Hydrogeochemical models locating sulfate-methane transition zone in marine sediments overlying black shales: A new tool to locate biogenic methane?

Precise hydrogeochemical modeling of early diagenesis is a key in the reconstruction of sedimentary basin models. This determines the mineralogical evolution of the sediment and consequently the porosity of the rock. During early diagenesis also part of the initial organic matter is converted into biogenic gas: CH₄ CO₂, and H₂S. These processes are part of complex reaction chains during sedimentation, and biogeochemical reactions leave different signals that can be observed today. In this work, we reproduce the early diagenetic processes as integrated signals over geological times in sediments of the Demerara Rise by applying chemical thermodynamics using the PHREEQC (version 2) computer code. The investigated sediments are characterized by the presence of black shales in 410–490 mbsf and by a diagenetic barite layer above in 300–350 mbsf at depth of sulfate-methane transition (SMT). We determine the parameters that influence the location of diagenetic barite peaks in sediments overlying black shales by means of a novel modeling approach. Crucial parameters are the amount of bacterial organic matter mineralization, sedimentation rates and bottom water sulfate concentrations. All parameters are intertwining and influence the sulfate-methane cycle. They affect the location of the SMT visualized by diagenetic barite peaks. However, our model approach opens a wide field in exploring early diagenetic reactions, processes and products (such as biogenic methane) over geological times mirrored by diagenetic minerals and pore water concentration profiles that can be detected in present-day sediments.     

甲烷流体活动与沉积物中碳、氮同位素组成响应——南海东北部海洋IV号地区研究

南海东沙群岛东北部海洋IV号地区GC16站重力柱状沉积物中的有机碳、总氮,以及自生碳酸盐岩矿物的无机碳同位素组成分析结果表明,甲烷流体活动区硫酸盐甲烷转换带(SMTZ—sulfate methane transition zone)是重要的生物地球化学界面,该带内沉积物中有机碳、氮与无机碳同位素组成变化明显。GC16站SMTZ上界面以下的沉积物中有机碳、氮同位素组成分别比其上的沉积物负偏1.4‰PDB和0.93‰,反映该带内甲烷缺氧氧化作用(AOM—anaerobic oxidation of methane)与氨氧化作用发育。无机碳同位素组成表明SMTZ界面之下沉积物中的自生碳酸盐岩矿物为以微生物为媒介的甲烷驱动成因,地质历史时期(至柱状沉积物底部沉积时期)曾发生过2次较强烈的CH₄流体活动;目前该区甲烷流体活动较弱,甲烷流体影响深度与SMTZ上界面一致,未能到达海底。     

Authigenic carbonate precipitates from the NE Black Sea: a mineralogical,geochemical, and lipid biomarker study

Carbonate precipitates recovered from 2,000 m water depth at the Dolgovskoy Mound (Shatsky Ridge, north eastern Black Sea) were studied using mineralogical, geochemical and lipid biomarker analyses. The carbonates differ in shape from simple pavements to cavernous structures with thick microbial mats attached to their lower side and within cavities. Low δ¹³C values measured on carbonates (−41 to −32‰ V-PDB) and extracted lipid biomarkers indicate that anaerobic oxidation of methane (AOM) played a crucial role in precipitating these carbonates. The internal structure of the carbonates is dominated by finely laminated coccolith ooze and homogeneous clay layers, both cemented by micritic high-magnesium calcite (HMC), and pure, botryoidal, yellowish low-magnesium calcite (LMC) grown in direct contact to microbial mats. δ¹⁸O measurements suggest that the authigenic HMC precipitated in equilibrium with the Black Sea bottom water while the yellowish LMC rims have been growing in slightly ¹⁸O-depleted interstitial water. Although precipitated under significantly different environmental conditions, especially with respect to methane availability, all analysed carbonate samples show lipid patterns that are typical for ANME-1 dominated AOM consortia, in the case of the HMC samples with significant contributions of allochthonous components of marine and terrestrial origin, reflecting the hemipelagic nature of the primary sediment.     

Authigenic Gypsum in Gas-Hydrate Associated Sediments from the East Coast of India (Bay of Bengal)

Authigenic gypsum crystals, along with pyrite and carbonate mineralization, predominantly calcites were noticed in distinct intervals in a 32 m long piston core, collected in the gas hydrate- bearing sediments in the northern portion of the Krishna-Godavari basin, eastern continental margin of India at a water depth of 1691 m. X-ray diffraction and energy dispersive spectrum studies confirm presence of pyrite, gypsum, calcite, and other mineral aggregates. The occurrence of gypsum in such deep sea environment is intriguing, because gypsum is a classical evaporite mineral and is under saturated with respect to sea water. Sedimentological, geochemical evidences point to diagenetic formation of the gypsum due to oxidation of sulphide minerals (i.e. pyrite). Euhedral, transparent gypsum crystals, with pyrite inclusions are cemented with authigenic carbonates, possibly indicating that they were formed authigenically in situ in the gas hydrate-influenced environment due to late burial diagenesis involving sulphate reduction and anaerobic oxidation of methane (AOM). Therefore, the authigenic gypsums found in sediments of the Krishna-Godavari and Mahanadi offshore regions could be seen as one of the parameters to imply the presence of high methane flux possibly from gas hydrate at depth.     

Morphology and formation mechanism of pyrite induced by the anaerobic oxidation of methane from the continental slope of the NE South China Sea

In order to understand the response of authigenic pyrite to gas hydrate geo-systems, pyrite tubes or rods at the sulfate–methane transition (SMT) zone of core GC10 from the northern continental slope of the South China Sea (SCS) were investigated. In situ X-ray diffraction (XRD) results show that the pyrite tube consists of pyrite micro-crystals with trace amount of graphite in the inner tube. Scanning electron microscope (SEM) observations of pyrite tubes indicate various aggregations in the form of framboidal, euhedral, and colloidal pyrite microcrystals. Typical framboidal pyrite is considered as packing of octahedral microcrystals. Interestingly, many framboids in the tubes consist of round or irregular microcrystals and have an outer crust that consists of secondary pyrite. The size of the framboids in the inner wall of the tube is larger than that in the middle wall or foraminifer-filled pyrite. High-resolution transmission electron microscopic (HRTEM) images show marcasite lamellae defects in the spherulitic pyrite crystals, which reveal different solution conditions during the pyrite precipitation. Nano-foil-like graphitic carbon was observed to be closely associated with the pyrite spherules. The occurrence of both marcasite layers and nano-foil-like graphitic carbon suggest that the migration of methane from deep sediment. It is suggested that the formation of pyrite serves as a catalyst during the reaction from methane to elemental carbon under the anaerobic oxidation of methane. Meanwhile, this reaction results in local acidification of the solution inside the pyrite tubes, which favors marcasite lamellae growth on the host pyrite substrate.     

海洋沉积物甲烷厌氧氧化作用(AOM)及其对无机硫循环的影响

甲烷厌氧氧化作用(AOM)在调控全球甲烷收支平衡以及缓解因甲烷引起的温室效应等方面扮演着十分重要的角色,成为近些年来海洋生物地球化学领域的研究热点之一.一般而言,海洋沉积物孔隙水硫酸盐还原主要是通过2种反应途径来完成,即氧化有机质途径和AOM途径.长期以来,与有机质氧化途径相关的硫酸盐还原作用研究已有充分展示,而由AOM驱动的硫酸盐还原及其对自生硫化铁形成与埋藏的重要贡献却被严重低估.侧重从生物地球化学、同位素地球化学等角度,综述近些年来不同环境条件下海洋沉积物AOM作用发生的地球化学证据和AOM对沉积物孔隙水硫酸盐消耗比例的贡献大小及其调控因素.AOM过程产生的H2S会与沉积物中活性铁结合形成自生铁硫化物.与沉积物浅表层条件相比,AOM过程固定的自生铁硫化物不容易发生再氧化,更利于在沉积物中埋藏保存起来.AOM与海洋沉积物硫酸盐还原作用相偶联,由AOM驱动的硫酸盐还原过程对海底自生铁硫化物形成与埋藏的重要贡献不容忽视.该综述有助加深对海洋沉积物AOM作用的认识及其对硫循环的全面理解.     

深海冷泉古菌厌氧甲烷氧化研究进展

在富含甲烷水合物的海相冷泉沉积物中, 古菌厌氧甲烷氧化作用(anaerobic oxidation of methane, AOM)越来越受到人们的重视。目前普遍认为, AOM是由嗜甲烷古菌和硫酸盐还原菌共同调节的生物地球化学过程。16S rRNA基因分析表明, 包括AEME-1、AEME-2和AEME-3在内的多种甲烷古菌参与了AOM的过程, 它们广泛分布于全球大洋海底缺氧带。AOM过程与全球环境变化密切相关, 从深海底部冷泉区向上渗漏的甲烷气体, 绝大部分在穿透缺氧带沉积层过程中被甲烷氧化古菌所消耗, 有效减少了具有强烈温室效应的甲烷气体向大气的释放。对AOM生物地球化学过程的研究, 在认识冷泉系统碳酸盐的形成机理、控制强温室气体甲烷从海底的渗漏和开发可燃冰新能源等方面具有重要意义。     

华北地台串岭沟组砂脉中自生碳酸盐沉淀和自生黄铁矿——中元古代甲烷厌氧氧化的沉积证据

华北中元古界串岭沟组暗色页岩中普遍发育的砂脉构造被解释为气体逃逸形成的沉积构造。在砂脉和相邻围岩中发现有自生碳酸盐沉淀和自生黄铁矿。自生碳酸盐主要由白云石组成,表现为微晶质条带或隐晶质斑块状胶结物。微晶白云石条带多出现在有机质纹层密集发育的部位,沿微生物膜两侧发育;而隐晶质斑块多以胶结物形式充填粒间孔隙或沿砂脉外缘密集产出。围岩中共生草莓状黄铁矿,砂脉中有具环带黄铁矿。这种特殊的矿物组构以及富有机质沉积和丰富的微生物成因构造(MISS)均表明串岭沟组形成于缺氧环境。层面发育的气泡构造为砂脉的气体逃逸成因提供了支持,而自生碳酸盐和黄铁矿则是沉积浅层硫酸盐-甲烷转换带(SMTZ)甲烷厌氧氧化(AOM)和细菌硫酸盐还原(BSR)的产物;气体源于有机质的厌氧分解和甲烷菌活动。砂脉中发现有疑似微生物化石,围岩含古菌和硫细菌生物标志物,表明有发生AOM和BSR作用的条件。串岭沟组砂脉中识别的自生碳酸盐沉淀及其AOM成因对进一步认识中元古代古海洋与古气候条件具有重要意义。由于海洋的低硫酸盐浓度,SMTZ带很浅,海底沉积层的甲烷仅少量被AOM消耗;而大量甲烷进入大气必然导致中元古代强烈的温室气候效应。     

Anaerobic oxidation of methane in coastal sediment from Guishan Island (Pearl River Estuary), South China Sea

The concentrations of CH₄, SO₄²⁻, σCO₂ and the carbon isotope compositions of ΣCO₂ and CH₄ in the pore-water of the GS sedimentary core collected from Guishan Island (Pearl River Estuary), South China Sea, were determined. The methane concentration in the pore-water shows dramatic changes and sulfate concentration gradients are linear at the base of the sulfate reduction zone for the station. The carbon isotope of methane becomes heavier at the sulfate-methane transition (SMT) likely because of the Raleigh distillation effect; ¹²CH₄ was oxidized faster than ¹³CH₄, and this caused the enrichment of residual methane δ ¹³C and δ ¹³C-ΣCO₂ minimum. The geochemical profiles of the pore-water support the existence of anaerobic oxidation of methane (AOM), which is mainly controlled by the quality and quantity of the sedimentary organic matter. As inferred from the index of δ ¹³C-TOC value and TOC/TN ratio, the organic matter is a mix of mainly refractory terrestrial component plus some labile alga marine-derived in the study area. A large amount of labile organic matter (mainly labile alga marine-derived) is consumed via the process of sedimentary organic matter diagenesis, and this reduces the amount of labile organic matter incorporated into the base of the sulfate reduction zone. Due to the scarcity of labile organic matter, the sulfate will in turn be consumed by its reaction with methane and therefore AOM takes place. Based on a diffussion model, the portion of pore-water sulfate reduction via AOM is 58.6%, and the percentage of ΣCO₂ in the pore-water derived from AOM is 41.4%. Thus, AOM plays an important role in the carbon and sulfur cycling in the marine sediments of Pearl River Estuary.