Journal of the Geological Society, London, Vol. 162, 2005, pp. 815–827. Printed in Great Britain.
Integrated petrographic and geochemical record of hydrocarbon seepage on the
Vøring Plateau
A . M A Z Z I N I 1,2 , G . A L O I S I 3 , G . G . A K H M A N OV 4, J. PA R N E L L 1 , B. T. C RO N I N 1 & P. M U R P H Y 5
Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, King’s College, Aberdeen
AB24 3UE, UK
2
Present address: Physics of Geological Processes, Department of Physics, University of Oslo, P.O. Box 1048, Blindern,
0316 Oslo, Norway (e-mail: adriano.mazzini@fys.uio.no)
3
Laboratoire de Paléoenvironnements et Paléobiosphère, Université Claude-Bernard, UMR 5125 CNRS, 2, rue Dubois,
69622 Villeurbanne cedex, France
4
UNESCO–MSU Centre for Marine Geology and Geophysics, Faculty of Geology, Moscow State University, Vorobjevy
Gory, Moscow, 119992, Russia
5
School of Earth Sciences and Geography, Kingston University, Kingston upon Thames KT1 2EE, UK
1
Abstract: Authigenic carbonate crusts, nodules and chemoherms were sampled from pockmarks and mud
diapirs on the southern part of the Vøring Plateau during the TTR-8 and TTR-10 marine expeditions. A
petrographic and geochemical study was carried out to investigate their possible relationship with the seepage
of hydrocarbon fluids. All authigenic carbonates are depleted in 13 C (31.6‰ , 13 C , 52‰) indicating
that methane is the primary source of the carbonate carbon. Furthermore, pyrite framboids are often associated
with these samples, indicating that sulphate reduction is spatially coupled with methane oxidation and
implying that the carbonates are formed through the anaerobic oxidation of methane. The oxygen stable
isotope composition of the near-subsurface carbonates (3.1‰ , 18 O , 4.9‰) suggests a precipitation
temperature very close to the one recorded on the sea floor (between 1 and 2 8C), which is consistent with
their stratigraphic position, and a recent (Holocene?) age of formation. Carbonates sampled from greater
depths (up to 5.5 m below the sea floor) are richer in 18 O (4.6‰ , 18 O , 6.2‰), which is interpreted as a
result of precipitation from an 18 O-rich fluid. The occurrence of different carbonate mineral phases (aragonite,
calcite, dolomite) is possibly related to varying dissolved sulphate concentrations in the diagenetic
environment. Fluid inclusion microthermometry and Raman spectroscopy indicate the presence of an aqueous
þ hydrocarbon mixture inside the inclusions. This seepage mixture was almost certainly immiscible, resulting
in heterogeneous trapping.
Keywords: Vøring Plateau, cold seeps, authigenic minerals, carbonate, methane.
Savard 1992; Kauffman et al. 1996; Jonk et al. 2003; Mazzini et
al. 2003). These hydrocarbon-rich fluids commonly originate
from deep leaky reservoirs, the dissociation of gas hydrate layers,
or from areas of biogenic methane production (Barnard &
Bastow 1991; Bouriak et al. 2000; Sassen et al. 2003; Berndt et
al. 2004; Formolo et al. 2004).
The Norwegian Atlantic margin exhibits many of the
features described above and therefore is attracting wide
attention from oil companies and academic researchers. During
two Training Through Research marine expeditions (TTR-8 in
1998 and TTR-10 in 2000) the southern edge of the Vøring
Plateau (Fig. 1) was investigated, including sampling and
acoustic investigations, and locating and targeting modern
pockmarks and other seepage-related features (Kenyon et al.
1999, 2001). During both expeditions authigenic carbonates
were retrieved. These represent the first documented carbonate
deposits retrieved from this region and could support the results
of previous geophysical investigations that showed the presence
of gas-charged sediments. In this paper we investigate these
authigenic carbonates using petrographic and stable isotopic
data to understand if they are indicators of fluid seepage. These
investigations are complemented by fluid inclusion microthermometry and Raman spectroscopy, which are both novel
The seepage of hydrocarbon-rich fluids at the sea floor (cold
seeps) is a phenomenon frequently characterized by the presence
of methane-derived carbonate deposits and chemosynthetic communities (Hovland et al. 1987; Embley et al. 1990; Kulm &
Suess 1990; Corselli & Basso 1996; Sibuet & Olu 1998;
Peckmann et al. 2002). These chemosynthetic authigenic carbonates are the result of microbially mediated processes that
involve the oxidation of methane and incorporation of carbon
molecules within carbonate minerals (e.g. Ritger et al. 1987;
Roberts et al. 1993; Boetius et al. 2000; Elvert et al. 2000).
The record of cold seeps extends back at least to the Devonian
(Aharon 1994; Campbell & Bottjer 1995; Campbell et al. 2002),
and is substantial in the Quaternary, when they occur in both
active margins (Boulegue et al. 1987; Ritger et al. 1987; Le
Pichon et al. 1990; Limonov et al. 1994; Henry et al. 2002) and
passive margins (Brooks et al. 1986; Hovland et al. 1987; Paull
et al. 1992; MacDonald et al. 1994; Roberts & Aharon 1994).
Mud diapirs (Brown 1990; Hovland 1990), pockmarks (Hovland
& Judd 1988; Hovland 1992) and mud volcanoes (Henry et al.
1996; Olu et al. 1997; Aloisi et al. 2000; Peckmann et al. 2001;
Mazzini et al. 2004) are common features at seepage sites, where
various pathways such as faults, other fractures and sedimentary
discontinuities act as conduits for fluid seepage (Beauchamp &
815
-5°
-5°
00
30
0°
0
20
500
10
m
m
m
00
5°
5°
200 m
Voring
Plateau
m
m
00
00
25
20 00 m
15
m
0°
10°
10°
NO
15°
15°
60°
62°
64°
66°
68°
70°
AT-327G
AT-328G
2
AT-115G
AT-113G
AT-114G
AT-112G
AT-111G
AT-137G AT-142B
AT-329G
AT-143B
AT-330Gr
AT-117G
AT-119G
AT-325GR
AT-116G
AT-326G AT-118G
1
AT-141B
AT-122G
AT-121G
AT-140G
AT-138G
3
AT-324G
AT-323Gr
AT-120G
AT-139G
Fig. 1. Map of the Norwegian Sea (inset) and OKEAN sidescan sonar mosaic of the study area on the southern Vøring Plateau in the vicinity of the Storegga Slide (Kenyon et al. 2001). Boxes indicate the
main areas of occurrence of pockmarks and diapirs. Black dashed line defines the interpreted boundary with the Storegga Slide to the south. Sampling stations collected during TTR-8 and TTR-10 cruises
are included.
60°
62°
64°
66°
68°
70°
RW
AY
816
A. MAZZINI ET AL.
H Y D RO C A R B O N S E E PAG E O N T H E V Ø R I N G P L AT F O R M
additions to the range of techniques used to study cold seep
carbonates.
Geological setting
The Vøring Basin started to develop in the Late Jurassic with a
main phase of extension related to rifting, followed by continental break-up between Scandinavia and Greenland in the Early
Eocene (Mørk et al. 2001). The Vøring marginal high was
uplifted and became the site of subaerial flow basalts in the Late
Paleocene–Early Eocene (Eldholm et al. 1989; Skogseid &
Eldholm 1989). Post-break-up thermal subsidence during the
Cretaceous resulted in the formation of a 10 km thick sedimentary basin fill (Brekke 2000). Between the Late Eocene and MidMiocene, compressional phases generated dome structures
(Skogseid & Eldholm 1989; Doré & Lundin 1996) that are
potential hydrocarbon traps or accumulations. After the Neogene
uplift of the Norwegian mainland (e.g. Hjelstuen et al. 1999) and
the Plio-Pleistocene glacial–interglacial cycles, large amounts of
sediment were deposited on the shelf edge (Henriksen & Vorren
1996). Two sedimentary successions are the most important: the
Miocene–Pliocene Kai Formation (hemipelagic oozes faulted by
polygonal systems) and the Plio-Pleistocene Naust Formation
(alternating debris flow deposits and hemipelagic sediments)
(Rokoengen et al. 1995; Bunz et al. 2003). The Quaternary
Storegga Slide, adjacent to the Vøring marginal high, cuts into
the sediments of the Naust Formation (Bugge et al. 1987). The
slide reaches a length of 800 km including a total volume of
5500 km3 of Tertiary–Quaternary sediments (Bugge et al. 1987;
Bouriak et al. 2000). The slide has been active on several
occasions, and there have been at least three main stages of
sliding (Bugge et al. 1987; Bouriak et al. 2000). The triggering
factors for the slides have been debated in several papers. Bugge
et al. (1987) suggested earthquake loading with a contribution of
ice loading and gas hydrate dissociation after earthquake-induced
change of the pore pressure distribution. Other workers (Bouriak
et al. 2000) suggested that sliding was not necessarily associated
with the dissociation of gas hydrates in the area, and further
research is required to investigate the possible triggering mechanisms of the Storegga Slide.
For the last two decades the Vøring Plateau and the adjacent
Storegga Slide offshore Norway (province I according to Bunz
et al. 2003) have been the target of several geophysical and
geological surveys (Bugge et al. 1987; Evans et al. 1996;
Mienert et al. 1998; Posewang & Mienert 1999; Vogt et al. 1999;
Bunz et al. 2003). The presence of gas hydrates is inferred by
the record of bottom simulating reflectors (BSR) in the vicinity
of the main scarp of the Storegga Slide (Bugge et al. 1987;
Mienert et al. 1998; Kenyon et al. 1999, 2001; Bouriak et al.
2000; Bunz et al. 2003; Berndt et al. 2004). In particular, Bunz
et al. (2003) defined the boundaries of the BSR area and the
inferred distribution of gas hydrates that are characteristic of the
Storegga Slide and the Vøring Plateau region. Results show that
the BSR is shallowest in the vicinity of the headwall where most
pockmark fields are located. The BSR area is underlain by an
extensive polygonal fault system present in the Kai Formation
(Berndt et al. 2003). These faults are presumably acting as
pathways for the fluids (Henriet et al. 1991) involved in the
formation of gas hydrates (Berndt et al. 2003). Berndt et al.
(2003) also highlighted that the pipes observed in the seismic
profile underneath the pockmark fields mostly originate from the
base of the gas-hydrate stability zone. In fact, evidence of fluid
activity is recorded by pockmark fields at the southern edge of
the Vøring Plateau adjacent to the Storegga Slide (e.g. Evans
817
et al. 1996; Bouriak et al. 2000). The localized dissociation of
the gas hydrates or the overpressure caused by continued
accumulation of rising fluids is likely to lead to the formation of
pipes, which in turn generate the pockmark and diapiric(?)
features that are observed on the sea floor.
Analytical methods
The initial sidescan sonar mosaic was obtained with the R.V. Professor
Logachev operated by the Polar Marine Geosurvey Expedition. Polished
slabs and thin sections were studied using standard petrographic fluorescence and cathodoluminescence (CL) techniques. SEM analyses were
performed using an ISI ABT-55 SEM. Semiquantitative mineralogical
composition of carbonate phase was determined by X-ray diffraction
(XRD) analyses performed using a Siemens D-500 instrument (Cu KÆ,
Ni-filtered radiation). The weight percentages of minerals were estimated
using the peak heights and manocalcimeter values. The estimated error is
5%. Authigenic (high-Mg) calcite was distinguished from pelagic (lowMg) calcite on the basis of d values of the (104) diffraction peak, which
are c. 2.998 Å and 3.028 Å, respectively (Aloisi et al. 2000).
Samples for microthermometric analysis of fluid inclusions were
prepared as doubly polished wafers and examined following established
guidelines and terminology (e.g. Goldstein 2001) using a calibrated
Linkam THM600 heating–freezing stage attached to a Nikon Optiphot2POL microscope. The stage was calibrated using organic crystals of
known melting points. Fluid inclusion measurements were performed
almost exclusively on inclusions observed in large acicular aragonite
crystals forming the botryoids. It was extremely difficult to distinguish
inclusions in other phases because of the small size of the crystals and
the presence of impurities. Several of the inclusions in aragonite crystals
showed irregular changes in liquid/vapour ratios during heating, indicating leakage as a result of the fragile structure of the mineral. In
particular, evidence of leakage was observed in the intercrystal inclusions. In all inclusions where leakage was suspected, measurements were
disregarded. A large number of measurements had to be rejected because
in several instances inclusions gave differing homogenization temperatures on repeated measurements on the same inclusion. Only inclusions
that gave reproducible results are reported in this paper; all the others
were disregarded.
To determine the stable isotope composition of authigenic cements
with a high spatial resolution, samples for stable isotope analyses were
taken from polished slabs using a hand-held microdrill. Samples for
isotopic analyses were selected from the different diagenetic textures
observed. Measurements were performed liberating CO2 by the standard
phosphoric acid technique (McCrea 1950; Sharma & Clayton 1965).
Ratios were calculated with a Carbonate Prep System linked to an
AP2003 mass spectrometer. The 13 C and 18 O results are reported
relative to the V-PDB standard (Craig 1957; Coplen 1994); the precision
of the analyses is 0.2‰. The Friedman & O’Neil (1977) fractionation
equation was used to calculate the temperatures of carbonate precipitation.
Raman spectroscopy was performed on fluid inclusions in the doubly
polished wafers prepared for microthermometry, using a Renishaw
RM1000 Raman spectrometer, which is equipped with a thermoelectrically cooled CCD detector and a 514.5 nm Ar ion laser. Using a 1003
objective lens, the laser was focused to a spot size of c. 1 ìm. Peak fitting
was performed using Galactic GRAMS/32 software, which uses a mixed
Gaussian–Lorentzian curve function.
Results
Geophysical investigations and sea-floor observations
The sidescan sonar mosaic of all the profiles acquired in the
region during the two cruises (Kenyon et al. 1999, 2001) showed
the rough morphology of the Storegga Slide, markedly distinct
from the sea floor that characterizes the Vøring Plateau. Here
three main pockmark and mud diapir fields were observed (Fig.
1). In particular, in the central and eastern part of the region, the
818
A. MAZZINI ET AL.
results of both cruises suggested the presence of free gas in the
sediments, revealed by the presence of a BSR and vertical
acoustically transparent zones on seismic and sub-bottom profiler
records. Figure 2 shows a portion of the sidescan sonar profile
run along the easternmost part of the studied region, mostly
comprising an area north of the Storegga Slide scar (Area 3). On
the right side of the image the irregular morphology that
characterizes the Storegga Slide is visible, whereas on the left
side a relatively flat sea floor is characterized by the presence of
diffuse pockmarks and mud diapirs, which were targeted for
sampling.
Spectacular images were recorded from the easternmost area,
where a TV survey explored the pockmarks and the inferred
seepage features. The video record showed a relatively flat seafloor morphology characterized by laterally extensive carbonate
deposits. These deposits could be responsible for the strong
reflections observed on the sidescan sonar images at these sites.
Slabs were occasionally interrupted in localized zones where low
edifices were elevated from the sea floor, and where soft
hemipelagic sediment filled the intervening depressions. Fluids
were occasionally observed seeping through the sediments.
Associated macrofaunal taxa including shrimps, sea-spiders,
ahermatyphic corals, gastropods and clams were present. For the
first time in this area, a large chemoherm block measuring
0.5 m3 was observed with the underwater TV camera and was
collected (station AT-323Gr, Fig. 2) with a remote-controlled TV
grab from an inferred mud volcanic structure named ‘Tobic’
(Kenyon et al. 2001).
Authigenic carbonates of the Vøring Plateau
During the two cruises, a total of 27 sampling stations were
completed using gravity cores and TV remote-controlled grabs to
groundtruth the acoustic data acquired. The most interesting
samples were collected from the central and the eastern part of
the studied region (Areas 1 and 3) from mud diapiric and
N
S
1 km
Tobic Mud Volcano
pockmark features (Tables 1 and 2). Here several gravity cores
recovered homogeneous or bioturbated stiff silty clays with
occasional rock clasts and a strong smell of H2 S. Pogonophora
worm aggregates and bivalves were occasionally observed in the
top part of the cores. In several instances sedimentary features
indicating gas saturation (e.g. gas expansion pockets, flame-like
gas escape structures and expansion cracks) were observed.
Pockmark features revealed the presence of carbonate nodules
and tabular-shaped crusts in the uppermost part and up to 5.5 m
below the surface. The cored mud diapirs revealed the presence
of carbonate nodules in the mud breccia and in the hemipelagic
veneer occasionally capping the mud flows, where the authigenic
carbonates usually occur with an elongated flame-like shape. At
the sea floor the mud diapirs are associated with carbonatecemented bivalve deposits.
The retrieved large chemoherm sample (Fig. 3) was very
porous, releasing a strong smell of H2 S. Microbial mats of
pinkish and brownish colour were observed covering the smooth
external surface of the sample and in the internal part of the
block. The carbonate block consisted of precipitated carbonate
minerals, skeletal bivalve remains, siliciclastic sediment composed of clay and small clasts of varying lithology. The dominant
chemosynthetic species lithified within the carbonate crust were
bivalves belonging to the families Vesicomyidae and Mytilidae.
They average 2 cm in length, some reaching a length of 6 cm.
Their shells were commonly still articulated. Other symbiontcontaining species included Pogonophora worms and Cladorhizidae sponges (Mazzini et al. 2001). Geopetal infillings in the
internal cavity of the shells were observed and represented by
poorly lithified micritic and aragonitic cement. The geopetal
structures in the shell cavities appeared to have differing orientations suggesting that, after the infill, most of the molluscs were
gently oriented without disturbing the attachment of the two
valves. This also indicates that the cementation process occurred
relatively quickly. In the lower part of the sample fine clayey
sand and silty clay were observed.
50 m
Fig. 2. Results of 30 kHz OREtech sidescan sonar and 3.5 kHz sub-bottom profiler
with sampling station and structures
indicated (Kenyon et al. 2001).
2.914
2.91
3.034
2.995
2.987
2.982
2.975
3.034
2.998
3.003
3.006
2.995
2.995
2.986
2.991
2.989
2.991
0
12.6
18
0
0
0
0
0
0
0
0
0
0
0
0
0
0
23
43.4
57.6
38.4
0
0
0
0
0
0
0
0
0
11
49.3
22.5
19.6
4.4
5.6
73
50
55
73
62
60
63
7
49
11
63
65
63
62
44
73
50
55
73
62
60
53
7
49
Bulk
Bulk
Upper part
Lower part
Bulk
Cement
Bulk
Bulk
Bulk
Bulk
Bulk
Bulk
Bulk
Exterior part
Interior part
Hemipelagic clay
Nodule
Tabular crust
Tabular crust
Nodule
Bivalve aggregates
Nodule
Nodule
Nodule
Nodule
Nodule
Nodule
Nodule
Nodule
Nodule
Chemoherm
Chemoherm
1056
802
805
805
805
734
734
734
734
734
730
730
730
734
734
723
723
BSF, below sea floor.
64840.4199N
64847.0809N
64846.1059N
64846.1059N
64846.1059N
64840.8759N
64840.8759N
64840.8759N
64840.8759N
64840.8759N
64840.5869N
64840.5869N
64840.5869N
64840.8789N
64840.8789N
64840.8769N
64840.9079N
AT 111G 40 cm
AT117G 445 cm
AT118G 514 cm
AT118G 514 cm
AT118G 535 cm
AT120G 0–3 cm
AT120G 60–90 cm
AT120G 71–79 cm
AT120G 120–118 cm
AT120G 120–118 cm
AT121G 5–10 cm
AT121G 25–50 cm—1
AT121G 25–50 cm—2
AT122G 60–70 cm
AT122G 60–70 cm
AT123Gr
AT123Gr
819
Mineralogy and petrography
4835.6409E
4848.3799E
4848.6599E
4848.6599E
4848.6599E
5815.8159E
5815.8159E
5815.8159E
5815.8159E
5815.8159E
5814.7179E
5814.7179E
5814.7179E
5815.8089E
5815.8089E
5815.8149E
5815.7499E
Facies, part of
sample analysed
Lithology
Water
depth
(m)
Longitude
Latitude
Station number, sampling
depth (BSF)
Table 1. Sampling station coordinates and mineralogical composition of samples selected for XRD analyses
Amount of
carbonate
(% normalized)
%
Calcite
%
%
Aragonite Dolomite
d (104)
calcite
(Å)
d (104)
dolomite
(Å)
H Y D RO C A R B O N S E E PAG E O N T H E V Ø R I N G P L AT F O R M
XRD analyses (Table 1) completed on the authigenic carbonates
from the cores revealed the presence of aragonite, calcite (as
high-magnesium calcite (HMC) and low-magnesium calcite
(LMC)), and dolomite. Samples containing aragonite were found
in the shallow subsurface at pockmark sites associated with
bivalve aggregates and Pogonophora present in the benthic
surface, whereas LMC, HMC and dolomite were also present at
greater depths on both pockmarks and mud diapirs.
Petrographic analyses revealed the presence of well-defined
phases of differing diagenetic carbonate cements within the
chemoherm block (Mazzini 2004). Thin-section and SEM analyses revealed that there are two main cements that bind the crust
together: pervasive Mg-calcite (as micrite and sparite) and
acicular aragonite. Micritic cement appears widely distributed
and is mostly present inside the shells as a geopetal filling, where
it is usually rich in clay content, pyrite framboids and ovoidshaped pellets (Fig. 4a). Pellets, ranging from 20 to 400 ìm in
size, may locally represent 80% of the micrite, indicating
extensive bioturbation within the sediment. A distinct micriterich layer can be distinguished in contact with the shell bodies.
The better preserved microfossils are present within the micritecemented clayey sediment whereas shells in contact with the
micritic cement show evidence of possible microbial remains.
Sparite is the less abundant cement. It usually appears in micrite
embayments as botryoidal calcite (Fig. 4b) showing the ghost of
recrystallized needles (former aragonite) and displaying undulose
extinctions under crossed Nicols, or can appear as a pore filling
inside the pervasive micrite. Aragonite is the most common
cement. This type of cement is often observed projecting directly
from the micrite. The base of the aragonite phase is often
characterized by the presence of a continuous organic matterand pyrite-rich layer (P/B layers; see below for details). Aragonite also nucleates from calcite crystals or peloids (Fig. 4c and
d), or inside the bivalve cavities, often directly on the shells. It
usually forms clear botryoids that progressively infill the porosity
of the rock. Aragonite represents the latest cementation phase, as
is also confirmed by the occasional displacement of micritic and
pelitic sediment by the botryoids. The detrital sediment observed
consists of a clayey matrix and rounded and angular quartz and
feldspar grains. It mainly occurs filling the internal cavity of
shells and bioturbation burrows. Aragonitic and micritic cements
were often observed coating siliciclastic grains or filling the
voids between them. Sequences of alternating cementation
phases are primarily visible on the convex part of the bivalve
shells. Figure 5 shows a common example of a sequence of the
different cements described above. Two phases of micritic cement (C1 and C2), both pyrite- and pellet-rich, follow each other
adjacent to the bivalve shell. The second micritic phase (C2)
shows pore-filling sparite (C3) and aragonite that is interpreted
as coeval with the first aragonitic phase (C4). A thin dark brown
layer (P/B1 ) defines the boundary between the micrite and the
first clear aragonite phase (C4). A similar layer (P/B2 ) separates
this phase from the most recent aragonite phase (C5). The P/B
layers consist of organic matter and small (2–4 ìm) pyrite
framboids. Aragonite crystals (C4 and C5) commonly nucleate
on this dark brown material, as was also observed in some
samples recovered from the Alaminos Canyon (Roberts et al.
1993), from Prince Patrick Island, Canada (Beauchamp & Savard
1992), and from the North Sea (see Hovland et al. 1987, fig. 6).
The dark brown layer observed in several samples defining the
boundary between different cementation phases, and often coating the last aragonite phase, is interpreted as microbial deposits
820
A. MAZZINI ET AL.
Table 2. Setting of the sampling stations and summary of carbon ( 13 C) and oxygen ( 18 O) stable isotope results and short sample description
Station number, sampling
depth (BSF)
Setting or feature
Description
AT 111G 40 cm
AT117G 445 cm
AT118G 514 cm
Upper slope
Pockmark
Pockmark
Hemipelagic clay
Mélange calcite–dolomite, nodule bulk
Mélange calcite–dolomite, upper surface of
tabular crust
Calcite, lower surface of tabular crust
Aragonite, nodule bulk
Aragonite, bivalve aggregates
Calcite, nodule bulk
Calcite, nodule bulk
Calcite, nodule bulk
Calcite, nodule bulk
Calcite, nodule bulk
Calcite, nodule bulk
Calcite, nodule bulk
Calcite, nodule external part
Calcite, nodule internal part
Aragonite botryoid
Aragonite botryoid
Aragonite botryoid
Aragonite botryoid
Aragonite botryoid
Mollusc shell
Mollusc shell
Micritic geopetal filling
Micritic geopetal filling
Micritic geopetal filling
Calcite, light, internal part of chemoherm
Calcite, darker, internal part of chemoherm
Calcite, internal part of chemoherm
AT118G 514 cm
AT118G 535 cm
AT120G 0–3 cm
AT120G 60–90 cm
AT120G 71–79 cm
AT120G 120 cm—1
AT120G 120 cm—2
AT121G 5–10 cm
AT121G 25–50 cm—1
AT121G 25–50 cm—2
AT122G 60–70 cm
AT122G 60–70 cm
AT323Gr
AT323Gr
AT323Gr
AT323Gr
AT323Gr
AT323Gr
AT323Gr
AT323Gr
AT323Gr
AT323Gr
AT323Gr
AT323Gr
AT323Gr
Mud diapir
Mud diapir
Mud diapir
Mud volcano or pockmark
18 O ‰
(V-PDB)
13 C ‰
(V-PDB)
0.2
5.9
6.2
0.3
43.1
37.4
5.8
6.8
5.6
5.3
4.1
5.3
2.6
5.1
5.3
5.8
6
5.9
4.6
5.8
4
4.5
4.4
4.3
4.5
3.3
3.1
4.5
4.4
3.3
3.8
3.6
4.9
49.3
48.4
38.3
49.9
31.6
47.2
50
51.1
51.5
50.6
43.2
49.6
49.6
49.8
47.4
49
48.9
13.9
27.2
45.9
45.4
44.3
41.3
47.3
52
4.6
3.5
0.9
3.5
7
2.9
3.5
5.3
6.1
5.8
1
5.3
1.2
0.7
0.2
0.1
0.7
4
4.8
0.7
0.2
4
2
2.8
2.1
Calculated
precipitation
temperature
(8C)
Calculated precipitation temperatures assume isotopic equilibrium with actual sea-floor fluids (18 O ¼ 0:2‰SMOW ).
Fig. 3. Polished surface of cross-section through a portion of the
chemoherm block packed with associated chemosymbiotic fauna. (Note
the large bivalve shells.)
mixed with pyrite framboids (Fig. 6a and b). CL microscopy
showed an overall poor luminescence. Aragonite and sparite
showed no luminescence, although some zoning was distinguishable as a result of the luminescence of the pelitic and terrigenous
(mostly clayey) admixture contained in the micritic cement. The
brightest luminescence was observed in the geopetal infillings
(mainly consisting of clay-rich micrite containing peloids and
microfossil fragments) and where a high amount of detrital
sediment was present. Framboidal pyrite up to 25 ìm in diameter
has a pervasive distribution through the carbonate cements, with
many small (up to 5 ìm) framboids found in yellowish, micritic,
peloid-rich cement. This texture is common in other cold seep
deposits from passive margins (e.g. Commeau et al. 1987;
Hovland et al. 1987). Framboids are commonly observed as
localized aggregates in the micritic, sparitic and aragonitic
cements, or concentrated in layers (Fig. 5, P/B layer; Fig. 6a and
b) mixed with dark brown microbial biofilm in direct contact
with aragonite crystals. The largest framboids are usually sited
between the acicular crystals of aragonite (Fig. 6c) in the internal
part of bivalves. Microbial coatings were observed adjacent to
and in contact with these framboids in the internal part of the
carbonate block. Pyrite and aragonite needles appear to be coated
by the brownish organic material. Microbial mat was observed
on the external surface of the carbonate block at the time of
sample retrieval. SEM images of rock fragments from the
external part of the samples show a mucous biofilm containing
discrete filaments adhering to the sediment surface and elongated
needles (Fig. 6d).
Carbon and oxygen stable isotope composition
Oxygen and carbon stable isotope analyses and a brief sample
description are recorded in Table 2 and Figure 7. The hemipelagic mud sample analysed yielded values of 13 C of 0.3‰
H Y D RO C A R B O N S E E PAG E O N T H E V Ø R I N G P L AT F O R M
821
Fig. 4. Thin-section images of different carbonate phases. (a) Micritic cement, pyrite-rich with pellets; (b) sparitic calcite (former aragonite) in micrite
embayment (clotted in darker colour); (c) aragonite crystals nucleating around peloids; (d) SEM image of acicular aragonite growing from clumps of
loose peloids close to quartz grain coated with pelitic sediment.
and 18 O of 0.2‰. The carbonates drilled from the shells
reveal the least depleted 13 C values (27.2‰ , 13 C , 8‰).
The different authigenic carbonates revealed carbon isotope
compositions with a strong depletion relative to marine carbonate, showing 13 C ranging from 31.6‰ to 52‰. A trend
can be observed between the hemipelagic mud and the calcite
and the geopetal fillings, suggesting the presence of varying
quantities of hemipelagic mud in the carbonates. 18 O results
vary from 2.5‰ to 6.2‰; the carbonates sampled in the near
subsurface show lower enrichment in 18 O (3.1‰ , 18 O ,
4.9‰) compared with those sampled from deeper intervals (up to
5.5 m below the sea floor, 4.6‰ , 18 O , 6.2‰).
Microthermometry and Raman spectroscopy of fluid
inclusions
Outline details of the fluid inclusion study were presented by
Parnell et al. (2002). Results of microthermometry measurements
indicate three populations (Table 3). The first population comprises two-phase (liquid- and vapour-filled) inclusions, situated
inside the aragonite crystals, homogenizing to the gaseous phase
at a mean Th of 28 8C and to the liquid phase at a mean Th of
37 8C (Fig. 8a). The second population includes two-phase
(liquid- and vapour-filled) inclusions situated in intercrystal sites,
homogenizing to the liquid phase at a mean Th of 70 8C. The
third population consists of intracrystal aqueous inclusions,
which are monophase (liquid-filled) at room temperature, but
which on cooling (at c. 10 to 15 8C) nucleate a vapour phase
that homogenizes at temperatures between 0 and 2 8C. This type
of inclusion usually has a subrounded shape, unlike the other
inclusions, which can be more elongated.
The largest fluid inclusions were subjected to Raman spectroscopic analyses. Figure 8b shows one example of a Raman
spectrum obtained from a 4 ìm 3 10 ìm inclusion. Three main
features are evident: the strong, sharp band of the aragonite host
at 1085 cm1 , a minor, broad band around 1450 cm1 , and a
stronger broad band between 2850 and 2950 cm1 . The last two
frequencies are characteristic of H–C–H bending and C–H
stretching bonds in hydrocarbons, respectively (Pironon 1993),
and the broad form of the bands indicates a range of hydrocarbons of different chain lengths. Methane should show a single,
sharp band at 2917 cm1 , which is not recognized. The frequency range covered by the Raman analysis would also cover
other possible gases such as CO2, N2 and H2 S if they were
present in the inclusions. The possibility of surface contamination contributing to the hydrocarbon signal was ruled out by
analyses of host aragonite close to the inclusions.
Pironon (1993) showed that the intensity ratio of Raman bands
at two frequencies in the C–H stretching region (R ¼
I(2854)=I(2935)) and two in the H–C–H bending region
(r ¼ I(1440)=I(1455)) could be used to approximate the CH2 /
CH3 ratio of oils in fluid inclusions. Although the technique is
only approximate (it can be affected, for example, by temperature
and by dilution in non-polar solvents, and it assumes only
822
A. MAZZINI ET AL.
QZ
MF
T
S
C1
C2
C4
P/B1
P/B1
C3
C4
C5
P/B2
500 µm
Fig. 5. Thin-section image showing an example of different generations
of carbonate cement. The external part of the shell (S) is coated by
different types of cement: C1, micrite (pellets- and pyrite-rich); C2,
mostly represented by micrite (pellets- and pyrite-rich) with pore-filling
botryoidal aragonite (C4) and sparite (C3); P/B1 , thin layer consisting of
pyrite framboids and of dark brown microbial coating; C4, aragonite
forming large crystals rising from the P/B1 layer and from large pelitic or
micritic fragments; P/B2 , thin layer, pyrite- and organic matter-rich,
similar to P/B1 separating two different aragonite layers; C5, youngest
layer of small crystals of aragonite filling a void containing detrital
sediment (T). A portion of a microfossil (MF) and a quartz grain (QZ)
are visible in the upper part of the image.
calcite precipitation is inhibited (Naehr et al. 2000; Aloisi et al.
2002).
The material with slightly brighter CL is distinguished by the
different contents of clayey and detrital admixture (more luminescent) mixed with the carbonate cements. The clayey fraction
is commonly mixed with the micritic cement. This suggests that
the micrite grew within pre-existing fine sediment or that clay
and carbonate were mixed by intense biological activity (e.g.
faecal pellets and fossil remains). The origin of this detrital
sediment is uncertain, but could be either ice-rafted or contourite-transported sediment, or material transported to the surface
from deeper layers during mud volcanic activity.
The occurrence of pyrite within the cement has commonly
been interpreted as an indicator of carbonate precipitation in the
sulphate reduction zone (e.g. Commeau et al. 1987; Hovland
et al. 1987; Beauchamp & Savard 1992). The samples show
pyrite-rich layers that suggest episodes of iron sulphide precipitation prior to carbonate precipitation. Furthermore, the framboids
observed on the edge of the acicular aragonite crystals were
coated with organic matter of probable microbial origin. This
suggests that the microbially mediated reaction (promoted by
sulphate-reducing bacteria, SRB) had been present or might still
be continuing.
The small spheres observed adhering to the pyrite framboids
and the aragonite crystals are in the size range of the Archaea–
bacteria aggregates described in the literature (Boetius et al.
2000; Michaelis et al. 2002). However, our SEM study is not
sufficient to confirm that the consortium operating the anaerobic
oxidation of methane (AOM) is present in our samples.
The seepage fluids and authigenic carbonate precipitation
aliphatic, not aromatic hydrocarbons), it allows an estimate of
the average chain length. In the case of the example shown in
Figure 8b, after removal of background fluorescence the intensity
ratios are R ¼ 1:061 and r ¼ 0:931. According to the calibration
curves of Pironon (1993) these both equate to n-alkanes where n
is c. 7. Although the weak Raman signal for these inclusions
adds considerable uncertainty to this estimate, it is clear that the
hydrocarbon present is not simple methane but a heavier, longerchained oil.
Discussion
Diagenetic processes and microbial activity
A temporal sequence of the different carbonate cements observed in the chemoherm has been established. In several
instances the pattern in Figure 5 was observed. The micrite is
broadly distributed binding together pellets and a siliciclastic
admixture. C1 and C2 appear to represent the first two cementation phases, containing a later pore-filling sparite (C3). The
sparitic phase is likely to overlap the first aragonite precipitation
phase (C4), as indicated by partial recrystallization of the
aragonite cement observed in some places in the micrite embayments (Fig. 4b). Aragonite appears to represent the last cementation episode and occurred as two distinct phases (C4 and C5). In
the cored samples, Mg calcite was mostly retrieved in the deeper
subsurface (down to 530 cm). Aragonite occurred instead in
sedimentary layers closer to the subsurface (,14 cm), mostly on
bivalves and Pogonophora-rich cements developing on the
benthic surface. This is consistent with aragonite precipitation
taking place in sulphate-rich diagenetic environments where
Evidence of hydrocarbon fluid seepage is clearly indicated by
the carbon isotope results. The range of data seems to be
approximately aligned along a trend line that represents a linear
regression in the 13 C content and a gradual increase in 18 O
content. A similar trend was observed in other cold seep-related
carbonates, where it has been ascribed to the mixing of varying
proportions of primary carbonate shells and methane-derived
authigenic carbonate (Schmidt et al. 2002). The two end
members of the trend line in Figure 7 represent hemipelagic
seawater-derived carbonate (13 C 0.3%) and methane-derived
carbonates (13 C 52%), respectively. The trend is produced by
the presence of variable amounts of hemipelagic mud in the
authigenic carbonates. The depletion in 13 C suggests a strong
component of biogenic methane seeping during the precipitation
of these carbonates. Samples that fall in the central part of the
line suggest a component of carbon derived from the oxidation
of methane in addition to heavier carbon from the seawater (e.g.
mollusc shells). Despite the selective drilling of the different
cementation phases present in the chemoherm block, most of the
data consist of clustered values for oxygen and carbon, indicating
that there were no substantial variations in the fluid composition
during the seepage and that all the cements precipitated near the
subsurface.
The precipitation temperatures of the carbonates have been
calculated applying the Friedman & O’Neil (1977) fractionation
equation assuming an isotopic equilibrium with a 18 O fluid
composition of 0.2‰SMOW (Tchernia 1978; Aloisi 2000). Calculated equilibration temperatures for the two extreme 18 O values
measured on the carbonate samples (2.6‰ and 6.2‰) reveal a
temperature range from 6.8 8C to 7 8C approximately (Table 2).
All the samples analysed from the chemoherm block revealed
precipitation temperatures in agreement and mostly closer to the
H Y D RO C A R B O N S E E PAG E O N T H E V Ø R I N G P L AT F O R M
823
Fig. 6. Aragonite and microbial coating in the chemoherm block. (a) Aragonite acicules in the internal part of a shell and dark coating consisting of small
pyrite framboids and microbial coating; (b) detail of area framed in (a) (note the framboids attached to the aragonite aciculae coated by organic remains);
(c) SEM image of pyrite framboids (arrowed) associated with microbial coating in the internal part of the carbonate block where aragonite botryoids
nucleated in the internal part of a shell; (d) SEM image of remains of mucous biofilm and organic remains on the external surface of the carbonate block.
ä 18O (‰ V- PDB)
-1
0
1
2
3
4
5
6
7
10
ä 13C (‰ V-PDB)
0
-10
-20
-30
-40
-50
aragonite
calcite
hemipelagic mud
melange calcite/dolomite
mollusc shell
micritic geopetal filling
-60
Fig. 7. Cross-plot of carbon (13 C) and oxygen (18 O) stable isotope
compositions for carbonate cements (see Table 2 for sample
identification).
are unrealistic and indicate anomalous enrichment in 18 O of the
diagenetic fluids. At other cold seep sites 18 O-rich fluids evolve
by clay mineral diagenetic reactions such as transformation of
smectite into illite (Dahlmann & de Lange 2003).
The importance of this as well as of other processes such as
gas hydrate destabilization in producing the observed oxygen
isotopic signature of the Vøring cold seep fluids cannot be
established with our dataset. Seismic studies obtained from this
region (Kenyon et al. 1999, 2001; Bouriak et al. 2000; Bunz et
al. 2003) revealed the presence of gas hydrates in the sedimentary section beneath the sampled sites. The dissociation of the
hydrates could provide 18 O-rich fluids. Nevertheless, the P–T
conditions regulating these gas hydrate accumulations are not
known and it is not possible to discriminate between the gas
hydrate source and other sources (e.g. clay mineral transformations, etc.).
Implications from fluid inclusions
modern sea-floor temperature (c. 0 8C) (Tchernia 1978; Thiede et
al. 1989; Mienert et al. 1998, M. Hovland, pers. comm.),
whereas most of the samples found at greater depth in the cores
reveal precipitation temperatures below 0 8C. These temperatures
Microthermometry and fluid inclusion analysis show three
populations of fluid inclusions. In several instances the inclusions
do not totally freeze until temperatures lower than 100 8C,
suggesting that they may contain a mixture of water and
hydrocarbons. Nevertheless, the high temperatures of homogeni-
824
A. MAZZINI ET AL.
Table 3. Summary of primary fluid inclusion populations
Contents at room
temperature
Host
LAþH
VAþH
LAþH
VAþH
LH
Aragonite
intracrystal
Aragonite
intercrystal
Aragonite
intracrystal
Size (ìm)
Morphology
Degree of fill
,10
1
,6
Subrounded,
irregular
Cubic
,5
Subrounded
1
0.7
Th (8C)
Tf (8C)
n
27–30 (Vap)
27–50 (Liq)
65–75 (Liq)
, 100
9
, 100
15
–
6
0–2 (Liq)
Th, homogenization temperature; Tf , freezing temperature; n, number of measurements; Liq, homogenization to liquid; Vap, homogenization to vapour; Degree of fill, proportion
of liquid in fluid (where fluid includes liquid and vapour); L, liquid; V, vapour; A, aqueous; H, hydrocarbons.
guished from the other hydrocarbons. Given the uncertainty of
the exact composition of the hydrocarbons, and the limited data
for complex systems even if the exact composition were known,
it is not possible to calculate temperature and pressure conditions
for the immiscibility of water and the hydrocarbon. However, it
seems reasonable to assume that, at the low temperatures
believed to be present, the water and hydrocarbons would have
been immiscible, making the heterogeneous trapping hypothesis
plausible. The combination of water and hydrocarbons, and the
probable heterogeneous trapping, means that we cannot simply
interpret the homogenization temperatures to directly indicate
trapping temperatures. Although the study of fluid inclusions in
these environments may not provide reliable temperature and
pressure data, it still offers information on the composition of the
fluid at the time of carbonate precipitation. The third population
of inclusions, which are monophase (liquid-filled) at room
temperature, nucleated a bubble on cooling (Th 2–4 8C).
Although this temperature is consistent with the Norwegian Sea
bottom water temperature, the vapour phase, which nucleated at
temperatures below 0 8C, is a metastable phase and therefore the
temperature at which it homogenizes might not be a true
reflection of the temperature at which the inclusion was trapped.
However, the fact that the inclusions are monophase (liquidfilled) at room temperature indicates that they were entrapped
from a low-temperature fluid that is consistent with the aragonite
forming close to the sea floor.
20 µm
A
2840cm-1
Aragonite
1085cm-1
12000
3000cm-1
14000
8000
1450cm-1
Intensity
10000
6000
Hydrocarbon
C-H bands
4000
2000
B
0
1000
The origin of the methane-rich fluids
1500
2000
2500
3000
3500
-1
Frequency (cm )
Fig. 8. (a) Fluid inclusions in aragonite cements in intracrystal sites; (b)
Raman spectrum for inclusion in chemosynthetic aragonite; noteworthy
features are the strong, sharp band of the aragonite host at 1085 cm1 , a
minor, broad band around 1450 cm1 , and a stronger broad band between
2850 and 2950 cm1 . The last band indicates a range of hydrocarbons of
different chain lengths.
zation recorded are not consistent with known conditions
(typically low temperatures) that characterize cold seeps at the
sea floor. The variability is likely to be the result of trapping of
variable proportions of aqueous liquid and hydrocarbon (vapour
or liquid), resulting in spurious homogenization temperatures that
are unrelated to the temperature of trapping.
Supporting evidence is provided by the Raman spectroscopy,
which indicated the presence of a mixture of hydrocarbons
within the inclusions. Methane, if present, could not be distin-
Doré & Lundin (1996) and Bunz et al. (2003) proposed that the
gas that is involved in the formation of gas hydrates (and thus
later seeping towards the surface) moves by advection from the
Tertiary dome structures at greater depth. This gas has been
proposed to be of thermogenic origin (Posewang & Mienert
1999; Andreassen et al. 2000). However, carbon isotopic
analyses from the carbonates retrieved from the sea floor indicate
significant depletion (as low as 52‰), suggesting a component
of biogenic methane. Ocean Drilling Program (ODP) studies
(Kvenvolden et al. 1989) performed in this region (Leg 104)
showed that the highest content of methane (.99.9% among the
gases detected) was observed at Site 644 located in the inner
Vøring Plateau. Headspace analyses (Whiticar & Faber 1989)
from Site 644 indicate the presence of mainly biogenic gas. This
is supported by carbon isotope analyses (average 13 C 76‰) of
methane, likely to be generated at depth (at c. 40 m below sea
floor (mbsf) and at c. 230 mbsf) through the reduction of CO2
from organic matter (Kvenvolden et al. 1989; Whiticar & Faber
1989). Kvenvolden & McDonald (1989) and McDonald et al.
(1989) indicated that the origin of the organic matter extracted
H Y D RO C A R B O N S E E PAG E O N T H E V Ø R I N G P L AT F O R M
from the three ODP sites is terrestrial, possibly ice rafted. Based
on their analyses, Whiticar & Faber (1989) concluded that at Site
644 a mixture of diagenetic and biogenic gases is likely to occur,
although a component of more mature thermogenic gas, migrating from deeper sources, cannot be excluded. The combined data
suggest that in the study area thermogenic gas, migrating through
the polygonal faults of the Kai Formation and forming the gas
hydrates, could mix with the biogenic gas generated at shallower
depth.
Conclusions
The different analytical methods applied in this study confirm the
presence of hydrocarbons seeping in the study area. Three main
types of carbonate cements were observed within the described
samples. Micrite occurs as pellet-rich, pyrite-rich and detrital
sediment-rich, representing the first cementation phase, whereas
aragonite, associated with pyrite and microbial biofilm, represents
the last phase of carbonate precipitation. Isotopic analyses on the
cements show depleted 13 C values, indicating a direct link with
hydrocarbon seepage, and thus confirming that the carbonates
derive predominantly from the microbial oxidation of methane
(probably a combination of biogenic and thermogenic?). Oxygen
stable isotope data revealed that part of the carbonates precipitated
in the presence of fluids enriched in 18 O with respect to modern
sea-floor waters. With the dataset available it is not possible to
define precisely the source of these fluids (e.g. gas hydrate
dissociation, clay mineral transformations). The framboidal pyrite
represents an early diagenetic product linked with the activity of
sulphate-reducing bacteria in the anoxic sulphidic zone, as
supported by the intimate association with microbial biofilm.
Further evidence of hydrocarbon-rich fluids seeping is provided
by fluid inclusion microthermometry and confirmed by Raman
spectroscopy. Moreover, hydrocarbons are not represented by
simple methane, but they consist of a complex mixture of longerchain hydrocarbons that were heterogeneously trapped, resulting
in microthermometric measurements that cannot be interpreted
simply.
We are grateful for the contributions of M. Fowler, M. Schmidt and an
anonymous reviewer, which improved the manuscript during its review.
We are particularly grateful to M. Ivanov for allowing the data to be
collected and for his constant support during the preparation of this
manuscript. We would like to thank the crew and the Scientific Party of
the TTR-8 and TR-10 cruises and the UNESCO–MSU Centre for Marine
Geosciences. We acknowledge the Intergovernmental Oceanographic
Commission of UNESCO, the Ministry of Natural Resources of the
Russian Federation, and all the organizations that supported the TTR-8
and TTR-10 cruises and that continue to support the TTR programme. I.
Belenkaya is thanked for her support onboard and for fruitful discussions.
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Received 21 October 2004; revised typescript accepted 16 March 2005.
Scientific editing by Mike Fowler