Seep Deposits Research Articles

Evolutionarily ancient deep‐water seep communities in the Eocene Tethys: examples from Buje (Croatia)

AbstractWe report a newly discovered hydrocarbon seep deposit from the Eocene bathyal flysch, exposed in the town of Buje in Istria, Croatia. Molecular fossils of methane‐oxidizing prokaryotes and abundant banded botryoidal cements indicate strong fluid flux at this site. We systematically describe the fauna of this and another seep deposit previously reported from Buje. The faunal assemblages are composed of eight species, these being an unidentified solemyid protobranch bivalve, the nuculid Nucula bowerbanki?, the nuculanid Nuculana? sp., the mytilid Brachidontes? amanoi sp. nov., the two thyasirids Channelaxinus dinaricus sp. nov. and Thyasira histriaensis sp. nov., the lucinid bivalve Amanocina bujensis sp. nov., and a possible provannid gastropod. The two assemblages are of low diversity (4 and 5 species, respectively), and are dominated by chemosymbiotic species whose occurrence is largely restricted to seeps. Despite their spatial and stratigraphic proximity, the two deposits share only a single species, Channelaxinus dinaricus, probably due to different fluid flux regimes at both seeps. The Buje seep assemblages are among the very few Late Cretaceous to Palaeogene chemosynthesis‐based faunal assemblages from the Tethys Ocean (the others being Late Cretaceous vent assemblages from Cyprus). From an evolutionary perspective, the Buje seep communities consist of genera with Mesozoic origins but lack Cenozoic novelties such as bathymodiolin mussels and vesicomyid clams, which are known from coeval deposits from the Pacific and dominate vents and seeps today. Thus, the Buje seep fauna support previous assertions that the Eocene Tethyan seep faunas preserved an ancient aspect, whereas evolutionary novelties arose in the Pacific.

Fossilized giant sulfide-oxidizing bacteria from the Devonian Hollard Mound seep deposit, Morocco.

The giant sulfide-oxidizing bacteria are particularly prone to preservation in the rock record, and their fossils have been identified in ancient phosphorites, cherts, and carbonates. This study reports putative spherical fossils preserved in the Devonian Hollard Mound hydrocarbon-seep deposit. Based on petrographical, mineralogical, and geochemical evidence the putative microfossils are interpreted as sulfide-oxidizing bacteria similar to the present-day genus Thiomargarita, which is also found at modern hydrocarbon seeps. The morphology, distribution, size, and occurrence of the fossilized cells show a large degree of similarity to their modern counterparts. Some of the spherical fossils adhere to worm tubes analogous to the occurrence of modern Thiomargarita on the tubes of seep-dwelling siboglinid worms. Fluorapatite crystals were identified within the fossilized cell walls, suggesting the intercellular storage of phosphorus analogous to modern Thiomargarita cells. The preservation of large sulfide-oxidizing bacteria was probably linked to changing biogeochemical processes at the Hollard Mound seep or, alternatively, may have been favored by the sulfide-oxidizing bacteria performing nitrate-dependent sulfide oxidation-a process known to induce carbonate precipitation. The presence of sulfide-oxidizing bacteria at a Devonian hydrocarbon seep highlights the similarities of past and present chemosynthesis-based ecosystems and provides valuable insight into the antiquity of biogeochemical processes and element cycling at Phanerozoic seeps.

The earliest record of a sclerobiont assemblage from a methane-seep ecosystem (upper Silurian, Morocco)

Sclerobionts colonizing brachiopod shells derived from the oldest-known methane seep deposit (upper Silurian, Moroccan Meseta) are reported for the first time. Despite a large number of the brachiopod shells collected, all representing the atrypide species Septatrypa lantenoisi Termier, very few showed the presence of sclerobionts. The majority of the shells were taphonomically altered: worn, exfoliated and corroded. The sclerobionts are represented by cornulitid tentaculitoids and less common auloporid tabulate corals and epibiontic brachiopods. Although the observed assemblage certainly does not reflect the original taxonomic composition of the sclerobiont assemblage, it provides the first insight into the Palaeozoic encrusters inhabiting methane-seep ecosystems. As implied by the moderately 13C-depleted carbon isotope composition and simple, micrite-dominated paragenetic succession of the associated seep carbonates, the investigated community most likely represents a period of slow, diffuse fluid seepage, an interpretation consistent with the common perception of the seep-related brachiopods as non-chemosymbiotic, perhaps opportunistic seep-dwellers unable to colonize sites with more vigorous fluid flow. Likewise, the studied encrusters most likely were non-obligate inhabitants of the seep ecosystem, using the brachiopod shells as firm attachment sites and elevation above the seafloor. This colonization pattern would have probably improved their access to the prolific suspended organic particles and potentially limited their exposure to the highest environmental toxicity. As in the case of geologically much younger serpulid polychaetes, also the cornulitid tubeworms colonized the methane seeps relatively shortly after their first, Middle Ordovician appearance in the fossil record.

Mass occurrence of echinoids in an Oligocene hydrocarbon-seep limestone from the Olympic Peninsula, Washington State, USA

AbstractLoose limestone blocks of a newly recognized hydrocarbon-seep deposit from the lower Oligocene Jansen Creek Member of the Makah Formation were collected on a beach terrace close to the mouth of Bullman Creek in Washington State, USA. The limestone consists largely of authigenic carbonate phases, including 13C-depleted fibrous cement forming banded and botryoidal crystal aggregates with δ13C values as low as –23.5 ‰. Lipids extracted from the limestone yielded molecular fossils of anaerobic methanotrophic archaea (ANME), dominated by compounds of an ANME-2/DSS consortium with δ13C values as low as −106 ‰, indicating formation at an ancient methane seep. The fossil inventory of the seep deposit is remarkable, consisting almost solely of echinoid remains, whereas typical seep biota are absent. Varying preservation of the echinoid fossils indicates parautochthonous deposition, corroborated by evidence for high fluid flow at the ancient seep, possibly responsible for displacement of echinoids after death. Although a full taxonomic description of the echinoids cannot be given, almost all fossils were assigned to one taxon of irregular spatangoids, except for a single regular echinoid. Abundance and lifestyle of the irregular spatangoids in the Bullman Creek echinoid seep deposit resemble those of the fossil Tithonia oxfordiana from an upper Jurassic seep deposit in France and extant Sarsiaster griegii from modern seeps in the Gulf of Mexico. The Bullman Creek echinoid deposit probably represents a fossil analogue of the Gulf of Mexico Sarsiaster mass occurrence, indicating that the adaptation of spatangoid echinoids to chemosynthesis-based ecosystems ranges back at least to the earliest Oligocene.

Well-preserved clotted micrite in a diagenetically altered Jurassic seep deposit

Geochemical analyses of ancient seep carbonates provide information on past environmental conditions and the chemical composition of seep fluids, allowing for the reconstruction of the evolution of chemosynthesis-based faunal communities in the Phanerozoic. However, diagenetic processes commonly mask the primary composition of ancient seep carbonates, impeding a reliable reconstruction of formation environments. The potential for the preservation of primary element patterns in microcrystalline carbonate, which is typically the volumetrically dominant phase in seep limestones, is poorly constrained to date. This study investigates Jurassic seep limestones from the Blue Mountains of eastern Oregon, USA. The seep limestones are mostly composed of three carbonate phases, (1) clotted microcrystalline calcite (clotted micrite), (2) banded and botryoidal, fibrous calcite cement, and (3) equant calcite cement. Mineralogically, these three carbonate phases are now calcite characterized by low magnesium/calcium (Mg/Ca) ratios. The fibrous crystal habit and flat crystal terminations indicate that the precursor mineral of the banded and botryoidal cement was originally aragonite. Cathodoluminescence imaging of banded and botryoidal cement shows partially dull to non-luminescence, suggesting that only parts of this phase experienced diagenetic alteration. However, low strontium (Sr) content and positive europium (Eu) anomalies of botryoidal cement indicate that this phase was affected by diagenetic alteration. Equant calcite exhibits the largest crystal size and the highest manganese/strontium (Mn/Sr) ratios, suggesting that this phase was the product of later-stage diagenesis, probably having precipitated during progressive burial. High Sr contents and low Mn/Sr ratios of clotted micrite indicate that this phase was less affected by diagenetic alteration and has the highest preservation potential with respect to the original seep fluid composition. Shale-normalized rare earth element and yttrium (REY) patterns of clotted micrite are close to those of modern seawater, agreeing with a primary composition of clotted micrite. In spite of its small crystal size, which is generally believed to favor recrystallization, our study suggests that clotted micrite has the potential to reliably record information on fluid composition. Given the dominance of microcrystalline carbonate in many seep limestones, this phase may prove to be a valuable archive – ideally alongside other authigenic minerals – in cases where its pristine composition can be confirmed by independent petrographical and geochemical proxies.

Methane seeps following Early Permian (Sakmarian) deglaciation, interior East Gondwana, Western Australia: Multiphase carbonate cements, distinct carbon-isotope signatures, extraordinary biota

Shallow-marine methane seeps, described here for the first time from the Australian Upper Paleozoic, are among few reported globally from the Carboniferous–Permian. Carbonate-cement fabrics, δ13C values and biota indicate that concentrations of nodules scattered in a narrow stratigraphic interval within the lower Holmwood Shale in the Irwin Basin formed in methane seeps. Based on ammonoids, this zone belongs within the Sakmarian. The Holmwood Shale overlies the glaciogenic Nangetty Formation that sits on crystalline Precambrian basement. The Holmwood is little deformed, shows low thermal maturation and has been covered by less than 1000 m of Permian and Neogene strata. The nodules are cementstone with complex multiphase cement fabric and δ13C values (mostly, ‐25 – -45‰ VPDB) below the ranges for normal-marine limestone and carbonate nodules common in marine shale elsewhere. A biota preserved in nodules of the seep deposits includes representatives of three ecosystems: seep, pelagic, and coastal plain. The seep biota, new to the Australian Permian, consists of small thickets with a framework of tubeworms, less common algal-like Tubiphytes, and problematica (? algae or sponges). Other components include microbial mats, macrophyte alga Litostroma (first report from Gondwana); foraminifers; sponges with siliceous monaxon spicules; a new group of small, lightly ornamented rostroconchs; microgastropods and rare larger types; ostracods and minute scolecodonts from polychaete jaws. Elements of the pelagic ecosystem are ammonoids (abundant in enclosing shale) and marine microphytoplankton. Probable vertebrate bone (possibly amphibians) and diverse spores and pollen from land plants come from the adjoining coastal plain. The surrounding mudstone lacks benthic macrofauna but includes an unusual assemblage of siliceous agglutinated foraminifers. The seeps were oases of high organic productivity on an otherwise barren muddy seafloor. The Holmwood Shale's seep carbonates have close similarities to modern seep deposits in shallow-marine settings. They more closely resemble the modern deposits than some of the few others interpreted elsewhere from the Carboniferous–Permian, but seem close to equivalents in shale of latest Pennsylvanian (Gzhelian) age overlying glaciogenic deposits in the Namibian sector of East Gondwana. In both regions, deglaciation of Pennsylvanian ice sheets that covered large parts of East Gondwana, dewatering of melt-water from the glaciogenic sediment pile after burial, and associated release of probable biogenic methane seems to have coincided with formation of seeps.

Methane seeps as refugia during ash falls in the Late Cretaceous Western Interior Seaway of North America

Abstract Methane seeps host rich biotic communities, forming patchy yet highly productive ecosystems across the global ocean. Persistent hydrocarbon emissions fuel chemosynthetic food webs at seeps. Methane seeps were abundant in the Western Interior Seaway of North America during the Late Cretaceous. This area also experienced intermittent ash falls, which negatively impacted the marine fauna. We propose that methane seeps acted as refugia during these environmental perturbations. We report a laterally continuous bentonite within the upper Campanian Baculites compressus Zone of the Pierre Shale in southwestern South Dakota (USA) that fortuitously cuts across a methane seep deposit. We compare the macroinvertebrate record below and above the bentonite at seep and non-seep sites. Our results reveal that the paleocommunity (measured by abundance and diversity) was largely unaffected by the ash fall at the seep site, whereas it was significantly altered at the non-seep site. Thus, methane seeps in the Western Interior Seaway may have provided refuges or served as oases in the aftermath of severe environmental perturbations.

A first notice of the goniodromitid crab from the Cenomanian (Upper Cretaceous) cold seep deposit of Hokkaido, Japan, with the redescription of Sabellidromites inflata (Collins and Karasawa, 1993) (Decapoda: Goniodromitidae)

Goniodromites sp. (Brachyura: Dromiacea: Goniodromitidae) is described from the Cenomanian (Upper Cretaceous) cold seep sediment in the Middle Yezo Group of Hokkaido, Japan. This species is the first recognition of the genus and family from the Cretaceous chemosynthetic communities, and represents the second record of a decapod from the fossil chemosynthetic communities in Japan. In addition, Sabellidromites inflata (Collins and Karasawa, 1993), a poorly known goniodromitid crab is redescribed.

Sr–Nd isotope decoupling at Cretaceous hydrocarbon seeps of the Basque-Cantabrian Basin (Spain): Implications for tracing volcanic-influenced fluids in sedimented rifts

Strontium isotopes have seen common use as a tracer of volcanic-influenced fluids at hydrothermal vents and cold seeps hosted by sedimented rifts. However, for some fluid emissions, no apparent contribution of volcanogenic Sr has been observed, despite geological or seismic evidence suggesting magmatic involvement in the fluid expulsion. Here, we explore the behaviour of Sr during fluid-rock interactions in a sedimented rift based on the isotope and elemental composition of Albian hydrocarbon-seep carbonates of the Basque-Cantabrian Basin, Pyrenees. The basin represents a peri-cratonic rift formed during the opening of the Bay of Biscay and infilled with organic-rich siliciclastics affected by subvolcanic intrusions. The intrusions stimulated thermogenic hydrocarbon generation in the sediments and resulted in enrichment of the associated seep carbonates in volcanic-derived, radiogenic Nd. In contrast, none of the 87Sr/86Srcarbonate ratios observed in this study provide evidence for subseafloor interactions between the seeping fluids and igneous materials. Instead, all four studied seep deposits show various degrees of 87Sr-enrichment, bearing the fingerprint of interactions with the rift-filling siliciclastics, rather than a volcanogenic signature. Furthermore, for three deposits, the observed unusual, positive correlation between the Sr and Nd isotope ratios attests to a presence of an evolved, deep-seated fluid end-member enriched in both 87Sr and 143Nd. This isotope decoupling can be explained by the different responses of Nd and Sr to subseafloor fluid-volcanic interactions, with a moderate degree of elemental exchange able to modify the fluid Nd isotope signal, but insufficient to affect the fluid Sr isotope signature. The study shows that for many sedimented seeps and vents, and especially for low-temperature, hydrocarbon-dominated emissions, volcanic-influenced fluids can be more reliably traced using the Nd isotope system, whereas Sr isotopes cannot provide standalone evidence for their absence.

Molybdenum isotope composition of seep carbonates – Constraints on sediment biogeochemistry in seepage environments

Methane seepage causes significant changes in the local diagenetic environment and impacts the marine sedimentary cycles of sulfur, carbon, and molybdenum (Mo), which are intimately linked to each other. Molybdenum isotopic compositions were determined for authigenic carbonate nodules and bulk sediments of a 230-m long sediment core collected at a methane seep site in the South China Sea. Authigenic carbonate nodules occur at different sediment depths and display low δ13C values around −50‰ revealing that carbon predominantly derived from biogenic methane. Moreover, seep carbonate nodules are substantially enriched in Mo (Al-normalized enrichment factors MoEF between 27 and 70), and display nearly constant δ98Mo values around an average of +1.9‰ (relative to NIST3134 = +0.25‰), close to the Mo isotopic composition of modern seawater (+2.3‰). Linear positive correlations between δ98Mo values and pyrite sulfur contents and Mo contents were identified for seep carbonate samples. This implies that Mo scavenging into pyrite is likely controlled by hydrogen sulfide availability derived from sulfate-driven anaerobic oxidation of methane (SD-AOM). The essentially constant and high δ98Mo values suggest that Mo species were nearly quantitatively incorporated into the authigenic carbonates under highly sulfidic conditions, probably at times when the sulfate-methane transition zones (SMTZ) was situated at shallow sediment depth at times of high seepage activity. In contrast, bulk sediments that exhibit typical marine δ13C values close to 0‰ (versus VPDB), hence devoid of methane-derived seep carbonates, are characterized by low Mo contents (MoEF = ~0.9). Their lower δ98Mo values mostly ranging from +0.2 to +0.8‰, in accord with low Mo contents, likely reflect input of detrital Mo. Bulk sediments bearing seep carbonate nodules display variable δ13C values lower than −5‰ and are characterized by a wide range in Mo contents (MoEF from 1.5 to 45.3) and δ98Mo values (+0.7 to +3.0‰). The high δ98Mo values of some sediment samples, relative to the seep carbonate signature, might indicate an additional accumulation of a 98Mo enriched pool of molybdenum during later SD-AOM at a deeper SMTZ postdating initial seep carbonate formation; such 98Mo enrichment was probably caused by preceding preferential removal of lighter Mo isotopes from pore fluids. Our results indicate that Mo isotope systematics of seep deposits can provide constraints on the dynamics of the biogeochemical cycling of carbon, sulfur, and trace elements in methane seep environments. A potential for authigenic carbonates to archive the Mo isotopic composition of seawater is indicated, but will have to be assessed in future studies.

A thyasirid-dominated methane-seep deposit from Montañita, southwestern Ecuador, from the Oligocene-Miocene boundary

A mass occurrence of the thyasirid bivalve Thyasira montanita in a limestone bed, exposed at Punta Montañita on the northern side of the Santa Elena peninsula in southeastern Ecuador, is here identified as an ancient methane-seep deposit. The massive to nodular limestone shows carbonate phases and microfabrics typical of seep limestones, such as banded and botryoidal cements. Its δ13Ccarbonate values, as low as −52.6‰, suggest a biogenic methane source. Furthermore, the carbonate microfabrics and biomarkers typical of ANME-2 archaea indicate high seepage rates. Tubular fossils previously interpreted as worm tubes show cone-shaped projections and an internal structure consistent with that of crustacean cuticle and are here interpreted as crustacean fragments. Seep deposits with mass occurrences of thyasirid bivalves have so far only been reported from the Cretaceous, making the Montañita seep deposit the first such example from the Cenozoic. The faunal and biomarker inventories of this deposit indicate deposition on an open shelf; the oysters on the upper surface of the limestone are likely secondary encrusters that arrived only after the emergence of this deposit in shallower water.

Massive seafloor mounds depict potential for seafloor mineral deposits in the Great South Basin (GSB) offshore New Zealand

Seafloor mounds are enigmatic features along many continental margins and are often interpreted as gas hydrate pingoes, seep deposits, mud volcanoes, or hydrothermal mounds. When such mounds occur in basins with past volcanic activities, they have the potential to host seafloor metalliferous deposits, which is generally overlooked. Using geophysical datasets, we document the fluid plumbing systems that promoted the formation of seafloor mounds in the Great South Basin (GSB), offshore New Zealand. We also investigate these mounds as potential seafloor metalliferous deposits. Our results reveal 9 seafloor mounds (~ 137 m high) above gigantic (~ 5.4 km high) fluid escape pipes that are associated with deeper crystalline rocks. The structural make-up of the mounds, their geospatial relationships with the pipes and intrusive rocks, and geophysical properties suggest a primary volcanic or hydrothermal origin for the culpable fluids and mounds respectively. Fluids derived from deeper coal beds and shallow foraminiferal oozes in the basin constitute secondary fluid sources focused along polygonal faults and lateral flow cells. A main sub-vertical and minor lateral fluid plumbing patterns are proposed. The relationship between the mounds, pipes, underlying intrusive rocks, and upward routing of mineral-rich fluids could have implications for the formation of ore-grade mineral deposits on the seafloor in the GSB.

The role of magmatism in hydrocarbon generation in sedimented rifts: A Nd isotope perspective from mid-Cretaceous methane-seep deposits of the Basque-Cantabrian Basin, Spain

Studies on the involvement of intrusive magmatism in hydrocarbon generation within sedimentary basins have gained momentum owing to increasing appraisal of the role that such processes may play in controlling global carbon cycle perturbations, and the exploration potential of the volcanic sedimentary basins. Nevertheless, for many areas the causal link between the intrusions and surrounding hydrocarbon systems remains disputed, encouraging a search for methods that could aid in identifying different hydrocarbon sources. Here, we have performed a multi-proxy geochemical study of the middle Cretaceous methane-seep deposits of the Basque-Cantabrian Basin, an early-stage, peri-cratonic rift marking the Mesozoic opening of the Bay of Biscay. Infilled by a thick sedimentary succession intruded by shallow-level igneous bodies, the basin shares analogies with modern young, sedimented rifts that sustain hydrocarbon seepage. We have applied a novel approach that uses the Nd isotope composition of the seep deposits to constrain the relationship between hydrocarbon seepage and igneous activity, and to explore the general potential of Nd isotopes to trace magmatic-influenced fluids in volcanic sedimentary basins. The Nd isotope data have been combined with rare earth element analyses and carbon and oxygen isotope measurements, providing broad insight into the former composition of the seeping fluids. For three out of four investigated seeps, the Nd isotope ratios observed in authigenic seep carbonates include signatures markedly more radiogenic than that reconstructed for background seawater-derived pore waters. The level of this 143Nd-enrichment varies both between and within individual deposits, reflecting spatial and temporal differences in fluid composition typical of seep-related environments. The radiogenic Nd isotope signals provide evidence of subseafloor interactions between the seeping fluids and mafic igneous materials, supporting the model of an igneous control on the mid-Cretaceous methane expulsion in the Basque-Cantabrian Basin. The thermogenic origin of the methane is in accord with the moderately negative δ13C values and paragenetic successions observed in the studied seep carbonates. For a single deposit, its relatively unradiogenic Nd isotope composition can be attributed to the smallest size and shallowest emplacement depth of the underlying intrusion, likely resulting in a short-lived character and limited hydrocarbon-generation potential of the associated contact metamorphism. The study demonstrates that Nd isotope analyses of seep carbonates offer a tool in disentangling methane fluxes from different organic matter alteration pathways for the numerous, both fossil and modern sedimented rifts for which the involvement of various methane sources remains insufficiently understood.

Temperatures of Late Cretaceous (Campanian) methane-derived authigenic carbonates from the Western Interior Seaway, South Dakota, USA, using clumped isotopes

Abstract Methane seep deposits, comprising large, carbonate-rich mounds formed from hydrocarbon seepage, were widely distributed in the Late Cretaceous Western Interior Seaway (WIS) of North America. Well-preserved, methane-derived authigenic carbonates (MDACs) from these deposits have been shown to retain petrological, paleontological, and geochemical imprints of their ancient depositional setting, all of which are important for understanding the dynamics and evolution of the shallow, epeiric WIS. To better characterize the environmental conditions of WIS seeps, we applied clumped isotope paleothermometry to magnesium calcite MDAC samples from five seep localities in the upper Campanian Pierre Shale, South Dakota, USA. We measured 21 subsamples, including 18 micritic carbonates and demonstrated apparent clumped isotope equilibrium between MDACs and WIS bottom waters. Extreme 13C depletion in most samples (δ13C ranging to −45.44‰) indicates they were precipitated with oxidized methane as a major source of dissolved inorganic carbon, which itself implies a close association with ancient methanotrophic metabolism. The average clumped isotope paleotemperature from the micritic carbonates is 23 ± 7 °C (1σ standard deviation), which agrees with bottom water paleotemperatures inferred from δ18O measurements of MDACs and well-preserved mollusk shells at similar localities in the WIS. The calculated average δ18Ow value for these samples is −0.5 ± 1.7‰ (1σ SD), which is indistinguishable from previously reported calculation on Campanian seawater δ18Ow from fossil mollusk shells, but elevated above younger fossils collected from other locations in the WIS. Our conclusions are inconsistent with previously hypothesized disequilibrium for WIS MDAC clumped isotope and therefore we propose that fossil MDAC deposits may be used as paleotemperature archives.

Miocene Seep-Carbonates of the Northern Apennines (Emilia to Umbria, Italy): An Overview

The natural emission of methane-rich fluids from the seafloor, known as cold seepage, is a widespread process at modern continental margins. The studies on present-day cold seepages provide high-resolution datasets regarding the fluid plumbing system, biogeochemical processes in the sediment, seafloor seepage distribution and ecosystems. However, the long-term (hundreds of thousands to millions of years) evolution of cold seepage remains elusive. The identification and study of outcrop analogous now exposed on land represent a valuable method for better understanding the effects of geological processes and climate forcing on the development of cold seepage systems. Here, we provide an overview on Miocene seep-carbonate deposits of the northern Apennines (from Emilia to the Umbria-Marchean sector, Italy), based on decades of field research integrated with detailed sedimentological and geochemical investigations. We report a total of 13 seep-carbonate outcrops, which formed in three different structural settings of the paleo-accretionary wedge corresponding to wedge-top basins, outer slope and intrabasinal highs at the deformational front. We discuss the recurring lithostratigraphic occurrence of seep deposits and the main compositional features (carbonate facies, carbon and oxygen stable isotopes) in order to interpret the seepage dynamics, duration and infer the contribution of methane-rich fluids released by paleo-gas hydrates. The datasets presented in this study represent a valuable complete record of cold seepage spanning ~12 Myr, that can be used to better understand factors controlling the regional-scale spatial and temporal evolution of cold seepage systems at modern active continental margins.

First report of the megathyridid (Terebratellidina) brachiopod Argyrotheca from a hydrocarbon seep deposit, middle Eocene Humptulips Formation, Washington State, USA

First report of the megathyridid (Terebratellidina) brachiopod Argyrotheca from a hydrocarbon seep deposit, middle Eocene Humptulips Formation, Washington State, USA

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.

Mineral authigenesis within chemosynthetic microbial mats: Coated grain formation and phosphogenesis at a Cretaceous hydrocarbon seep, New Zealand

AbstractMarine hydrocarbon seeps are sites of chemosynthetic microbial activity and authigenic carbonate formation. Seep limestones are typified by a range of geochemical signatures of microbial hydrocarbon oxidation, but only few seep deposits reveal mesofabrics that can be regarded as evidence of microbial activity. A Cretaceous methane‐seep limestone from Waipiro Bay, New Zealand, exhibits a fabric composed of cryptocrystalline carbonate fluorapatite between carbonate coated grains and spheroidal calcite. To understand the unusual Waipiro deposit, a paragenetic sequence has been derived for coated grains, spheroidal calcite and carbonate fluorapatite using thin section petrography, scanning electron microscopy, Fourier‐transform infrared spectroscopy and stable isotope geochemistry. The formation of 13C‐depleted coated grains (δ13C values as low as −15.8‰ Vienna‐Pee Dee Belemnite) and spheroidal calcite (δ13C values as low as −21.3‰) was favoured by hydrocarbon oxidation. Fibrous banded and botryoidal cement, a typical early diagenetic phase of hydrocarbon‐seep deposits, features δ13C values as low as −22.9‰. Coated grains and spheroidal calcite grew by displacive growth in a gel‐like medium, probably a microbial mat. Phosphorus is a mobile element and marine pore waters are typically undersaturated with respect to carbonate fluorapatite. Specific conditions are consequently required to retain sufficient concentrations to precipitate carbonate fluorapatite. Possible sources of phosphorus for the formation of the 13C‐depleted Waipiro carbonate fluorapatite (mean δ13C value of −15.4‰) include (a) the oxidation of sedimentary organic matter by organoclastic sulphate reduction, (b) the degradation of the microbial mat itself and (c) the active release of polyphosphate by sulphide‐oxidizing bacteria. This study suggests that the formation of the Cretaceous Waipiro seep limestone involved an interplay of biogeochemical processes including sulphate‐driven anaerobic oxidation of methane, organoclastic sulphate reduction and possibly nitrate‐dependent sulphide oxidation. It further demonstrates that coated grains resembling oncoids are not restricted to shallow water, photosynthesis‐dependent ecosystems.

LATE CRETACEOUS METHANE SEEPS AS HABITATS FOR NEWLY HATCHED AMMONITES

ABSTRACT Cold methane seeps were common in the Late Cretaceous Western Interior Seaway of North America. They provided a habitat for a diverse array of fauna including ammonites. Recent research has demonstrated that ammonites lived at these sites. However, it is still unknown if they hatched at the seeps or only arrived there later in ontogeny. To answer this question, we documented the abundance and size distribution of small specimens of Baculites and Hoploscaphites at eight seep sites in the Pierre Shale of South Dakota. The specimens of Hoploscaphites range from 0.8 to 8.1 mm in shell diameter, with most of them falling between 1 and 1.5 mm. The specimens of Baculites range from 0.7 to 19.2 mm in length, with most specimens falling between 6 and 8 mm. The small size and morphology of these specimens indicate that they are neanoconchs, that is, newly hatched individuals that lived for a short time after hatching. We also analyzed the isotope composition (δ13C and δ18O) of 12 small specimens of Baculites and one specimen of Hoploscaphites with excellent shell preservation from one seep deposit. The values of δ13C and δ18O range from -16.3 to -2.5‰ and -3.0 to -0.9‰, respectively. The values of δ18O translate into temperatures of 19–28°C, which are comparable to previous estimates of the temperatures of the Western Interior Seaway. The low values of δ13C suggest that the tiny animals incorporated carbon derived from anaerobic oxidation of 12C-enriched methane into their shells. Evidently, they must have lived in close proximity to seep fluids emerging at the sediment-water interface and the associated microbial food web. However, this may have contributed to their demise if they were exposed to elevated concentrations of H2S derived from the anaerobic oxidation of methane.

Mollusks from Pliocene and Pleistocene seep deposits in Leyte, Philippines

Mollusks from Pliocene and Pleistocene seep deposits in Leyte, Philippines