室温条件下青海湖水中镁离子浓度对沉淀碳酸钙同质多像类型的调控

青海湖是我国唯一报道过的现代湖底沉积物中白云石、方解石和文石等多种碳酸盐矿物共存的高原内陆咸水湖泊。以青海湖水和除菌青海湖水作为载体,以CaCl_2和MgCl_2·6 H_2O作为反应原料,在实验室常温条件下采取控制变量法制备出不同浓度Mg~(2+)参与下的钙质沉淀物,探讨Mg~(2+)浓度对沉淀物类型的影响。仅添加CaCl_2时,青海湖水中的沉淀物主要是石膏(Ca SO_4·2 H_2O)和球霰石(CaCO_3);在添加CaCl_2的同时添加MgCl_2·6 H_2O,沉淀物的石膏消失,完全转变成碳酸盐矿物,包括方解石和球霰石;当湖水中Mg~(2+)浓度为0.62 mol/L时,球霰石消失,沉淀物变为方解石和文石;随着Mg~(2+)浓度继续升高,文石含量稳步增加,方解石含量则逐渐减少,当Mg~(2+)浓度达到1.22 mol/L或更高时,方解石全部消失,沉淀物仅剩文石。实验结果表明,青海湖水中较高浓度的SO_4~(2-)对碳酸钙晶体生长有抑制作用,而额外加入的Mg~(2+)可以解除SO_4~(2-)的抑制作用,使得Ca~(2+)与HCO_3~-和CO_3~(2-)结合形成碳酸钙。此外,碳酸钙的同质多像类型也明显受到Mg~(2+)浓度的控制,随着湖水中Mg~(2+)浓度增加,方解石、球霰石不再稳定,而文石逐渐占主导地位,当Mg/Ca值达到6.1时,反应产物中仅有文石稳定存在。     

甲烷水合物生成过程中海水常量离子浓度的变化规律

本文自行研制了一套甲烷水合物合成装置,模拟海洋环境甲烷水合物的生成过程,对该过程水合物生成位置、形态、反应时间、环境温压条件进行观测,同时连续测试体系海水中常量离子K+、Na+、Ca2+、Mg2+、C1-、SO42-的浓度及海水盐度,探讨水合物生成过程的温压变化及离子浓度变化之间的关系和离子浓度的变化规律.结果表明,海水中甲烷水合物生成具有很大的随机性,在相同的初始条件下可能有不同的水合物成核、聚集过程;甲烷水合物在生成过程中,耗气量不断增加,孔隙水的盐度和海水中常量阴阳离子的浓度也在不断增加,这种变化具有较高的线性相关性(相关系数为0.9848～0.9950),且不受甲烷水合物生成位置及状态的影响;在水合物生成过程的微环境下耗气量相同时,离子浓度存在细微的差异.这些特征为通过测定海底水合物周围孔隙水中常量离子的浓度初步推算水合物的甲烷耗气量提供了依据.     

输气管线中天然气水合物形成的地球化学控制因素

对于纯气体形成水合物，甲烷形成的温压条件最高，丁烷最低，乙烷和丙烷介于二之间。在甲烷-乙烷和甲烷-丙烷的二元混合体系中，形成水合物时随甲烷含量增高，形成温度降低、压力增高；对于自然产出天然气的多组分混合体系，随丙烷 丁烷含量增加，水合物形成压力降低，温度增高。增高水合物体系的盐度或加入阻凝剂，水合物的形成压力增高，温度降低。在确定的环境温度条件下，根据天然气成分可以确定天然气形成水合物的最大压力，从而能最大限度地提高输气设备的工作效率；加入阻凝剂或提高体系的盐度也可大大提高输气设备的工作效率。     

南海东沙海域浅表层柱状沉积物孔隙水地球化学特征及对冷泉流体活动的指示

海底沉积物孔隙水地球化学特征能够快速响应有机质硫酸盐还原作用(OSR)和甲烷缺氧氧化作用(AOM)引起的变化,并记录冷泉渗漏活动的特征。本文对采集自南海东沙海域天然气水合物发育区的重力柱状沉积物孔隙水样品(D17-2015)进行了阴阳离子(SO_4~(2-)、Ca~(2+)、Mg~(2+))、微量元素(Sr~(2+)、Ba~(2+))、溶解无机碳(DIC)及其δ~(13)C_(DIC)等地球化学分析。在深度剖面上硫酸根离子浓度呈直线降低,确定的硫酸盐-甲烷转换带(SMTZ)约为海底下7.0 m,在紧邻SMTZ界面之下的溶解Ba~(2+)浓度突然增加,同时δ~(13)C_(DIC)(-38.8‰)值极端负偏,说明此站位存在强烈的AOM作用。利用反应-运移模型模拟计算获得的硫酸根向下通量为35.3 mmol/(m~2·a)、甲烷向上的通量为30.0 mmol/(m~2·a)、OSR和AOM消耗硫酸盐的相对比例分别约为15%和85%。这些结果表明AOM作用是占主导作用的生物地球化学过程。     

ODP/IODP典型水合物站位稳定带界限和水合物分布的地球化学识别标志

本文采用ODP/IODP典型站位样品实测和相关数据搜集对比研究的方法,进行了多种游离烃甲烷及相关指标在水合物形成过程中的地质作用和地球化学特征研究,以及酸解烃甲烷与围岩组分和沉积中自生碳酸盐、浊积岩的关系及地球化学特征的研究,据此筛选出判识水合物稳定带(GHSZ)的地球化学标志。结果显示:(1)下伏地层中游离甲烷异常是天然气水合物稳定带孔隙中赋存动态甲烷的反映,由于指标测试方法和沉积物状态的差异,深部水合物稳定带上呈现出特定的两种游离烃甲烷指标HS和VAC正负拆离分隔的异常模式;(2)酸解烃与地层吸附甲烷能力和围岩组分性质密切相关,对应于水合物最佳赋存条件的自生碳酸盐和浊流岩,酸解烃甲烷具有显著的量化异常特征;(3)水合物形成过程中甲烷通量促进了甲烷厌氧氧化反应(AMO)和自生碳酸盐的生成。酸解烃甲烷与浅层自生碳酸盐具有显著的对应关系,弥补了浅部沉积中游离烃甲烷异常具多源和多解性的不足;(4)游离烃甲烷异常组合模式和酸解烃甲烷量化异常分布是GHSZ和有利水合物赋存条件的综合反映,能够为水合物稳定带和带内水合物聚集的判识提供依据。     

张英1，郭依群2，莫午零3，胡玉波4

南海北部神狐海域钻探获得的水合物中天然气组分以甲烷为主,为典型干气,气体甲烷碳氢同位素组成揭示天然气为典型的生物成因,为二氧化碳还原形成。南海北部地区在硫酸盐-甲烷还原界面（SMI）以下进入生物甲烷生成阶段,盐度适中,适宜产甲烷菌等菌群的生存和生物甲烷气的生成,埋深200～1500 m层段是生物甲烷的主要生成阶段。中新世中晚期、上新世和第四纪沉积物以泥为主,部分层段为砂泥岩互层,有机质丰度较高,类型好,热演化程度低,生物气生成条件优越,可为浅部天然气水合物的形成提供充足的气源。     

海洋天然气水合物系统硫同位素研究进展

在海洋天然气水合物的地质系统中,甲烷的渗漏作用形成了独特的地球化学微环境。渗漏的甲烷在硫酸根-甲烷氧化还原界面与硫酸根之间发生厌氧氧化反应,同时硫酸盐发生还原反应,形成具有特殊同位素组成的自生碳酸盐、硫化物（AVS、黄铁矿等）和硫酸盐（重晶石、石膏）等。反应过程中硫酸盐还原菌的作用使得产物中硫的同位素发生了强烈分馏,具体表现为低δ34S值的硫化物矿物和高δ³⁴S值的硫酸盐矿物的形成。沉积物中这种独特的硫同位素特征与海洋天然气水合物系统中独特地球化学微环境有关,是硫酸盐还原反应过程中细菌控制的硫酸盐分馏和厌氧细菌对硫的歧化反应（disproportionation）的共同作用结果。     

浅海Mg~(2+)和SO_4~(2-)对微生物诱导形成锰碳酸盐的影响

为了模拟浅海环境下锰氧化物微生物还原作用诱导碳酸盐沉淀的过程,选取最常见的锰氧化物-水钠锰矿(K_(0.33)Mn_7O_(14)·7H_2O)为研究对象,在不同种类与浓度盐离子(Mg~(2+)、SO_4~(2-))存在条件下开展异化锰还原菌Dietzia cercidiphylli 45-1b好氧还原水钠锰矿的实验研究.通过测试体系蛋白、Mn~(2+)等离子浓度变化,利用X射线衍射(XRD)和X射线吸收谱(XAS)表征反应前后矿物结构变化,来探讨不同初始Mg~(2+)和SO_4~(2-)浓度对于菌株45-1b还原水钠锰矿及诱导碳酸盐矿物沉淀的影响.结果显示体系pH值在4天内从7.0迅速上升至9.3,Mn~(2+)浓度在2天内迅速上升至166μmol/L,随后迅速下降至8μmol/L(第4天),其好氧还原产物为菱锰矿(MnCO3),且其产生量随Mg~(2+)浓度的升高而降低,随SO_4~(2-)浓度的升高而升高.上述实验结果表明好氧环境下菌株45-1b能够利用乙酸为电子供体,水钠锰矿为电子受体还原水钠锰矿释放Mn~(2+),最终转化有机碳为无机碳酸盐矿物菱锰矿.Mg~(2+)通过影响微生物生长和菱锰矿成核对水钠锰矿的还原及菱锰矿沉淀产生抑制作用,而SO_4~(2-)可以缓解Mg~(2+)的抑制作用并促进水钠锰矿的还原及菱锰矿沉淀.     

南海天然气水合物稳定带的影响因素

文章利用南海所积累的大量热流、海底温度和地温梯度数据，针对地温梯度的变化，对地温梯度数据进行了初步校正。分情况研究了纯甲烷，甲烷、乙烷、丙烷混合物分别在纯水、海水条件下形成的天然气水合物在南海的可能分布范围；进而对影响天然气水合物分布的影响因素进行了讨论。研究表明，随着天然气中重烃含量的增加，孔隙水盐度的降低，水合物稳定带在平面上的分布范围越来越大，水合物稳定带的厚度也越来越大。比较而言，气体组成的影响要比孔隙水盐度的大。同时，天然气水合物稳定带的厚度与热流有一定的负相关关系。在南海2000m水深范围之内，由于受海底温度的控制，水合物稳定带的厚度与水深呈明显的正相关关系。     

榆林北部气田山2段地层水化学特征与天然气聚集关系

通过对榆林北部气田山2段地层水样品测试结果的统计与分析,可知目的层段地层水以偏酸性为主,矿化度较高,以CaCl_2水型为主,总体反映了地层水封闭条件较好,处于还原的阻滞-停滞水文地质状态,有利于天然气的聚集与保存。地层水化学特征与天然气藏关系研究表明:地层水总矿化度高值区、CaCl_2型水分布区、rNa~+/Cl~-低值区(0.5)、rMg~(2+)/Ca~(2+)低值区(0.1)、r(Cl~--Na~+)/2Mg~(2+)高值区(10)等均与天然气富集区有较好的对应关系。     

The methane hydrate formation and the resource estimate resulting from free gas migration in seeping seafloor hydrate stability zone

It is a typical multiphase flow process for hydrate formation in seeping seafloor sediments. Free gas can not only be present but also take part in formation of hydrate. The volume fraction of free gas in local pore of hydrate stable zone (HSZ) influences the formation of hydrate in seeping seafloor area, and methane flux determines the abundance and resource of hydrate-bearing reservoirs. In this paper, a multiphase flow model including water (dissolved methane and salt)-free gas hydrate has been established to describe this kind of flow-transfer-reaction process where there exists a large scale of free gas migration and transform in seafloor pore. In the order of three different scenarios, the conversions among permeability, capillary pressure, phase saturations and salinity along with the formation of hydrate have been deducted. Furthermore, the influence of four sorts of free gas saturations and three classes of methane fluxes on hydrate formation and the resource has also been analyzed and compared. Based on the rules drawn from the simulation, and combined information gotten from drills in field, the methane hydrate(MH) formation in Shenhu area of South China Sea has been forecasted. It has been speculated that there may breed a moderate methane flux below this seafloor HSZ. If the flux is about 0.5 kg m⁻² a⁻¹, then it will go on to evolve about 2700 ka until the hydrate saturation in pore will arrive its peak (about 75%). Approximately 1.47 × 10⁹ m³ MH has been reckoned in this marine basin finally, is about 13 times over preliminary estimate.     

Rising methane gas bubbles form massive hydrate layers at the seafloor

Extensive methane hydrate layers are formed in the near-surface sediments of the Cascadia margin. An undissociated section of such a layer was recovered at the base of a gravity core (i.e. at a sediment depth of 120 cm) at the southern summit of Hydrate Ridge. As a result of salt exclusion during methane hydrate formation, the associated pore waters show a highly elevated chloride concentration of 809 mM. In comparison, the average background value is 543 mM.A simple transport-reaction model was developed to reproduce the Cl⁻ observations and quantify processes such as hydrate formation, methane demand, and fluid flow. From this first field observation of a positive Cl⁻ anomaly, high hydrate formation rates (0.15-1.08 mol cm⁻² a⁻¹) were calculated. Our model results also suggest that the fluid flow rate at the Cascadia accretionary margin is constrained to 45-300 cm a⁻¹. The amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water. Thus, most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from CH₄ dissolved in the pore water.     

Low temperature behaviour of natural saline fluid inclusions in saddle dolomite (Paleozoic,NW Spain)

The origin of many dolomites is still a matter of debate because of many possible chemical and hydrological conditions of formation. Fluid inclusion studies have been applied in order to improve knowledge about paleofluids responsible for the precipitation of dolomite, and used to define temperatures and salinities. The combination of Raman Spectroscopy and microthermometry is tested here to improve the analytical method to identify the main ion species present in individual inclusions. Natural samples of saddle dolomite from the Cambrian Láncara Fm., Cantabrian Mountains (NW Spain), contain zoned crystals with two‐phase aqueous fluid inclusions (liquid‐rich). The most stable phase assemblage in these inclusions at −150 °C consists of ice, hydrohalite and an unknown salt hydrate. The latter melts between −47 and −41 °C, probably representing a eutectic temperature. Subsequently, ice melts in the range of −32.5 to −29 °C and, finally, hydrohalite melts between −9 and −3.5 °C. Salinities can be calculated in the fluid system H₂O–NaCl with addition of another salt, either CaCl₂ or MgCl₂, and result in 7.5–10.6 eq. mass% NaCl and 17.0–21.0 eq. mass% CaCl₂. Dependent on the rate of cooling runs, three different types of metastability may occur, i.e. the absence of hydrohalite, the unknown salt‐hydrate is not formed, and the nucleation of only ice. Salinity calculations from those melting temperatures differ substantially from equilibrium behaviour values. The unknown salt‐hydrate needs to be further specified by comparison to standard solutions. The method gives an opportunity to characterize the major compounds in complex fluid systems active during dolomitization, thus contributing to a better understanding of the ‘dolomite problem’.     

Geochemical characteristics of sediment pore water from Site XS-01 in the Xisha trough of South China Sea and their significance for gas hydrate occurrence

Gas hydrate is a recently-found new source of energy that mostly exists in marine sediments. In recent years, we have conducted gas hydrate exploration in the South China Sea. The Xisha trough, one of the promising target areas for gas hydrate, is located in the northern margin of the South China Sea, adjacent to several large oil and gas fields. The Xisha trough extends 420 km long with the water depth of 1 500 m in the west part and 3 400 m in the east part and deposits thick sediments with organic matter content of 0.41%–1.02%. Previous studies on topographical features, geological P-T conditions, structural geology, sedimentary geology and geophysical bottom simulating reflectors (BSR) in the Xisha trough suggest that this area is favorable for the formation and accumulation of gas hydrate. In this paper, we present geochemical analyses for the sediment and pore water from a piston core at Site XS-01 in the Xisha trough. Seven pore water samples were analyzed for their anion (Cl⁻, SO₄ ²⁻, Br⁻, I⁻) contents, cation (Na, K, Ca, Mg) contents and trace element (Li, B, Sr, Ba, Rb, Mn) contents. Eight sediment samples were analyzed for stable carbon and oxygen isotopic compositions. A number of geochemical anomalies such as anions (e.g. Cl⁻, SO₄ ²⁻), cations (e.g. Ca, Mg) and trace elements (e.g. Sr, Ba, B) were found in this study. For example, the concentrations of Cl⁻ and SO₄ ²⁻ in pore water show a decreasing trend with depth. The estimated sulfate/methane interface (SMI) is only 18 m, which is quite similar to the SMI value of 23 m in the ODP164 Leg 997 at Blake Ridge. The Ca, Mg and Sr concentrations of pore water also decrease with depth, but concentrations of Ba, and Mg/Ca and Sr/Ca ratios increase with depth. These geochemical anomalies are quite similar to those found in gas hydrate locations in the world such as the Blake Ridge and may be related to the formation and dissociation of gas hydrates. The salt exclusion effect during the gas hydrate formation will cause an increase in major ion concentrations in the pore waters that diffused upward such as Cl. The anaerobic methane oxidation (AMO) may lead to the change of SO₄ ²⁻ and other cations such as Ca, Mg, Sr and Ba in pore water. Low δ ¹³C value of authigenic carbonates is a good indicator for gas hydrate occurrence. However, the bulk sediment samples we analyzed all show normal δ ¹³C values similar to biogenic marine carbonates, and this may also suggest that no gas hydrate-related authigenic carbonates exist or their amount is so small that they are not detectable by using this bulk analytical method. In conclusion, we suggest that the Site XS-01 in the Xisha trough of the northern margin of the South China Sea is a potential target for further gas hydrate exploration. Translated from Quaternary Sciences, 2006, 26(3): 442–448 [译自: 第四纪研究]     

Metasomatism and ore formation at contacts of dolerite with saliferous rocks in the sedimentary cover of the southern Siberian platform

The data on the mineral composition and crystallization conditions of magnesian skarn and magnetite ore at contacts of dolerite with rock salt and dolomite in ore-bearing volcanic—tectonic structures of the Angara—Ilim type have been integrated and systematized. Optical microscopy, scanning and transmission electron microscopy, electron microprobe analysis, electron paramagnetic resonance, Raman and IR spectroscopy, and methods of mineralogical thermometry were used for studying minerals and inclusions contained therein. The most diverse products of metasomatic reactions are found in the vicinity of a shallow-seated magma chamber that was formed in Lower Cambrian carbonate and saliferous rocks under a screen of terrigenous sequences. Conformable lodes of spinel-forsterite skarn and calciphyre impregnated with magnesian magnetite replaced dolomite near the central magma conduit and apical portions of igneous bodies. At the postmagmatic stage, the following mineral assemblages were formed at contacts of dolerite with dolomite: (1) spinel + fassaite + forsterite + magnetite (T = 820?740°C), (2) phlogopite + titanite + pargasite + magnetite (T = 600–500°C), And (3) clinochlore + serpentine + pyrrhotite (T = 450°C and lower). Rock salt is transformed at the contact into halitite as an analogue of calciphyre. The specific features of sedimentary, contact-metasomatic, and hydrothermal generations of halite have been established. The primary sedimentary halite contains solid inclusions of sylvite, carnallite, anhydrite, polyhalite, quartz, astrakhanite, and antarcticite; nitrogen, methane, and complex hydrocarbons have been detected in gas inclusions; and the liquid inclusions are largely aqueous, with local hydrocarbon films. The contact-metasomatic halite is distinguished by a fine-grained structure and the occurrence of anhydrous salt phases (CaCl₂ · KCl, CaCl₂, nMgCl₂ · mCaCl₂) and high-density gases (CO₂, H₂S, N₂, CH₄, etc.) as inclusions. The low-temperature hydrothermal halite, which occurs in skarnified and unaltered silicate rocks and in ore, is characterized by a low salinity of aqueous inclusions and the absence of solid inclusions. The composition and aggregative state of inclusions in halite and forsterite indicate that salt melt-solution as a product of melting and dissolution of salt was the main agent of high-temperature metasomatism. Its total salinity was not lower than 60%. The composition and microstructure of magnetite systematically change in different mineral assemblages. Magnetite is formed as a result of extraction of iron together with silicon and phosphorus from dolerite. The first generation of magnetite is represented by mixed crystals, products of exsolution in the Fe-Mg-Al-Ti-Mn-O system. The Ti content is higher at the contact of dolerite with rock salt, whereas, at the contact with dolomite, magnetite is enriched in Mg. The second generation of magnetite does not contain structural admixtures. The distribution of boron minerals and complex crystal hydrates shows that connate water of sedimentary rocks could have participated in hydrothermal metasomatic processes.     

Salting-out of methane in single-salt solutions at 25°C and below 800 psia

Aqueous solubilities of methane at 25°C have been determined in single-salt solutions equilibrated with a CH₄ gas phase at 350, 550, and 750 psia. Measurements were made over a range of ionic strengths in NaCl, KCl, CaCl₂, MgCl₂, Na₂SO₄, K₂SO₄, MgSO₄, Na₂CO₃, K₂CO₃, NaHCO₃, and KHCO₃ aqueous solutions.At 25°C and constant pressure and methane fugacity, methane solubilities were largely controlled by the stoichiometric ionic strength, I, and the cation of the salt. Except for an increased salting-out due to HCO₃^?, the anion effect was relatively insignificant. Different but consistent solubility trends were followed in monovalent and divalent cation salt solutions as a function of I. Solubilities increased in salt solutions having a common anion in the following cation sequence: Na⁺ < K⁺ ? Ca²⁺ < Mg²⁺.The molal salting coefficient, k_m, for each salt was constant under the experimental conditions of the study, k_m is defined by $\text{log}\text{γ}\text{ch}{}_4\text{I}$ where $\text{γ}\text{ch}{}_4$, the molal activity coefficient, is the methane solubility ratio () measured at constant temperature, pressure, and CH₄ fugacity. Single-salt k_m values are as follows: 0.121, NaCl (4m); 0.121, Na₂SO₄ (1m); 0.118, Na₂CO₃ (1.5m); 0.146, NaHCO₃ (0.5m); 0.101, KCl (4m); 0.108, K₂SO₄ (0.5m); 0.111, K₂CO₃ (2m); 0.145, KHCO₃ (0.5m); 0.071, CaCl₂ (2m); 0.063, MgCl₂ (2m); and 0.066, MgSO₄ (1.5m) where the molalities in parentheses refer to the maximum salt concentrations used in this study.     

Amorphous silica solubilities—II. Effect of aqueous salt solutions at 25°C

The solubility of amorphous silica was measured at 25°C in ten separate sets of aqueous salt solutions—potassium chloride, potassium nitrate, sodium chloride, lithium chloride, lithium nitrate, magnesium chloride, calcium chloride, magnesium sulfate, sodium bicarbonate and sodium sulfate. The concentrations of the salts were varied from zero to saturation with both salt and amorphous silica. With increasing concentration of salt, the solubility of amorphous silica always decreased as expected from an average value of 0.00218 m in water. Nevertheless, the extent of decrease differed greatly from a 6% decrease in a solution saturated with NaHCO₃ to a 95.7% decrease in a solution saturated with CaCl₂. A striking correlation was observed: In the 1-1 and 2-1 electrolyte salt solutions at a given molality the effect on the solubility of silica depended upon the cation in the order Mg²⁺, Ca²⁺ > Li⁺ > Na⁺ > K ⁺.     

Geochemical studies of pore fluid in surface sediment on the Daini Atsumi Knoll

We collected sediment samples and pore water samples from the surface sediment on the Daini Atsumi Knoll, and analyzed the sediments for CH₄, C₂H₆, and δ¹³C_CH4, and the pore fluids for CH₄, C₂H₆, δ¹³C_CH4, Cl⁻, SO₄²⁻, δ¹⁸O_H2O, and δD_H2O, respectively. A comparison of the measured concentration and isotopic composition of methane in pore water samples with those in sediment samples revealed that methane was present in the sediment samples at a higher concentration and was isotopically heavier than those in the pore water samples. It suggests that the effect of the release of a sorbed gas bound to organic particles when heated prior to analysis of hydrocarbons was larger than that of the degassing process. A large amount of a sorbed gas would be a significant source of natural gas. Two striking features are the chemical and isotopic composition of the pore water samples taken from the different sites around the Daini Atsumi Knoll. In the KL09, KL10, and KP07 samples, Cl⁻ concentrations in the pore water samples showed depletion to a minimum of 460 mmol/kg, correspond to  17% dilution of seawater, however the latter was not enriched in CH₄. The isotopic compositions of pore water samples suggested the low-Cl⁻ fluids in the pore water were not derived from dissociation of methane hydrate, but were derived from input of meteoric water. In contrast, in the KP05 samples from the north flank of the Daini Atsumi Knoll, pore water were characterized by CH₄ enrichment more than 370 μmol/kg, but not depleted in Cl⁻ concentrations. The observed methane concentration in the KP05 samples is not sufficient for methane hydrate to form in situ, indicating that the existence of methane hydrate in the surface sediment is negligible, as supported by Cl⁻ concentration. Based on the stable carbon isotope ratio of methane in the pore fluid from the KP05 site (δ¹³C_CH4 < − 50‰PDB), methane is thought to be of microbial origin. The pore waters in the surface sediments in the north flank of the Daini Atsumi Knoll were not directly influenced by upward fluid bearing methane of thermogenic origin from a deeper part of the sedimentary layer. However, extremely high methane concentration in the north flank site as compared with the concentration of pore water taken from the normal seafloor suggests that the north flank site is not the normal seafloor. We hypothesize that upward migration of chemically-reduced fluids from a deeper zone of the sedimentary layer reduces chemically-oxidized solutes in the surface sediment. As a consequence methane production replaced sulfate reduction as the microbial metabolism in the reduced environment of the surface sediment.     

Thermodynamic and pore water halogen constraints on gas hydrate distribution at ODP Site 997 (Blake Ridge)

Marine sediment sequences with CH₄ hydrate are characterized by an atypical depth profile in dissolved Cl⁻ squeezed from pore space: a shallow subsurface Cl⁻ maximum overlies a lengthy and pronounced Cl⁻ minimum. This pore water Cl⁻ profile represents a combination of multiple processes including glacial–interglacial variations in ocean salinity, advection and diffusion of ions that are excluded during gas hydrate formation at depth, and release of fresh water from dissociation of hydrate during core recovery. In situ quantities of gas hydrate can be determined from a measured pore water Cl⁻ profile provided the in situ pore water signature prior to core recovery can be separated. Ocean Drilling Program (ODP) Site 997 was drilled into a large CH₄ hydrate reservoir on the Blake Ridge in the western Atlantic Ocean. Previously we have constructed a high-resolution pore water Cl⁻ profile at this location; here we present a `coupled chloride-hydrate' numerical model to explain basic trends in the Cl⁻ profile and to isolate in situ Cl⁻ concentrations. The model is based on thermodynamic and ecological considerations, and uses established equations for describing chemical behavior in marine sediment–pore water systems. The model incorporates four key concepts: (1) most gas hydrate is formed immediately below the SO₄²⁻ reduction zone; (2) fluid, dissolved ions and gas advect upward through the sediment column; (3) CH₄ hydrate dissociates at the base of hydrate stability conditions; and (4) seawater salinity fluctuates during glacial–interglacial cycles of the late Pliocene and Quaternary. Rates of upward advection in the model are sufficient to account for measured Br⁻ and I⁻ concentrations as well as CH₄ oxidation at the base of the SO₄²⁻ reduction zone. In situ pore water Cl⁻ inferred from the model is similar to that determined by limited direct sampling; in situ CH₄ hydrate amounts inferred from the model (an average of about 4% of porosity) are broadly consistent with those determined by direct gas sampling and indirect geophysical techniques. The model also predicts production of substantial quantities of free CH₄ gas bubbles (>2.5% of porosity) at a depth immediately below the lowest accumulation of CH₄ hydrate in the sediment column. Our explanation for the pore water Cl⁻ profile at Site 997 is important because it provides a theoretical mechanism for understanding the distribution of interstitial water Cl⁻, gas hydrate, and free gas in a marine sediment column.