Seep Carbonates Research Articles

Sulfur bacteria promote dissolution of authigenic carbonates at marine methane seeps

Carbonate rocks at marine methane seeps are commonly colonized by sulfur-oxidizing bacteria that co-occur with etch pits that suggest active dissolution. We show that sulfur-oxidizing bacteria are abundant on the surface of an exemplar seep carbonate collected from Del Mar East Methane Seep Field, USA. We then used bioreactors containing aragonite mineral coupons that simulate certain seep conditions to investigate plausible in situ rates of carbonate dissolution associated with sulfur-oxidizing bacteria. Bioreactors inoculated with a sulfur-oxidizing bacterial strain, Celeribacter baekdonensis LH4, growing on aragonite coupons induced dissolution rates in sulfidic, heterotrophic, and abiotic conditions of 1773.97 (±324.35), 152.81 (±123.27), and 272.99 (±249.96) μmol CaCO3 • cm−2 • yr−1, respectively. Steep gradients in pH were also measured within carbonate-attached biofilms using pH-sensitive fluorophores. Together, these results show that the production of acidic microenvironments in biofilms of sulfur-oxidizing bacteria are capable of dissolving carbonate rocks, even under well-buffered marine conditions. Our results support the hypothesis that authigenic carbonate rock dissolution driven by lithotrophic sulfur-oxidation constitutes a previously unknown carbon flux from the rock reservoir to the ocean and atmosphere.

Behavior of Mg isotopes during precipitation of methane-derived carbonate: Evidence from tubular seep carbonates from the South China Sea

The magnesium (Mg) isotopic composition of marine carbonate has been used as a proxy to constrain the geochemical composition of seawater and to trace the global Mg cycle. Sulfate-driven anaerobic oxidation of methane (SD-AOM) is a prominent process that facilitates the formation of marine authigenic carbonate. Yet the understanding of the impact of SD-AOM on the Mg isotope composition of authigenic carbonate under natural conditions is limited. Here we present the analysis of two tubular methane-seep carbonates from the South China Sea. The tubular carbonates reveal a trend of decreasing δ13C values and increasing carbonate contents from the periphery (i.e., the outer surface of the carbonate) to the inner portion (i.e., close to the inner surface of the carbonate pipes). The tubular carbonates are interpreted as former fluid conduits, indicating intense and lasting seepage that apparently increased with time. The observed variability in composition probably results from changes in the intensity of SD-AOM and correlating carbonate precipitation rates. Trends of enrichment of middle rare earth elements (MREEs), molybdenum (Mo), and uranium (U) from the periphery inward indicate more reducing formation conditions toward the inner surface of the former conduit. Along the same transects, δ26Mg values decrease from periphery to inner portion in one tubular carbonate (−2.63‰ to −3.24‰), while δ26Mg values are close to constant for the second tubular carbonate (−3.42‰ to −3.37‰); such patterns highlight the complexity of Mg isotope fractionation during seep carbonate formation. A positive correlation between δ26Mg and δ13C values and a negative correlation between δ26Mg and Mg/Ca molar ratios suggests that enhanced SD-AOM facilitates the incorporation of light Mg isotopes into the carbonate lattice. However, such assumption contradicts with the fact that higher carbonate precipitation rates reduce Mg isotope fractionation due to incomplete dehydration of Mg ions. It is hypothesized that hydrogen sulfide produced by SD-AOM may enlarge the extent of fractionation by weakening the hydration of Mg. If this interpretation is correct, the effect of sulfide on the fractionation of Mg isotopes can even offset or exceed the effect of precipitation rate. Our study indicates that SD-AOM has an effect on the Mg isotope fractionation during carbonate precipitation. Similar effects may have occurred in geological periods with more widespread carbonate authigenesis. A better understanding of the factors affecting the Mg isotopic fractionation during marine carbonate authigenesis is needed to efficiently use this archive of paleoenvironments and element cycling.

Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities

Cold seep carbonates are important archives of pore water chemistry and ancient methane seepage activity. They also provide an important contribution to the global carbon sink. Seep carbonates at three sediment layers (3.0, 52.1, and 53.6 mbsf) were collected at site W08B in the Qiongdongnan Basin of the South China Sea. This study investigated the mineralogy, microstructure, stable carbon and oxygen isotopes, trace elements, and U-Th dates of these carbonates to identify the relationship between methane flux and authigenic carbonate precipitation. The results showed that the δ13C and δ18O values of all carbonates are similar, indicating that the carbon source for shallow carbonates and deep carbonates has remained constant over time and included biogenic and thermogenic methane. Although carbonates were found in three sediment layers, the two main stages of methane seepage events were discernible, which was likely caused by the dissociation of gas hydrates. The first methane seep took place at 131.1–136.3 ka BP. During a dramatic drop in the sea level, the seep carbonate at 52.1 mbsf formed at 136.3 ka BP through the anaerobic oxidation of methane (AOM). The carbonate at 53.6 mbsf resulted from the vertical downward movement of the sulfate-methane transition zone with decreasing methane flux at 131.1 ka BP. This is the reason for the age of carbonates at 52.1 mbsf being older than the age of carbonates at 53.6 mbsf. The second methane seep took place at 12.2 ka BP. Shallow carbonate formed at that time via AOM and is now located at 3 mbsf. Moreover, thin-section photomicrographs of deep carbonate mainly consisted of matrix micrite and biological debris and acicular aragonite occurred as vein cement filling the pore spaces between the matrix micrite. The acicular aragonite was mainly influenced by the timing of the carbonate precipitation of minerals. This research identified a long history of methane seep activity reflected by the vertical distribution of carbonates.

Methane transport and sources in an Arctic deep-water cold seep offshore NW Svalbard (Vestnesa Ridge, 79°N)

We investigate the uppermost 60 cm of sediment in active pockmarks of a deep-water methane seep site from Vestnesa Ridge offshore NW Svalbard. Using video guided core sampling with a remotely operated vehicle we collected push cores directly from bacterial mats within two active pockmarks, Lunde and Lomvi. Pore water analyses show very shallow sulphate methane transition zones and transport-reaction modelling suggests a considerable amount of dissolved methane passing through the sediment water interface due to upwards advection of an aqueous fluid not previously reported from Vestnesa Ridge. In addition, we show that the amount of methane that bypasses the benthic methane filter greatly increases with higher aqueous fluid advection rate. Recent changes in methane flux are evident from lipid biomarker, seep carbonate, and δ13C-organic carbon profiles in both pockmarks. Hydrocarbons at this cold seep site are supplied both by deep thermogenic sources from below the gas hydrate stability zone but also to a significant degree by microbial methanogenesis which dominates the signature in our shallow sediment cores with δ13C–CH4 values as low as −77‰.

Formation, diagenesis and fauna of cold seep carbonates from the Miocene Taishu Group of Tsushima (Japan)

AbstractThe studied seep carbonates from Tsushima, Japan, are embedded within marine siliciclastics of the lower Miocene Taishu Group and represent the earliest evidence of hydrocarbon seepage in the Sea of Japan. In contrast to Miocene and Pliocene examples from Honshu, which are often found above anticlines, the seeps from Tsushima formed within a pull-apart basin before major anticlines had formed. The three carbonates from Fukuzaki, Kanoura and Tanohama are composed chiefly of calcite, with significant admixture of ankerite only at Kanoura. The stable carbon isotope composition of calcites (δ13C as low as −40.2 ‰ VPDB for Fukuzaki, −41.8 ‰ VPDB for Kanoura, and −52.8 ‰ VPDB for Tanohama) indicate methanogenic origin of the carbonates. Textures of these deposits, including radiaxial and yellow cements, are indicative of formation at a methane seep. The stable oxygen isotope composition of calcites (δ18O values as low as −14.4 ‰ VPDB for Fukuzaki, −14.5 ‰ VPDB for Kanoura and −13.9 ‰ VPDB for Tanohama) indicate that they were influenced by burial fluids. Burial diagenesis is also indicated by the stable isotopic compositions of ankerite (δ13C ranging from −19.1 ‰ to −7.1 ‰ VPDB, δ18O from −11.1 ‰ to −9.7 ‰ VPDB). Molecular fossils from Tanohama comprise n-alkanes with short-chain predominance, interpreted to have formed due to thermal cracking of organic matter. The carbonates yield a chemosynthesis-based community comprising vesicomyids Pleurophopsis chitanii, P. cf. hamuroi, the bathymodiolin ‘Bathymodiolus’ akanudaensis, the lucinid Lucinoma sp. and the provannid Provanna? sp., which have never been hitherto identified. ‘Bathymodiolus’ akanudaensis, Lucinoma sp. and Provanna? sp. are the oldest records of these taxa in the Sea of Japan.

Fluid expulsion system and tectonic architecture of the incipient Cascadia convergent margin as revealed by Nd, Sr and stable isotope composition of mid-Eocene methane seep carbonates

Stratigraphic and structural constraints on the initiation and early evolution of the Cascadia convergent margin, following accretion of the igneous Siletzia terrane at 50–45 Ma, remain elusive. This study applies a novel approach based on the combination of Nd, Sr, C and O isotope analyses of the oldest-known methane-seep carbonates (Humptulips Formation, middle Eocene, 42.5–40.5 Ma) of Cascadia to provide insight into its early hydrogeological regime and incipient subduction history. Both anomalously high εNd and low 87Sr/86Sr signatures of the studied carbonates attest to interactions between the seeping fluids and mafic, igneous components of the forearc basement. Moderately negative δ13Ccarbonate values imply a predominantly thermogenic origin of hydrocarbons at three out of four studied seeps, with likely an important contribution of biogenic methane at a site that was closer to the ancient shoreline. When combined with structural constraints, the recorded signals point to discharge of fluids originating mostly from deep portions of the young subduction wedge underthrust beneath the accreted Siletzia terrane, and their channelled ascent through its thick volcanic succession. These results document the presence of a fluid generation and migration system typical of active convergent margins prior to maturation of typical, calc-alkaline arc magmatism in the Cascades at 40 Ma. The strongly pronounced involvement of the exotic, 143Nd-enriched and 87Sr-depleted fluids reflects the distinctive structural architecture of the newly reorganised subduction zone. Of particular importance was the combination of a relatively thin sedimentary cover of the infant forearc, its extensional tectonics, and the proximity of the young trench to the volcanic backstop that the deep, subducting plate-derived fluids must have interacted with before their seepage at the seafloor.

Palaeoecological analysis of a methane seep deposit from the Upper Cretaceous (Maastrichtian) of the U.S. Western Interior

Methane seeps were a common feature in the Late Cretaceous Western Interior Seaway of the United States. We document the occurrence of methane seep deposits in the Pierre Shale on the Cedar Creek Anticline in east-central Montana for the first time. The seep deposits occur in the lowermost part of the Baculites baculus Zone (the Endocostea typica Zone), corresponding to the lowermost Maastrichtian. They are therefore the youngest seeps yet described from the Western Interior Seaway. We conducted a detailed faunal analysis of a single seep deposit, together with geochemical investigation of both seep carbonates and molluscan shell material to determine palaeoenvironmental conditions. Oxygen isotope analysis of well-preserved molluscan shell material reveals water temperatures of between 19 and 27°C, while depleted carbon isotope values of seep carbonates are indicative of the anaerobic oxidation of methane. The morphology of the seep deposit suggests a strong advective flux of methane to the sediment–water interface. Comparison to a nearby contemporaneous non-seep site reveals that similar groups of organisms occur in both settings, albeit with varying relative abundances – the seep is numerically dominated by the lucinid bivalve Nymphalucina occidentalis. Substrate appears to be the major control on the diversity and palaeoecological composition of both seep and non-seep sites.

Insights into the REY inventory of seep carbonates from the Northern Norwegian margin using geochemical screening

Rare earth element and yttrium (REY) systematics of authigenic seep carbonates can provide insights into the physico-chemical characteristics of seep systems and allow discrimination of carbonate precipitation under seawater- or porewater-dominated fluid regimes. However, care must be taken when interpreting their REY systematics, since seep carbonates comprise a mixture of detrital silicates and authigenic carbonate cement. Since concentrations of trace elements, including Rb, Zr, Th and REY, are considerably lower in the carbonate fraction than in the detrital silicate fraction, leaching of these trace elements from detrital components (particularly aluminosilicates) even during a rather “mild” sample decomposition approach with, for example, nitric acid (HNO3) can effectively mask the authigenic carbonate REY signal. To assess this effect, trace element concentrations were determined in seep carbonates and detrital sediments from two sites off the Norwegian margin (Lofoten-Vesterålen margin and Vestnesa Ridge). Seep carbonate samples included bulk crusts and nodules, and individual microfacies (microcrystalline aragonite and/or Mg-calcite cementing detrital sediment and void-filling fibrous aragonite cement) microdrilled from crusts. A screening procedure based on Rb, Zr, Th and REY reveals effective masking of the carbonate REY systematics by leaching of trace elements from even minute amounts (0.1–1%) of detrital components. The latter cause elevated trace element concentrations and flat shale-normalized REY patterns. They are prominent in the bulk crusts and nodules, and in microcrystalline carbonate cementing sediment microfacies, which makes these unsuitable as archives of the REY inventory of the carbonate-precipitating fluid even when digested with HNO3. Some void-filling cements, however, contain little detrital material and show REY signatures characterized by light REY-depletion in shale-normalized (subscript SN) patterns and small negative CeSN anomalies.Comparison of the seep carbonate REYSN patterns with seawater and seep porewater REESN patterns indicate that the detrital contribution considerably masks putative pore water REYSN patterns of microcrystalline carbonate cement. Seawater-like REYSN patterns of void-filling fibrous cements, however, agree with precipitation of this microfacies in an open system dominated largely by seawater near the sediment-water-interface.

Microbial utilization of rare earth elements at cold seeps related to aerobic methane oxidation

A major breakthrough in the field of rare earth element (REE) geochemistry has been the recent discovery of their utility to microbial life, as essential metalloenzymes catalyzing the oxidation of methanol to formaldehyde. Lanthanide-dependent bacteria are thought to be ubiquitous in marine and terrestrial environments, but direct field evidence of preferential microbial utilization of REE in natural systems is still lacking. In this study, we report on the REE and trace element composition of the tube of a siboglinid worm collected at a methane seep in the Gulf of Guinea; a tube-dwelling annelid that thrives in deep-sea chemosynthetic ecosystems. High-resolution trace element profiles along the chitin tube indicate marked enrichments of lanthanum (La) and cerium (Ce) in its oxic part, resulting in REE distribution patterns that depart significantly from the ambient seawater signature. Combined with various geochemical and microbiological evidence, this observation provides direct support for an active consumption of light-REE at cold seeps, associated with the aerobic microbial oxidation of methane. To further evaluate this hypothesis, we also re-examine the available set of REE data for modern seep carbonates worldwide. While most carbonate concretions at cold seeps generally display REE distribution patterns very similar to those for reduced pore waters in marine sediments, we find that seafloor carbonate pavements composed of aragonite commonly exhibit pronounced light-REE enrichments, as inferred from high shale-normalized La/Gd ratio (>~0.8), interpreted here as possibly reflecting the signature of lanthanide-dependent methanotrophic activity. This finding opens new perspectives for revisiting REE systematics in ancient seep carbonates and other microbialites throughout the Earth's history. In particular, the geochemical imprint of aerobic methane oxidation could be possibly traced using REE in Archaean stromatolites and other archives of Precambrian seawater chemistry, potentially providing new insights into the oxygenation of early Earth's oceans and associated microbiogeochemical processes.

Methane seepage patterns during the middle Pleistocene inferred from molybdenum enrichments of seep carbonates in the South China Sea

Seepage of methane-rich fluids, are widespread along the continental margins of the world. Intense seepages of methane may lead to the emission of this powerful greenhouse gas into the hydrosphere and atmosphere with implications in the understanding of past dramatic marine environment changes and global warming. However, euxinic environments with sulfidic bottom water as a result of vigorous seepages at ancient cold seeps remain unidentified. Here we report trace element geochemistry and associated mineralogical and carbon and oxygen isotopic data in seep carbonates collected from long drill cores in the South China Sea. Three distinct stages of methane seepages events probably caused by the dissociation of gas hydrates are discernible. Carbonates formed in stage 2 during the Marine Isotope Stage 5 (MIS5) are mainly occurred as aragonite and show extremely high molybdenum (Mo) contents (up to 118 μg/g), and strong enrichments of authigenic Mo in relative to uranium. These results suggest the presence of intermittent or even permanent euxinic conditions generated by sulfidic bottom water that result from intensive methane seepages. In contrast, mineral compositions and trace element patterns indicate that the carbonates before and after the MIS 5 (stages 1 and 3) form within sulfidic conditions restricted to pore water under relatively lower methane fluxes. Our results highlight extremely intense methane seepages that may potentially result in localized euxinic environments. The inferred euxinic environments imply more amount of massive methane release into the water column than a previous estimation, which was triggered by large-scale dissociation of gas hydrates in the South China Sea during MIS 5.

Phosphorus Species in Deep-Sea Carbonate Deposits: Implications for Phosphorus Cycling in Cold Seep Environments

Phosphorus (P) is an important nutrient for biological communities in cold seeps. However, our knowledge on the source, species, and cycling of P in cold seep environments is limited. In this study, the concentration, species, and micro to nanometer scale distribution of P in seep carbonates were examined at three deep-sea cold seeps in the South China Sea and East China Sea. The Ca-P accounts for the largest proportion of P—followed by detrital-P, Fe-P, organic-P, and exchangeable-P. The distribution patterns of Ca-P, detrital-P, and organic-P in the seep carbonates differ from one another, as shown by elemental mapping with NanoSIMS and scanning electron microscopy. The covariation of P with Ca and C reveals that Ca-P co-precipitates with Ca-carbonate, which is linked to the process of sulfate-driven anaerobic oxidation of methane. Organic-P is also observed within biofilm-like organic carbon aggregates, revealing the microbial enrichment of P by fluids in the process of anaerobic oxidation of methane. P with a granulated morphology was identified as detrital-P derived from deep sediments. Most importantly, it is evident that Ca-P is positively correlated to the Fe content in all the seep carbonates. This indicates the likelihood that the dissolved P in cold-seep fluids is released primarily from Fe oxides through Fe-driven anaerobic oxidation of methane in deep sediments. These processes associated with different species of P may have significant implications for P geochemical cycling and anaerobic oxidation of methane impelled by Fe and sulfate reduction in cold seep environments.

Molecular indicators of methane metabolisms at cold seeps along the United States Atlantic Margin

Anaerobic oxidation of methane (AOM) and the environmental conditions supporting AOM on continental margins is an essential component to global methane budgets. Diagnostic lipid biomarkers and their compound specific isotope analysis preserved in authigenic carbonates at cold seeps can serve as “fingerprints” to archaeal−bacterial consortia involved in AOM. However, despite the discovery of several hundreds of seeps along the United States Atlantic Margin (USAM), there are relatively few biomarker investigations of cold seep carbonates along this passive margin. A lipid biomarker, carbon isotope, and DNA marker gene study was therefore undertaken to determine the microbial origins of authigenic carbonates from two USAM seeps, Norfolk and the Baltimore Canyon seep fields. Results from this study capture a distinct archaeal lipid signature from putative methanotrophic archaea, including archaeol (I), sn-2-hydroxyarchaeol, 2,6,10,15,19-pentamethylicosane (PMI), and crocetane. The 13C-depleted AOM-related archaeal lipid samples (i.e., archaeol: −91.6‰, sn-2-hydroxyarchaeol: −129.2‰, PMI −92.8‰, and crocetane: −70.9‰) confirm the dominance of methane assimilation and isotope fractionation during AOM. These results are consistent with the detection of archaeal anaerobic methanotrophs (ANMEs) based on 16S rRNA gene sequencing. The Norfolk authigenic carbonate contained ANME-1a, -1b, 2a-2b, and 2c whereas only the ANME-2 clade was detected at Baltimore and present as the subclusters 2a-2b and -2b. The ANME-2d clade may also be present, particularly at the Baltimore seep site, given the high abundance of Candidatus Methanoperedens nitroreducens detected in the mcrA gene sequencing. The presence of terminally branched fatty acids, antesio- and iso-C15:0 components, as well as C16:1ω7 with δ13C values as low as −107.6‰, are indicative of sulfate-reducing bacteria (SRB) at the Norfolk seep site and supports syntrophy of SRB with methane-oxidizing archaea. In contrast, nitrate-driven AOM in syntrophy M. nitroreducens at the Baltimore seep site is consistent with elevated fatty acid δ13C values and lack of Deltaproteobacteria at the Baltimore seep site. Taken together, the range in lipid composition, distribution, and carbon isotopic composition observed at the Norfolk and Baltimore seep sites suggests AOM is performed by multiple archaea instead of a single species and may be paired with either or both nitrate- and sulfate-reduction. Given the heterogeneous nature of cold seep ecosystems, this study fills a critical spatial gap in our knowledge of AOM activity at two seep sites along a passive margin.

Origin and water depth of a newly identified seep carbonate and paleoecology of Bathymodiolus in the Miocene Taishu Group, southwestern Japan

The Fukuzaki Limestone is a small limestone lens embedded in siliciclastic Taishu Group rocks on Tsushima Island in the southwestern Japan Sea. A fossil assemblage in this limestone consisting of the bivalve mollusc Bathymodiolus sp. (Bathymodiolinae) was previously inferred, on the basis of the limestone's isolated lenticular occurrence and the chemosynthetic nature of modern Bathymodiolinae, to rely on chemosynthetic production, but conclusive evidence was not provided. Carbon, nitrogen, oxygen, and sulfur isotope ratios in the limestone and its host rock mudstone were measured to clarify the paleoecology of the fossil assemblage and the formation environment of the limestone. The low δ13C (−39.8‰ to −31.7‰) and δ18O (−13.8‰ to −11.9‰) values of carbonate of the limestone lens strongly suggest that the limestone is composed of authigenic carbonate rock precipitated from bicarbonate derived from anaerobic oxidation of biogenic or thermogenic methane. The fossil assemblage in the limestone lens is consistent with a chemosynthesis-based animal assemblage harboring thioautotrophic (sulfur-oxidizing) symbionts, and the low δ13C and δ34S values of total organic carbon and sulfide sulfur, respectively, in the limestone lens and mud layers suggest that they reflect chemosynthetic production using hydrogen sulfide derived from anaerobic methane oxidation by microbes. Furthermore, the stratigraphic position of the lens in the upper part of the Lower Unit of the Taishu Group indicates that it was precipitated at a water depth >800 m (deeper than the lower middle bathyal zone). These findings indicate that a deep-sea basin locally supplied with methane had already formed in the southwestern Japan Sea by rapid subsidence during the opening stage of the Japan Sea (16–17 Ma).

Sr and Nd isotopes of cold seep carbonates from the northern South China sea as proxies for fluid sources

Abstract In this study, Sr isotopes combined with Nd isotopes of modern cold seep carbonates were studied to provide reliable proxies for sources and pathways of seep fluids. We presented Sr and Nd isotopes together with stable carbon and oxygen isotope data of cold seep carbonates recovered from the northern continental slope of the South China Sea (SCS). Based on the evidence for negligible detrital contamination and diagenetic alteration, carbonate samples are considered dependable archives which record Sr and Nd isotope signals of the original pore waters from which carbonates have precipitated. Excellent correlation between 87Sr/86Sr ratios and 1/Sr observed in carbonate samples indicates two end-members with distinct Sr isotopic signatures, including modern seawater as a major source and a lesser contribution from methane-rich seep fluid driven from a deep source. Besides these two end-members, release of Nd from organic matter and Fe–Mn oxyhydroxide is an additional contribution to pore water Nd. Furthermore, Sr and Nd isotopic compositions suggest water-rock interactions between upward seep fluid and detrital component in sediments, which resulted in the isotopic shift towards pronounced 143Nd depleted and slightly 87Sr enriched in the fluid phase. This study provides insight into the knowledge for tracing seepage fluid circulation by Sr–Nd isotope system at modern seep systems.

A new approach to discern the hydrocarbon sources (oil vs. methane) of authigenic carbonates forming at marine seeps

Numerous marine hydrocarbon seeps have been discovered in the past three decades, the majority of which are dominated by methane-rich fluids. However, an increasing number of modern oil seeps and a few ancient oil-seep deposits have been recognized in recent years. Oil seepage exerts significant control on the composition of the seep-dwelling fauna and may have impacted the marine carbon cycle through geological time to a greater extent than previously recognized. Yet, distinguishing oil-seep from methane-seep deposits is difficult in cases where δ13Ccarb values are higher than approximately −30‰ due to mixing of different carbon sources. Here, we present a comparative study of authigenic carbonates from oil-dominated (site GC232) and methane-dominated (site GC852) seep environments of the northern Gulf of Mexico, aiming to determine the geochemical characteristics of the two types of seep carbonates. We analyzed (1) major and trace element compositions of carbonates, (2) total organic carbon (TOC), total nitrogen (TN) and carbon isotope (δ13CTOC) of residue after decalcification, (3) sulfur isotope signatures of chromium reducible sulfur (CRS, δ34SCRS) and residue after CRS extraction (δ34STOS), as well as (4) sulfur contents (TOS) of residue after CRS extraction. Carbonates from the studied oil seep are dominated by aragonite and exhibit lower δ34SCRS values, suggesting carbonate precipitation close to the sediment surface. In addition, oil-seep carbonates are characterized by higher TOC and TOS contents and higher TOC/TN ratios, as well as less negative δ13CTOC values compared to methane-seep carbonates, probably reflecting a contribution of residual crude oil enclosed in oil-seep carbonates. Very low δ13CTOC values (as low as −68.7‰, VPDB) and low TOC/TN ratios of methane-seep carbonates indicate that the enclosed organic matter is derived mainly from the biomass of methanotrophic biota. This study presents new geochemical data that will allow the discrimination of oil-seep from methane-seep deposits. Although some of the geochemical patterns are likely to be affected by late diagenesis, if applied with caution, such patterns can be used to discern the two end-member types of seepage – oil seeps and methane seeps – in the geological record.

Gas Hydrate Dissociation During Sea‐Level Highstand Inferred From U/Th Dating of Seep Carbonate From the South China Sea

AbstractGas hydrates represent a huge reservoir of methane in marine sediments, prone to dissociation in response to environmental changes. There is consensus that past events of gas hydrate dissociation in the marine environment mainly occurred during periods of low sea level. Here, we report geochemical data for 2‐m‐thick layers of seep carbonate collected from a hydrate‐bearing drill core from ~800‐m water depth in the northern South China Sea. The aragonite‐rich carbonates reveal positive δ18O values, confirming a genetic link with gas hydrate dissociation. Uranium‐thorium dating of seep carbonates indicates that gas hydrates at the study site dissociated between 133,300 and 112,700 years BP, hence coinciding with the Last Interglacial (MIS 5e) sea‐level highstand. We put forward the concept that a climate‐driven increase in temperature was responsible for a period of pronounced gas hydrate dissociation.

Cold-Water Corals in Gas Hydrate Drilling Cores from the South China Sea: Occurrences, Geochemical Characteristics and Their Relationship to Methane Seepages

Cold-water corals (CWCs) are frequently found at cold seep areas. However, the relationship between fluid seepage and CWC development is not clear. Here, for the first time, we report the occurrences, species identification, mineralogy, carbon and oxygen isotopes, as well as elemental compositions of fossil CWC skeletons from gas-hydrate-bearing sediment in drilling cores from the South China Sea (SCS). Three sites (GMGS-08, GMGS-09B, and GMGS-16) were investigated but CWCs were only found at one site (GMGS-09B). Interestingly, the CWCs were found in three horizons and they were all embedded with authigenic carbonates. Three genera of fossil CWCs (Crispatotrochus sp., Solenosmilia sp. and Enallopsammia sp.) were identified. The CWC fragments are predominantly aragonite. The CWCs exhibit δ13C values between −8.4‰ and −0.6‰ that are significantly higher than δ13C values of the associated seep carbonates (δ13C values with an average of −55.6‰, n = 19), which indicates a carbon source other than methane for the CWCs. It appears that authigenic carbonates provide a substratum for coral colonization. Bathymetric high points, appropriate water temperature and stronger bottom-water currents at site GMGS-09B might be crucial to keep conditions favorable for the growth of CWCs in the studied area. In addition, high trace-element concentrations of Cr, Ni, Pb, U, Ba, Th, and Sr suggest that the CWCs are influenced by strong fluid seepage that can reach the water-sediment interface, and associated microbial activity. Hence, it also becomes evident that CWCs in hydrocarbon-rich seepage areas not only provide a critical constraint on the impact of fluid emission on the bottom water chemistry, but also are likely to be very precise recorders of the end time of cold seep activity.

Archaeal lipid biomarker as a tool to constrain the origin of methane at ancient methane seeps: Insight into subsurface fluid flow in the geological past

The origin of methane discharged at ancient methane seeps is a key to understand subsurface fluid-flow processes in sedimentary basins. The carbon isotopic composition of methane-seep carbonates can be used to estimate the origin of methane, but mixing of multiple carbon sources results in ambiguous interpretation. We here constrained the origin of methane at ancient seeps by using the carbon isotopic composition and the isotope fractionation of lipid biomarkers of anaerobic methane-oxidizing archaea. We estimated the carbon-isotope fractionation between methane and biomarkers as the offset between their δ13C values at modern seep sites in the Japan Sea and other regions of the world. The biomarker pentamethylicosane (PMI) extracted from a modern seep carbonate of the Japan Sea shows a 44‰ offset from the seep methane. On the basis of literature data, we calculated the carbon-isotope offset as mostly ~20–60‰ for PMI at modern seeps across the world. We also assessed the relationship of the δ13C values between methane and biomarkers by regression analysis on the data taken at the modern seeps, obtaining regression formulas to estimate the δ13C values of methane from those of biomarkers. For ancient seeps, we examined Miocene to Pleistocene methane-seep carbonate rocks collected from Japan Sea sediments. PMI extracted from these ancient carbonates have δ13C values ranging from −137‰ to −93‰. Using the δ13C values of PMI and the carbon-isotope offset and regression formulas, we estimated that the methane at the ancient seeps was mainly of biogenic origin and migrated from the shallow subsurface.

Mineralogical and geochemical records of seafloor cold seepage history in the northern Okinawa Trough, East China Sea

Cold seep carbonate represents a faithful record of the ancient methane seepage and provides a nonnegligible contribution to the global carbon reservoir. On the western slope of the Northern Okinawa Trough (NOT), a recent seafloor visualized survey has discovered widespread crust of cold seep carbonate. Here we study mineralogy and geochemistry of these authigenic carbonate to investigate source origin and reconstruct its growth history. Mineralogically, the carbonate crusts are mainly composed of micritic aragonite, with botryoidal aragonite, framboidal pyrite, and microcrystalline authigenic gypsum. Petrographic characteristic unambiguously indicates that this carbonate precipitates in relatively open systems due to a considerable rate of sulfate-dependent anaerobic oxidation of methane (AOM). Regarding geochemistry, strongly 13C-depleted carbon isotope values (as low as −56.1‰, V-PDB) demonstrate that the carbon in the carbonate crusts is mainly derived from biogenic methane coupled with AOM. In contrast, the δ18O enrichment (up to +2.7‰, V-PDB) suggests that the fluid flow from which carbonate precipitated is sourced from dissociation of underlying natural gas hydrates. The U–Th ages of authigenic carbonates fall in the timescale of 22.8–55.7 ka BP, consistent with the period of sea-level lowstand in the late Pleistocene. Overall, several lines of evidence of this study indicate that extensive methane was released by gas hydrate decomposition during sea level fall, consequently resulting in the precipitations of carbonate crust in the NOT. Furthermore, the obviously episodic methane seepages led to the constant accretion from the interior to the exterior within the preformed crust, ultimately inducing the carbonate blocks, slabs and crusts to be exposed on the seafloor. The existence of large-scale carbonate crusts represents a good trapper of the later released carbon especially the isotopically light methane from the deep.

<strong>Crinoids lived around the Cretaceous seeps: the second example from cold-seep deposit in the Yezo Group in Hokkaido, Japan</strong>

Cold seep deposits are characterized by authigenic carbonates with very low δ13C signatures and specific fossils of chemosynthetic community members. These members are mainly composed of mollusks, whereas only a few occurrences of fossil echinoderms from cold seep deposits have been reported. The information of paleoecology of fossil echinoderms in or near cold seep environments is also sporadic. Allochthonous columnal fossils of an Isocrinina crinoid species were found in a boulder of Cretaceous cold seep carbonate from the Yezo Group in northern Hokkaido. The stable carbon isotope of the fossil crinoid specimens showed very low values, approximately −30‰ (VPDB) or much lower value. The low δ13C signatures of the seep crinoids and their mode of occurrences suggest that the fossil crinoids lived near the seep environment, and they assimilated low δ13C organic particles shifted from there.