Methane-derived Authigenic Carbonates Research Articles

Methane-derived authigenic carbonates from active hydrocarbon seeps of the St. Lawrence Estuary, Canada

Nearly 2000 pockmarks with diameters ranging from a few tens of meters up to 700 m are present on the seafloor of the St. Lawrence Estuary in eastern Canada. Coring of some pockmarks resulted in the recovery of various-sized and shaped carbonate concretions in a predominantly silty mud matrix. Petrographic and geochemical data on four authigenic carbonate concretions are reported as well as data from shell material in the unconsolidated sediment. Video observations and echo-sounder images indicate that the sampled pockmarks are actively gas venting. The video images show significant look-alike microbial mats in areas where gas is venting. The carbonate concretions are primarily made up of carbonate cements with varying percentage of shell fragments, micrite particles and fine-grained clastics. Orthorhombic crystal morphology and diagenetic fabrics including isopachous layers and botryoids characterize the aragonite cement. Oxygen isotopes ratios for the cement crusts do not record any thermal anomaly at the site of precipitation with δ 18O VPDB ratios (+3 ‰) in equilibrium with cold (5 °C) deep marine waters, whereas significant negative δ 13C VPDB ratios (−9.9 to −33.5 ‰) for cement and shell material within concretions indicate that the carbonates largely derive from the microbial oxidation of methane. The δ 13C VPDB ratios of aragonite shells (−2.7 to −5.6 ‰) taken from unconsolidated sediments at some distance from the concretions/vents show variable dilution of HCO 3 − with negative δ 13C VPDB ratios derived from microbial oxidation of methane with isotopically normal (0 ‰) marine bicarbonate. These results are in agreement with other lines of evidence suggesting that pockmarks formed through the recent and still active release of gas from a reservoir within the Paleozoic sedimentary succession.

Methane-derived authigenic carbonates from the Ulleung basin sediments, East Sea of Korea

Authigenic carbonates were sampled in methane-enriched piston core sediments collected from gas venting sites on the western continental slope of the Ulleung Basin, East Sea of Korea. Multidisciplinary investigations on these carbonates, including the scanning electronic microscope (SEM) observations and mineralogical-geochemical compositions, were carried out to identify the carbon and oxygen sources and the forming mechanism of these carbonates. The authigenic carbonates from the study area correspond to semi-consolidated, compact concretions or nodules ranging from 2 to 9 cm in size. X-ray diffraction and electron microprobe analyses showed that most of the sampled carbonate concretions were composed of almost purely authigenic high-Mg calcite (10.7–14.3 mol% MgCO 3). Characteristically, microbial structures such as filaments and rods, which were probably associated with the authigenic minerals, were abundantly observed within the carbonate matrix. The carbonates were strongly depleted in δ 13C (−33.85‰ to −39.53‰ Peedee Belemnite (PDB)) and were enriched in δ 18O (5.16–5.60‰ PDB), indicating that the primary source of carbon is mainly derived from the anaerobic oxidation of methane. Such methane probably originated from the destabilization of the underlying gas hydrates as strongly supporting from the enriched 18O levels. Furthermore, the strongly depleted δ 13C values (−60.7‰ to −61.6‰ PDB) of the sediment void gases demonstrate that the majority of the gas venting at the Ulleung Basin is microbial methane by CO 2 reduction. This study provides another example for the formation mechanism of methane-derived authigenic carbonates associated with gas-hydrate decomposition in gas-seeping pockmark environments.

Growth and size-structure of Stegophiura sp. (Echinodermata: Ophiuroidea) on the continental slope off central Chile: a comparison between cold seep and non-seep sites

The growth and size-structure of the bathyal ophiuroid brittle star, Stegophiura sp., were analysed from skeletal growth bands and disc diameter frequencies. Specimens were collected in trawl samples taken on the continental slope off central Chile (~36°S) at two sites within the recently discovered Concepción Methane Seep Area (CMSA) and at two control non-seep sites. Growth bands were measured as radii of vertebral ossicles from scanning electron microscope (SEM) micrographs and used to provide size-at-age data. The von Bertalanffy and the Gompertz growth models provided good fit to size-at-age data. The size-structure distributions observed in the study area suggest that small-bodied (<10 mm disc diameter) individuals of Stegophiura sp. are more abundant near seep sites, probably attracted there by the presence of methane-derived authigenic carbonates, which provide a preferred habitat for ophiuroids and benthic fauna in general. Furthermore, size-at-age data from measurements of the ossicle growth bands indicate relatively rapid growth of Stegophiura sp. populations at seep sites. Assuming that the growth rings are annual, the maximum Stegophiura sp. age was estimated to be 15 years. The growth performance of this species falls within the range of values reported for sub-Antarctic and bathyal species.

Multidisciplinary investigations exploring indicators of gas hydrate occurrence in the Krishna–Godavari Basin offshore, east coast of India

We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002–2003 in the Krishna–Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW–ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO4 2−) concentrations (28.7 to <4 mM), increasing downcore methane (CH4) concentrations (0–20 nM) and relatively high total organic carbon contents (1–2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments. Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gas-escape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the Krishna–Godavari Basin offshore.

Anomalous sea-floor backscatter patterns in methane venting areas, Dnepr paleo-delta, NW Black Sea

The relation between acoustic sea-floor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single-beam echosounder. This study is further supported by side-scan sonar recordings, high-resolution 5 kHz seismic data, pore-water analysis, grain-size analysis and visual sea-floor observations. The datasets were acquired during the 2003 and 2004 expeditions of the EC-funded CRIMEA project in the Dnepr paleo-delta area, northwestern Black Sea. More than 600 active methane seeps were hydroacoustically detected within a small (3.96 km by 3.72 km) area on the continental shelf of the Dnepr paleo-delta in water depths ranging from − 72 m to − 156 m. Multibeam and side-scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area. Seeps generally occur within medium- to high-backscatter areas which often coincide with pockmarks. High-resolution seismic data reveals the presence of an undulating gas front, i.e. the top of the free gas in the subsurface, which domes up towards and intersects the sea floor at locations where gas seeps and medium- to high-backscatter values are detected. Pore-water analysis of 4 multi-cores, taken at different backscatter intensity sites, shows a clear correlation between backscatter intensity and dissolved methane fluxes. All analyzed chemical species indicate increasing anaerobic oxidation of methane (AOM) from medium- to high-backscatter locations. This is confirmed by visual sea-floor observations, showing bacterial mats and authigenic carbonates formed by AOM. Grain-size analysis of the 4 multi-cores only reveals negligible variations between the different backscatter sites. Integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane-derived authigenic carbonates (MDACs) caused by AOM. The carbonate formation also appears to lead to a gradual (self)-sealing of the seeps by cementing fluid pathways/horizons followed by a relocation of the bubble-releasing locations.

Association among active seafloor deformation, mound formation, and gas hydrate growth and accumulation within the seafloor of the Santa Monica Basin, offshore California

Seafloor blister-like mounds, methane migration and gas hydrate formation were investigated through detailed seafloor surveys in Santa Monica Basin, offshore of Los Angeles, California. Two distinct deep-water (≥ 800 m water depth) topographic mounds were surveyed using an autonomous underwater vehicle (carrying a multibeam sonar and a chirp sub-bottom profiler) and one of these was explored with the remotely operated vehicle Tiburon. The mounds are > 10 m high and > 100 m wide dome-shaped bathymetric features. These mounds protrude from crests of broad anticlines (~ 20 m high and 1 to 3 km long) formed within latest Quaternary-aged seafloor sediment associated with compression between lateral offsets in regional faults. No allochthonous sediments were observed on the mounds, except slumped material off the steep slopes of the mounds. Continuous streams of methane gas bubbles emanate from the crest of the northeastern mound, and extensive methane-derived authigenic carbonate pavements and chemosynthetic communities mantle the mound surface. The large local vertical displacements needed to produce these mounds suggests a corresponding net mass accumulation has occurred within the immediate subsurface. Formation and accumulation of pure gas hydrate lenses in the subsurface is proposed as a mechanism to blister the seafloor and form these mounds.

Rates of anaerobic oxidation of methane and authigenic carbonate mineralization in methane-rich deep-sea sediments inferred from models and geochemical profiles

Pore water chemical data obtained from a 10.5-m long giant gravity core collected in methane-rich sediments from 647 m water depth in the northern Gulf of Mexico (N 28°04.00′ W 89°43.15′) defines sub-bottom gradients in unprecedented detail. This core penetrated the sulfate-methane interface (SMI) at ∼ 300 cm below the seafloor (cmbsf). At the SMI dissolved inorganic carbon (DIC) concentrations reach a maximum (13.5 mM) and pore water δ 13C DIC (− 63.2‰ PDB) and δ 13C methane (− 89.5‰ PDB) values are most negative. Below the SMI pore water sulfate is nearly depleted, methane concentrations rise sharply with simultaneous occurrence of a bubble-textured sediment, and fine-grained methane-derived authigenic carbonate nodules and cements are common. The sharp peaks in DIC concentration and isotope values centered at the SMI indicate that DIC is being produced by anaerobic oxidation of methane (AOM) within a narrow zone centered at the SMI. The detailed sulfate and DIC concentration profiles, and DIC δ 13C values have enabled geochemical models to be constructed that explore the rate of DIC formation by AOM and its effect on pore water DIC δ 13C values. Model results closely match measured DIC concentration and δ 13C isotope profiles and indicate that microbiological conversion of methane carbon to DIC is rapid in geologic terms and that AOM is occurring at the present position of the SMI. Isotope values for authigenic carbonate found immediately below the present-day SMI ( δ 13C = − 60.2 ± 0.7‰ PDB at 440 cmbsf) are consistent with derivation of the carbonate carbon from methane via AOM at the former location of a SMI. These observations and model results suggest that AOM is occurring at rates that would generate the observed profiles and begin the precipitation of methane-derived carbonate occur on time-scales of centuries. Model results also show that the time needed to produce the resulting authigenic cements is an order of magnitude greater than that for AOM to produce the observed DIC profiles. The metabolic rates for DIC production by AOM inferred from modeling the geochemical profiles compare favorably with available rate data obtained from laboratory microbial incubations and radiolabeled tracer experiments.

Origin of pockmarks and chimney structures on the flanks of the Storegga Slide, offshore Norway

Seafloor pockmarks and subsurface chimney structures are common on the Norwegian continental margin north of the Storegga Slide scar. Such features are generally inferred to be associated with fluid expulsion, and imply overpressures in the subsurface. Six long gravity and piston cores taken from the interior of three pockmarks were compared with four other cores taken from the same area but outside the pockmarks, in order to elucidate the origins and stratigraphy of these features and their possible association with the Storegga Slide event. Sulfate gradients in cores from within pockmarks are less steep than those in cores from outside the pockmarks, which indicates that the flux of methane to the seafloor is presently smaller within the pockmarks than in the adjacent undisturbed sediments. This suggests that these subsurface chimneys are not fluid flow conduits lined with gas hydrate. Methane-derived authigenic carbonates and Bathymodiolus shells obtained from a pockmark at >6.3 m below the seafloor indicate that methane was previously available to support a chemosynthetic community within the pockmark. AMS 14C measurements of planktonic Foraminifera overlying and interlayered with the shell-bearing sediment indicate that methane was present on the seafloor within the pockmark prior to 14 ka 14C years b.p., i.e., well before the last major Storegga Slide event (7.2 ka 14C years b.p., or 8.2 ka calendar years b.p.). These observations provide evidence that overpressured fluids existed within the continental margin sediments off Norway during the last major advance of Pleistocene glaciation.

Methane-derived authigenic carbonates from the northern Gulf of Mexico — MD02 Cruise

Authigenic carbonates were sampled in piston cores collected from both the Tunica Mound and the Mississippi Canyon area on the continental slope of the northern Gulf of Mexico during a Marion Dufresne cruise in July 2002. The carbonates are present as hardgrounds, porous crusts, concretions or nodules and shell fragments with or without carbonate cements. Carbonates occurred at gas venting sites which are likely to overlie gas hydrates bearing sediments. Electron microprobe, X-ray diffraction (XRD) and thinsection investigations show that these carbonates are high-Mg calcite (6–21 mol% MgCO 3), with significant presence of framboidal pyrite. All carbonates are depleted in 13C ( δ 13C = − 61.9 to − 31.5‰ PDB) indicating that the carbon is derived mainly from anaerobic methane oxidation (AMO). Age estimates based on 14C dating of shell fragments and on regional sedimentation rates indicate that these authigenic carbonates formed within the last 1000 yr in the Mississippi Canyon and within 5500 yr at the Tunica Mound. The oxygen isotopic composition of carbonates ranges from + 3.4 to + 5.9‰ PDB. Oxygen isotopic compositions and Mg 2+ contents of carbonates, and present in-situ temperatures of bottom seawater/sediments, show that some of these carbonates, especially from a core associated with underlying massive gas hydrates precipitated in or near equilibrium with bottom-water. On the other hand, those carbonates more enriched in 18O are interpreted to have precipitated from 18O-rich fluids which are thought to have been derived from the dissociation of gas hydrates. The dissociation of gas hydrates in the northern Gulf of Mexico within the last 5500 yr may be caused by nearby salt movement and related brines.

Extensive methane-derived authigenic carbonates in the Irish Sea

Extensive areas of methane-derived authigenic carbonate (MDAC) have been mapped in the Irish Sea. In the Irish Sector, 23 seabed mounds associated with the Codling Fault Zone were identified by multi-beam echo sounder mapping. Inspection by ROV-mounted video showed that these mounds are rocky features rising 5–10 m above the normal seabed; sampling showed that they are comprised of quartz grains bound together by carbonate cement, probably MDAC. Two separate locations have been mapped in the UK Sector. At Texel 11, seabed mounds and a 6–8 m high cliff were mapped geophysically (MBES, SSS and seismic profiler surveys). Video surveys showed that both the mounds and the cliff are rocky reefs colonised by a prolific fauna. Samples proved to be carbonate-cemented sediments, and carbon isotope analysis (δ13C −41 to −46% PDB) showed that the cement was MDAC. Similar surveys of the Holden’s Reefs area proved the presence of similar rocky reefs which are also cemented by MDAC. The total area covered by these two MDAC occurrences is estimated to be >500,000 m2. These MDAC occurrences are comparable in nature and formation to the ‘bubbling reefs’ of the Kattegat. As the bubbling reefs are “seabed features formed by leaking gas”, one of the marine habitats identified by the European Commission’s Habitats Directive as being sensitive and worthy of protection, it is suggested that the Irish Sea carbonate reefs should also be considered as special habitats.

Classification of sea-floor features associated with methane seeps along the Gulf of Cádiz continental margin

Based on recently gathered swath-bathymetry, high- to ultra-high-resolution seismic, and underwater camera data, along with dredging and coring samples, this paper examines the relationship between sea-floor features and the nature of hydrocarbon-enriched fluid and gas leaks from degassing of deeply buried sediments along the continental margin of the Gulf of Cádiz (eastern Central Atlantic). A classification into three main groups is proposed on the basis of the morphology and nature of deposits: (1) mud volcanoes, (2) methane-derived authigenic carbonates (MDAC) mounds, and (3) crater-like pockmarks. Mud volcanoes are, topographically, cone-shaped sea-floor edifices, built up from catastrophic mud and fluid degassing, intercalated with periods of inactivity. So far more than 25 mud volcanoes have been discovered in the Gulf of Cádiz, named in memory of deceased colleagues (e.g., Ginsburg and Baraza), or researchers’ birth places (e.g. Faro, Cibeles, Almazán, San Petersburgh, Yuma, Rabat, Bonjardim, Coruña, Gades). These structures range from 800 to 2500 m in diameter and tower 150–300 m above the seabed. The volcanoes consistently feature a well-defined outer ring or circular terrace and an inner dome. All mud volcanoes are built up of episodes of mud-breccia flows, intercalated with deep-current deposits, with evident indications of gas saturation: degassing structures, a strong H2S smell, and chemosynthetic fauna (such as Pogonophora sp. tube worms and Calyptogena sp.). Commonly observed carbonate crusts and slabs overlying some mud volcanoes are thought to have been formed by slow, diffuse venting during periods of inactivity or slower rates of fluid venting following the ejection of mud. A “fermentation” process, the result of microbial-mediated oxidation of hydrocarbon-enriched fluids, seems to play an important role in the growth of large deep-water carbonate mounds and chimneys during periods of low methane-seep fluid pressure. More than 400 crater-like pockmarks, a common sea-floor feature in the TASYO field, have been observed. Their seabed diameter ranges from 500 to 3000 m, with steep slopes ranging from 12° to 35°. According to their internal structure, pockmarks can be grouped into: (1) pockmarks with low central mounds and (2) crater-like pockmarks, apparently collapse structures lacking central mounds, with a flat to slightly undulating sea bottom surrounded by steep slopes (up to 25°).

Submarine pingoes: Indicators of shallow gas hydrates in a pockmark at Nyegga, Norwegian Sea

Complex pockmarks up to 300 m wide and 12 m deep are located in the Nyegga area in the Norwegian Sea. Bathymetric data and direct visual documentation and sampling with ROVs (remotely operated vehicles) have shown that these pockmarks contain abundant methane-derived authigenic carbonate rocks. Furthermore, geochemical results and the finding of seep-associated organisms, including tubeworms and bacteria shows that the pockmarks are still active fluid flow locations [Hovland, M., Svensen, H., Forsberg, C.F., Johansen, H., Fichler, C., Fosså, J.H., Jonsson, R., Rueslåtten, H., 2005. Complex pockmarks with carbonate-ridges off mid-Norway: Products of sediment degassing. Marine Geology, 218, 191–206.]. Here we report the discovery of localized pingo-like sediment mounds up to 1 m high and 4 m wide. They occur inside one of the Nyegga complex pockmarks, ‘G11.’ All of the seven structures we investigated have four characteristics in common. (1) They have a positive topography (rounded mounds and cones). (2) They are partly covered in bacterial mats (indicating ongoing fluid flow). (3) They are partly covered in a carpet of small, living tubeworms (polychaetes, which utilize methane). (4) They have distinct corrosion pits on their surfaces, indicating fluidization and point-source corrosion of the covering sediments (probably caused by localized sub-surface hydrate dissociation). We interpret the features as true submarine pingoes, formed by the local accumulation of hydrate (ice) below the sediment surface. It is inferred that the pingoes are formed as documented hydrocarbon gases, methane, ethane, propane, and butane migrate upwards through distinctive sub-surface channels or conduits inside the pockmark. We suggest that these submarine hydrate-pingoes manifest the exact locations where fluid flow through the seafloor is currently active, and that they can therefore be used as small-scale indicators of active seepage.

Methane emission and consumption at a North Sea gas seep (Tommeliten area)

Abstract. The Tommeliten seepage area is part of the Greater Ekofisk area, which is situated above the Tommeliten Delta salt diapir in the central North Sea (56°29.90' N, 2°59.80' E, Norwegian Block 1/9, 75 m water depth). Here, cracks in a buried marl horizon allow methane to migrate into overlying clay-silt and sandy sediments. Hydroacoustic sediment echosounding showed several venting spots coinciding with the apex of marl domes where methane is released into the water column and potentially to the atmosphere. In the vicinity of the gas seeps, sea floor observations showed small mats of giant sulphide-oxidizing bacteria above patches of black sediments as well as carbonate crusts, which are exposed 10 to 50 cm above seafloor forming small reefs. These Methane-Derived Authigenic Carbonates (MDACs) contain 13C-depleted, archaeal lipids indicating previous gas seepage and AOM activity. High amounts of sn2-hydroxyarchaeol relative to archaeol and low abundances of biphytanes in the crusts give evidence that ANaerobic MEthane-oxidising archaea (ANME) of the phylogenetic cluster ANME-2 were the potential mediators of Anaerobic Oxidation of Methane (AOM) at the time of carbonate formation. Small pieces of MDACs were also found subsurface at about 1.7 m sediment depth, associated with the AOM zone. This zone is characterized by elevated AOM and Sulphate Reduction (SR) rates, increased concentrations of 13C-depleted tetraether derived biphytanes, and specific bacterial Fatty Acids (FA). Further biomarker and 16S rDNA based analyses of this horizon give evidence that AOM is mediated by archaea belonging to the ANME-1b group and Sulphate Reducing Bacteria (SRB) most likely belonging to the Seep-SRB1 cluster. The zone of active methane consumption was restricted to a distinct horizon of about 20 cm. Concentrations of 13C-depleted lipid biomarkers (e.g. 500 ng g-dw−1 biphythanes, 140 ng g-dw−1 fatty acid ai-C15:0), cell numbers (1.5×108 cells cm−3), AOM and SR rates (3 nmol cm−3 d−1) in the Tommeliten AOM zone are 2–3 orders of magnitude lower compared to AOM zones of highly active deep water cold seeps such as Hydrate Ridge or the Gulf of Mexico.

Complex pockmarks with carbonate-ridges off mid-Norway: Products of sediment degassing

A new type of pockmark has been discovered off mid-Norway. They contain up to 10 m high ridges of methane-derived 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 (C 1–C 5) in clay-rich sediments. The methane δ 13C values range from − 54‰ to − 69‰ 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 (δ 13C = − 52‰ to − 58‰ PDB). We suggest that the complex pockmarks formed by sudden (‘catastrophic’) fluid flow and that the large carbonate ridges represent dislodged MDACs originally formed subsurface, prior to the event. Subsequent micro-seepage 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 from the complex pockmarks. Thus our data rather suggest that the ‘catastrophic’ formation of the complex pockmarks may be a one-time-event.

Pall Corp, USA

Plumbing systems of marine seeps are complex pathways along which hydrocarbon-rich fluids move upward through the sedimentary column. In the shallow subsurface, seep plumbing systems may become successively filled by methane-derived authigenic carbonates resulting in mineralized conduits. This work investigates mineralized conduits in the uppermost plumbing network of Oligocene seep deposits from the Lincoln Creek Formation in Washington State that consist of a paragenetic sequence of authigenic mineral phases. At two study sites the pipes grade into the base of seep deposits. The studied samples of concentrically-mineralized conduits are cylindrical in shape, measure 2–3 cm in diameter, and are up to 15 cm in length; the mineralized conduits are referred to as pipes hereafter. The earliest mineral phase in the paragenetic sequence is matrix micrite, making up the outer part of the pipes. Toward their centers the pipes are filled by two intercalated cement phases, (1) clear, banded and botryoidal aragonite and (2) micro-to cryptocrystalline yellow aragonite. The innermost portions of the pipes are filled by brownish calcite, microspar, or pipe-filling micrite. The observed paragenetic sequences archive successions of biogeochemical processes. The δ13C values of clear and yellow aragonite cements range from −50.6 to −41.2‰, suggesting that their main carbonate source was provided by anaerobic oxidation of methane. In contrast, the later phases (brownish calcite, microspar, pipe-filling micrite) are relatively enriched in 13C (δ13C values: −2.3 to 3.9‰), pointing to carbonate precipitation from fluids affected by methanogenesis. The size and morphology of the pipes suggest that initial excavation was caused by burrowing organisms. The burrows subsequently acted as preferred fluid pathways, suggesting that the transport of fluids within near-surface sediments at the Oligocene seeps was strongly influenced by the activity of the local fauna. Possible producers of the burrows were sediment-dwelling crustaceans, specifically callianassid decapods, as well as bivalves.

Methane-derived authigenic carbonates formed by subduction-induced pore-water expulsion along the Oregon/Washington margin: Discussion and reply

Methane-derived authigenic carbonates in terrigenous sediments described from the Pacific Ocean along the Oregon/Washington margin (Ritger and others, 1987) have their counterparts in the outer continentalshelf setting of the western Atlantic Ocean. Lithified sandstones, containing a modern microfauna and resulting from submarine cementation, underlie extensive stretches of the shelf between New Jersey and Florida (Allen and others, 1969; Friedman, 1975; Friedman and others, 1971; Friedman and Sanders, 1978, esp. p. 157-158). In a dredged shelly molluscan sandstone, the radiocarbon age of the molluscan shells is 4,390 ± 120 yr and that of cement is 15,600 ± 250 yr. At first sight, the radiocarbon ages give what appears to be an impossible result: cementation of young shells by old cement. To explain these ages, one may invoke the idea that the actual precipitation of the cement occurred less than 4,390 yr ago but that the carbon in the aragonite did not come directly out of sea water (in which case, the carbon would be new, that is, 4,390 yr old) and rather was derived from a source of older carbon. Analyses of stable-carbon isotopes show that the cement is enriched in the light stable isotope 12C; its S13C value is -44.8. This contrasts with a S13C value of -8.4 for the molluscan shells of the rock. The strongly negative 613C value of the cement suggests that carbon of the aragonite cement came from methane (CH4) (Allen and others, 1969; Friedman and others, 1971; Friedman, 1975); no other material is known in which the lighter carbon isotope has been that much enriched. The stable isotopic compositions of cement and skeletal carbonates overlap with those of the Oregon/Washington margin (Fig. 7, p. 151 of Ritger and others, 1987).

Methane-derived authigenic carbonates formed by subduction-induced pore-water expulsion along the Oregon/Washington margin

Authigenic magnesian calcite, dolomite, and aragonite are precipitated in the uppermost terrigenous sediments of the Washington/Oregon accretionary prism by subduction-induced dewatering. These distinctive carbonates are methane-derived and occur at sites of concentrated pore-water expulsion. Unique biologic communities that subsist, at least indirectly, on methane (Suess et al., 1985) are also found at some of these sites. The methane, which is dominantly biogenic, is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement. Carbonate precipitation occurs below the oxic layer, probably no deeper than several centimetres to a few metres below the seabed. Cementation may be induced by three factors: (1) increased carbonate alkalinity resulting from microbial sulfate reduction, (2) decreased σCO 2 solubility resulting from a pressure decrease when the pore water escapes the prism, and/or (3) the addition of Ca 2+ and Mg 2+ ions from sea water near the sediment/water interface. The convergent margin setting engenders precipitation of authigenic carbonates in several ways. Compressive stresses induce anomalously rapid compaction and dewatering rates, and they may cause overpressuring in migrating pore water, thereby delaying precipitation of carbonates until pressure is released near the sediment-water interface. Structural deformation of the accretionary prism creates pathways (such as fault zones), secondary fracture porosity, and dipping permeable layers (often exposed by mass movement) for efficient advection and expulsion of methane-enriched pore water. These characteristic conditions, which lead to the precipitation of methane-derived carbonates, may be found at other convergent margins.