A contribution to the reconstruction of Miocene seepage from authigenic carbonates of the northern Apennines (Italy)

Authigenic carbonates from outcrops of the northern Apennines consist of small and irregular lenses and exhibit numerous features indicative of cold-seep settings. Detailed petrographic, mineralogical and geochemical studies from two Miocene deposits are presented. The first carbonate outcrop, named Fosso Riconi, is located in the foredeep basin of the Apenninic chain, whereas the second deposit represents a satellite basin called Sarsetta. The stable isotope data from specific carbonate minerals show a wide range of values well known from other palaeoseeps of the Apennine Mountains. The majority of seep carbonates are formed by low-Mg calcite and ankerite. Those minerals have δ¹³C values between ?7 and ?23‰ V-PDB, suggesting variable amounts of carbonate derived from oxidized methane, seawater (dissolved inorganic carbon) and sedimentary organic matter. Dolomite samples have the lowest δ¹³C values (?30.8 to ?39.0‰ V-PDB), indicating methane as the main carbon source. The findings suggest an evolutionary formation of the seeps and development of authigenic carbonates influenced by the activity of chemosynthetic organisms, of which large lucinid clams are preserved. Bioirrigation by the clams controlled the sediment–water exchange, and is here considered as an explanation for the anomalous Mg content of the calcite. We hypothesize that the seep carbonates were formed during periods of active methane-rich seepage, whereas during periods of slow seepage carbonate formation was reduced. Despite different geological settings, the two examined deposits of Sarsetta and Fosso Riconi show similar features, suggesting that a common pattern of fluid circulation played a major role in carbonate formation at both seep sites.     

Exhumed hydrocarbon-seep authigenic carbonates from Zakynthos Island (Greece): Concretions not archaeological remains

In Zakynthos Island (Greece), authigenic cementation of marine sediment has formed pipe-like, disc and doughnut-shaped concretions. The concretions are mostly composed of authigenic ferroan dolomite accompanied by pyrite. Samples with >80% dolomite, have stable isotope compositions in two groups. The more indurated concretions have δ¹⁸O around +4‰ and δ¹³C values between −8 and −29‰ indicating dolomite forming from anaerobic oxidation of thermogenic methane (hydrocarbon seep), in the sulphate-methane transition zone. The outer surfaces of some concretions, and the less-cemented concretions, typically have slightly heavier isotopic compositions and may indicate that concretion growth progressed from the outer margin in the ambient microbially-modified marine pore fluids, inward toward the central conduit where the isotopic compositions were more heavily influenced by the seep fluid. Sr isotope data suggest the concretions are fossil features, possibly of Pliocene age and represent an exhumed hydrocarbon seep plumbing system. Exposure on the modern seabed in the shallow subtidal zone has caused confusion, as concretion morphology resembles archaeological stonework of the Hellenic period.     

Authigenic minerals from the Paola Ridge (southern Tyrrhenian Sea): Evidences of episodic methane seepage

Paola Ridge, along the NW Calabrian margin (southern Tyrrhenian Sea), is one of the few reported deep sea sites of precipitation of authigenic carbonates in the Tyrrhenian Sea. Here, the changing composition of the seeping fluids and the dynamic nature of the seepage induced the precipitation of pyrite, siderite and other carbonate phases. The occurrence of this array of authigenic precipitates is thought to be related to fluctuation of the sulfate-methane transition zone (SMTZ).Concretions of authigenic minerals formed in the near sub-bottom sediments of the Paola Ridge were investigated for their geochemical and isotopic composition. These concretions were collected in an area characterized by the presence of two alleged mud volcanoes and three mud diapirs. The mud diapirs are dotted by pockmarks and dissected by normal faults, and are known for having been a site of fluid seepage for at least the past 40 kyrs. Present-day venting activity occurs alongside the two alleged mud volcanoes and is dominated by CO₂-rich discharging fluids. This discover led us to question the hypothesis of the mud volcanoes and investigate the origin of the fluids in each different domed structure of the study area.In this study, we used stable isotopes (carbon and oxygen) of carbonates coupled with rare earth element (REE) composition of different carbonate and non-carbonate phases for tracing fluid composition and early diagenesis of authigenic precipitates. The analyses on authigenic precipitates were coupled with chemical investigation of venting gas and sea-water.Authigenic calcite/aragonite concretions, from surficial sediments on diapiric structures, have depleted ¹³C isotopic composition and slightly positive δ¹⁸O values. By contrast, siderite concretions, generally found within the first 6 m of sediments on the alleged mud volcanoes, yielded positive δ¹³C and δ¹⁸O values. The siderite REE pattern shows consistent LREE (light REE) fractionation, MREE (medium REE) enrichment and positive Gd and La anomalies. As shown by the REE distribution, the ¹³C-depleted composition and their association with chemosymbiotic fauna, calcite/aragonite precipitated at time of moderate to high methane flux close to the seafloor, under the influence of bottom seawater. Authigenic siderite, on the other hand, formed in the subseafloor, during periods of lower gas discharges under prolonged anoxic conditions within sediments in equilibrium with ¹³C-rich dissolved inorganic carbon (DIC) and ¹⁸O-rich water, likely related to methanogenesis and intermittent venting of deep-sourced CO₂.     

Authigenic carbonates related to active seepage of methane-rich hot brines at the Cheops mud volcano,Menes caldera (Nile deep-sea fan,eastern Mediterranean Sea)

On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20–30 m above seafloor, 3,010–3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δ¹³C values (–42.6 to –24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δ¹³C (up to –5.2‰) and low δ¹⁸O values (down to –2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by ¹³C-rich CO₂ and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals.     

Authigenic Mg-calcite at a cold methane seep site in the Laptev Sea

Authigenic minerals were studied in Holocene shelf sediments of the Laptev Sea (cold methane seep site, water depth 71 m). The study presents the first finds of large hard carbonate concretions with Mg-calcite cement in recent sediments of the Arctic shelf seas. These concretions differ from previously reported glendonites and concretions from bottom sediments of the White Sea, Kara Sea, Sea of Okhotsk, etc. A study of the morphology, microstructure, and composition of these newly reported concretions revealed the multistage formation of carbonates (structural varieties of Mg-calcite and aragonite). It was shown that organic matter played an important role in the formation of authigenic carbonates, i.e., in the formation of sedimentary–diagenetic Mg-calcite. The role of methane as a possible source for authigenic carbonate formation was estimated. It was found that methane-derived Mg-calcite accounts for 17–35% of concretion materials. Mg-calcite had δ¹³С-С_carb values between–24 and–23‰ and δ¹³С-С_org values between–44.5 and–88.5‰.     

Gas flux and carbonate occurrence at a shallow seep of thermogenic natural gas

The Coal Oil Point seep field located offshore Santa Barbara, CA, consists of dozens of named seeps, including a peripheral ～200 m² area known as Brian Seep, located in 10 m water depth. A single comprehensive survey of gas flux at Brian Seep yielded a methane release rate of ～450 moles of CH₄ per day, originating from 68 persistent gas vents and 23 intermittent vents, with gas flux among persistent vents displaying a log normal frequency distribution. A subsequent series of 33 repeat surveys conducted over a period of 6 months tracked eight persistent vents, and revealed substantial temporal variability in gas venting, with flux from each individual vent varying by more than a factor of 4. During wintertime surveys sediment was largely absent from the site, and carbonate concretions were exposed at the seafloor. The presence of the carbonates was unexpected, as the thermogenic seep gas contains 6.7% CO₂, which should act to dissolve carbonates. The average δ¹³C of the carbonates was ?29.2?±?2.8‰ VPDB, compared to a range of ?1.0 to +7.8‰ for CO₂ in the seep gas, indicating that CO₂ from the seep gas is quantitatively not as important as ¹³C-depleted bicarbonate derived from methane oxidation. Methane, with a δ¹³C of approximately ?43‰, is oxidized and the resulting inorganic carbon precipitates as high-magnesium calcite and other carbonate minerals. This finding is supported by ¹³C-depleted biomarkers typically associated with anaerobic methanotrophic archaea and their bacterial syntrophic partners in the carbonates (lipid biomarker δ¹³C ranged from ?84 to ?25‰). The inconsistency in δ¹³C between the carbonates and the seeping CO₂ was resolved by discovering pockets of gas trapped near the base of the sediment column with δ¹³C-CO₂ values ranging from ?26.9 to ?11.6‰. A mechanism of carbonate formation is proposed in which carbonates form near the sediment–bedrock interface during times of sufficient sediment coverage, in which anaerobic oxidation of methane is favored. Precipitation occurs at a sufficient distance from active venting for the molecular and isotopic composition of seep gas to be masked by the generation of carbonate alkalinity from anaerobic methane oxidation.

The impact of organic fluids on the carbon isotopic compositions of carbonate-rich reservoirs: Case study of the Lucaogou Formation in the Jimusaer Sag,Junggar Basin,NW China

The processes involved in the interaction between organic fluids and carbonates, and the resulting effect on reservoir quality during the evolution and maturation of organic matter remain unclear despite the fact that these processes influence the carbon and oxygen isotopic compositions of carbonates. Here, we provide new insights into these processes using data obtained from a detailed analysis of a mixed dolomitic–clastic and organic-rich sedimentary sequence within the middle Permian Lucaogou Formation in the Junggar Basin of NW China. The techniques used during this study include drillcore observations, thin section petrography, scanning electron microscopy (SEM) and electron probe microanalysis, and carbon and oxygen isotope analyses. Oil grades and total organic carbon (TOC) contents represent the amount of oil charging and the abundance of organic fluids within a reservoir, respectively, and both negatively correlate with the whole-rock δ¹³C and δ¹⁸O of the carbonates in the study area, indicating that organic fluids have affected the reservoir rocks. Secondary carbonates, including sparry calcite and dolomite overgrowths and cements, are common within the Lucaogou Formation. Well-developed sparry calcite is present within dark mudstone whereas the other two forms of secondary carbonates are present within the dolomite-rich reservoir rocks in this formation. Comparing thin section petrology with δ¹³C compositions suggests that the carbon isotopic composition of matrix carbonates varies little over small distances within a given horizon but varies significantly with stratigraphic height as a result of the development of secondary carbonates. The net change in whole-rock δ¹³C as a result of these secondary carbonates ranges from 1.8‰ to 4.6‰, with the secondary carbonates having calculated δ¹³C compositions from −18.6‰ to −8.5‰ that are indicative of an organic origin. The positive correlation between the concentration of Fe within matrix and secondary carbonates within one of the samples suggests that the diagenetic system within the Lucaogou Formation was relatively closed. The correlation between δ¹³C and δ¹⁸O in carbonates is commonly thought to be strengthened by the influence of meteoric water as well as organic fluids. However, good initial correlation between δ¹³C and δ¹⁸O of whole rock carbonates within the Lucaogou Formation (resulted from the evaporitic sedimentary environment) was reduced by organic fluids to some extent. Consequently, the δ¹³C–δ¹⁸O covariations within these sediments are not always reliable indicators of diagenetic alteration by organic fluids or meteoric water.The characteristics and δ¹³C compositions of the sparry calcite within the formation is indicative of a genetic relationship with organic acids as a result of the addition of organic CO₂ to the reservoir. Further analysis suggests that both carbonate and feldspar were dissolved by interaction with organic CO₂. However, dissolved carbonate reprecipitated as secondary carbonates, meaning that the interaction between organic fluids and dolomites did not directly improve reservoir quality, although this process did enhance the dissolution of feldspar and increase porosity. This indicates that the δ¹³C and δ¹⁸O of secondary carbonates and their influence on whole-rock carbonate isotopic values can be used to geochemically identify the effect of organic fluids on closed carbonate-rich reservoir systems.     

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5?m length indicates a maximum age of about 60,000–80,000?years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0?<?δ¹³C ‰ V-PDB?<?–40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4?<?δ¹⁸O ‰ V-PDB?<?+6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ¹⁸O values might reflect the contribution of ¹⁸O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca²⁺ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ¹⁸O–δ¹³C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.     

Mineral-forming and diagenetic processes related to Tertiary hydrocarbon seepage at the Bohemian Massif/Outer Western Carpathians interface: Evidence from the Hrabůvka quarry,Moravia, Czech Republic

Ancient hydrocarbon seepage occurred in the Hrabůvka quarry at the boundary between the basement of the Bohemian Massif (represented by folded Lower Carboniferous siliciclastics of the Culm facies) and Tertiary sedimentary cover of the Carpathian Foredeep (formed by Lower Badenian siliciclastics and calcareous clays). The unconsolidated Lower Badenian sediments contain lithified domains composed of limestone and breccias with limestone cement, whereas the basement rocks are cut by subvertical neptunic dykes filled up by limestone and calcite-marcasite-pyrite veinlets representing sealed fluid conduits. The deeply negative δ¹³C values of both vein calcite and limestone (down to −38.1‰ V-PDB) indicate that oxidation of hydrocarbons was the major source of carbon for authigenic mineralization. A fluid inclusion study suggests low fluid temperatures (<50 °C) and low and variable salinities of aqueous fluids associated with hydrocarbons (0.7–6.7 wt. % NaCl eq.). The variability of δ¹⁸O values of authigenic carbonates (−1.7 to −8.2‰ V-PDB) could reflect either slight changes in temperature of escaping fluids (mostly within 15 °C), and/or some mixing with meteoric waters. The low δ³⁴S values of vein marcasite (∼–20‰ V-CDT) are consistent with bacterial reduction of sulfate in the hydrothermal system. Low C₁/(C₂+C₃) ratios in hydrocarbon gas extracted from authigenic carbonates (9.9 and 5.8) as well as the high δ¹³C values of methane (−31.8 and −32.4‰ V-PDB) are compatible with a thermogenic source of hydrocarbons. REE data indicate sequestration of REE from finely dispersed detrital material in the apical part of the hydrothermal system. The available data are compatible with two possible scenarios of fluid origin. The hydrocarbons could have been leached from underlying Paleozoic sedimentary sequence by aqueous fluids that infiltrated into the basement after Tertiary tectonic reactivation. Alternatively, an external source of hydrocarbon-bearing fluids can be found in the adjacent Outer Western Carpathians flysch nappes containing petroleum-producing lithologies. Nevertheless, a regional flow of hydrocarbon-bearing fluids is evidenced by the occurrence of very similar hydrocarbon-bearing vein mineralizations in a wider area.     

Petrographic evidence of the past occurrence of gas hydrates in the Tertiary Piedmont Basin (NW Italy)

Methane-derived rocks in Monferrato and the Tertiary Piedmont Basin (NW Italy) consist of seep carbonates, formed by gas seepage at the seafloor, and macroconcretions resulting from the cementation of buried sediments crossed by gas-rich fluids. These rocks are characterized by both negative δ¹³C values and a marked enrichment in δ¹⁸O. Petrographic features not commonly described and that point to enigmatic depositional and diagenetic conditions have been observed in both types of rocks: inhomogeneous distribution of cements within cavities; dolomite crystals floating within cavity-filling calcite spar; non-gravitational fabrics of internal sediments plastering cavity walls; open framework within microbial crusts. These features suggest the former presence of gas hydrates in sediments. During their dissociation, new space was formed and filled with authigenic carbonates or injected sediments. Analogous mechanisms of clathrate freeze-and-thaw processes have been inferred for the genesis of zebra and stromatactis structures and particular kinds of carbonate breccias. The term melt-seal structure is proposed for this kind of diagenetic structure. The fabrics of gas hydrates and the geochemical conditions of sediments, in turn depending on the relative rates of supply of methane-rich fluids and normal seawater, conditioned the final aspect of the rocks.     

Authigenic barite nodules and carbonate concretions in the Upper Devonian shale succession of western New York – A record of variable methane flux during burial

Authigenic barite nodules associated with modestly ¹³C-depleted calcium carbonate concretions and ³⁴S-enriched pyrite at the bottom of the Upper Devonian Hanover Shale of western New York provide evidence of sulfate reduction coupled with anaerobic oxidation of methane (AOM). The methane, much of it biogenic in origin, may have diffused upward from Middle Devonian Marcellus Shale and perhaps the Upper Ordovician Utica Shale. Strong ³⁴S enrichment and high δ³⁴S/δ¹⁸O values of the barite nodules reflect: (1) substantial kinetic fractionation induced by microbial sulfate reduction perhaps intensified by a low seawater sulfate recharge rate and (2) upward delivery of Ba²⁺- and CH₄- bearing pore fluid sourced within underlying sulfate-depleted deposits. However, the association of authigenic calcium carbonate and barite in the same stratigraphic interval, especially the presence of barite overgrowths on carbonate concretions, is not consistent with what is known of AOM-related mineralization of a sediment column passing downward through the sulfate–methane transition (SMT). The documented early formation of authigenic carbonate followed by barite observed relations may reflect a diminished rate of methanogenesis and/or CH₄ supply. The tempered methane flux would have induced the SMT to descend the sediment column enabling barite to form within the same stratigraphic horizon that ¹³C-depleted calcium carbonate had most recently precipitated. Diminished methane flux may have been caused by burial-related passage of the organic-rich Marcellus Shale below the depth of peak biogenic methane generation and its replacement at that depth interval by organic-lean deposits of the upper part of the Hamilton Group. Subsidence of the SMT would have increased the preservation potential of authigenic barite. However, continued survival of the labile barite as it eventually moved through the SMT suggests that the underlying sulfate-depleted zone was strongly enriched in Ba²⁺.     

Laboratory experiment and numerical simulation on authigenic mineral formation induced by seabed methane seeps

In natural gas hydrate marine settings, cold seepage of methane fluid is a widely observed phenomenon, where authigenic minerals serve as an indication of potential gas hydrate-bearing reservoirs at depth. In this study, based on the data from the site HD196 near Dongsha Island, northern continental slope of South China Sea, laboratory experiments and numerical simulation studies were conducted to investigate the biogeochemical processes of authigenic mineral formation induced by methane seepage. The bioreactor experimental results show that in response to methane flux, pH increased to 8.5 after 20 days of reaction, and Eh declined rapidly first and then remained unchanged at about 100 mV. The decrease in SO₄²⁻, HS⁻ and HCO₃⁻ concentrations indicated the occurrence of anaerobic oxidation of methane coupled with sulfate reduction (AOM-SR). The depletion of Fe²⁺ implied the formation of iron-bearing minerals, e.g., iron sulfides. Carbonate minerals were also identified in the experimental phase of this study. Most iron sulfides existed as massive pieces, and in some cases as spherical or rod-shape pieces. The calcium carbonates were observed as blocky pieces. Numerical simulations were also performed to reproduce the biogeochemical reactions that occurred in the reactor experiments. Based on experimental data, kinetic parameters associated with the observed reactions were calibrated. The model simulated results are general consistent with those obtained by the experiments conducted in this study. The combination of simulation and experimental studies provided a powerful tool to investigate the biogeochemical processes in the methane leakage environment at different temporal and spatial scales. This study gave a new perspective to understand the formation of cold seep authigenic minerals in marine sediments, and was significant for future investigations on the effects of hydrate decomposition.     

Molecular signals for anaerobic methane oxidation in Black Sea seep carbonates and a microbial mat

Linked to gas seeps on the Ukrainian shelf (northwestern Black Sea), massive authigenic carbonates form as a result of anaerobic methane oxidation. Lipid distributions in these ‘cold seep’ carbonates and an associated microbial mat were investigated for process markers reflecting the presence and metabolic activity of distinctive methane-related biota. The samples contain free, irregular isoprenoid hydrocarbons, namely the tail-to-tail linked acyclic C₂₀-isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), its C₂₅-homologue 2,6,10,15,19-pentamethylicosane (PMI), and several unsaturated derivatives thereof. Furthermore, specific acyclic and cyclic C₄₀-isoprenoids were released upon ether cleavage of the polar fraction from the carbonate. The abundance of these compounds indicates a pronounced role of particular Archaea in the biogeochemical cycling of carbon at methane seeps. Stable carbon isotopic analyses of these lipids reveal extraordinary depletions in ¹³C corresponding to δ-values in the range of −100±30‰ PDB, whereas other compounds show isotopic compositions normally observed for marine lipids (around −30‰ PDB). The isotope data imply that the biosynthesis of the archaeal isoprenoids occurred in situ and involved the utilization of isotopically depleted, i.e. methane-derived, carbon. Apart from archaeal markers, the carbonate and the mat contain authigenic, framboidal pyrite and isotopically depleted fatty acids, namely iso-, and anteiso-branched compounds most likely derived from sulphate-reducing bacteria (SRB). The indications for a tight association of these normally competitive organisms support a model invoking a syntrophic relationship of SRB with Archaea responsible for the anaerobic oxidation of methane. The biomarker patterns obtained from the Black Sea samples were further compared to those from a Oligocene seep carbonate (Lincoln Creek Formation, WA, USA) in order to evaluate their biomarker potential for ancient settings. The prominent occurrence of isotopically light crocetane (−112‰) and PMI (−120‰) meets the findings for the contemporary materials. Thus, isotopically depleted isoprenoids provide diagenetically stable fingerprints for the reconstruction of carbon cycling in both, modern and ancient methane seep systems.     

Enrichment of adsorbed methane in authigenic carbonate concretions of the Japan Trench

Substantial amounts of adsorbed methane were detected in authigenic carbonate concretions recovered from sedimentary layers from depths between 245 and 1,108 m below seafloor during Ocean Drilling Program Leg 186 to ODP sites 1150 and 1151 on the deep-sea terrace of the Japan Trench. Methane contents were almost two orders of magnitude higher in the concretions (291–4,528 nmol/g wet wt) than in the surrounding bulk sediments (5–93 nmol/g wet wt), whereas methane/ethane ratios and stable carbon isotopic compositions were very similar. Carbonate content of surrounding bulk sediments (0.02–3.2 wet wt%) and methane content of the surrounding bulk sediments correlated positively. Extrapolation of the carbonate contents of bulk sediments suggests that 100 wt% carbonate would correspond to 1,886±732 nmol methane per g bulk sediment, which is similar to the average value observed in the carbonate concretions (1,321±1,067 nmol/g wet wt, n = 13). These data support the hypothesis that, in sediments, adsorbed hydrocarbon gases are strongly associated with authigenic carbonates.     

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ¹³C values of DIC become heavier with increasing subbottom depth, and vary between ?9.5 and +21.4‰ PDB. Enrichment of DIC in ¹³C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO₂-reduction) were revealed by determining the offset of δ¹³C between dissolved CH₄ and CO₂, and also by using δ¹³C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.     

When mud volcanoes sleep: Insight from seep geochemistry at the Dashgil mud volcano,Azerbaijan

The worlds >1500 mud volcanoes are normally in a dormant stage due to the short duration of eruptions. Their dormant stage activity is often characterized by vigorous seepage of water, gas, and petroleum. However, the source of the fluids and the fluid–rock interactions within the mud volcano conduit remain poorly understood. In order to investigate this type of activity, we have combined satellite images with fieldwork and extensive sampling of water and gas at seeping gryphons, pools and salsa lakes at the Dashgil mud volcano in Azerbaijan. We find that caldera collapse faults and E–W oriented faults determine the location of the seeps. The seeping gas is dominated by methane (94.9–99.6%), with a δ¹³C (‰ V-PDB) in the −43.9 to −40.4‰ range, consistent throughout the 12 analysed seeps. Ethane and carbon dioxide occur in minor amounts. Seventeen samples of seeping water show a wide range in solute content and isotopic composition. Pools and salsa lakes have the highest salinities (up to 101,043 ppm Cl) and the lowest δ¹⁸O (‰ V-SMOW) values (1–4‰). The mud-rich gryphons have low salinities (<18,000 ppm Cl) and are enriched in ¹⁸O (δ¹⁸O = 4–6‰).     

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 δ¹⁸O_VPDB ratios (+3‰) in equilibrium with cold (5 °C) deep marine waters, whereas significant negative δ¹³C_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 δ¹³C_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 HCO₃⁻ with negative δ¹³C_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.     

Hydrocarbon-derived carbonate buildups of the northern Gulf of Mexico continental slope: A review of submersible investigations

Hydrocarbon-derived and microbially mediated authigenic carbonates occur over the entire depth range of the northern Gulf of Mexico slope. These carbonates consist of nodules and incipient nodules in surface sediments, hardgrounds and isolated slabs, and moundlike buildups of up to 10–20 m relief above the surrounding seafloor. The authigenic carbonates are characterized by ¹³C negative values in the range –18 to –55 (PDB) suggesting mixing of seawater carbon with¹³C-depleted carbon sources ranging from crude oil to biogenic methane. Near the shelf edge, carbonates are diluted with biogenic material produced by reefs—bioherms developed at low sea level stands. Fossil-poor carbonates over salt diapirs of the upper and middle slope formed in the shallow subsurface and have been exhumed by the combined processes of uplift and physical erosion. Middle and lower slope carbonates are generally rich in fossil shells of chemosynthetic organisms. Mg calcite pelloidal matrix and acicular to botryoidal aragonitic void-filling cements are common petrographic features of these hydrocarbonderived carbonates. At two sites carbonates are mixed with barite.     

Molecular and isotopic characteristics of gas hydrate-bound hydrocarbons in southern and central Lake Baikal

We investigated the molecular composition (methane, ethane, and propane) and stable isotope composition (methane and ethane) of hydrate-bound gas in sediments of Lake Baikal. Hydrate-bearing sediment cores were retrieved from eight gas seep sites, located in the southern and central Baikal basins. Empirical classification of the methane stable isotopes (δ¹³C and δD) for all the seep sites indicated the dominant microbial origin of methane via methyl-type fermentation; however, a mixture of thermogenic and microbial gases resulted in relatively high methane δ¹³C signatures at two sites where ethane δ¹³C indicated a typical thermogenic origin. At one of the sites in the southern Baikal basin, we found gas hydrates of enclathrated microbial ethane in which ¹³C and deuterium were both highly depleted (mean δ¹³C and δD of –61.6‰ V-PDB and –285.4‰ V-SMOW, respectively). To the best of our knowledge, this is the first report of C₂ δ¹³C–δD classification for hydrate-bound gas in either freshwater or marine environments.     

南海北部表层沉积物碳酸盐含量和δ18O及δ13C的空间与粒径分布特征及其控制因素

为深入研究南海北部表层沉积物中碳酸盐的来源及其控制因素,采用湿筛法,对南海北部珠江口、陆架、陆坡及海盆表层沉积物中碳酸盐的含量、δ18O和δ13C及其粒度分布特征进行了研究。结果表明,南海北部表层沉积物中的碳酸盐可能主要以海洋自生碳酸盐为主,其含量在空间上以陆坡显著高于海盆、陆架及珠江口为特征。粒度上,陆架(水深80 m)和浅陆坡(水深550 m)处碳酸盐的粒度分布相对平均,但在珠江口及730 m以深,76.9%~84.5%的碳酸盐分布于小于32 μm的粒级中。另外,珠江口和陆架与陆坡和海盆碳酸盐δ18O和δ13C的空间及粒度分布特征显著不同。珠江口陆架区,在空间上从珠江口向陆架,全样碳酸盐的δ18O和δ13C由轻变重。粒度上,碳酸盐的δ18O和δ13C随着粒径的增大而变重,并且δ18O和δ13C之间正相关,主要受珠江淡水的δ18O及其δ13C_DIC控制。陆坡海盆区,空间上,从陆坡向海盆,δ18O略有变轻,δ13C基本不变。粒度上,碳酸盐的δ18O和δ13C随着粒度的增大没有明显的变化趋势,并且δ18O与δ13C之间不相关或负相关。其δ18O可能主要受表层海水的温度控制,其δ13C可能主要受海水δ13C_DIC控制。