8 1 5
Integrated petrographic and geochemical record of hydrocarbon seepage on the
Vøring Plateau
A . M A Z Z I N I1,2, G . A L O I S I3, G . G . A K H M A N OV4, J. PA R N E L L1, B. T. C RO N I N1 & P. M U R P H Y5 1Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, King’s College, Aberdeen
AB24 3UE, UK
2Present 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)
3Laboratoire de Pale´oenvironnements et Pale´obiosphe`re, Universite´ Claude-Bernard, UMR 5125 CNRS, 2, rue Dubois,
69622 Villeurbanne cedex, France
4UNESCO–MSU Centre for Marine Geology and Geophysics, Faculty of Geology, Moscow State University, Vorobjevy
Gory, Moscow, 119992, Russia
5School of Earth Sciences and Geography, Kingston University, Kingston upon Thames KT1 2EE, UK
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 in13C (31.6‰,ä13C,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‰ , ä18O, 4.9‰) suggests a precipitation
temperature very close to the one recorded on the sea floor (between1 and 28C), 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 in18O (4.6‰,ä18O,6.2‰), which is interpreted as a
result of precipitation from an18O-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 þhydrocarbon mixture inside the inclusions. This seepage mixture was almost certainly immiscible, resulting in heterogeneous trapping.
Keywords:Vøring Plateau, cold seeps, authigenic minerals, carbonate, methane.
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 com-munities (Hovland et al. 1987; Embley et al. 1990; Kulm & Suess 1990; Corselli & Basso 1996; Sibuet & Olu 1998; Peckmannet al. 2002). These chemosynthetic authigenic carbo-nates 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; Robertset al. 1993; Boetiuset al. 2000; Elvertet al. 2000).
The record of cold seeps extends back at least to the Devonian (Aharon 1994; Campbell & Bottjer 1995; Campbellet al. 2002), and is substantial in the Quaternary, when they occur in both active margins (Boulegue et al. 1987; Ritger et al. 1987; Le Pichonet al. 1990; Limonovet al. 1994; Henryet al. 2002) and passive margins (Brookset al. 1986; Hovlandet 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 (Henryet al. 1996; Oluet al. 1997; Aloisiet al. 2000; Peckmannet al. 2001; Mazziniet 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 &
Savard 1992; Kauffmanet al. 1996; Jonket al. 2003; Mazziniet 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; Bouriaket al. 2000; Sassenet al. 2003; Berndtet al. 2004; Formoloet al. 2004).
1
3
2
60° 60°
62° 62°
64° 64°
66° 66°
68° 68°
70° 70°
-5° -5°
0° 0°
5° 5°
10° 10°
15° 15°
200 m
500m 1000
m 1500m
2000 m 2500
m 300
0m
200m
NOR WAY
Voring Plateau
AT-111G
AT-112G AT-113G
AT-114G AT-115G
AT-116G AT-117G
AT-118G AT-119G
AT-120G AT-121GAT-122G AT-137G
AT-138G AT-139G AT-140G
AT-141B AT-142B
AT-143B
AT-323Gr AT-324G AT-325GR
AT-326G AT-327G
AT-328G
AT-329G AT-330Gr
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 (Kenyonet 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.
A.
MA
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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 continen-tal 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 sedimen-tary basin fill (Brekke 2000). Between the Late Eocene and Mid-Miocene, compressional phases generated dome structures (Skogseid & Eldholm 1989; Dore´ & Lundin 1996) that are potential hydrocarbon traps or accumulations. After the Neogene uplift of the Norwegian mainland (e.g. Hjelstuenet 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 (Buggeet 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 (Buggeet al. 1987; Bouriaket 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 me-chanisms 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; Mienertet al. 1998; Posewang & Mienert 1999; Vogtet 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; Bunzet al. 2003; Berndtet 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
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 Logachevoperated by the Polar Marine Geosurvey Expedition. Polished slabs and thin sections were studied using standard petrographic fluores-cence and cathodoluminesfluores-cence (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 (low-Mg) calcite on the basis ofdvalues of the (104) diffraction peak, which arec. 2.998 A˚ and 3.028 A˚, respectively (Aloisiet 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 Optiphot2-POL 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, indi-cating leakage as a result of the fragile structure of the mineral. In particular, evidence of leakage was observed in the intercrystal inclu-sions. 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 tempera-tures 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 CO2by 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 ä13C and ä18O results are reported
relative to the V-PDB standard (Craig 1957; Coplen 1994); the precision of the analyses is0.2‰. The Friedman & O’Neil (1977) fractionation equation was used to calculate the temperatures of carbonate precipita-tion.
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 thermoelectri-cally cooled CCD detector and a 514.5 nm Ar ion laser. Using a 1003 objective lens, the laser was focused to a spot size ofc. 1ìm. Peak fitting
was performed using Galactic GRAMS/32 software, which uses a mixed Gaussian–Lorentzian curve function.
Results
Geophysical investigations and sea-floor observations
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 sea-floor 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’ (Kenyonet 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
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 H2S.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 carbonate-cemented bivalve deposits.
The retrieved large chemoherm sample (Fig. 3) was very porous, releasing a strong smell of H2S. 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 com-posed 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 symbiont-containing species includedPogonophoraworms and Cladorhizi-dae 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 orienta-tions 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.
Tobic Mud Volcano
1k
m
50 m
N
S
Mineralogy and petrography
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 ana-lyses 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 ovoid-shaped pellets (Fig. 4a). Pellets, ranging from 20 to 400ìm in
size, may locally represent 80% of the micrite, indicating extensive bioturbation within the sediment. A distinct micrite-rich layer can be distinguished in contact with the shell bodies. The better preserved microfossils are present within the micrite-cemented 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 matter-and pyrite-rich layer (P/B layers; see below for details). Arago-nite 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 ce-ment (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 (2–4ìm) 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 Hovlandet al. 1987, fig. 6). The dark brown layer observed in several samples defining the boundary between different cementation phases, and often coat-ing the last aragonite phase, is interpreted as microbial deposits
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 distinguish-able 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 25ìm in diameter
has a pervasive distribution through the carbonate cements, with many small (up to 5ìm) 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 hemi-pelagic mud sample analysed yielded values ofä13C of 0.3‰
Table 2. Setting of the sampling stations and summary of carbon (13C) and oxygen (18O) stable isotope results and short sample description
Station number, sampling depth (BSF)
Setting or feature Description ä18O ‰
(V-PDB)
ä13C ‰
(V-PDB)
Calculated precipitation
temperature (8C)
AT 111G 40 cm Upper slope Hemipelagic clay 0.2 0.3
AT117G 445 cm Pockmark Me´lange calcite–dolomite, nodule bulk 5.9 43.1 5.8
AT118G 514 cm Pockmark Me´lange calcite–dolomite, upper surface of
tabular crust
6.2 37.4 6.8
AT118G 514 cm Calcite, lower surface of tabular crust 5.6 49.3 4.6
AT118G 535 cm Aragonite, nodule bulk 5.3 48.4 3.5
AT120G 0–3 cm Mud diapir Aragonite, bivalve aggregates 4.1 38.3 0.9
AT120G 60–90 cm Calcite, nodule bulk 5.3 49.9 3.5
AT120G 71–79 cm Calcite, nodule bulk 2.6 31.6 7
AT120G 120 cm—1 Calcite, nodule bulk 5.1 47.2 2.9
AT120G 120 cm—2 Calcite, nodule bulk 5.3 50 3.5
AT121G 5–10 cm Mud diapir Calcite, nodule bulk 5.8 51.1 5.3
AT121G 25–50 cm—1 Calcite, nodule bulk 6 51.5 6.1
AT121G 25–50 cm—2 Calcite, nodule bulk 5.9 50.6 5.8
AT122G 60–70 cm Mud diapir Calcite, nodule external part 4.6 43.2 1
AT122G 60–70 cm Calcite, nodule internal part 5.8 49.6 5.3
AT323Gr Mud volcano or pockmark Aragonite botryoid 4 49.6 1.2
AT323Gr Aragonite botryoid 4.5 49.8 0.7
AT323Gr Aragonite botryoid 4.4 47.4 0.2
AT323Gr Aragonite botryoid 4.3 49 0.1
AT323Gr Aragonite botryoid 4.5 48.9 0.7
AT323Gr Mollusc shell 3.3 13.9 4
AT323Gr Mollusc shell 3.1 27.2 4.8
AT323Gr Micritic geopetal filling 4.5 45.9 0.7
AT323Gr Micritic geopetal filling 4.4 45.4 0.2
AT323Gr Micritic geopetal filling 3.3 44.3 4
AT323Gr Calcite, light, internal part of chemoherm 3.8 41.3 2
AT323Gr Calcite, darker, internal part of chemoherm 3.6 47.3 2.8
AT323Gr Calcite, internal part of chemoherm 4.9 52 2.1
Calculated precipitation temperatures assume isotopic equilibrium with actual sea-floor fluids (ä18O¼0:2‰
SMOW).
and ä18O of 0.2‰. The carbonates drilled from the shells
reveal the least depleted13C values (27.2‰,ä13C,8‰).
The different authigenic carbonates revealed carbon isotope compositions with a strong depletion relative to marine carbo-nate, showing ä13C 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. ä18O results
vary from 2.5‰ to 6.2‰; the carbonates sampled in the near subsurface show lower enrichment in 18O (3.1‰ , ä18O ,
4.9‰) compared with those sampled from deeper intervals (up to 5.5 m below the sea floor, 4.6‰,ä18O,6.2‰).
Microthermometry and Raman spectroscopy of fluid inclusions
Outline details of the fluid inclusion study were presented by Parnellet al. (2002). Results of microthermometry measurements indicate three populations (Table 3). The first population com-prises two-phase (liquid- and vapour-filled) inclusions, situated inside the aragonite crystals, homogenizing to the gaseous phase at a mean Th of 288C and to the liquid phase at a mean Thof
378C (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 708C. The
third population consists of intracrystal aqueous inclusions, which are monophase (liquid-filled) at room temperature, but
which on cooling (atc.10 to158C) nucleate a vapour phase
that homogenizes at temperatures between 0 and 28C. 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 spectro-scopic analyses. Figure 8b shows one example of a Raman spectrum obtained from a 4ìm310ìm 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 hydro-carbons of different chain lengths. Methane should show a single, sharp band at 2917 cm1, which is not recognized. The
fre-quency range covered by the Raman analysis would also cover other possible gases such as CO2, N2 and H2S if they were
present in the inclusions. The possibility of surface contamina-tion 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 (R¼
I(2854)=I(2935)) and two in the H–C–H bending region (r¼I(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
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 areR¼1:061 andr¼0:931. According to the calibration curves of Pironon (1993) these both equate ton-alkanes wheren
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, longer-chained oil.
Discussion
Diagenetic processes and microbial activity
A temporal sequence of the different carbonate cements ob-served 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 cementa-tion 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 embay-ments (Fig. 4b). Aragonite appears to represent the last cementa-tion 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
calcite precipitation is inhibited (Naehret al. 2000; Aloisiet al. 2002).
The material with slightly brighter CL is distinguished by the different contents of clayey and detrital admixture (more lumi-nescent) 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 contour-ite-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 precipita-tion prior to carbonate precipitaprecipita-tion. 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
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 13C content and a gradual increase in 18O
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 (ä13C 0.3%) and methane-derived
carbonates (ä13C52%), respectively. The trend is produced by
the presence of variable amounts of hemipelagic mud in the authigenic carbonates. The depletion in 13C 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 ä18O fluid
composition of 0.2‰SMOW (Tchernia 1978; Aloisi 2000).
Calcu-lated equilibration temperatures for the two extremeä18O values
measured on the carbonate samples (2.6‰ and 6.2‰) reveal a temperature range from6.88C to 78C approximately (Table 2). All the samples analysed from the chemoherm block revealed precipitation temperatures in agreement and mostly closer to the
S
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/B1layer and from large pelitic or
micritic fragments; P/B2, thin layer, pyrite- and organic matter-rich,
similar to P/B1separating two different aragonite layers; C5, youngest
modern sea-floor temperature (c. 08C) (Tchernia 1978; Thiedeet 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 08C. These temperatures
are unrealistic and indicate anomalous enrichment in18O of the
diagenetic fluids. At other cold seep sites 18O-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 (Kenyonet al. 1999, 2001; Bouriaket al. 2000; Bunzet al. 2003) revealed the presence of gas hydrates in the sedimen-tary section beneath the sampled sites. The dissociation of the hydrates could provide 18O-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 transforma-tions, etc.).
Implications from fluid inclusions
Microthermometry and fluid inclusion analysis show three populations of fluid inclusions. In several instances the inclusions do not totally freeze until temperatures lower than 1008C, suggesting that they may contain a mixture of water and hydrocarbons. Nevertheless, the high temperatures of
homogeni-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.
13C
(
V
-PD
B)
ä18O ( V- PDB)
-60 -50 -40 -30 -20 -10 0 10
-1 0 1 2 3 4 5 6 7
aragonite calcite hemipelagic mud melange calcite/dolomite mollusc shell micritic geopetal filling
ä
Fig. 7. Cross-plot of carbon (13C) and oxygen (18O) stable isotope
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-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 2–48C).
Although this temperature is consistent with the Norwegian Sea bottom water temperature, the vapour phase, which nucleated at temperatures below 08C, 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 (liquid-filled) 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.
The origin of the methane-rich fluids
Dore´ & Lundin (1996) and Bunzet 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 as52‰), 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 (averageä13C76‰) 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 (Kvenvoldenet al. 1989; Whiticar & Faber 1989). Kvenvolden & McDonald (1989) and McDonald et al. (1989) indicated that the origin of the organic matter extracted
Table 3. Summary of primary fluid inclusion populations
Contents at room temperature
Host Size (ìm) Morphology Degree of fill Th(8C) Tf(8C) n
LAþH Aragonite ,10 Subrounded, 1 27–30 (Vap) ,100 9
VAþH intracrystal irregular 27–50 (Liq)
LAþH Aragonite ,6 Cubic 0.7 65–75 (Liq) ,100 15
VAþH intercrystal
LH Aragonite ,5 Subrounded 1 0–2 (Liq) – 6
intracrystal
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.
20 µm
1000 1500 2000 2500 3000 3500
Frequency (cm-1
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
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, migrat-ing 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 depleted13C 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 in18O 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 longer-chain 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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