Terrestrial Seeps Research Articles

Carbon and hydrogen stable isotope fractionation due to monooxygenation of short-chain alkanes by butane monooxygenase of Thauera butanivorans Bu-B1211.

Multi element compound-specific stable isotope analysis (ME-CSIA) is a tool to assess (bio)chemical reactions of molecules in the environment based on their isotopic fingerprints. To that effect, ME-CSIA concepts are initially developed with laboratory model experiments to determine the isotope fractionation factors specific for distinct (bio)chemical reactions. Here, we determined for the first time the carbon and hydrogen isotope fractionation factors for the monooxygenation of the short-chain alkanes ethane, propane, and butane. As model organism we used Thauera butanivorans strain Bu-B1211 which employs a non-haem iron monooxygenase (butane monooxygenase) to activate alkanes. Monooxygenation of alkanes was associated with strong carbon and hydrogen isotope effects: εbulkC = -2.95 ± 0.5 ‰ for ethane, -2.68 ± 0.1 ‰ for propane, -1.19 ± 0.18 ‰ for butane; εbulkH = -56.3 ± 15 ‰ for ethane, -40.5 ± 2.3 ‰ for propane, -14.6 ± 3.6 ‰ for butane. This resulted in lambda (Λ ≈ εHbulk/εCbulk) values of 16.2 ± 3.7 for ethane, 13.2 ± 0.7 for propane, and 11.4 ± 2.8 for butane. The results show that ME-CSIA can be used to track the occurrence and impact of monooxygenase-dependent aerobic processes converting short-chain alkanes in natural settings like marine and terrestrial seeps, gas reservoirs, and other geological formations impacted by natural gas.

Methane in West Siberia terrestrial seeps: Origin, transport, and metabolic pathways of production.

The expansive plains of West Siberia contain globally significant carbon stocks, with Earth's most extensive peatland complex overlying the world's largest-known hydrocarbon basin. Numerous terrestrial methane seeps have recently been discovered on this landscape, located along the floodplains of the Ob and Irtysh Rivers in hotspots covering more than 2500 km2 . We articulated three hypotheses to explain the origin and migration pathways of methane within these seeps: (H1) uplift of Cretaceous-aged methane from deep petroleum reservoirs along faults and fractures, (H2) release of Oligocene-aged methane capped or trapped by degrading permafrost, and (H3) horizontal migration of Holocene-aged methane from surrounding peatlands. We tested these hypotheses using a range of geochemical tools on gas and water samples extracted from seeps, peatlands, and aquifers across the 120,000 km2 study area. Seep-gas composition, radiocarbon age, and stable isotope fingerprints favor the peatland hypothesis of seep-methane origin (H3). Organic matter in raised bogs is the primary source of seep methane, but observed variability in stable isotope composition and concentration suggest production in two divergent biogeochemical settings that support distinct metabolic pathways of methanogenesis. Comparison of these parameters in raised bogs and seeps indicates that the first is bogs, via CO2 reduction methanogenesis. The second setting is likely groundwater, where dissolved organic carbon from bogs is degraded via chemolithotrophic acetogenesis followed by acetate fermentation methanogenesis. Our findings highlight the importance of methane lateral migration in West Siberia's bog-dominated landscapes via intimate groundwater connections. The same phenomenon could occur in similar landscapes across the boreal-taiga biome, thereby making groundwater-fed rivers and springs potent methane sources.

Aerobic microbial oxidation of hydrocarbon gases: Implications for oil and gas exploration

Abstract The molecular and carbon and hydrogen isotope compositions associated with the aerobic consumption of methane (Cl), ethane (C2) and propane (C3) were determined from incubations of marine sediments collected from the seafloor surface of the gas- and oil-bearing Bohai Bay in eastern China. The fresh sediment samples were incubated aerobically at 28 °C and treated with three types of hydrocarbon gas mixtures as follows: mixture I (C1:C2:C3 = 100:0:0), II (C1:C2:C3 = 99.00:0.70:0.30) and III (C1:C2:C3 = 79.96:10.04:10.00), corresponding to incubations i, ii and iii, respectively. All the hydrocarbons are of thermogenic origin, except C1 (microbial) in incubation ii. The oxidation of C1-C3 was observed in all three treatments, with variations in carbon isotope fractionation factors (eC) of C1 and average values of −10.5 ± 0.1, −5.7 ± 1.6, and −10.9 ± 2.9, respectively. The hydrogen isotope fractionation factors (eH) of C1 in the three incubations also display variation, with average values of −118.4 ± 3.0, −68.3 ± 15.0 and −56.3 ± 8.4, respectively. Experiments with thermogenic C1 display greater carbon fractionation than those with microbial C1. The eC and eH values for C2 and C3 were obtained only in incubation iii. Compared to C1, C2 and C3 exhibit weaker carbon isotope fractionation but stronger hydrogen isotope fractionation. The fractionation factors presented in this study can be applied to estimate the extent of C1-C3 oxidation in natural environments, especially at seafloor and terrestrial seeps where aerobic microbes are commonly present. Our study has important implications in oil and gas exploration, with an alert for exercising caution when using the molecular and isotopic characteristics of hydrocarbon gases to evaluate the gas sources because these are susceptible to aerobic microbial oxidation.

An examination by GC×GC-TOFMS of organic molecules present in highly degraded oils emerging from Caribbean terrestrial seeps of Cretaceous age

For our ancestors, oil seeps were both a fascination and a resource but as the planet's reserves of high quality low density oil becomes increasingly depleted, so there is now a renewed interest in heavier, biodegraded oils such as those encountered in terrestrial seeps. One such seep is Pitch Lake in the Caribbean island of Trinidad, which is the largest natural deposit of asphalt in the world. At the northern end of the Caribbean, oil emerges along a tectonic contact on the island on Cuba. The sources of the oils from these seeps are relatively recent and both are subject to intense weathering due to the tropical conditions. When analysed by gas chromatography (GC) both oils appear as unresolved complex mixtures (UCM) and show a very high degree of biodegradation thus presenting an analytical challenge. In this case study, these two Caribbean seep oils were analysed by comprehensive two dimensional GC with time of flight mass spectrometry (GC×GC-TOFMS) to expose many thousands of the individual compounds that comprise the UCM. The high chromatographic resolution of the GC×GC-TOFMS produced good quality mass spectra allowing many compounds including molecular fossil ‘biomarkers’ to be identified. Compound classes included diamondoid hydrocarbons, demethylated hopanes and seco-hopanes, mono- and tri-aromatic steroids. D-ring aromatised structures of the 8,14-seco-hopanes, including demethylated forms were present in both oils but further demethylation, probably at position C-25 during biodegradation, was only observed in the Pitch Lake oil. Many polycyclic aromatic hydrocarbons (PAHs) were absent although the fungal-derived pentacyclic PAH perylene was present in both oils. The presence of the angiosperm biomarker lupane in the Pitch Lake oil constrained the age to the Late Cretaceous. The higher degree of biodegradation observed in the Cuban oil was likely due to relatively slow anaerobic processes whereas oil within Pitch Lake was probably subject to additional more rapid aerobic metabolism within the lake.

Evidence for archaeal methanogenesis within veins at the onshore serpentinite-hosted Chimaera seeps, Turkey

Serpentinite-hosted ecosystems are potential sites where life may first have evolved on Earth. Serpentinization reactions produce strongly reducing and highly alkaline fluids that are typified by high concentrations of molecular hydrogen (H2) and methane (CH4), which can be used as an energy source by chemosynthetic life. Low-temperature serpentinization at slow-spreading mid-ocean ridges provides an ideal environment for rich microbial communities. Similar environments have also been discovered on land, where present-day low temperature serpentinization occurs during the circulation of groundwater through exposed ophiolites, triggering the production of CH4 and H2, as well as the precipitation of secondary carbonate minerals. The rock samples analyzed here are from the Chimaera seeps in Turkey, representing serpentinized peridotites that are cross-cut by veins composed of brucite and hydromagnesite. Hydromagnesite features a mean δ13C value of −19.8‰ caused by kinetic isotope fractionation during air-groundwater exchange of CO2, followed by CO2 hydroxylation to bicarbonate within the groundwater. Geochemical modeling revealed that mixing of Mg- and Ca-rich groundwaters is required for hydromagnesite formation at the expense of brucite. Within the carbonate-hydroxide veins the lipid biomarkers pentamethylicosane (PMI) and squalane with δ13C values of +10‰ and +14‰, respectively, and unsaturated derivatives thereof were identified. Archaeol, sn2-hydroxyarchaeol, and sn3-hydroxyarchaeol are other prominent archaeal biomarkers in the veins, also revealing high δ13C values from +6 to +13‰. These isotope patterns combined with the absence of crocetane – a biomarker for methanotrophic archaea – reveal that the microbial communities of the Chimaera seeps performed methanogenesis from a CO2-limited pool rather than methanotrophy. Moreover, bacterial dialkyl glycerol diethers (DAGEs) with unusually high δ13C values (−9 to −2‰) and minor monoalkyl glycerol monoethers (MAGEs) were identified, suggesting that bacterial sulfate reduction is also active at the Chimaera site. This study reveals that archaeal methanogenesis and bacterial sulfate reduction may be prominent at onshore peridotite-hosted sites, and that biogenic CH4 may contribute to abiotic CH4 emissions from terrestrial seeps.

Minimal geological methane emissions during the Younger Dryas-Preboreal abrupt warming event.

Methane (CH4) is a powerful greenhouse gas and plays a key part in global atmospheric chemistry. Natural geological emissions (fossil methane vented naturally from marine and terrestrial seeps and mud volcanoes) are thought to contribute around 52 teragrams of methane per year to the global methane source, about 10 per cent of the total, but both bottom-up methods (measuring emissions) and top-down approaches (measuring atmospheric mole fractions and isotopes) for constraining these geological emissions have been associated with large uncertainties. Here we use ice core measurements to quantify the absolute amount of radiocarbon-containing methane (14CH4) in the past atmosphere and show that geological methane emissions were no higher than 15.4 teragrams per year (95 per cent confidence), averaged over the abrupt warming event that occurred between the Younger Dryas and Preboreal intervals, approximately 11,600 years ago. Assuming that past geological methane emissions were no lower than today, our results indicate that current estimates of today's natural geological methane emissions (about 52 teragrams per year) are too high and, by extension, that current estimates of anthropogenic fossil methane emissions are too low. Our results also improve on and confirm earlier findings that the rapid increase of about 50 per cent in mole fraction of atmospheric methane at the Younger Dryas-Preboreal event was driven by contemporaneous methane from sources such as wetlands; our findings constrain the contribution from old carbon reservoirs (marine methane hydrates, permafrost and methane trapped under ice) to 19 per cent or less (95 per cent confidence). To the extent that the characteristics of the most recent deglaciation and the Younger Dryas-Preboreal warming are comparable to those of the current anthropogenic warming, our measurements suggest that large future atmospheric releases of methane from old carbon sources are unlikely to occur.

Sourcing archaeological asphaltum (bitumen) from the California Channel Islands to submarine seeps

Asphaltum, often referred to as bitumen, is a naturally occurring form of petroleum that was used by ancient cultures for thousands of years. Asphaltum deposits are found throughout the world and occur both on land and submerged under water. Ethnohistoric accounts of native Californians suggest that asphaltum from terrestrial seeps was shaped by hand into cakes and traded throughout Southern California and was the only grade of asphaltum used to manufacture plank-canoes. While there are no terrestrial seeps on the California Channel Islands, drift asphaltum exuded from submarine seeps can frequently be found washed up on the shore. It remains unclear to what extent prehistoric island populations relied on this drift asphaltum and whether or not they acquired terrestrial asphaltum through trade. This study combines gas chromatography/mass spectrometry (GC/MS) and liquid chromatography coupled with carbon isotopic analysis in an effort to identify the sources of six archaeological bituminous mixtures from San Nicolas and San Miguel islands. We compare the archaeological asphaltum to four modern samples collected from marine tarballs and a mainland terrestrial seep. Further, we compare our results to a USGS chemometric database to determine if our archaeological samples match extant sources. Our results show that prehistoric peoples on the Channel Islands utilized drift asphaltum from submarine seeps in a variety of technological applications throughout the Holocene. The methods used in our study are globally applicable and can be used to address a variety of broad anthropological questions.