Marine Geology 218 (2005) 191 – 206
www.elsevier.com/locate/margeo
Complex pockmarks with carbonate-ridges off mid-Norway:
Products of sediment degassing
Martin Hovlanda,*, Henrik Svensenb, Carl Fredrik Forsbergc, Harald Johansend,
Christine Fichlere, Jan Helge Fossåf, René Jonssong, Håkon Rueslåttene
b
a
Statoil, N-4035, Stavanger, Norway
Physics of Geological Processes (PGP), University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway
c
Norwegian Geotechnical Institute, P.O. Box 3930 Ullevaal Stadion, 0806 Oslo, Norway
d
Institute of Energy, IFE, P.O. Box 40, N-2027 Kjeller, Norway
e
Statoil, R&D Department, N-7005 Trondheim, Norway
f
Institute of Marine Research, P.O. Box 1870, N-5817 Nordnes, Norway
g
DeepOcean, P.O. Box 2144, N-5504 Haugesund, Norway
Received 8 June 2004; received in revised form 22 March 2005; accepted 4 April 2005
Abstract
A new type of pockmark has been discovered off mid-Norway. They contain up to 10 m high ridges of methanederived authigenic carbonate (MDAC) rock. The pockmarks were investigated using ROVs (remotely operated vehicles)
in 2003 and 2004. The complex pockmarks are located well within the gas hydrate pressure/temperature stability field,
and are associated with a regional BSR (bottom simulating reflector). Our high-resolution seismic data demonstrate that
the near-surface sedimentary bedding dips into vertical pipe structures underlying the pockmarks, and that the pipes
are deeply rooted. The pockmarks are circular in plan view and contain up to 190 m long and 40 m wide ridges
formed by up to 4 m wide carbonate rocks piled on top of each other in irregular piles. Other key features of the
complex pockmarks include: 1) a distinct fauna with local bacterial mats, small tubeworms, stalked crinoids, and
pycnogonids (sea spiders); and 2) the presence of light hydrocarbon gases (C1–C5) in clay-rich sediments. The methane
y13C values range from 54x to 69x PDB, suggesting the presence of both bacterial and thermogenic gases. During our
two-day observation periods only micro-seepage occurred, without ebullition or other visual fluid flow. Petrography and
geochemistry of carbonate blocks from the pockmarks suggest precipitation of methane-derived aragonite within the sediments
(y13C = 52x to 58x PDB). We suggest that the complex pockmarks formed by sudden (dcatastrophicT) fluid flow and that
the large carbonate ridges represent dislodged MDACs originally formed subsurface, prior to the event. Subsequent microseepage is still active, and may indicate that the pipes represent fluid flow conduits which transport both bacterial and
thermogenic hydrocarbons to the water column. Our data do not suggest pressure buildup and do not predict future eruptions
* Corresponding author.
E-mail address: mhovland@statoil.com (M. Hovland).
0025-3227/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.margeo.2005.04.005
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M. Hovland et al. / Marine Geology 218 (2005) 191–206
from the complex pockmarks. Thus our data rather suggest that the dcatastrophicT formation of the complex pockmarks
may be a one-time-event.
D 2005 Elsevier B.V. All rights reserved.
Keywords: complex pockmark; conduit; degassing; dewatering; gas hydrates; authigenic carbonates
1. Introduction
Over the last decades pockmarks have proven to be
important seabed features that provide information
about fluid flow on continental margins, even if
their formation and dynamics are still poorly constrained. Pockmarks were first discovered off Nova
Scotia, Canada, and classified as seabed gas and porewater escape features by King and MacLean (1970).
Further documentation of a fluid flow origin of pockmarks came with the discovery of methane-related
carbonate crusts and slabs inside North Sea pockmarks (Hovland et al., 1985). This proved that at
least some of the depressions form by local mass
wasting by escaping hydrocarbon gases and porewaters (Hovland et al., 1985; Hovland and Judd,
1988; Judd and Sim, 1998). The source of the gases
and waters are in many cases poorly constrained,
although some studies during recent years have
demonstrated a genetic relationship with destabilization of gas hydrates (Vogt et al., 1999a,b; Paull et al.,
2000; Wood et al., 2002; Bünz et al., 2003). It is
suspected that pockmarks form by sudden local gas/
porewater/sediment eruption, and that they periodically have short outbursts followed by long periods
of quiescence or microseepage. Their detailed formation mechanism and dynamics is still largely unknown
because of deficiency in long-term monitoring (c.f.,
Hovland and Judd, 1988).
In this paper, we present new data from some
complex pockmarks, partly associated with gas
hydrates in deep water off mid-Norway. Our findings
may have implications for pockmark dynamics in
general.
2. Methods
During spring, 2003, and summer, 2004, detailed
surveys were conducted along the northeastern flank
of the Storegga Slide, in the so-called Nyegga area
(approx. 648 40V 2W N, 058 17V 32W E) (Fig. 1). They
were undertaken in order to investigate suspected
localized fluid flow through the seabed. Geophysical
mapping, visual inspection, and seabed sampling was
performed by use of the DeepOcean ROV dHiRovTsystems, operated from the survey/ROV-support vessels dNormand TonjerT in 2003 and dEdda FonnT in
2004. Two 3 m long, 10 cm diameter sediment samples were acquired, with the vessel’s 800 kg gravity
coring system. The ROV was equipped with video
cameras, multi-beam echosounder (MBE), side scan
sonar (SSS), sub-bottom profiler (SBP), grabber arms
(manipulators), and 50 cm long, 6 cm diameter sediment sampling tubes.
Two carbonate rocks were sampled with the ROV’s
manipulators, in addition to four ROV sediment core
samples. Geochemical and isotopic analyses of sediment-hosted hydrocarbon gases were conducted on
300 g sediment sub-samples. These sub-samples
were stored frozen in plastic bags before analysis at
Geolab Nor, Trondheim, Norway. Standard gas chromatography and mass spectrometry procedures were
used to determine the amount of occluded (interstitial)
and adsorbed (acid released) methane (C1) to hexane
(C6) and the stable isotope values, respectively. The
initial survey of the Nyegga pockmarks was performed by use of the ROV SSS and MBE. The
ROV-mounted geophysical systems were run at an
altitude of 20 m above the seabed.
Detailed geophysical records and maps from these
surveys, defined the approach to the pockmarks with
visual ground-contact with the aim of not disturbing
sediments too much (due to visibility). First, a carbonate pile in pockmark A and later in pockmarks G8
and G11 were visited. Here we concentrate most of
our description on findings and sampling in pockmark
G11, which was visited also in 2004. The ROV
manipulators were used to find out whether the blocks
observed were fragile, i.e., consisting of unlithified
sediments or whether they were consolidated/lithified.
Most of the jumbled piles of blocky material consisted
M. Hovland et al. / Marine Geology 218 (2005) 191–206
193
Fig. 1. The Nyegga study area is located between the Ormen Lange oil and gas field (OL) and a cluster of fields in the Åsgard/Kristin area (K).
The Storegga slide scar is evident on this general location map by the contours (contour interval 100 m). The Nyegga area (yellow square) is
located on the intact continental slope, between 650 and 750 m water depth. In the little map, dTT denotes Trondheim. Colour code on the fields:
Green = oil, red = gas, violet = condensate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
of solid rock (Fig. 6a). However, several blocks of a
different nature were also found. Some of these
resembled piles of exposed and corroded apparently
drhythmically layered sedimentsT or layered, possibly
lithified sediments (Fig. 6c). Although they were
found to be brittle, they could not be sampled with
manipulators due to crushing. Their composition and
true physical properties are not known.
Isotopic analyses on carbonate samples were performed at the Institute of Energy Research (IFE), Kjeller, Norway. Stable isotope analysis of carbonates is
based on reaction with H3PO4 in vacuum. Organic
material was removed prior to carbonate stable isotope
analysis by vacuum heating the samples in a furnace
for 4 h at 400 8C. Calcite/aragonite is reacted for 2 h at
25.0 8C. The released CO2 is cryogenically purified
and transferred to a Finnigan MAT 251 isotope ratio
mass spectrometer (IRMS) with a dual inlet and triple
collector, for determination of y13C and y18O. The
analyses were controlled by house standards of calcite
and aragonite. Data are reported in per mil relative to
the VPDB (Vienna Pedee Belemnite) standard. The
precision for y13C is 0.1 per mil and for y18O 0.2
per mil.
Mineralogical and petrographical analyses of collected limestone samples were done using a JEOL
JSM 840 scanning electron microscope (SEM) at the
Department of Geosciences, University of Oslo.
Microprobe analysis of carbonate was done with a
CAMECA SX 100 electron microprobe, with an
accelerating voltage of 15 kV and a beam current of
10 nA, using synthetic and natural standards.
X-ray diffraction analyses were performed at
the University of Oslo (Mineralogical–Geological
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M. Hovland et al. / Marine Geology 218 (2005) 191–206
Museum) on bulk carbonate samples to determine
the dominant carbonate phase. Micrograms of the
samples were crushed in ethanol in a mortar prior
to mounting and analysis on a Siemens 5005
spectrometer.
3. Geological setting
The seabed of our study area at the Nyegga region,
has a general slope angle of 18 towards the west and
represents the dshoulderT of the continental slope leading down to abyssal depths of about 3000 m in the
Norwegian Sea Basin. The region we studied (Fig. 2)
Fig. 2. A shaded colour relief map of the general study area, based
on multibeam echosounder data. The most pronounced bathymetric
feature on this image, is the Storegga Slide scarp seen crossing
from E to W in the lower part, with numerous rotated slump blocks.
The pink arrow points at the complex pockmark G11, discussed in
this study. Numerous similar pockmarks can also be seen on the
map. The warmest colour in this image (red, upper right) represents
about 550 m water depth, and the dark blue, lower left, represents
about 1100 m. The approximate location of the 2D-line shown in
Fig. 3 is indicated on this map (its exact location cannot be disclosed
due to commercial reasons). (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version
of this article.)
lies at the border between two large sedimentary
basins: the Møre Basin to the south, and the Vøring
Basin to the north (Bünz et al., 2003). The basins
probably developed as a result of several rifting episodes leading to Late Paleocene/Early Eocene continental break-up and thermal subsidence (cf. Skogseid
and Eldholm, 1989; Brekke, 2000). The sediment
infill reaches up to 10 km (cf. Brekke, 2000). Several
dome-shaped structures, suspected to represent contractional structures of Late Eocene and Mid Miocene
age occur on this margin and some are known to be
hydrocarbon reservoirs, such as that of the Ormen
Lange dome structure (Brekke and Riis, 1987; Doré
and Lundin, 1996; Vågnes et al., 1998), to the south of
Nyegga.
The seafloor of the Nyegga region is generally
smooth, and consists of a few-cm-thick layer of Holocene sediments overlying the Naust, Kai, and Brygge
Formations. The Naust Formation is of Plio–Pleistocene age, representing glacial–interglacial successions
of debris flow deposits and hemipelagic and/or contouritic drift sediments (Stuevold and Eldholm, 1996;
Vorren et al., 1998; Hjelstuen et al., 1999; Evans et al.,
2002) (Fig. 3). The giant retrogressive submarine
Storegga Slide is the last of a series of slide events
coupled to Pleistocene climatic fluctuations (Bugge,
1983; Bugge et al., 1987; Mienert et al., 1998; Haflidason et al., 2003; Bryn et al., 2003). Beneath the
Naust Formation the sedimentary succession includes
the Brygge Formation, of the Eocene/Oligocene and
the Miocene/earliest Pliocene Kai Formation, generally characterized by fine-grained hemipelagic oozes
(Rokoengen et al., 1995; Haflidason et al., 2001).
According to Bünz et al. (2003), the border of glacigenic debris flow deposits lies immediately to the
north of the Nyegga region. The main depocentre
for these sediments lies about 50 km to the NNE,
with a total accumulated glacigenic debris flow thickness of over 300 m.
3.1. BSR, polygonal faults and fluid flow
A prominent BSR occurs in the Nyegga region and
is also observed north, west, and south of our study
area (Mienert et al., 1998; Gravdal, 1999; Bouriak et
al., 2000). Of particular interest to our study is the
finding of polygonal faults through the acoustically
parallel layered sediments of the Oligocene and Mio-
M. Hovland et al. / Marine Geology 218 (2005) 191–206
195
Fig. 3. A multi-channel 2D-seismic section showing the general geological features described in this article. The triangles on the seafloor,
marked with arrows and dPmT denote pockmark locations on the seafloor associated with sub-surface dpipesT. The regional bottom simulating
reflector (BSR) is indicated with arrows, and is probably apparent because of free gas below the GHSZ (gas hydrate stability zone).
Megapolygonal faults are indicated on this interpreted section.
cene successions (the uppermost Brygge and Kai
Formations) of the Vøring/Møre basins (Berndt et
al., 2003; Bünz et al., 2003). The polygonal shape
of these faults in plan view has been mapped (cf.
Hjelstuen et al., 1997; Cartwright and Dewhurst,
1998). The faults commonly occur in at least two
tiers (Berndt et al., 2003). The density of surface
and shallow sub-surface fluid escape features in the
Møre and Vøring sedimentary basins is highest in the
Nyegga region (Gravdal et al., 2003), and is suspected
by Berndt et al. (2003) to be associated with the
polygonal fault system (Bünz et al., 2003; Bünz et
al., 2005).
The pockmarks on the Vøring Plateau and in
the Nyegga area, were first recognised on 2Dseismic records as local topographic anomalies
associated with BSRs (Bugge, 1983). Because of
their partly positive relief, these features were at first
interpreted as possible dmud diapirsT, and later, based
on side scan sonar and high-resolution seismic
records, as pockmarks, mud diapirs, and mud volcanoes (Mienert et al., 1998; Gravdal, 1999; Laberg et
al., 2001). Berndt et al. (2003), showed that pockmarks and other fluid flow features at Nyegga are
associated with pipes and also with polygonal faults.
According to Bünz et al. (2005), 3D seismic data
provide geophysical evidence that gas has leaked
from the nearby Ormen Lange reservoir and has
migrated upwards into the shallow geosphere. Furthermore, they surmise that sediments with increased gas
content might have liquefied during the mobilization
of the slides. The complex pockmarks and associated
vertical pipes that we describe are thus suspected to
represent sub-features in a large regional fluid flow
system.
4. Oceanographical setting
The oceanographic conditions at Nyegga are rather
special due to very cold Arctic bottom water in the
Norwegian–Greenland deep sea basin, to the west of
Nyegga. A distinct mid-water temperature gradient
with a temperature decline of about 6 8C over a
depth interval of only 200 m exists in the water
column above the Nyegga survey area. This thermocline separates the warm, upper Atlantic water mass
(7–9 8C), from the cold ( 0.5 to 1.5 8C) Norwegian
Sea Basin water mass (Fig. 4). Furthermore, the thermocline seasonally varies in depth as well as over
longer, decadal time periods (Mork and Blindheim,
2000; Orvik and Skagseth, 2003), a factor, which may
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M. Hovland et al. / Marine Geology 218 (2005) 191–206
Fig. 4. The seawater temperature gradient measured during the ROV-dives in the summer of 2004. Note the deep thermocline located between
300 and 600 m.
influence the stability of underlying sediment-hosted
gas hydrates.
During our survey, the pockmarks, located at 600–
750 m water depth, were situated in ambient bottom
water temperatures of 0.5 8C to 0.7 8C. It is
estimated that a 7–10 8C warming is needed to dissociate gas hydrates located at the seafloor in the
600–750 m water depth range (cf. Dickens et al.,
1997).
5. Morphology of pockmarks
The studied pockmarks are morphologically more
complex than dnormalT seabed pockmarks (Hovland
and Judd, 1988), and occur as near-circular, up to 15
m deep and 320 m wide depressions. Their most
distinctive feature is the occurrence of chaotic heaps
of large carbonate rocks and slabs, which protrude
from the central part of the depressions up to the mean
surrounding seafloor level or, even slightly higher.
A total of four large pockmarks, named: A, C, G8,
and G11 were investigated. Characteristic dimensions
of depressions and interior carbonate piles are given in
Table 1.
Pockmark A is ellipsoidal in plan view. It has a
distinct (asymmetric) curved ridge, crossing slightly
off the depression centre. The maximum height difference between top of ridge and bottom of depression is 9.5 m (Fig. 5a). Scattered, up to 3 m high
smaller carbonate piles also occur on the pockmark’s
slopes.
Pockmark C is very similar to A, although it is
symmetrical, with a maximum height difference
Table 1
Characteristic sizes and heights, relating to the Nyegga complex pockmarks
Loctn.
w
L
Max d
Min d
Pockm d
Ridge w
Ridge L
Ridge h
No piles
A
C
G8
G11
210
200
230
260
270
200
320
310
694
664
729
738
684
657
722
723
10
7
7
15
40
30
35
40
190
80
100
130
8
5
5
10
9
7
10
12
Dimentions are in m. d = depth, w = width, L = length, h = height.
M. Hovland et al. / Marine Geology 218 (2005) 191–206
197
Fig. 5. Shaded relief and perspective (oblique view) images of the four Nyegga complex pockmarks surveyed (see Table 1 for dimensions). The
vertical scale has been enhanced by a factor of 5 vs the horizontal scale. The swath bathymetry (multi-beam echosounder) data has been gridded
at a grid size of 1 m by 1 m. Note the location of sediment sample (bacterial mat location) marked with an asterix, and location of authigenic
carbonate slab-sample (G11-B) marked with a circle.
between top of ridge and depression base of 7 m
(Fig. 5b). Furthermore, C has 7 distinct, and up to 2 m
high isolated carbonate piles along its outer perimeter
(Fig. 5b).
Pockmarks G8 and G11 are adjacent with a distance of 500 m. G11 is near-circular, G8 is ellipsoidal in plan view. G8 resembles A and C, in that
it also has 10 distinct, up to 2 m high isolated
carbonate piles along its perimeter. It has an elongated dome-shaped carbonate pile, slightly offset
the pockmark centre (Fig. 5c). Pockmark G11 is
the deepest and most spectacular, with two irregular ridges, divided by a deep central basin.
Additional small interior basins and piles add to
its chaotic appearance (Fig. 5d). The largest individual carbonate block seen inside any of the four pockmarks measures about 4 m by 3 m by 2 m, i.e., a
volume of about 24 m3, and is located inside G11
(Fig. 5d).
6. Seabed observations
Our survey documents the existence of an apparently pockmark-specific micro- and macrofauna
(Figs. 6 and 7), which includes bacterial mats
(probably Beggiatoa sp), fields of small tube-worms
(probably polychaetes and pogonophorans) and
large (15 cm) pycnogonids (sea-spiders, suspected
to be of the species Colossendeis proboscidea
(T. Brattegard, personal communication, 2004). The
bacterial mats were located in the deepest, soft
sediment-covered portions of the G11-pockmark,
and were also observed at one of the sediment
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M. Hovland et al. / Marine Geology 218 (2005) 191–206
Fig. 6. Underwater colour images grabbed from the ROV-acquired video footage from complex pockmark G11 (better picture resolution is
unfortunately not possible). a) Up to 24 m3 large carbonate slabs occur inside the pockmark. Note the basket star (lower middle), stalked
crinoids, sea-anemone, and unidentified organism (cnidaria?) lower middle (right). Notice that most of these macrofaunal organisms are perched
on the lower side of the carbonate rock (see text). b) Five stalked crinoids perched on top of an adjacent slab to that shown in a). Note the fish
(eel pout?) resting next to one of the crinoids (inset detail). c) Layered and friable (crisp and fragile) exposed sediment structure is suspected to
represent carbonate-cemented sediments. The organisms include basket stars, crinoids, and unidentified macro-fauna. d) Five exposed thin
wafer-like carbonate rocks found in the deepest portion of G11. The one standing vertically next to the basket star (right) was sampled (Sample
G11-A). It is shown lying on the deck of dNormand TonjerT in the inset (lower left). e) A sea spider (pycnogonid suspected to be a Collossendeis
sp.) located on one of the large slabs near that shown in a). f) A pycnogonid ambling near the sediment sampling location (bacterial mat location
shown in Fig. 7). The animal measures about 15 cm across (between tips of legs).
core sample locations (Fig. 7). In addition to the
typical discoloration of the seafloor, slimy and
bfluffyQ filaments were seen undulating in the
currents set up by the ROV. Up to 30 cm tall stalked
crinoids (Fig. 6b) and large (up to 1 m diameter)
ophiurids (basket stars, Fig. 6a) are abundant on the
carbonate rocks. These, together with hydrozoa, and
numerous other unidentified sessile organisms were
perched at specific locations on the large carbonate
rocks.
7. Sub-bottom geology and active seeps
High-resolution SBP records were acquired
across three of the pockmarks: A, C, and G8.
M. Hovland et al. / Marine Geology 218 (2005) 191–206
199
Fig. 7. Dark- and white-stained seabed sediments caused by microseepage and bacterial mat development on the surface. The area is about 1 m
by 1 m and shows small tube-worms and small pogonophorans growing round the stained sediments. This is sampling location G11-2, also
called the dbacterial matT location, shown in Fig. 5d (Tables 2 and 3). The small inset image shows the site after coring with ROV.
These demonstrate that the fine acoustic upper
layering gives way to a zone of unlayered (disturbed) acoustically transparent sediments inside
the pockmarks. Furthermore, the finely horizontally
layered sediments clearly dip down around the perimeter of the structures, indicating some sort of
central subsidence or collapse. From the SBPrecords it is clear that dyoungerT material has been
deposited on top of these dipping layers (see Fig. 8).
We suggest that these dyoungerT sediments in fact
represent older material that has erupted from the
central conduit-structures (acoustically transparent
zones) and has become redistributed after possibly
having undergone lithification.
Results from the geochemical investigations show
that the occluded (interstitial) hydrocarbon gases in
the pockmark-sediments, range from methane (C1) up
to pentane (C5) (Table 3). The relatively high content
of typical thermogenic hydrocarbons (C3+) indicates a
prolific and probably long-term hydrocarbon fluid
flow. Table 3 shows that the highest occluded gas
concentrations are found in pockmarks A and G11.
However, the occluded methane value was about 5
times higher in G11 (sample G11-2) at a bacterial mat
Fig. 8. High-resolution, sub-bottom profiler (SBP) image from G8 (see Fig. 5c). The upper image presents the raw data, whereas the lower
image contains interpretations. Acoustically turbid (opaque) sediment occupy most of the sub-surface of this complex pockmark. Notice the
funnel-shape, formed by the boundaries between acoustically coherent reflections (layered) and amorphous (turbid) reflections. This suggests
that transport of sediments and fluids have occurred up to the near-surface and out to both sides at the surface.
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M. Hovland et al. / Marine Geology 218 (2005) 191–206
Table 2
The isotopic character of authigenic carbonates and hydrocarbons
sampled in the Nyegga complex pockmarks
Sample
C-3
G11-2
Carbonate B
Occl.
C1
69.3
yC13
yC13
yC13
yC13
Ads. C1 Ads. C2 Ads. C3
53.9
58.3
33.8
35.9
yO18
30.2
32.1
52.1 6.4
Occl.C1 = Occluded methane, Ads.C1 = adsorbed methane. C2 =
ethane, C3 = butane. Units are in per mil VPDB.
than in pockmark A. However, for sample G11-1,
taken only some metres away from G11-2, the
occluded methane value is much lower, even though
the ethane, propane, and butane values are similar as
for G11-2 (Table 3). For occluded ethane the values
are similar and for propane, butane, and pentane, the
values were slightly higher for A, compared to G11.
For adsorbed hydrocarbons (Table 3), the G11 bacterial mat location (Sample G11-2) had the highest
hydrocarbon values.
The y13C stable isotope values (Table 2) of the
occluded methane indicate a mixed biogenic/thermogenic gas origin (Whiticar, 1999; Hovland and Judd,
1988).
8. Carbonate structure and composition
Two carbonate rocks were sampled, both of them
from pockmark G11. These rocks represent two different types of carbonate lithology. However, XRD
results show that both samples are dominated by
aragonite. Sample A is one of at least three about 1
cm thick by 30 cm by 20 cm wide dlithified beddingT
wafer-shaped rocks found protruding out of the
seabed inside G11 (Fig. 6d). Sample B was acquired
from one of the large heaps of rough carbonate blocks
and is thought to represent a typical sample from the
chaotic carbonate rock ridges and probably represents
the bulk of carbonate rock observed inside the complex pockmarks.
ple is horizontally layered, where the texturally oldest
zone is composed of granular and fibrous aragonite.
The fibrous aragonite typically occurs in radiating
aggregates or bundles (Fig. 9). Both morphologies
are typical for aragonite from seep localities (e.g.,
Ritger et al., 1987; Hovland et al., 1985; Bohrmann
et al., 1998; Aloisi et al., 2000).
There are two layers in the lower half of the sample
that have a more complex mineralogy. In addition to
aragonite, they contain zoned aggregates of Mg-calcite and dolomite (Table 2). Interestingly, these two
phases occur within the same crystals (Fig. 9).
Microprobe analysis shows that the calcite contains
14–15 mol% Mg, and the dolomite between 37 and 41
mol% Mg. The calcite is thus Mg-calcite, and the
dolomite is classified as protodolomite/dolomite.
Small grains composed of iron and sulphur (probably
pyrite) were also found in these layers.
8.2. Carbonate sample B
The irregularly shaped carbonate sample B has a
mineralogy and structure which strongly contrasts
with the G11-A wafer-shaped sample. Firstly, its
main textural component consists of curved or blocky
aggregates of aragonite-cemented clastic material,
overgrown by fibrous aragonite with some of the
individual crystals being longer than 500 Am (Fig.
9). Secondly, neither Mg-calcite nor dolomite, were
found in the sample. The clastic content (Fig. 9)
includes quartz, feldspar, clay minerals, and silicious
microfossils. These are cemented and intergrown by
fibrous to granular aragonite.
Other authigenic minerals in this sample contain Fe
and S (probably pyrite), and Fe phases (Fig. 9). The
Fe phase occurs as rims on the Fe and S-phase, and
represents alteration to Fe oxides or hydroxides. The
sulphide mineral (pyrite) is abundant in the zone
containing fibrous aragonite, but is also found in the
clastic-rich zone of the sample.
9. Discussion
8.1. Carbonate sample A (dwaferT)
9.1. Fauna
Texturally, carbonate sample A is composed of
three distinct layers (Fig. 9), all of which differ in
porosity, crystal shape, and mineral content. The sam-
The relatively high density of macro-benthos and
characteristic composition of the observed fauna are
M. Hovland et al. / Marine Geology 218 (2005) 191–206
201
Fig. 9. a) Fibrous interlocking crystals of aragonite from the G11-A slate sample. Viewed with an optical microscope with crossed polars. Note
the finegrained nucleation point in the centre of the radiating aragonite. b) SEM backscatter image of carbonate from the G11-A slate sample.
Note the zoned single crystals of Mg-calcite and dolomite. c) SEM backscatter image of sample G11-B slab. Note the two distinct textures of
aragonite: 1) cementing clastic fragments, and 2) secondary growth along the edges of the sample. d) SEM backscatter image of aragonite
crystals and pyrite from sample G11-B slab. The pyrite is slightly oxidized—probably due to exposure at the sea floor.
surprising when considering the sub-zero ( 0.7 8C)
ambient seawater temperatures of the area. This fauna
was only found inside the pockmarks, despite extensive visual surveys on the general seafloor of the
region. However, a similar faunal composition has
previously been reported from the Håkon Mosby
Mud Volcano (HMMV), in the Barents Sea (Vogt et
al., 1999a; Gebruk et al., 2003). We infer that the
primary trophic stage within the pockmarks partly
consists of a chemosynthetic micro-organic community, which is fuelled by advecting hydrocarbon-associated fluids, much in analogy with the HMMVenvironment, even though there is no visual seepage
at the Nyegga complex pockmarks. The pockmark
specific fauna may therefore be important, in that it
signifies an active fluid flow through the pockmarks,
as is also indicated by our geochemical results (Tables
2 and 3).
9.2. Suspected gas hydrates
Considering the water depth where the Nyegga
pockmarks are located (between 600 and 750 m),
and the water temperature (generally between 1
and + 5 8C), there is no doubt that methane hydrates
are stable at seabed and also down to at least 200 m
sub-seabed (Mienert and Posewang, 1999; Posewang
and Mienert, 1999). But, because potential hydrates
would become chemically unstable and dissociate
because the ambient seawater is depleted in methane
202
M. Hovland et al. / Marine Geology 218 (2005) 191–206
Table 3
Sample
C1
C2
C3
C4
C5
Sum C2–5
(a) The concentration of occluded (interstitial free) gases in sediments sampled inside the Nyegga complex pockmarks
C-3
48.51
2.13
1.13
0.537
0.421
G8-5
41.19
2.63
1.23
0.469
0.165
G11-1
11.78
1.19
0.887
0.268
0.140
G11-2
256.75
3.18
0.688
0.262
0.077
4.218
4.494
2.485
4.207
(b) The concentration of adsorbed (particle-bound) gases in sediments sampled inside the Nyegga complex pockmarks
C-3
262.70
50.61
28.50
12.75
3.783
G8-5
153.53
14.37
6.44
2.69
0.727
G11-1
202.74
37.68
24.60
12.54
4.533
G11-2
275.54
55.13
36.17
18.20
6.204
95.643
24.227
79.353
115.704
Units are in mg gas per g sediment. C1 = methane, C2 = ethane, C3 = propane C4 = butane, C5 = pentane.
and because the seepage of hydrocarbons is very slow
and weak (i.e., microseepage), hydrates would only be
expected to occur below the sediment surface. The
fact that many of the macro organisms observed on
large carbonate rocks are perched on the underside of
overhanging and steep-walled rocks (Fig. 6a), supports our suspicion that microseepage is continuously
occurring. It is therefore suspected that any gas
hydrates at the Nyegga pockmarks will only occur
below the seafloor sediments where the porewaters are
charged with hydrocarbons by upward migration, as
shown in Tables 2 and 3. The porewaters currently
seeping out at distinct locations inside the pockmarks
are charged with light hydrocarbons and are most
probably anoxic as indicated by the occurrence of
(Beggiatoa?) bacterial mats.
9.3. Carbonates
From Table 2, it can be seen that the y13C and y18O
values are typical for MDACs. Lithification must
therefore have occurred sub-seabed in a zone where
porewater has been charged with methane. The textural equilibrium between pyrite and aragonite suggests a precipitation within the sulphate reduction
zone, where the carbon is ultimately derived from
microbial oxidation of methane. Carbonates from the
same area collected during two Teaching Through
Research (TTR) cruises in 1998 and 2000 (Mazzini,
2004) show comparable values for seep calcite and
aragonite (y13C of 51x to 42x PDB).
If carbonate sample G11-B is considered representative for the blocky carbonate ridges within the pockmarks, it implies that the rocks are formed from
authigenic aragonite precipitation within porous sediments. From Table 1, it can be seen that the y13C and
y18O values are typical for MDACs. Lithification
must therefore have occurred sub-seabed in a zone
where porewater has been charged by methane. This is
also consistent with aragonite precipitation taking
place in sulphide-rich diagenetic environments
where calcite precipitation is inhibited (Burton,
1993; Naehr et al., 2000; Aloisi et al., 2002). We
infer that the carbonate formation zone must be within
the vertical conduit zones previously described.
Even though microseepage can explain how large
blocks of authigenic carbonate rocks form in subseabed sediments (Roberts et al., 1992), this quiescent
(mechanically feeble) process cannot explain neither
the dramatic topography of the complex pockmarks,
nor the formation of jumbled piles of large carbonate slabs. To explain these findings, much more
energy and physical force is required than that
excerted by microseepage. A viable complex pockmark formation model has to account for all their
characteristic features.
10. Suggested complex pockmark formation mode
Similar fluid escape features interpreted as large
deep-water pockmarks associated with BSRs, have
been described and discussed from offshore western
Canada (Wood et al., 2002) and fossilized features
have been described from the Meiklejohn Peak,
Nevada (Krause, 2001). These latter ones consist of
dpiled authigenic limestonesT suspected to be of cryogenic (freeze/thaw) origin.
M. Hovland et al. / Marine Geology 218 (2005) 191–206
Those from offshore Canada occur in the Cascadia
Accretionary prism. The explanation provided by Riedel et al. (2001) and by Wood et al. (2002) for these
features is fluid flow caused by a local disintegration
of gas hydrates. It is theorized by Wood et al. (2002)
that gas hydrates within the sediments dissociate
locally due to anomalously high temperatures. However, the researchers fail to account for how the high
temperature zones should develop, even though the
idea is tantalizing. At the second, Meiklejohn, location, the cause of piled authigenic carbonates is
suspected by Krause (2001) to be of cryogenic origin
(freeze/thaw) associated with destabilization of gas
hydrates. If there is a mechanism, which induces
periodic temperature variations in the water column
and seabed, this freeze/thaw idea could also account
for the Nyegga features. Therefore, the periodic dissociation and re-formation of gas hydrates, due to
periodic heatflow at Nyegga, might be a viable alternative for the complex pockmark formation.
Berndt et al. (2003), concluded that the fluid
escape features at Nyegga are attributed to de-watering through a polygonal fault system from finegrained hemipelagic sediments of the Kai Formation.
We basically agree with this concept, but in addition,
we suggest that the fluids are mixed with thermogenic
hydrocarbons of deeper origin, in agreement with
results from the 3D-analysis performed by Bünz et
al. (2005) and with the higher hydrocarbons we found.
We hereby suggest a model that relies on the longterm focused migration of gas-charged porewater and
also free gas.
As prerequisite conditions, we consider the
following:
1) High fluid content oozes have been deposited in
relatively thick layers 2–3 km below surface.
2) The porefluids dewatering from these sediments
migrate through a microfracture system, manisfested by the megapolygonal fault pattern, documented by others.
3) Thermogenic hydrocarbons have been generated in
source rocks located beneath the oozes. The lightest of these components (C1–C5) also migrate
through the same ddrainage systemT.
4) A set of dpipesT, some of which lead up to the
seafloor, have also been established over time.
Through these pipes, the migration of porewater
203
and gases is more rapid than through the rest of the
drainage system.
5) Because the hydrocarbon flux is suspected to be
higher through the pipes than through the adjacent sediment, either i) a temperature anomaly
may develop in the pipes, ii) a water depletion
anomaly, or iii) a chlorinity anomaly may
develop in the pipes.
6) The general GHSZ of the area (GHSZg) is about
200 m thick. However, for some still unknown
reason, the GHSZ of the pipes (GHSZp) are
suspected to be only a fraction of this depth.
At this stage, we do not intend to speculate on any
of the three possibilities for why the pipes underlying
the complex pockmarks may form, but accept the
possibility that the GHSZp is much less than the
regional thickness. During our short period of observation, there were evidently no violent eruptions—the
activity we observed was one of microseepage. Based
on the marine organisms observed, we suspect this
mode to be the normal mode of activity over relatively
long time periods (years?). However, the amount of
seeping fluids transported through the pockmarks can
be relatively significant, and it is expected that in the
transformation zone, between free gas and gas
hydrates, perhaps only some metres below seabed,
there are actively forming carbonates.
This would mean that the sediments within the
pipes were gradually being sealed by carbonates
(Hovland, 2002), before an eruption occurred,
whereby the large carbonate piles were formed inside
the pockmarks. A precursor period, whereby gases
and gas-charged porewaters have migrated upwards
through the pipes to the seawater column, is inferred.
Over some time, the amount of carbonate cementation, near the seafloor would temporarily seal off the
pipes, and a column of free gas could form underneath
this seal. Consider the moment when carbonates have
completely sealed the pipe and no more seepage
occurs. Then it is expected to be only a matter of
time before the pressure difference between surface
hydrostatic pressure and the breaking capacity of the
seal is balanced, so that the seal breaks abruptly and
releases the trapped gases together with fine-grained
sediments and porewaters. At the same time, the
carbonate dcorkT will be dislodged and broken, so
that the remains are pushed upwards to form the
204
M. Hovland et al. / Marine Geology 218 (2005) 191–206
observed carbonate ridges and piles. This process will
probably also move some of the fine-grained sediments in the lower portion of the pipe, in which
case some fine-grained sediment is lost to the surrounding water and seafloor. This would result in the
observed collapse depression forming. Thus, while
lower parallel bedding may acquire an upwards-dipping attitude, the near-surface bedding will be downward dipping (Fig. 10).
We have found no indications suggesting that a
bblowoutQ process, as described above is repetitive or
periodic. In case of periodic eruptions, it would be
expected that some of the carbonates should have
contained a brecciated structure. We therefore conclude that the complex pockmarks at Nyegga have
only had one violent episode, i.e., the one that formed
the circular features. Subsequent seepage is regarded
as having been of a microseepage nature. However,
only future observations, and long-term monitoring of
one or two of these fascinating features will provide
us with a proper answer to their mode of operation.
11. Conclusions
The complex pockmarks at Nyegga contain up to
190 m long irregular ridges consisting of piled-up
carbonate blocks. They are associated with microseepage of light hydrocarbon gases (C1–C5) and a
distinct fauna, including bacterial mats, tubeworms,
stalked crinoids, and pycnogonids (sea spiders). Petrography and geochemistry of carbonate blocks from
the pockmarks suggest precipitation of methanederived aragonite within the sediments (y13C =
52x to 58x PDB). The pockmarks are located
well within the gas hydrate stability field, and are
associated with a regional BSR. We suggest that the
formation of the pockmarks was sudden (catastrophic), followed by micro-seepage. During our
observation period (only 2 days) no ebullition or
other visual fluid flow was seen. Based on a synthesis
of all gathered visual, geophysical, geochemical, and
mineralogical data, we conclude that the complex
pockmarks at Nyegga have only had one violent
episode, i.e., the one that formed the circular complex
features. Subsequent seepage is regarded as having
been of a micro-seepage nature. Our data do not
suggest any current pressure build-up and therefore
do not predict future eruptions from the complex
pockmarks.
Acknowledgements
Fig. 10. A sketch showing the conceptual model of the inferred
structure and geological habitat of the Nyegga complex pockmarks.
Whereas the general gas hydrate stability zone (GHSZg) causes a
regional bottom simulating reflector (BSR), either anomalously high
temperature and/or chlorinity in the vertically disturbed dpipeT sediments (pockmark conduit) causes a suspected shallow GHSZ in the
pipe. It is also suggested that a relatively high column of free gas
(up to 180 m?) can build up under gas hydrates and authigenic
carbonates inside the pipe. This local overpressuring is suspected to
lead to a blow-out type of eruption through the pockmark.
We would like to thank Ian Grieves (DeepOcean) in help to assemble the data, and to all others
onboard the dNormand TonjerT and dEdda FonnT who
have helped. We acknowledge the release of seismic
data from the licencees of the Ellida prospect, and also
all help from our seniors in Statoil, especially Tor Inge
Tjelta, who have provided the opportunity to develop
this model and who have given permission to publish
the results. Finally, we thank Maarten Vanneste and
one anonymous reviewer for their constructive comments to an early version of the manuscript.
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