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High-resolution seismic reflection profiles (3·5 kHz) have revealed the presence of extensive interstitial gas accumulation within the sedimentary sequences of Loch Tay, Scotland, as identified by acoustic turbidity masking the seismic stratigraphy. Within the central section of the loch, in the deepest water area directly above the zone of the seismically active Loch Tay Fault, focused flows of gas through the sediment pile to the loch bed via chimneys and pockmarks, together with gas seeps within the water column, have been identified. Microbiological observations indicate that the gas is biogenic CH4 , produced by both chemoautotrophic (which use CO2 as a source of carbon and H2 as a source of energy) and aceticlastic species (which use acetate as a source of carbon and energy) of methanogens in the fine-grained, organic rich deposits that have been focused into the zone of accumulation in the deep central part of the loch. The spatial distribution of the gas escape features suggests that earthquake movements along the Loch Tay Fault are responsible for facilitating focused gas escape in this part of the loch, by the creation of new pathways and conduits through the sediment pile, along which gas can migrate upwards and exit into the water column. Relict pockmarks and associated chimneys identified in the seismic records indicate that gas escape has been taking place since Pleistocene times though the precise timings cannot be ascertained. This is the first time that such features have been reported from a lake in the UK. 相似文献
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Root‐related rhodochrosite and concretionary siderite formation in oxygen‐deficient conditions induced by a ground‐water table rise 
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下载免费PDF全文 Maciej J. Bojanowski Elżbieta Jaroszewicz Adrijan Košir Maciej Łoziński Leszek Marynowski Anna Wysocka Arkadiusz Derkowski 《Sedimentology》2016,63(3):523-551
Sedimentological, mineralogical, stable carbon and oxygen isotope determinations and biomarker analyses were performed on siderite concretions occurring in terrestrial silts to understand their formation and to characterize the sedimentary and diagenetic conditions favouring their growth. High δ13C values (6·4‰ on average) indicate that siderite precipitated in an anoxic environment where bacterial methanogenesis operated. The development of anoxic conditions during shallow burial was induced by a change in sedimentary environment from flood plain to swamp, related to a rise of the ground‐water table. Large amounts of decaying plant debris led to efficient oxygen consumption within the pore‐water in the peat. Oxygen depletion, in combination with a decrease in sedimentation rate, promoted anoxic diagenetic conditions under the swamp and favoured abundant siderite precipitation. This shows how a change in sedimentary conditions can have a profound impact on the early‐diagenetic environment and carbonate authigenesis. The concretions contain numerous rhizoliths; they are cemented with calcium‐rhodochrosite, a feature which has not been reported before. The rhodochrosite cement has negative δ13C values (?16·5‰ on average) and precipitated in suboxic conditions due to microbial degradation of roots coupled to manganese reduction. The exceptional preservation of the epidermis/exodermis and xylem vessels of former root tissues indicates that the rhodochrosite formed shortly after the death of a root in water‐logged sediments. Rhodochrosite precipitated during the initial stages of concretionary growth in suboxic microenvironments within roots, while siderite cementation occurred simultaneously around them in anoxic conditions. These suboxic microenvironments developed because oxygen was transported from the overlying oxygenated soil into sediments saturated with anoxic water via roots acting as permeable conduits. This model explains how separate generations of carbonate cements having different mineralogy and isotopic compositions, which would conventionally be regarded as cements precipitated sequentially in different diagenetic zones during gradual burial, can form simultaneously in shallow burial settings where strong redox gradients exist around vertically oriented permeable root structures. 相似文献
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新疆乌鲁木齐二叠系湖相微生物白云岩成因 总被引：3，自引：0，他引：3
新疆乌鲁木齐地区养牛场剖面中二叠统芦草沟组以发育浅湖至半深湖背景下的中层深灰色、灰色碳酸盐岩、粉砂岩、细砂岩与中、厚层灰黑色泥岩、油页岩的互层沉积为特点.湖相碳酸盐岩以微晶白云岩为主体,其次为微晶灰岩.微晶白云岩主要由白云石、铁白云石及少量方解石等组成,常混有泥质组分且富含有机质.镜下观察白云石主要为微晶(＜4μm)及微亮晶(4-10 μm),极少数为亮晶(＞10μm).扫描电镜分析发现微晶白云岩中存在微球状(直径约9μm)、微棒状(长度约0.3～1.2μm)及微米级它形(＜ 5μm)等3种微形貌的白云石,其中微米级它形白云石在白云岩中占绝大多数.在微晶灰岩中还发现了直径约70-150nm,形态与球菌相似的纳米微粒,具有微生物矿化的特征.研究区白云岩Sr丰度及Sr/Ca比总体持平或略高于微晶灰岩,Mn丰度远高于微晶灰岩,C、O同位素均高于微晶灰岩,暗示了白云岩可能形成于比微晶灰岩更深及盐度更大的水下还原环境,二者之间缺乏明显的交代关系.芦草沟组白云岩的δ13CPDB介于9.2 ‰～15.6‰,强烈正偏的δ13CPDB可能是产甲烷古菌的代谢活动引起有机质碳同位素分馏的结果.以上特征表明,研究区芦草沟组白云石的沉淀可能与产烷带厌氧微生物的代谢活动引起的甲烷生成作用有关. 相似文献
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Seasonal Study of Methane and Nitrous Oxide in the Coastal Waters of the Southern Baltic Sea 总被引：2，自引：0，他引：2
H.W. Bange S. Dahlke R. Ramesh L.-A. Meyer-Reil S. Rapsomanikis M.O. Andreae 《Estuarine, Coastal and Shelf Science》1998,47(6):807-817
Atmospheric and dissolved methane (CH4) and nitrous oxide (N2O) were measured in the unique coastal ecosystem of theBoddenwaters, including the western Oder estuary, (southern Baltic Sea) during five campaigns between 1994 and 1997. The CH4saturations, ranging from 105–15 500%, showed great spatial and temporal variability with maximum values in September and minimum values in December. The N2O saturations were in the range of 91–312% with a maximum in March. Enhanced concentrations of both gases were observed only in the western Oder estuary near the mouth of the Peene River. Thus, we conclude that the distributions of CH4and N2O in the investigatedBoddenwaters are, directly or indirectly, linked to the Peene River runoff and not to the Oder River. Our estimate of the annual CH4emissions from theBoddenwaters to the atmosphere indicates a significant contribution (c. 17%) to the overall CH4emissions from the Baltic Sea. In contrast, theBoddenwaters represent only a small source for atmospheric N2O.CH4production rates estimated from sediment slurry experiments revealed a significant spatial variability and indicated that methanogenic activity was related to acetate consumption in the surface sediment layer. Sedimentary CH4production might depend on different amounts of accumulation of organic material. 相似文献
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湖相碳酸盐岩产状多样,多以夹层形式分布在碎屑岩剖面中,也见以结核或钙质微体化石等薄层赋存于泥岩、页岩等碎屑岩中,具有层数多、单层薄、呈韵律性变化等特点。受构造背景、物源输入和古环境等因素控制,中国湖相碳酸盐沉积最早出现于二叠纪,主要发育时段为古近纪,具有沉积时间跨度大、分布面积广的特征。湖相碳酸盐岩按成因可分为原生沉积型、成岩改造型和热液喷流型3类。中国湖相碳酸盐岩的碳氧同位素特征可揭示原生和成岩过程中水介质环境的差异性: (1)δ13C和 δ18O 密切相关,且δ13C多正偏,指示封闭型咸水、半咸水湖泊环境;(2)δ13C和 δ18O 无相关性,且δ13C多负偏,指示开放型湖泊环境;(3)δ13C严重正偏,指示成岩作用时发酵带的环境受到古细菌参与的甲烷生成作用的影响。 相似文献
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Salinity‐induced increase of the hydraulic conductivity in the hyporheic zone of coastal wetlands 下载免费PDF全文
下载免费PDF全文 Gijs van Dijk Jelmer J. Nijp Klaas Metselaar Leon P.M. Lamers Alfons J.P. Smolders 《水文研究》2017,31(4):880-890
In coastal zones globally, salinization is rapidly taking place due to the combined effects of sea level rise, land subsidence, altered hydrology, and climate change. Although increased salinity levels are known to have a great impact on both biogeochemical and hydrological processes in aquatic sediments, only few studies have included both types of processes and their potential interactions. In the present paper, we used a controlled 3‐year experimental mesocosm approach to test salinity induced interactions and discuss mechanisms explaining the observed hydrological changes. Surface water salinity was experimentally increased from 14 to 140 mmol Cl per L (0.9 and 9 PSU) by adding sea salt which increased pore water salinity but also increased sulfate reduction rates, leading to higher sulfide, and lower methane concentrations. By analyzing slug test data with different slug test analysis methods, we were able to show that hydraulic conductivity of the hyporheic zone increased 2.8 times by salinization. Based on our hydrological and biogeochemical measurements, we conclude that the combination of pore dilation and decreased methane production rates were major controls on the observed increase in hydraulic conductivity. The slug test analysis method comparison allowed to conclude that the adjusted Bouwer and Rice method results in the most reliable estimate of the hydraulic conductivity for hyporheic zones. Our work shows that both physical and biogeochemical processes are vital to explain and predict hydrological changes related to the salinization of hyporheic zones in coastal wetlands and provides a robust methodological approach for doing so. 相似文献
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鄂尔多斯盆地南部铜川地区露头区中三叠统延长组长7段细砂岩、粉砂岩、泥质粉砂岩、粉砂质泥岩层序中发育有大量圆球状、椭球状、扁球状碳酸盐岩结核,对于其成因存在争议.对铜川地区多个剖面的结核开展宏观描述,在微观薄片观察基础上开展岩石矿物组成、稳定同位素、簇同位素以及微区同位素等分析,特别是选取一个结核开展了详细的解剖.结果显示结核成分以方解石为主,少量白云石;内部结构有差异,在球体横切面上明显分为两个圈层;碳同位素值正偏,由内部圈层向外部圈层降低;氧同位素与碳同位素变化趋势一致,计算结核形成古温度变化区间为28.6～76.5℃;簇同位素揭示结核中部形成温度为42.9℃.认为该类结核为早期发酵带内细菌参与甲烷生成作用形成,甲烷菌活动引起有机质碳同位素分馏,碳酸盐结核中方解石和白云石圆球粒可能是蓝细菌细胞方解石化或白云石化的结果.根据埋藏史,认为该类结核形成于成岩作用早期,后期在成岩过程中叠加生长,同心环层结构明显. 相似文献
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Methane concentrations and selected chemical parameters in interstitial water were examined along subsurface flowpath in two subsystems (hyporheic and parafluvial sediments) in the Sitka stream, Czech Republic. Interstitial methane concentrations exhibited a distinct spatial pattern. In the hyporheic downwelling zone where the sediments are relatively well oxygenated due to high hydrologic exchange with the surface water, low interstitial methane concentrations, averaging 9.3 μg CH4/l, were found. In contrast, upwelling sediments and parafluvial sediments (active channel sediments lateral to the wetted channel) had significantly higher methane concentrations (p < 0.05, and p < 0.01, respectively), averaging 43.2 μg CH4/l and 160.5 μg CH4/l, respectively. Dissolved oxygen was the highest where surface water entered hyporheic/parafluvial sediments and decreased with water residence time in the sediments (p < 0.01). Nitrate concentrations decreased along the flowpath and were significantly lower at downstream end of the riffle (p < 0.001). Sulfate concentrations also show a slight decline with the water residence time, but differences were not significant. Effect of both nitrate and sulfate on methanogenesis is also discussed. The interstitial methane concentration significantly increased with surface water temperature (p < 0.001) and was negatively correlated with redox potential (p < 0.01) and dissolved oxygen (p < 0.05). 相似文献