Methane Release Events Research Articles

Ebullition drives high methane emissions from a eutrophic coastal basin

The production of methane in coastal sediments and its release to the water column is intensified by anthropogenic eutrophication and bottom water hypoxia, and it is still uncertain whether methane emissions to the atmosphere will be enhanced. Here, we assess seasonal variations in methane dynamics in a eutrophic, seasonally euxinic coastal basin (Scharendijke, Lake Grevelingen). In-situ benthic chamber incubations reveal high rates of methane release from the sediment to the water column (74–163 mmol m−2 d−1) during monthly measurements between March and October 2021. Comparison of these in-situ total benthic methane fluxes and calculated diffusive fluxes point towards a major role for ebullition. In spring and fall, when the water column was oxic, microbial removal of dissolved methane occurred aerobically in the bottom water. In summer, in contrast, dissolved methane accumulated to concentrations of up to 67 μmol L−1 below the oxycline. Shifts in δ13C–CH4 and δD-CH4 towards higher values and the abundant presence of methane oxidizing bacteria point towards removal of methane around the oxycline, likely through both aerobic and anaerobic pathways, with the latter possibly linked to iron oxide reduction. Shifts in δ13C–CH4 and δD-CH4 to lower values above the oxycline indicate that bubble dissolution contributed to dissolved methane. Methane emissions to the atmosphere were observed in all seasons with the highest, in-situ measured diffusive fluxes (1.2 mmol m−2 d−1) upon the onset of water column mixing at the end of summer. Methane release events during the measurement of in-situ water-air fluxes and model calculations point towards a flux of methane to the atmosphere in the form of bubbles, which bypass the microbial methane filter. The model calculations suggest a potential year-round ebullitive methane flux between 30 and 120 mmol m−2 d−1. We conclude that methane emissions from eutrophic coastal systems may be much higher than previously thought because of ebullition.

Methane release effects on foraminiferal tests in northern South China Sea

Upward diffusing methane in gas hydrate geological systems is consumed in the sediment and water column by a series of biogeochemical reactions, which not only affect living foraminifera but also lead to early diagenetic alteration of buried foraminiferal tests. Previous studies of the impact of methane release events on fossil foraminifera focused mainly on carbon and oxygen isotopes, with little attention given to other geochemical proxies to date. Here, we examine the test wall microstructure and analyze the elemental and stable carbon-oxygen isotopic compositions of buried foraminifera from gas hydrate-bearing sediments at Site GMGS2-16 in the northern South China Sea. Our results show that diagenetic alteration of foraminiferal tests at Site GMGS2-16 is mainly linked to high-Mg calcite overgrowths. Test δ13C covaries negatively with the degree of diagenetic alteration (based on petrographic characteristics) but positively with Mg/Ca ratios. With increasing diagenetic alteration, Ba/Ca, Mn/Ca, Fe/Ca, Mo/Ca, U/Ca, V/Ca, Ni/Ca and Co/Ca also generally increase, but Sr/Ca and Cu/Ca exhibit variable changes. We infer that foraminiferal tests are highly susceptible to alteration by methane-bearing fluids, and that the elemental ratios of diagenetically altered tests are potentially reliable proxies for paleo-methane release events. At Site GMGS2-16, the overall pattern of diagenetic alteration of foraminiferal tests records frequent vertical fluctuations of the sulfate-methane transition zone (SMTZ) caused by variable fluid flux and methane release rates, with two discrete large-scale methane release events having maximum ages of ~0.47 ka and ~170 ka.

Geochemical record of methane seepage in carbon cycling and possible correlation with climate events in the Qiongdongnan basin, South China Sea

At cold seeps, sulfate-driven anaerobic oxidation of methane (SD-AOM) is the dominant biogeochemical process, which can significantly affect the marine carbon-sulfur cycle. In this paper, multiple sedimentary indicators in core QS-1 were reported for methane seepage from Haima seep of the South China Sea (SCS). By combining total carbon (TC), total organic carbon (TOC), total sulfur (TS), total inorganic carbon (TIC), as well as the carbon and oxygen isotope compositions of benthic foraminifera Uvigerina spp. In sediments, our aim was to explore the geochemical evidence on methane seepage activities in this area. We identified three methane release events (MREs) with different intensities and timescales during MIS 1–2, characterized by particular anomalies of negative δ13CTIC (as low as −41.42‰) and δ13CU.spp (as low as −5.50‰), high contents of TS and high S/C ratios, simultaneously. High S/C ratios and negative values of δ13CTIC effectively indicate the intense SD-AOM and SD-AOM-related process in core QS-1. Three MREs, occurred at high or low sea level, may due to the climate-driven increase of hydrostatic pressure and seafloor temperature. The insights for sediment carbon-sulfur trace element, carbon isotopes of TIC contents and benthic foraminifera have obvious implications for tracing past methane seepages and its possible link with climate changes.

Episodic massive release of methane during the mid-Cretaceous greenhouse

Abstract Methane-derived carbonates (MDCs) are common along modern and ancient continental margins, and the majority of such formations are associated with seafloor cold seeps. Here, we document petrographic, rare earth element + yttrium (REE + Y), carbonate clumped isotope temperature (TΔ47), and carbon-isotopic evidence from a shale succession in southern Tibet spanning a ~28 m.y. interval (ca. 113–85 Ma) that coincided with the mid-Cretaceous greenhouse event. At least nine limestone nodule-bearing horizons exhibit seep-associated sedimentary structures, including carbonate fabrics (e.g., micritic crusts, crystal fans, and botryoidal textures) and 13C-depleted isotopic compositions (δ13Ccarb < –32.3‰), which are indicative of methane-derived carbon sources. Along with sedimentary evidence, the patterns of TΔ47–δ13Ccarb–δ18Ocarb support precipitation of these MDCs over a large temperature range. The REE + Y compositions and europium (Eu) anomalies indicate that the release of methane was associated with hydrothermal fluids. Methane may have been derived from both thermogenic and biogenic sources based on the inorganic carbon isotopic signatures of the carbonate. These nodular carbonate horizons document multiple episodes of seafloor methane release during the mid-Cretaceous and represent exceptionally long-lived, active methane seepage. Massive methane release events may have played a role in generating the greenhouse climate of the mid-Cretaceous.

Impact of methane seepage dynamics on the abundance of benthic foraminifera in gas hydrate bearing sediments: New insights from the South China Sea

The stable isotopic composition of benthic foraminifera tests is widely used to trace past methane release events. However, gaps in our knowledge on the ecology of benthic foraminifera at methane seeps impede the utility of test composition as proxy of methane emission histories and the influence of redox zonation on the local fauna. Here, we use sediment geochemistry and foraminiferal assemblages of a seep area of the South China Sea to reveal that seepage duration and intensity affect benthic foraminifera. Taxa tolerating eutrophic conditions like Uvigerina thrive at times of reduced seepage when aerobic methanotrophy is favored. In contrast, intense methane seepage leads to high sulfide levels, and a positioning of the sulfate-methane transition zone close to the seafloor. At these times, bottom waters may become oxygen depleted and benthic foraminifera will decline. This study provides (1) the means to understand how seepage dynamics control the distribution of benthic foraminifera and (2) new constraints on the use of foraminifera as paleoceanographic proxies, including possible mechanisms of mass mortality in the geological past.

Elemental and isotopic response of different carbon components to anaerobic oxidation of methane: A case study of marine sediments in the Shenhu region, northern South China Sea

• Evidences show that the SMTZs are located at 300–360 and 440–480 cmbsf in core SH-CL52. • Under high methane flux and short methane-seep duration conditions, carbon isotope of TOC can indicate the location of SMTZs. • In the SMTZ, the positive excursion of TOC isotope is possibly due to the relatively high methanogenic rate. • TOC isotope has the potential to be a novel proxy of SMTZ. In the cold seep region, the carbonate rocks/nodules help to reveal the source of methane and gas-hydrate decomposition. Conventionally, authigenic carbonate rocks/nodules are great proxy of AOM, but it only forms under an intermediate methane flow rate. It is thus vital to explore another carbon component proxy of AOM. In this paper, we investigate different carbon component features from a 5.2 m-long sediment core (SH-CL52) in the Shenhu region. We constrain the location of SMTZ based on the natural remanent magnetization (NRM) characteristic, pyrite content, TS/TOC and sulfur isotope, after that we discuss carbon component’s isotopic response to AOM. Pyrite increases in 300–360 and 440–480 cmbsf, negative (NRM AF60mT - NRM AF80mT )/NRM values occurrence in 300–360 and 440–480 cmbsf, bulk-sediment δ 34 S values have positive excursion in 300–360 and 440–480 cmbsf and pyrite δ 34 S positive excursions appear in 300–360 and 440–480 cmbsf. All these reflect that at least two methane release events (MREs) are identified in 300–360 and 440–480 cmbsf in the core SH-CL52, respectively. In these zones, δ 13 C TC , δ 13 C TIC , δ 13 C FC and δ 13 C FGC variety is not discernable, but δ 13 C TOC has positive excursions which are most likely interpreted to the relatively higher methanogenesis rate in SMTZs. Our results suggest that δ 13 C TOC represents a potential proxy of SMTZ under high methane flux and shortduration condition.

Tracing the paleo-methane seepage activity over the past 20,000 years in the sediments of Qiongdongnan Basin, northwestern South China Sea

The seepage activity of paleo-methane has been proven to exist in geological history and play an important role in the evolution of the Earth's environment. This study reports grain size, major/trace elements, total nitrogen (TN), total carbon (TC), total organic carbon (TOC), total sulfur (TS), magnetic susceptibility and AMS-14C measurements for sediment samples in a drill core (~282 cm long; Site Q6 at ~1400 m water depth) from the ‘Haima seep’ sedimentary area in the Qiongdongnan Basin, along the northern slope of the South China Sea (SCS). The alternative index system for paleo-methane seepage identification was established through testing and analysis, and three methane release events (MREs) were successfully identified: 254–282 cm (MRE1: 19.3–17.8 ka), 144–162 cm (MRE2: 11. 9–10.8 ka) and 90–124 cm (MRE3: 10.0–8.5 ka). When methane seepage occurred, the enrichment of authigenic carbonate minerals (high Mg-calcite or aragonite) led to the increase in the content of CaO and ‘dilution’ in the total rare earth element content (∑REE). However, the occurrence of authigenic pyrite resulted in a significant increase in TS and the total sulfur/total organic carbon ratio (S/C) in the sediments, and a synchronous decrease in the magnetic susceptibility. Meanwhile, the change of redox environment also induced the enrichment of Mo, U, and the positive anomaly of δCe. Based on the records of methane release events, the control mechanisms for the different time scales and different periods, such as sea level change, sea floor temperature change, and turbidity current activity were preliminarily discussed. Finally, the history of methane seepage activity was reconstructed, and conceptual diagrams were drawn of the study area encompassing the last 20 ka.

Geochemical evidence of methane seepage in the sediments of the Qiongdongnan Basin, South China Sea

In this paper, the QS-1 sediments (3.64 m long; at 1500 m water depth) collected from the Qiongdongnan Basin were used to analyze the relationship between sedimentary geochemical anomalies and methane seepage. Mo-U covariation, Ce anomaly, and Mg/Ca-Sr/Ca end-member model were analyzed to identify the variations of redox environment and authigenic carbonate precipitation, finally constructing the methane release events in this study area. These observations suggest that: 1) Significant Mo/U enrichments and Ba front indicate the presence of high-flux active methane seepage with SMI of 260 cmbsf in this study area. The depositional environment evolved as oxic-anoxic-reducing-sulfide environment from bottom to top, as evidenced by Mo-U covariation and Ce anomaly; 2) Based on Sr/Ca and Mg/Ca end-member model proposed by Bayon et al. (2007), the high-Mg calcite was the dominant authigenic carbonate currently which usually precipitated behind the methane seep events; and 3) The active cold seep can be divided into three methane release events (MREs): MRE I, the most intense methane seepage occurred at 22.1 ka BP; MRE II, three low-intensity methane seepage occurred at 19.3, 16.5, and 11.3 ka BP, respectively; and MRE III, the continuous methane seep began at 9.3 ka BP until now. The synthesis of geochemical indicators has significant implications for tracing methane seepage of varying intensity and duration.

Unique Authigenic Mineral Assemblages and Planktonic Foraminifera Reveal Dynamic Cold Seepage in the Southern South China Sea

Many cold seeps and gas hydrate areas have not been discovered beside the Beikan basin in the southern South China Sea (SCS), and their characteristics and histories also remain poorly known. Here we describe authigenic minerals and the carbon and oxygen isotopic composition of planktonic foraminifera Globigerinoides ruber from sediment core 2PC, recovered from the gas hydrate zone of the Nansha Trough, southern SCS, to elucidate its history of dynamic cold seepage. We infer that the occurrence of authigenic gypsum crystals and pyrite concretions, and anomalously negative δ13C values of Globigerinoides ruber, reflect paleo-methane seepage. Two major methane release events were identified, based on remarkable excursions in foraminifera δ13C at depths of 150–250 cm and 350–370 cm. Euhedral gypsum crystals and tubular pyrite concretions co-occur with extremely negative planktonic foraminifera δ13C values, indicating a shift in the sulfate methane transition zone and a change in the methane flux. Our data suggest that authigenic mineral assemblages and δ13C values of planktonic foraminifera provide a valuable tool in elucidating the characteristics of dynamic methane seepage in a marine environment.

The effects of diagenetic processes and fluid migration on rare earth element and organic matter distribution in seep-related sediments: A case study from the South China Sea

Methane seepage is an important factor governing microbial activity in marine sediments, although its impact on the distribution of rare-earth elements (REEs) and organic matter in sediments is unclear. Here, we report isotope signatures of organic (δ13CTOC) and inorganic (δ13CTIC) carbon as well as total REEs contents of core sediments collected from a well-characterized seep area on the northern South China Sea. Previous studies have identified sediments corresponding to three methane-release events (MREs) in the studied core: MRE I at 4.0–5.5 m, MRE II at 7.4–8.2 m, and MRE III at 10.0–11.5 m depth intervals. Increased methane flux and reducing bottom waters during MRE I resulted in a higher accumulation of marine-sourced organic matter, as indicated by lower C/N ratios and higher δ13CTOC values compared with those of MREs II and III. Sediments deposited during MRE I have relatively low total REE contents, which may be related to reduced REE adsorption on Mn oxyhydroxides. However, the absence of Ce anomaly at MRE I is more likely due to the detrital input that obscures the original Ce anomaly. In the MRE III sediments, a weak negative Eu anomaly relative to that in the MRE I and II may be attributed to the reduction of Eu3+ to Eu2+ under strongly reducing conditions that caused by intense methane seepage. These observations demonstrate that fluid flow may have an important effect on the distribution of both organic matter and REEs in methane-rich marine sediments.

Using chemical compositions of sediments to constrain methane seepage dynamics: A case study from Haima cold seeps of the South China Sea

Cold seeps frequently occur at the seafloor along continental margins. The dominant biogeochemical processes at cold seeps are the combined anaerobic oxidation of methane and sulfate reduction, which can significantly impact the global carbon and sulfur cycles. The circulation of methane-rich fluids at margins is highly variable in time and space, and assessing past seepage activity requires the use of specific geochemical markers. In this study, we report multiple sedimentary proxy records for three piston gravity cores (QDN-14A, QDN-14B, and QDN-31) from the Haima seep of the South China Sea (SCS). By combining total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), total sulfur (TS), acid insoluble carbon and sulfur isotope (δ13Corganic carbon and δ34Sacid-insoluble), and δ34S values of chromium reducibility sulfur (δ34SCRS), as well as carbon isotopes of TIC (δ13CTIC) in sediments, our aim was to provide constraints on methane seepage dynamics in this area. We identified three sediment layers at about 260–300 cm, 380–420 cm and 480–520 cm sediment depth, characterized by particular anomalies of low δ13CTIC values and high TS content, high TS and CRS contents, and high δ34Sacid-insoluble and δ34SCRS values, respectively. On this basis, we propose that these sediment horizons correspond to distinct methane release events preserved in the sediment record. While the exact mechanisms accounting for the presence (or absence) of these particular geochemical signals in the sediment are not known, we propose that they correspond to variations in methane flux and their duration through time. Overall, our results suggest that sedimentary carbon and sulfur and their isotopes are useful tracers for better understanding of methane seepage dynamics over time.

Paleo-cold seep activity in the southern South China Sea: Evidence from the geochemical and geophysical records of sediments

Over the past decade more than 30 cold seep sites have been reported along the continental slope of the South China Sea (SCS). These seep sites have all been located in the northern SCS, and no methane seepage has been discovered in the southern SCS. Here, we report on the analysis results of the geochemical and geophysical of the marine core sediments (58S) collected from the southern continental slope of the SCS. We suggest that the sedimentary total sulfur (TS), total inorganic carbon (TIC), δ34SCRS values of chromium reducible sulfur (CRS) and carbonate δ13CTIC values of the bulk sediments, coupled with the ratio of (NRMAF60mT-NRMAF80mT)/NRM can be used to trace the methane release events in a methane-rich environment. At least two methane release events were identified at depths of 5.5–6.2 m and 6.8–7.2 m. Moreover, according to the characteristic of the TS/TOC and (NRMAF60mT-NRMAF80mT)/NRM ratio of sediments, we suggest the methane flux has changed episodically. This inference is supported by the occurrence of the strongly negative carbonate δ13CTIC values of bulk sediments and the positive δ34SCRS values, which indicate the occurrence of anaerobic oxidation of methane (AOM) and the location of a past or present sulfate methane transition zone (SMTZ), respectively. Ultimately, these new data suggest that the combined geochemical and geophysical study of marine sediments provides a valuable tool with which to infer the temporal variations in methane seepage in a marine environment.

Using sediment geochemistry to infer temporal variation of methane flux at a cold seep in the South China Sea

Release of methane from the seafloor throughout the world's oceans and the biogeochemical processes involved may have significant effects on the marine sedimentary environment. Identification of such methane release events in marine sediment records can hence provide a window into the magnitude of ancient seeps. Here, we report on analysis of the geochemical composition of samples in a 12.3 m long sediment core (DH-5) collected from a seep site in the South China Sea (SCS). Our aim has been to investigate whether the evidence for the presence of methane release event within sediments is discernible from solid-phase sediment geochemistry. We show that sedimentary total sulfur (TS), δ34S values of chromium reducible sulfur (δ34SCRS) along with total organic carbon (TOC) and total inorganic carbon (TIC) content can be used to infer the presence of methane release events in cold seep settings. At least three methane release events were identified in the studied core (Unit I at 400–550 cm, Unit II at 740–820 cm, and Unit III at 1000–1150 cm). According to the characteristic of redox-sensitive elements (eg., Mo, U and Mn), we suggest that methane flux has been changed from relatively high (Unit I) to low (Unit II and III) rates. This inference is supported by the coupled occurrence of 34S-enriched sulfides in Unit II and III. AMS 14C dates from planktonic foraminifera in Unit I suggest that high methane flux event occurred at ∼15.4–24.8 kyr BP, which probably resulted in locally-focused aerobic methane oxidation. Overall, our results suggest that TS, TOC, TIC and δ34SCRS have potential for identifying present and fossil methane release events in marine sediments.

Gas hydrate destabilization and methane release events in the Krishna–Godavari Basin, Bay of Bengal

Methane release events have been linked to global warming, alteration of the carbon cycle and influence on biota. However, unequivocal evidence of paleomethane release events are limited. We report several negative carbon stable isotope excursions in planktic and benthic foraminifera in a core (MD161-8) from the Krishna-Godavari (K–G) Basin, Bay of Bengal. The most negative δ13C spikes are recorded during the marine isotope stages MIS-4 and at the transition of MIS-5 to 4. Occurrence of highly 13C depleted (average δ13C = −48 ± 2.4‰ VPDB) authigenic high magnesian calcite are also reported within this time window from the core MD161-8. In the present work an unequivocal explanation for the observed 13C depletion in the marine planktic and benthic foraminifera is difficult to achieve solely from the optical/electron microscopy or C–O stable isotope ratio analyses due to possible influence of diagenetic alteration. We attribute the observed episodic methane expulsion events, as inferred from the negative δ13C excursions and earlier reports on the occurrence chemosynthetic bivalves and Mo concentration anomaly to the destabilization of the base of gas hydrate stability zone (BGHSZ). Sea level drop and shale tectonics induced focused fluid flow are the two possible causes of hydrate destabilization discussed here. Shale tectonics were possibly responsible for creating fault systems which acted as the conduit for gas flow through the sediment column and subsequent seepage. Shale and salt tectonics in the passive continental margins being a globally observed phenomenon, its role as an important driving force for enhanced methane emission needs detailed investigation to understand the climatic perturbations through geologic time. Additional evidence of methane emission from site MD161-15 further supports the link between shale tectonics and methane emission.

Past methane release events and environmental conditions at the upper continental slope of the South China Sea: constraints by seep carbonates

Authigenic carbonates and seep biota are archives of seepage history and record paleo-environmental conditions at seep sites. We obtained the timing of past methane release events at the northeastern slope of the South China Sea based on U/Th dating of seep carbonates and seep bivalve fragments from three sites located at 22°02′–22°09′N, 118°43′–118°52′E (water depths from 473 to 785 m). Also, we were able to reconstruct the paleo-bottom water temperatures by calculating the equilibrium temperature using the ages, the corresponding past δ18O of seawater (δ18Osw) and the δ18O of the selected samples formed in contact with bottom seawater with negligible deep fluid influence. A criterion consists of mineralogy, redox-sensitive trace elements and U/Th-isotope systematics is proposed to identify whether the samples were formed from pore water or have been influenced by deep fluid. Our results show that all methane release events occurred between 11.5 ± 0.2 and 144.5 ± 12.7 ka, when sea level was about 62–104 m lower than today. Enhanced methane release during low sea-level stands seems to be modulated by reduced hydrostatic pressure, increased incision of canyons and increased sediment loads. The calculated past bottom water temperature at one site (Site 3; water depth: 767–771 m) during low sea-level stands 11.5 and 65 ka ago ranges from 3.3 to 4.0 °C, i.e., 1.3 to 2.2 °C colder than at present. The reliability of δ18O of seep carbonates and bivalve shells as a proxy for bottom water temperatures is critically assessed in light of 18O-enriched fluids that might be emitted from gas hydrate and/or clay dehydration. Our approach provides for the first time an independent estimate of past bottom water temperatures of the upper continental slope of the South China Sea.

Origin and nature of cold seep in northeastern Dongsha area, South China Sea: Evidence from chimney-like seep carbonates

The occurrence of seep carbonates is one of the characteristic features for cold seep sites at continental margins. The carbonates documented the venting history of methane-rich fluid. Compared to the chemoherm carbonates and carbonate pavements which formed on the sediment-water interface, chimney-like seep carbonates precipitated around fluid conduits below the sediment-water interface therefore better recording information of the past fluid flow and composition. Here the chimney-like seep carbonate samples from the northeastern Dongsha area of the South China Sea were studied to understand the origin and nature of the venting fluids and their potential relationship with gas hydrate deposits underneath the seafloor. Based on the occurrence, morphology, petrology, mineralogy and C- and O- isotope compositions, combined with present and past bottom water temperatures and the timing of methane release events, the oxygen isotopic fractionation between calcite and water were used to estimate the equilibrium δ 18O values of the precipitating fluids. The δ 13C values ranging from −56.33‰ to −42.70‰ V-PDB and thus clearly show that the studied chimneys were mainly derived from biogenic methane oxidation. The calculated equilibrium δ 18O values of the precipitating fluids ranged from 1.9‰ ± 0.3‰ to 0.6‰ ± 0.3‰ V-SMOW, with an average of 1.4‰ ± 0.3‰ V-SMOW which is heavier than those of seawater even at the last glacial maximum. It is considered that the formation of chimney-like carbonates was closely related to methane hydrate dissociation in the area. The methane hydrates contributed as much as 45.7% of water to the venting fluids. It is suggested that the climate and environmental changes (e.g. sea-level lowering, down-cutting canyons and mass wasting) are the major mechanisms maybe responsible for the destabilization of methane hydrates in the study area. The extensive occurrence of seep carbonates indicates that a large amount of the methane released from methane hydrate dissociation has been effectively captured and sequestered by microbial anaerobic oxidation of methane (AOM) before it escapes into the water column.

Carbon and oxygen isotope characteristics of foraminiferan from northern South China Sea sediments and their significance to late Quaternary hydrate decomposition

Carbon and oxygen isotope and dating analyses of foraminiferan in sediment cores collected from three different areas of the northern slope of the South China Sea were conducted, in order to examine the records of the gas hydrate decomposition events since the late Quaternary under the conditions of methane seepage. The results show that: 1) the δ 13C values of the benthic foraminiferan Uvigerina spp. (size range of 0.25–0.35 mm) are from −0.212% to −0.021% and the δ 18O values of the planktonic foraminiferan Globigerinoides ruber (size range of 0.25–0.35 mm) are from −0.311% to −0.060%; 2) three cores (ZD2, ZD3 and ZS5) from the bottom of a hole are aged for 11 814, 26 616 and 64 090 a corresponding to the early oxygen isotope stage (MIS) I, III and IV final period, respectively; 3) a negative-skewed layer of carbon isotope corresponds to that of MIS II (cold period), whose degree of negative bias is −0.2‰; and 4) the δ 13C compositions of foraminiferans are similar to those of the Blake Ridge and the Gulf of Mexico sediments of the late Quaternary. According to the analysis, the reasons for these results are that the studied area is a typical area of methane seep environment in the area during MIS II due to the global sea-level fall and sea pressure decrease. Gas hydrate is decomposed and released, and a large number of light carbon isotopes of methane are released into the ocean, dissolved to inorganic carbon (DIC) pool and recorded in the foraminiferan shells. A pyrite layer developed in the negative bias layers of the foraminiferans confirms that the δ 13C of foraminiferans is more affected by methane and less by the reduction of marine productivity and early diagenesis. The use of foraminiferan δ 13C could accurately determine late Quaternary hydrate release events and provide evidence for both reconstructing the geological history of methane release events and exploring natural gas hydrate.

Episodic methane release events from Last Glacial marginal sediments in the western North Pacific

According to recent observations of anomalous bottom‐simulating reflections (BSR), the northwest Pacific marginal sediments around Japan main islands bear large abundances of methane hydrate [Satoh, 2002]. During the Last Glacial, direct and indirect evidence accumulated from geochemical data suggests that methane episodically released from hydrate trapped in the seafloor sediments [Dickens et al., 1995; Hinrichs et al., 2003; Kennett et al., 2000]. Here we show that marginal sediments from the western North Pacific contain a hopanoid 17α(H), 21β(H)‐hop‐22(29)‐ene (diploptene) derived from the activity of methanotrophic bacteria in water column and/or surface sediment during a warming period (Interstadial 3) in the Last Glacial. The carbon isotopic compositions of diploptene range between −41.0‰ and −27.9‰ (relative to PDB). In the horizon indicative of a contribution of methanotrophic bacteria, foraminiferal isotope signals were also found with highly depleted 13C compositions of planktonic foraminifera (∼−1.9‰, PDB) and benthic foraminifera (∼−0.8‰, PDB), suggesting indirect records of enhanced incorporation of 13C‐depleted CO2 formed by methanotrophic process that use 12C‐enriched methane as their main source of carbon. From combined isotopic data of molecular (diploptene) and foraminifera, the most prominent signal of methane release was detected in the sediments deposited around 25.4 cal. kyr BP (∼100 year time span), corresponding to the Interstadial 3. This is the first evidence of methane hydrate instability in the open western North Pacific during the Last Glacial. Considering the glacial‐interglacial hydrographic conditions in this region, the instability of methane hydrate may be modulated by intermediate water warming and/or the lowering of sea level. Our results suggest that the western North Pacific marginal regions may be a profound effect on rapid global warming climate changes during the Last Glacial.

Methane Partitioning and Transport in Hydrated Carbon Nanotubes

The well defined shape and size of carbon nanotubes (CNTs) makes them attractive candidates for theoretical and experimental studies of various nanoscopic phenomena such as protection and confinement of molecular species as well as transport of molecules through their interior pores. Here we investigate solute partitioning and transport using molecular dynamics simulations of CNTs in mixtures of hydrophobic solutes and water. The hydrophobic pores of CNTs provide a favorable environment for partitioning of hydrophobic solutes. We find that the transfer of a methane molecule from aqueous solution into the CNT interior is favored by about 16 kJ/mol of free energy. In 50 molecular dynamics simulations, we observe that methane molecules replace water molecules initially inside the nanotubes, and completely fill their interior channels over a nanosecond time scale. Once filled with methane molecules, the nanotubes are able to transport methane from one end to the other through successive methane uptake and release events at the tube ends. We estimate a net rate of transport of about 11 methane molecules per nanotube and nanosecond for a 1 mol/L methane concentration gradient. This concentration-corrected rate of methane transport even exceeds that of water through nanotubes (∼1 per nanosecond at a 1 mol/L osmotic gradient). These results have implications for the design of molecule-selective CNT devices that may act through mechanisms similar to those of biological transmembrane channels.