of carbonate
cemented Unit 1 sometimes capped or interbedded by a
layer of carbonate Type MSa (cf. Section 3). The thickness
of these slabs can vary from a few centimetres up to
10–15 cm (Fig. 2). Microbial colonies were observed
thriving as a thin dark biofilm on the interface between
the carbonate cemented sedimentary layers. Thin section
observations are identical to the one described by Mazzini
et al. (2004) showing the fine laminations of alternated
clay- and coccolith-rich layers. Microscopy also shows
that carbonate precipitation occurs prevalently along the
coccolith-rich layers where pyrite framboids and microbial
remains are also highly concentrated (Figs. 2(C) and (D)).
The irregular features observed on the seafloor
(Fig. 3(A)) appear in the subsurface as deformed and
stacked carbonate cemented sedimentary intervals (Unit 1)
that encase carbonate-coated cavernous structures (up to
25–30 cm2 in size, Figs. 3(B) and (D)). These cavities are
cemented by carbonate Type U1 occasionally interbedded
by Type MS (high magnesium calcite). Approaching the
thick microbial mats in the internal part, the low
magnesium carbonate Type U1 is much better crystallized.
The internal part of these large voids is commonly coated
by light coloured sparitic calcite and by several centimetres
thick microbial colonies of differing colours (i.e. mainly
pinkish, but also whitish and yellowish, Figs. 3(D) and
(E)). Within these thick mats, millimetre scaled layers of
calcite were observed forming distinct horizons. XRD
analyses and thin section images of the carbonate coating
the subsurface chambers and growing within the microbial
mats show sparitic low magnesium calcite devoid of any
siliciclastic admixture (Fig. 3(F)).
Several types of ‘‘chimney-like’’ and tubular carbonate
samples are associated with these cavernous structures.
δ18O (%o V-PDB)
-2
-1
0
1
2
-10
-15
NIOZ MV
Odessa MV
-20
δ13C (%o V-PDB)
464
Dolgovskoy M
-25
-30
-35
-40
-45
-50
Fig. 5. Stable carbon and oxygen isotope values of authigenic carbonates
from Dolgovskoy Mound compared with values obtained from Odessa
and NIOZ Mud Volcanoes (Mazzini et al., 2004).
Chimneys erect vertically in the water column (Fig. 4(A)).
At these sites the carbonate thickness and vertical
deformation is prominent and a positive relief with
high concentrations of microbial mats are observed.
The internal part of these chimneys is often completely
filled by framboidal pyrite, but it is not uncommon to
observe a central void conduit that pierces the whole
chimney (Figs. 4(B) and (C)). XRD analyses and thin
section images confirm the sole presence of pyrite
aggregates together with microbial remains. Tubular
features (up to 5 cm in diameter) can also branch off
from the lower part of the cavernous structures, forming
a network that extends horizontally in the subsurface
(e.g. Fig. 3(C)).
5.3. Geochemical analyses
Carbon stable isotope measurements on the authigenic
carbonates (Table 1) reveal d13C varying from 27.4%
to 36.7% and d18O values from 0.8% to 0.2% (Fig. 5).
No significant variations of d13C and d18O were observed
in carbonate measured from the different samples.
The lightest carbon values are observed in carbonates
Fig. 6. Hele–Shaw sandbox analogue simulation of fluid induced deformation of an heterogeneous sedimentary package. (A) Initial experimental
conditions: the porous granular media (dark grey) is interlaid in the top part of the box by a impermeable thin layer of clay (black); the air inlet is indicated
by an arrow; (B) At P ¼ 2276 Pa diffusive air seepage through the porous granular media; once the air reaches the impermeable clayey layer the diffused
seepage is impeded and when the required overpressure is reached, a displacement occurs on the interface between the two media; (C, D) deformation
occurs when the overpressured fluids are gathered underneath the impermeable clayey layer; the deformation increases with pressure (pressure varies from
3530 to 3632 Pa); (E) the deformation persists and fluids continue to seep laterally. Microfractures and sediment unconformities develop, allowing part of
the fluid to breach the impermeable clayey media in the extensional zones (P ¼ 3415 Pa); (F) vertical seepage towards the open surface cannot cope with
the amount of fluids seeping from depth and is coupled with lateral seepage deformation; pressure drops to P ¼ 2960 Pa and (G) diagram showing pressure
reading and flow velocity of the fluid during the experiment. Corresponding figures are indicated by arrows.
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precipitating close to the microbial mats. To our knowledge no other stable isotope data from authigenic
carbonates from the Shatsky Ridge has been documented.
5.4. Hele–Shaw sandbox simulation
As previously described (cf. Section 3) localized variations and discontinuities exist in the thickness of the clayey
layers present in Unit 1. The working hypothesis is that
since some of the described impermeable clayey laminae
have thickness greater than centimetre scale, they
can impede more efficiently the seepage of fluids, hence
creating overpressure in the subsurface and further
deformation. This hypothesis was tested with the Hele–
Shaw sandbox experiment aiming to observe the fluid
behaviours through overpressured lithologically discontinuous units (Fig. 6(A)). Hence particularly significant were
the deformations occurring on the interface between the
granular and clayey media.
Constant monitoring of the pressure drop and the flow
velocity during the experiment (Fig. 6(G)) allowed to
couple visual observations with these physical parameters.
The sequence of images shows that at low air fluxes and at
pressure drop of 2276 Pa, the fluid seeps with a Darcian
flow distributing evenly through the permeable granular
media lying underneath the low permeability clayey layer
(Fig. 6(B)). The laterally continuous impermeable clayey
layer cannot accommodate the flow and impedes the
seepage of fluids towards the surface. As the fluid flux
increases, overpressure on the interface between the
granular and the clayey media is sufficient (i.e. pressure ¼
3530 Pa) to gradually lift vertically and deform the top
layers (Fig. 6(C)). As the overpressure build-up increases,
the bulging becomes more prominent as well as the lateral
seepage of fluids below the impermeable layer (Fig. 6(D)).
During this phase the pressure rises from 3530 to 3632 Pa.
During the vertical deformation, compression occurs
on the top part of the bulge and extension along the lower
flanks where weakness spots become more distinct
(Fig. 6(E)). To this abrupt fracturing corresponds a sudden
irreversible pressure drop (3415 Pa). This extension allows
the fracturing and thus the seepage of fluids through the
clayey layer, forming a subvertical conduit (Fig. 6(F)). At
this point the pressure drops further to 2960 Pa.
6. Discussion
6.1. Microbial reactions and differentiated authigenic
minerals precipitation
Ground truthing confirmed the interpretation of the
acoustic data. Sampling showed that a complex plumbing
system exists in the subsurface of the study area.
The carbonate 13C depletion revealed by stable isotope
analyses supports the idea that the extended microbial
colonies mediate the oxidation of hydrocarbon-rich fluid,
resulting in carbonate precipitation. This is also supported
by stable carbon isotopic analyses (d13 CCH4 as low as
75.1%) of free gas collected from the sediments in the TV
grab (V. Blinova, pers. comm.). The three described
carbonate types are interpreted as the result of three
different mechanisms of fluid seepage and carbonate
precipitation.
Striking mineralogical differences have been described
between the chimney features (entirely coated by framboidal pyrite) and the internal part of the cavernous features
(sparitic calcite). As already documented by other authors
these two minerals are, respectively, the by-product of
sulphate reducing bacteria (SRB) and methane oxidizing
(MO) archaea (e.g. Hovland et al., 1985; Ritger et al.,
1987; Boetius et al., 2000). Bahr et al. (2007) completed
detailed lipid biomarker analyses on the thick microbial
colonies in the cavernous features, detecting the presence of
SRB and MOAs. Our visual and petrography observations
could suggest that SRB thrive more abundantly in the
chimney and tubular features where focussed fluid seepage
occurs, while preferentially MO flourish where no sustained seepage occurs, forming thick mats and inducing
Fig. 7. Conceptual evolutionary cartoon of fluids seepage and carbonate precipitation in the subsurface. (A) Fluids rising from greater depth reach the
near subsurface moving by Darcian flow through parallel layered sediment deposits; (B) details from area framed in image A. When more sustained fluid
seepage occurs and when the seeping fluids reach thicker clayey layers, their rise is impeded and deform the soft sediment creating chambers where gas is
accumulated and (C) microbial colonies grow in the internal part of these chambers (pink framing line) and precipitation of authigenic carbonate occurs.
The overpressured fluids continue to move laterally, forming a network of pipes and tubular features that extends horizontally through the soft sediment.
Note: the fish has a purely cosmetic purpose for the cartoon, as we are considering anoxic conditions (i.e. water depth 2000 m).
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467
6.2. Gas hydrates fuelling from the subsurface?
precipitation of calcite (i.e. in the large voids in the
subsurface). However this is a speculative suggestion since
comparative analyses from the microbial colonies thriving
in the pyrite-rich chimneys are currently not available.
DM lies in the gas hydrate stability field. The hypothesis
that significant deposits of gas hydrates were present at this
P2
Glass beads
h
L
P1
Inlet
L=0.8 cm
Flow meter
Flow
control
2500
π1 = vc μh / (Ck), [1]
2000
1500
Deformation
1000
Static bed
500
0
0
10
20
30
40
π2 = h / L, [1]
50
60
70
80
Fig. 8. (A) Schematic drawing of the experimental set-up. The air is injected through the vertical pipe into the bed. The critical Darcy velocity vc and
pressure P ¼ P1–P2 is measured with sensors. The height h is measured from the surface down to the permeability contrast at which the sediment
deformation occurs. In the development of the phase diagram h is measured from the top of the inlet to the surface. In the experiment represented by the
pictures in Fig. 6, the main permeability contrast is between the surface and the bottom of the clay layer where the deformation occurs. (B) Dimensionless
phase diagram marking the onset of the fluid-induced deformation. The diamonds show the measurements of the normalized critical Darcy velocities, the
solid line shows the best fit to the measured values; g(p2) can be extrapolated and applied to similar cohesion controlled settings, such as the Dolgovskoy
Mound.
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468
location is certainly fascinating and could perfectly fit with
the morphology of the described features. The subsurface
cavities could represent the volume occupied by former gas
hydrate deposits that were subjected to dissociation.
Nevertheless this possibility might be ruled out since the
observed oxygen isotopic values are not consistent with
cements that precipitated from fluids enriched in 18O
(as it should be in case of gas hydrate dissolution).
The recorded isotopic range (0.8%od18Oo0.2%)
instead reveal that the cements precipitated from a
fluid in isotopic equilibrium with the seafloor temperature
that averages around 9 1C (Friedman and O’Neil,
1977; Swart, 1991). This is consistent with the absence of
typical features of gas hydrate dissociation in the grab
retrieved onboard.
6.3. The subsurface plumbing system: mechanisms of fluids
seepage and carbonate precipitation
Based on the observations and experimental results, and
taking into account the previously published data, the
following section suggests conceptual models of authigenic
carbonate formation and fluid seepage for the study
area. The Hele–Shaw sandbox simulation may decipher
some of the mechanisms of fluid seepage-induced matrix
deformation.
Where local fluid seepage is not sustained, Darcian flow
processes prevail and the formation of slab features occurs
(Fig. 7(A)). During this type of microseepage no significant
deformation occurs as observed on the carbonate cemented
slabs and during the sandbox experiments (Figs. 2 and
6(B)). The cementation process of almost identical slabs
has been studied in detail by Mazzini et al. (2004, Fig. 12)
after comparing a large collection of carbonate cemented
sedimentary units from different regions of the Black Sea.
The petrography observations presented in this paper
also confirm that the vertical seepage of hydrocarbon-rich
fluids is impeded by clayey layers. At this interface
microbial colonies thrive and induce precipitation of
micritic carbonate cement that extends within the more
porous coccolith-rich layers. This model is also strongly
supported by microbial mats observed thriving preferentially underneath thicker clayey intervals. The seepage of
fluids continues vertically through microfractures and
microvesicles (see Fig. 5(D) in Mazzini et al., 2004) until
another clayey layer will halt the fluid rise, initiating new
AOM and carbonate precipitation. The thickness of the
carbonate cemented slabs depends on the duration of the
fluid flow seepage and on local variations in the clayey
layer thickness.
A different mechanism involving a significant amount of
free gas seepage is necessary to explain the carbonatecemented decimetre scaled cavities and tubular features. At
these locations the thicker and more impermeable clayey
layers within Unit 1 (including also the Degens layers)
impede the immediate release of a more sustained seepage
of hydrocarbon-rich fluids (Fig. 7(B)). The gas saturated
fluids entrapped in the subsurface deform the soft
sedimentary layers, forming large, laterally elongated void
areas (Fig. 3). A similar behaviour is observed in the
Hele–Shaw experiment when sufficient overpressure is
accumulated to initiate the deformation (Fig. 6(C)). Below
the seafloor, microbial colonies grow inside these cavities,
inducing the precipitation of carbonate minerals via AOM.
Once the system reaches these conditions if more fluids are
provided (e.g. cf. Fig. 6(F) in the Hele–Shaw simulation)
focussed vertical seepage occurs through the chimneys,
releasing part of the overpressure created within the
cavernous features (Fig. 4). As carbonate and pyrite
precipitation continues, a solid cage inside and around
the cavernous structure forms, preventing further deformation of the sedimentary layers and, of course, further
reducing permeability of the seeping fluids. Additionally, a
progressive decrease of the chimney’s diameter (due to
continuous precipitation of authigenic minerals) induces
new internal overpressure. Since seepage of fluids at the
seafloor cannot cope with the amount of gas rising from
depth, the subsurface plumbing system has to further
adjust, forming complex networks of tubes, pipes and
cavernous structures that extend laterally (Figs. 7(C) and
3(C)). The described model is reproducible at various
depths (e.g. where the thicker clayey layers occur) and can
explain the presence of the superposed cavernous structures
collected from Unit 1.
6.4. Dimensional analysis from the Hele–Shaw sandbox
experiments
Numerous landers have been distributed at several
offshore seepage and venting sites with the aim to complete
long-term measurements of the seeping fluids (e.g. Leifer
et al., 2004; Marinaro et al., 2006; Person et al., 2006).
A problem with this type of monitoring is that it cannot
take into account the subsurface deformations or consider
Table 2
Physical parameters used in the Hele–Shaw sandbox and existing in natural seepage environments
Setting
C (Pa)
K (m2)
L (m)
m (Pa s)
h (m)
vc (m/s)
Hele–Shaw
Offshore seepsa
102
106
1.33 109
1013
3.8 103
L
17.6 106
104
h
101
vc
103
a
Average estimates.
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A. Mazzini et al. / Marine and Petroleum Geology 25 (2008) 457–472
469
h/L=10 000
12
0
Ultimate strength of materials
−2
log 10 (C), [Pa]
8
−3
Static bed
−4
6
−5
−6
4
−7
−8
2
−9
Deformation
0
−18
−16
log10 of critical Darcy velocity [m/s]
−1
C*
10
1m/s
−14
−12
−10
−8
−6
−4
−10 1mm/yr
2
log 10 (k/μ), [m /Pas]
h/L=10
12
0
Ultimate strength of materials
C*
−1
−2
Static bed
log 10 (C), [Pa]
8
−3
−4
deformation on
offshore seeps
6
−5
−6
4
−7
Deformation
log10 of critical Darcy velocity [m/s]
10
1m/s
−8
2
−9
0
−18
−16
−14
−12
−10
−8
−6
−4
−10 1mm/yr
log (k/μ ), [m 2 /Pas]
10
Fig. 9. Plots showing how the sediment cohesion C, which is the least constrained parameter, depends on sediment permeability over fluid viscosity k/m
when varying the critical Darcy velocity vc in two different extreme values where h/L ¼ 10,000 (image A) and h/L ¼ 10 (image B); plots based on phase
diagram in Fig. 8(B). The plots highlights that: (1) vc increases for increasing cohesion C and permeability k of the sediment, and decreasing fluid viscosity
m; (2) when increasing the size of the feeder channel L compared to the sediment height above the permeability contrast h, the bed deforms at lower vc at
given C and k/m. The dashed line represents the ultimate strength of any material, which acts as an absolute upper limit of the cohesion. The value C*
(i.e. 1010 Pa) corresponds to 1/10 of the shear modulus (Braeck and Podladchikov, 2007). Therefore, our estimates of cohesion are limited by this value.
the rheology of the sediments during the fluid seepage.
Sandbox experiments could provide further information in
order to quantify the mechanism of fluid seepage. In order
to estimate if the Hele–Shaw technique has relevance also
for natural seepage systems, we have used a quantitative
approach to the data collected.
In cohesion controlled systems the weight of the overburden can be neglected, when looking at the top
surface sediments (e.g. the topmost layers in marine
seepages). For this reason our analysis focused on five
dimensional parameters: the critical fluid Darcy velocity
vc (m/s) at which the deformation of the sediment occurs;
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A. Mazzini et al. / Marine and Petroleum Geology 25 (2008) 457–472
the cohesion C (Pa); the permeability of the bed over the
viscosity of the fluid k/m (m2/Pa s); the inlet diameter L (m)
and the depth of the permeability contrast at which
deformation occurs h (m). When using dimensional
analysis, the problem is reduced to two dimensionless
parameters p1 ¼ Ck/mhvc and p2 ¼ h/L. The parameter p1
is interpreted to be the ratio of the material strength C
and the fluid overpressure across the bed, while p2 is a
geometric conversion factor between the sediment height
and the inlet diameter. The dimensionless parameters can
also be written as
Ck
h
g
vc ¼
.
mh
L
The laboratory experiments were performed reproducing the simulation varying h and measuring vc as
described in Section 4 using a single media. Our results
(Fig. 8) show that g(p2) increased with (h/L)2. As g(p2) is
applicable also in natural seepage settings, and as L
represents a parameter that is measurable both in
laboratory and in the field, the width L of the feeder
channel could be extrapolated using the physical parameters existing in natural seepage settings (Table 2).
L was found to be in the order of 101 m. The value
obtained matches the size of the feeder channel observed
at the Shatsky Ridge seepage sites and is also typical
for many other marine locations (e.g. Kulm and
Suess, 1990; Michaelis et al., 2002; Diaz-del-Rio et al.,
2003).
Further extrapolations from the results summarized
in Fig. 8(B), can be obtained from Fig. 9 that shows
how the critical Darcy velocity vc correlates with
the variations of C and k/m. Fig. 9 also indicates the
field of deformation in natural settings (i.e. in clayey
sediments and in the close subsurface) where a critical
Darcy velocity is estimated at 103 m/s, confirming
other estimates (cf. Table 2). Finally the plot can be
used to constrain the sediment cohesion in any setting
where the permeability of the sediment, viscosity of
the fluid, the feeder width, height from the pressure
source onto the surface and the Darcy velocity are
measured.
Our results show that analogue experiments represent
a powerful tool to investigate the mechanisms of fluids
seepage in granular media. This type of calculation
represents an initial attempt to quantify the geometry
and the physical principles of the plumbing system.
However, further investigations have to be performed
to further corroborate the geological observations in
order to, e.g., prove the scaling dependency of the inlet
diameter.
7. Conclusions
Acoustic investigations and ground truthing of the DM
revealed the presence of seeping hydrocarbon-rich fluids
and ongoing AOM resulting in widespread carbonate
precipitation. Based on sample observations combined
with laboratory experiments, three types of carbonate
structures related to fluid seepage and mineralogical
precipitation have been identified.
(A) Slab: Darcian flow of hydrocarbon-rich fluids induces
precipitation of micritic carbonate cement within the
thin sedimentary laminae;
(B) Cavern: thick clayey laminae present inside Unit 1
impede sustained seepage of hydrocarbon-rich
fluids, inducing subsurface deformation and precipitation of carbonate cement within the large cavernous
features;
(C) Chimneys and tubes: can have vertical or horizontal
orientation (tubular structures). Focussed and vigorous seepage of fluids from the overpressured cavernous
features results in pipe structures where predominantly
pyrite precipitation occurs.
The data described indicates that fluids seep vigorously
not only vertically but also migrate horizontally in the
subsurface. The methanogenic origin of the carbonates is
supported by stable isotope data (d13C as low as 36.7%)
and by the thick microbial mats that thrive profusely at this
site. Mud breccia was not retrieved at any station; therefore
the inferred mud volcanic activity at DM cannot be proven
based on the existing data.
Sandbox simulations revealed to be a useful tool to
simulate the seepage of fluids and to understand the
deformation mechanisms occurring in the subsurface.
Dimensional analysis shows that the Hele–Shaw experiments can provide quantitative information of the natural
seeping systems.
Acknowledgements
This paper is dedicated to the memory of Leonid
Mazurenko, great friend and scientist. The authors are
grateful to Yuri Podladchikov for his support during
the preparation of the manuscript and to Andrey
Akhmetzhanov and Galen Gisler for their useful comments. David Roberts and two anonymous reviewers are
thanked for their constructive reviews. We would
like to acknowledge the Crew and the Scientific Party
of the TTR-15 Cruise and the UNESCO-MSU Centre
for Marine Geosciences. We gratefully acknowledge
BMBF-funded project METRO which paid ship time of
R/V Professor Logachev and support from a Centre of
Excellence grant to PGP, and a PETROMAKS grant
to Anders Malthe-Sørenssen, both from the Norwegian
Research Council.
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