南海神狐天然气水合物系统沉积物中自生黄铁矿的特征研究

南海神狐海域是中国最重要的天然气水合物调查研究区之一，为了解水合物存在对沉积物地球化学环境的影响，对采自神狐海域W19B井位的沉积物样品进行了矿物学和地球化学研究。X射线衍射分析和主量元素结果显示部分层位有异常高含量的硫化物（主要为黄铁矿）。扫描电镜结果表明随着样品深度的增加，黄铁矿的晶面、晶棱更加明显，且集合体形态呈现聚莓→单莓→细粒的变化趋势，扫描电镜还观察到草莓状黄铁矿向细粒自形黄铁矿转化的中间产物。在53.0 mbsf（meters below seafloor）和140.4 mbsf层位均发现异常高含量的黄铁矿。其中140.4 mbsf层位黄铁矿充填有孔虫壳体的现象普遍，并伴有大量柱状黄铁矿产出，可能与有机质和甲烷厌氧氧化相关，但主导作用应为甲烷厌氧氧化，该层位可能位于古硫酸根-甲烷界面（sulfate-methane interface，SMI）附近。根据所得结果，推测地质历史时期中甲烷异常渗漏事件的发生，致使向上的甲烷通量增加，推动SMI上移，导致53.0 mbsf和140.4 mbsf界面处因甲烷厌氧氧化而形成大量黄铁矿。多个黄铁矿富集层的存在可能表示沉积史中曾发生多期次的深部流体渗漏或者天然气水合物的分解活动。     

Dissolved inorganic carbon (DIC) and its carbon isotopic composition in sediment pore waters from the Shenhu area,northern South China Sea

The Shenhu area is one of the most favorable places for the occurrence of gas hydrates in the northern continental slope of the South China Sea. Pore water samples were collected in two piston cores (SH-A and SH-B) from this area, and the concentrations of sulfate and dissolved inorganic carbon (DIC) and its carbon isotopic composition were measured. The data revealed large DIC variations and very negative δ ¹³C-DIC values. Two reaction zones, 0–3 mbsf and below 3 mbsf, are identified in the sediment system. At site SH-A, the upper zone (0–3 mbsf) shows relatively constant sulfate and DIC concentrations and δ ¹³C-DIC values, possibly due to bioturbation and fluid advection. The lower zone (below 3 mbsf) displays good linear gradients for sulfate and DIC concentrations, and δ ¹³C-DIC values. At site SH-B, both zones show linear gradients, but the decreasing gradients for δ ¹³C-DIC and SO₄ ²⁻ in the lower zone below 3 mbsf are greater than those from the upper zone, 0–3 mbsf. The calculated sulfate-methane interface (SMI) depths of the two cores are 10.0 m and 11.1 m, respectively. The depth profiles of both DIC and δ ¹³C-DIC showed similar characteristics as those in other gas hydrate locations in the world oceans, such as the Blake Ridge. Overall, our results indicate an anaerobic methane oxidation (AMO) process in the sediments with large methane flux from depth in the studied area, which might be linked to the formation of gas hydrates in this area.     

Microscopic analysis of pyrite in the sediments of two Brazilian mangrove ecosystems

This study has used scanning electron microscopy coupled with X-ray micro-analysis to compare the morphology of diagenetic pyrite formed in the sediments of two contrasting Brazilian mangroves. The Sepetiba Bay site is influenced by marine conditions, and pore waters show lower dissolved iron and higher dissolved total sulfide concentrations. The Paraíba do Sul River site, on the other hand, is influenced by fluvial waters, and shows higher dissolved iron and lower dissolved sulfide concentrations in pore waters. At both sites, pore waters were oversaturated for pyrite, and pyrite morphology was similar. The finding that pyrite morphology was similar despite marked differences in pore-water chemistry can be explained by the low solubility product of pyrite and the daily exposure of surficial sediments to air, conditioned by tidal cycles. This implies that oxygen input to the sedimentary environment is a key factor in the pyrite formation mechanism.     

Molecular and isotopic signatures in sediments and gas hydrate of the central/southwestern Ulleung Basin: high alkalinity escape fuelled by biogenically sourced methane

Natural marine gas hydrate was discovered in Korean territorial waters during a 2007 KIGAM cruise to the central/southwestern Ulleung Basin, East Sea. The first data on the geochemical characterization of hydrate-bound water and gas are presented here for cold seep site 07GHP-10 in the central basin sector, together with analogous data for four sites (07GHP-01, 07GHP-02, 07GHP-03, and 07GHP-14) where no hydrates were detected in other cores from the central/southwestern sectors. Hydrate-bound water displayed very low concentrations of major ions (Cl⁻, SO₄²⁻, Na⁺, Mg²⁺, K⁺, and Ca²⁺), and more positive δD (15.5‰) and δ¹⁸O (2.3‰) signatures compared to seawater. Cl^– freshening and more positive isotopic values were also observed in the pore water at gas hydrate site 07GHP-10. The inferred sulfate–methane interface (SMI) was very shallow (<5 mbsf) at least at four sites, suggesting the widespread occurrence of anaerobic oxidation of methane (AOM) at shallow sediment depths, and possibly high methane flux. Around the SMI, pore water alkalinity was very high (>40 mM), but the carbon isotopic ratios of dissolved inorganic carbon (δ¹³C_DIC) did not show minimum values typical of AOM. Moreover, macroscopic authigenic carbonates were not observed at any of the core sites. This can plausibly be explained by carbon with high δ¹³C values diffusing upward from below the SMI, increasing alkalinity via deep methanogenesis and eventually escaping as alkalinity into the water column, with minor precipitation as solid phase. This contrasts, but is not inconsistent with recent reports of methane-fuelled carbonate formation at other sites in the southwestern basin sector. Methane was the main hydrocarbon component (>99.85%) of headspace, void, and hydrate-bound gases, C₁/C₂₊ ratios were at least 1,000, and δ¹³C_CH4 and δD_CH4 values were in the typical range of methane generated by microbial reduction of CO₂. This is supported by the δ¹³C_C2H6 signatures of void and hydrate-bound gases, and helps clarify some contradictory interpretations existing for the Ulleung Basin as a whole. In combination, these findings suggest that deep biogenic gas and pore waters migrate upward through pathways such as hydrofractures, and measurably influence the shallow carbon cycle. As a result, cation-adjusted alkalinity/removed sulfate diagrams cannot always serve to estimate the degree of alkalinity produced by sulfate reduction and AOM in high methane flux areas.     

Reactive iron and its buffering capacity towards dissolved sulfide in sediments of Jiaozhou Bay,China

Reactive iron (Fe) oxides in marine sediments play a critical role in removal of free sulfide. In this study, 0.5 and 6 N HCl-extractable Fe, acid volatile sulfide (AVS), and pyrite were examined in sediments at three sites of eutrophic Jiaozhou Bay to investigate the interactions of sulfur and Fe and possible influences of eutrophication on free sulfide removal. The results indicate that formation and accumulation of AVS and pyrite are limited by low availability of labile organic matter, despite eutrophication of the bay water. Quick buffering of free sulfide proceeded mainly via consumption of 0.5 N HCl-extractable Fe (labile Fe), however, the consumption did not result in a depletion of the Fe pool. High residual buffering capacity enables a quick removal of free sulfide in porewater, and thereby it is difficult for sulfide to accumulate and to cause detrimental effects on benthic organisms at the present steady state. Significant effects of eutrophication on Fe and sulfur geochemistry is restricted only to the estuarine sediments which were subject to direct wastewater discharges, whereas no such effects were observed in other sediments of the bay.     

珠江口淇澳岛海岸带沉积物中硫酸盐还原和不同形态硫的分布

对珠江口淇澳岛海岸带3个站位(QA-11,QA-9和QA-14)的沉积物中不同形态的还原硫(酸可挥发性硫,黄铁矿和有机硫)、总有机碳(TOC)和孔隙水中SO42-,甲烷浓度进行了测定,并且利用稳态扩散模型计算其中2个站位(QA-9和QA-14)硫酸盐还原通量[1.74和1.14 mmol/(m2.d)]和甲烷厌氧氧化通量[0.34和0.29 mmol/(m2.d)]。研究结果表明由于潮间带沉积物受到SO42-供给的限制,因此位于潮间带的QA-11站位硫酸盐还原带较浅(约16 cm);在潮下带的QA-9和QA-14站位,随离海岸距离和水深的增加,硫酸盐还原通量呈现减小的趋势,并且硫酸还原逐渐受到可利用活性有机质的限制;甲烷厌氧氧化对硫酸盐还原的贡献表现出增加的趋势,由19.2%增加至25.5%。三个站位沉积物中按不同形态还原硫含量由大到小列出,它们是有机硫(OS)、黄铁矿(DS)、酸可挥发性硫(AVS)。沉积物中AVS的空间分布与硫酸盐还原通量有正相关性。QA-11和QA-14站位的黄铁矿与AVS硫的含量比值大于3,分别为7.9和3.6,表明两个站位的黄铁矿形成可能受硫酸盐还原作用的控制;QA-9站位黄铁矿与AVS硫的含量比值为2.2,暗示AVS向黄铁矿转化受到可利用活性铁的限制。     

Numerical modeling study of mineralization induced by methane cold seep at the sea bottom

In order to investigate the response of authigenic minerals to gas hydrate geo-systems, the biogeochemical processes and its induced mineralization were predicted by employing the comprehensive reactive transport modeling approach. Based on the available data extracted from the northern continental slope area of the South China Sea, a 1-D vertical column model was developed. Three cases with different upward methane flux rates and three cases with different mineral compositions, i.e., a total of six cases were designed to investigate the effects of variations in the depth of sulfate methane transition zone (SMTZ) and in the mineral composition on the formation of authigenic minerals. The simulation results indicate that the SMTZ depth influenced by both the upward methane flux rate and the initial composition played an important role in the formation of authigenic minerals. The AOM reaction is intensive at the interface, and the precipitation amount of calcite is large, which is mainly controlled by AOM. When the methane leakage rate is 20 times higher than the base case, aragonite starts to precipitate. During the simulation, oligoclase, k-feldspar, smectite-Na, smectite-Ca, chlorite dissolved. Our study specific to this area as a starting point may provide a quantitative approach for investigating carbonate and pyrite formation in hydrate-bearing sediments accounting for methane oxidation and sulfate reduction. The method presented here and the model built in this study can be used for other sites with similar conditions. In addition, this study may serve as an indication for the potential natural gas hydrate reservoir in depth, and is also significant for marine carbon and sulfur cycle.     

南海北部琼东南盆地冷泉区表层沉积物微量元素的分析及其对甲烷渗漏的指示

Recent studies have shown that specific geochemical characteristics of sediments can be used to reconstruct past methane seepage events. In this work, the correlation between the Sr/Ca and Mg/Ca ratios of sediment samples is analyzed and the sulfate concentration profile in Site C14 from cold-seep sediments in the Qiongdongnan Basin in northern South China Sea is obtained. The results confirmed that, sulfate at 0–247 cm below sea floor(Unit I) is mainly consumed by organic matter sulfate reduction(OSR), while sulfate at 247–655 cm(Unit II) is consumed by both the OSR and the anaerobic oxidation of methane(AOM). In addition, the bottom sediment layer is affected by weak methane seepage. The Mo and U enrichment factors also exhibit similar trends in their respective depth profiles. The responses of trace elements, including Co/Al, Ni/Al, Cr/Al and Zn/Al ratios to methane seepage allowed the study of depositional conditions and methane seepage events. Based on the results, it is speculated that the depositional conditions of Unit II changed with depth from moderate conditions of sulfidic and oxic conditions to locally anoxic conditions, and finally to suboxic conditions due to methane fluid leakage. The stable isotope values of chromium-reducible sulfide produced by AOM and those of sulfide formed by OSR in the early diagenetic environment suffered serious depletion of 34 S. This was probably due to weak methane leakage, which caused the slower upward diffusion and the effect of early diagenesis on the samples. It is necessary to consider the effects of depositional environments and diagenesis on these geochemical parameters.     

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 (δ³⁴S_CRS) 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 δ³⁴S_CRS have potential for identifying present and fossil methane release events in marine sediments.     

Core lithologies and their constraints on gas-hydrate occurrence in the East Sea, offshore Korea: Results from the site UBGH1-9

Drilling at the site UBGH1-9, offshore Korea in 2007, revealed varied gas-hydrate saturation with depth and a wide variety of core litholgies, demonstrating how the variations in the lithology are linked with those in gas-hydrate saturation and morphology. Discrete excursions to low chlorinity values from in situ background chlorinity level occur between 63 and 151 mbsf. In this occurrence zone, gas-hydrate saturations estimated from the low chlorinity anomalies range up to 63.5% of pore volume with an average of 9.9% and do not show a clear depth-dependent trend. Sedimentary facies analysis based on grain-size distribution and sedimentary structures revealed nine sediment facies which mainly represent hemipelagic muds and fine- to medium-grained turbidites. According to the sedimentary facies distribution, the core sediments are divided into three facies associations (FA): FA I (0–98 mbsf) consisting mainly of alternating thin- to medium-bedded hemipelagic mud and turbidite sand or mud beds, FA II (98–126 mbsf) dominated by medium- to very thick-bedded turbidite sand or sandy debris flow beds, and FA III (126–178 mbsf) characterized by thick hemipelagic mud without intervening discrete turbidite sand layers. Thermal anomalies from IR scan, mousse-like and soupy structures on split-core surfaces, non-destructive measurements of pressure cores, and comparison of gas-hydrate saturations with sand contents of corresponding pore-water squeeze cakes, collectively suggest that the gas hydrate at the site UBGH1-9 generally occurs in two different types: “pore-filling” type preferentially associated with thin- to medium-turbidite sand beds in the FA I and “fracture-filling” type which occurs as hydrate veins or nodules in hemipelagic mud of the FA III. Gas-hydrate saturation in the FA II is generally anomalously low despite the dominance of turbidite sand or sandy debris flow beds, suggesting insufficient methane supply.     

南海神狐海域沉积物自生黄铁矿和石膏S同位素特征及对甲烷渗漏强度的示踪

The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments.Anaerobic oxidation of methane(AOM) coupled with bacterial sulfate reduction results in the precipitation of solid phase minerals in seepage sediment,including pyrite and gypsum.Abundant aggregates of pyrites and gypsums are observed between the depth of 667 and 850 cm below the seafloor(cmbsf) in the entire core sediment of HS328 from the northern South China Sea.Most pyrites are tubes consisting of framboidal cores and outer crusts.Gypsum aggregates occur as rosettes and spheroids consisting of plates.Some of them grow over pyrite,indicating that gypsum precipitation postdates pyrite formation.The sulfur isotopic values(δ~(34) S) of pyrite vary greatly(from –46.6‰ to –12.3‰ V-CDT) and increase with depth.Thus,the pyrite in the shallow sediments resulted from organoclastic sulfate reduction(OSR) and is influenced by AOM with depth.The relative high abundance and δ~(34) S values of pyrite in sediments at depths from 580 to 810 cmbsf indicate that this interval is the location of a paleo-sulfate methane transition zone(SMTZ).The sulfur isotopic composition of gypsum(from–25‰ to –20.7‰) is much lower than that of the seawater sulfate,indicating the existence of a 34 S-depletion source of sulfur species that most likely are products of the oxidation of pyrites formed in OSR.Pyrite oxidation is controlled by ambient electron acceptors such as MnO_2,iron(Ⅲ) and oxygen driven by the SMTZ location shift to great depths.The δ~(34) S values of gypsum at greater depth are lower than those of the associated pyrite,revealing downward diffusion of 34 S-depleted sulfate from the mixture of oxidation of pyrite derived by OSR and the seawater sulfate.These sulfates also lead to an increase of calcium ions from the dissolution of calcium carbonate mineral,which will be favor to the formation of gypsum.Overall,the mineralogy and sulfur isotopic composition of the pyrite and gypsum suggest variable redox conditions caused by reduced seepage intensities,and the pyrite and gypsum can be a recorder of the intensity evolution of methane seepage.     

Effects of natural organic matter from sediments on the growth of marine gas hydrates

Sediments recovered from 0 to 27 + meters below the seafloor (mbsf) of a gas-hydrate and gas-venting active area in the Gulf of Mexico were added to a hydrate growth test cell to determine the influence of the organic and inorganic sedimentary components on hydrate induction times and formation rates. Induction times were sixteen times shorter in the presence of sediment from approximately 18 mbsf (relative to sediment from 1 mbsf), and remained stable in the presence of sediment from 18 to 27 mbsf. Formation rates increased by a factor of 2.5 in the presence of sediments from approximately 18 mbsf and decreased somewhat in the presence of sediment from 18 to 27 mbsf. Selected samples (surface, 18 and 27 mbsf) were density fractionated and subjected to bulk elemental and X-ray photoelectron spectroscopy (XPS) analysis. XPS revealed the presence of iron in various chemical environments at depths of 18 and 27 mbsf. High Resolution Magic Angle Spinning Nuclear Magnetic Resonance (HR-MAS NMR) was used to characterize the organic component of sediments from selected depths. The discovery of intact proteinaceous material in the surface sediment was surprising due to the labile nature of these biopolymers, and potentially reflects microbial activity in these surface layers. This material was less abundant in sediment from increasing depths, where more lipid-like compounds were prominent. The results suggest that hydrate growth is inhibited by the presence of proteinaceous material but enhanced by lipid-like compounds associated with iron-bearing mineral surfaces.     

Numerical modeling of gas hydrate emplacements in oceanic sediments

We have implemented a 2-dimensional numerical model for simulating gas hydrate and free gas accumulation in marine sediments. The starting equations are those of the conservation of the transport of momentum, energy, and mass, as well as those of the thermodynamics of methane hydrate stability and methane solubility in the pore-fluid. These constitutive equations are then integrated into a finite element in space, finite-difference in time scheme. We are then able to examine the formation and distribution of methane hydrate and free gas in a simple geologic framework, with respect to the geothermal heat flow, fluid flow, the methane in-situ production and basal flux. Three simulations are performed, leading to the build up of hydrate emplacements largely linear through time. Models act primarily as free gas accumulators and are relatively inefficient with respect to hydrate emplacements: 26–33% of formed methane are converted to hydrate. Seepage of methane across the sea-floor is negligible for fluid flow below 2. 10⁻¹¹ kg/m²/s. At 5.625 10⁻¹¹ kg/m²/s however, 9.7% of the formed methane seeps out of the model. Moreover, along strike variation arising in the 2-dimensional model are outlined. In the absence of focused flow, the thermodynamics of hydrate accumulation are primarily one-dimensional. However, changes in free methane compressibility (density) and methane solubility (the intrinsic dissolved methane flux) subtlety impact on the formation of a free gas zone and the distribution of the hydrate emplacements in our 2-dimensional simulations.     

Reduced magnetization produced by increased methane flux at a gas hydrate vent

Magnetic susceptibility measurements on near-surface sediment cores from the North Cascadia accretionary sedimentary prism show that seismic blanking or wipe-out zones in the upper few hundred metres of sediments are associated with a prominent low magnetic susceptibility signature. Seismic blanking and low magnetization are both attributed to high upward methane flux within a vent zone, as evidenced by the presence of massive gas hydrate within the cores. Sedimentological analysis of these cores also reveals the presence of authigenic pyrite within the areas of magnetic susceptibility lows. This phenomenon is suspected to be produced by the reducing environment associated with the high upward methane flux and increased bacterial activity within the topmost sediments, resulting in diagenesis of highly magnetic detrital minerals such as magnetite into nearly non-magnetic pyrite. These low magnetic susceptibility zones may produce magnetic anomalies with a magnitude of 10–35 nT near the seafloor. Such anomalies might be detected using high-resolution near-bottom magnetometers to provide a means of mapping zones of methane venting.     

Pyrite formation under conditions approximating those in anoxic sediments: II. Influence of precursor iron minerals and organic matter

In the paper (Wang and Morse, 1996) that preceded this study, we presented results of experiments performed using a silica gel crystal growth technique to produce pyrite under conditions approximating those commonly occurring in anoxic marine sediments. The primary focus of that study was on the chemical pathways that pyrite formation follows and how differing conditions influenced reaction kinetics and morphology of pyrite crystals. In this paper, we present results of further long-term (up to 1 y) studies of pyrite formation, using the silica gel experimental technique, in which we investigated the role that different precursor iron (hydr)oxide minerals and marine organic matter play in pyrite formation. The minerals studied were akaganeite (β-FeOOH), ferrihydrite (Fe₅HO₈ · 4H₂O), goethite (α-FeOOH), hematite (α-Fe₂O₃), lepidocrocite (γ-FeOOH), and magnetite (Fe₃O₄). Marine organic matter used in this study was freeze-dried plankton collected from near-surface water in the Gulf of Mexico. The influence of precursor iron (hydr)oxide mineralogy, although important for initial iron sulfidization rates, was relatively minor compared to other variables, such as solution pH and sulfide concentration, in controlling the rate of pyrite formation. Consequently, major variations in the observed rate of pyritization of different iron (hydr)oxide minerals in sediments (e.g., Canfield and Berner, 1987) may reflect large differences in surface areas of the minerals rather than their intrinsic reactivity and is a confirmation of the estimates of Canfield et al. (1992) that most iron oxides have similar reactivity. The presence of marine organic matter (freeze-dried plankton) caused an increase in the sulfidization rate of goethite and a major (about 20 ×) decrease in the rate of pyrite formation. This can be interpreted as indicating that organic matter-iron interactions are important in both iron (hydr)oxide dissolution, and pyrite nucleation and growth. A possible explanation for this behavior is that dissolved organic matter produced during the long experiments (up to 1 year) increased the rate of goethite dissolution while inhibiting pyrite nucleation and growth by complexing iron. The lessons learned in the study of other mineral reaction kinetics (e.g., calcite and aragonite), that rates determined in pure inorganic systems, may not always be reliably applied to natural systems where organic matter can significantly influence mineral dissolution and growth rates, are, alas, repeated here for pyrite.     

Reactive iron in Black Sea Sediments: implications for iron cycling

The distribution of reactive iron in sediments of the northwestern shelf, the shelf edge and the abyssal part of the Black Sea has been studied. In the euxinic Black Sea, iron sulfides (pyrite and iron monosulfide) are formed in the upper part of the anoxic water column and sink to the deep-sea floor where they are buried in the sediment. This flux of iron sulfides from the water column is reflected in enhanced concentrations of highly reactive iron and a high degree of pyritization (0.57–0.80) for the deep-water sediments of the Black Sea. The iron enrichment of deep-water sediments is balanced by a loss of highly reactive iron from the oxic continental shelf. Calculations from a numerical diagenetic model and reported in situ flux measurements indicate that the dissolved iron flux out of the shelf sediments is more than sufficient to balance the enrichment in reactive iron in deep-sea sediments, and that the majority of the dissolved iron efflux is redeposited on the continental shelf. This iron mobilization mechanism likely operates in most shelf areas, but its net effect becomes only apparent when reactive iron is trapped in sulfidic water bodies as iron sulfides or when iron is incompletely oxidized in low oxygen zones of the ocean and transported over long distances.     

Seismic facies and specific character of the bottom simulating reflector on the western margin of Paramushir Island,Sea of Okhotsk

Seismic profiles from a venting area on the western margin of Paramushir Island (Sea of Okhotsk) reveal a local complex structure and an interesting, unusual pattern of the bottom simulating reflector (BSR). The BSR is gradual rising towards the venting area. The geothermal gradient and the bottom temperature confirmed the methane hydrate. The temperature appears to be the most important factor controlling the hydrate stability. A locally higher heat flow caused the upward migration of the hydrate stability field and the subsequent degradation of the hydrated sediments, causing gas vent formation and the flux of methane gas into the water column.     

Methane sources,sinks and fluxes in a temperate tidal Lagoon: The Arcachon lagoon (SW France)

The Arcachon lagoon is a 156 km² temperate mesotidal lagoon dominated by tidal flats (66% of the surface area). The methane (CH₄) sources, sinks and fluxes were estimated from water and pore water concentrations, from chamber flux measurements at the sediment–air (low tide), sediment–water and water–air (high tide) interfaces, and from potential oxidation and production rate measurements in sediments. CH₄ concentrations in waters were maximal (500–1000 nmol l⁻¹) in river waters and in tidal creeks at low tide, and minimal in the lagoon at high tide (<50 nmol l⁻¹). The major CH₄ sources are continental waters and the tidal pumping of sediment pore waters at low tide. Methanogenesis occurred in the tidal flat sediments, in which pore water concentrations were relatively high (2.5–8.0 μmol l⁻¹). Nevertheless, the sediment was a minor CH₄ source for the water column and the atmosphere because of a high degree of anaerobic and aerobic CH₄ oxidation in sediments. Atmospheric CH₄ fluxes at high and low tide were low compared to freshwater wetlands. Temperate tidal lagoons appear to be very minor contributor of CH₄ to global atmosphere and to open ocean.     

Carbonate system and CO2 degassing fluxes in the inner estuary of Changjiang (Yangtze) River,China

We examined the carbonate system, mainly the partial pressure of CO₂ (pCO₂), dissolved inorganic carbon (DIC) and total alkalinity (TAlk) in the Changjiang (Yangtze) River Estuary based on four field surveys conducted in Sep.–Oct. 2005, Dec. 2005, Jan. 2006 and Apr. 2006. Together with our reported pCO₂ data collected in Aug.–Sep. 2003, this study provides, for the first time, a full seasonal coverage with regards to CO₂ outgassing fluxes in this world major river–estuarine system. Surface pCO₂ ranged 650–1440 μatm in the upper reach of the Changjiang River Estuary, 1000–4600 μatm in the Huangpujiang River, an urbanized and major tributary of the Changjiang downstream which was characterized by a very high respiration rate, and 200–1000 μatm in the estuarine mixing zone. Both DIC and TAlk overall behaved conservatively during the estuarine mixing, and the seasonal coverage of these carbonate parameters allowed us to estimate the annual DIC export flux from the Changjiang River as ∼ 1.54 × 10¹² mol. The highly polluted Huangpujiang River appeared to have a significant impact on DIC, TAlk and pCO₂ in the lower reaches of the inner estuary. CO₂ emission flux from the main stream of the Changjiang Estuary was at a low level of 15.5–34.2 mol m^− 2 yr^− 1. Including the Huangpujiang River and the adjacent Shanghai inland waters, CO₂ degassing flux from the Changjiang Estuary may have represented only 2.0%–4.6% of the DIC exported from the Changjiang River into the East China Sea.     

Gas hydrate decomposition recorded by authigenic barite at pockmark sites of the northern Congo Fan

The geochemical cycling of barium was investigated in sediments of pockmarks of the northern Congo Fan, characterized by surface and subsurface gas hydrates, chemosynthetic fauna, and authigenic carbonates. Two gravity cores retrieved from the so-called Hydrate Hole and Worm Hole pockmarks were examined using high-resolution pore-water and solid-phase analyses. The results indicate that, although gas hydrates in the study area are stable with respect to pressure and temperature, they are and have been subject to dissolution due to methane-undersaturated pore waters. The process significantly driving dissolution is the anaerobic oxidation of methane (AOM) above the shallowest hydrate-bearing sediment layer. It is suggested that episodic seep events temporarily increase the upward flux of methane, and induce hydrate formation close to the sediment surface. AOM establishes at a sediment depth where the upward flux of methane from the uppermost hydrate layer counterbalances the downward flux of seawater sulfate. After seepage ceases, AOM continues to consume methane at the sulfate/methane transition (SMT) above the hydrates, thereby driving the progressive dissolution of the hydrates “from above”. As a result the SMT migrates downward, leaving behind enrichments of authigenic barite and carbonates that typically precipitate at this biogeochemical reaction front. Calculation of the time needed to produce the observed solid-phase barium enrichments above the present-day depths of the SMT served to track the net downward migration of the SMT and to estimate the total time of hydrate dissolution in the recovered sediments. Methane fluxes were higher, and the SMT was located closer to the sediment surface in the past at both sites. Active seepage and hydrate formation are inferred to have occurred only a few thousands of years ago at the Hydrate Hole site. By contrast, AOM-driven hydrate dissolution as a consequence of an overall net decrease in upward methane flux seems to have persisted for a considerably longer time at the Worm Hole site, amounting to a few tens of thousands of years.