Magnitude of dissociation of methane hydrate reservoir associate with climate change

Methane hydrates are found in enormous quantities along all the continental margins. The magnitude of the carbon pool locked in the methane hydrate reservoir is not known precisely, but estimates range from 10(3) to 10(6) Gt C. Methane hydrates are also of interest because of their potential role in climate change. When these marine methane hydrates begin to dissociate, the methane trapped in the methane hydrates is released into the ocean and the atmosphere. This review focuses on the dissociation mechanism of methane hydrates that are rather unique and involve a number of conditions, and presents current understanding of the effects of methane hydrate on the chemical and physical properties of seawater. We discuss past events about the dissociation of methane hydrate in the various geological periods. It will be shown how the previously mentioned sufficient flux of methane from hydrate dissociation can cause an oceanic anoxic event and a major global warming. These events maybe are occurring and will be believed to influence the climate change in the future

Geochemical characteristics of pore water in shallow sediments from north continental slope of South China Sea and their significance for natural gas hydrate occurrence

Northern slope of the South China Sea is a potential area of gas hydrates and research focus areas. Analyzed geochemical parameters about pore water of the anions and cations of major components and delta C-13(DIC) in the sediments from Shenhu sea area, Dongsha sea area and Southwest Taiwan Basin and collected by gravity piston. The results showed that Cl- concentration were no significant changes with depth at the three stations and their values were consistent with the normal sea water, SO42- concentration showed significant changes in gradient descent from core top to bottom at three stations and Ca2+, Mg2+, Ca2+/Mg2+ showed a similar downward trend with SO42- concentration, but the amplitude is inconsistent. The SMI of Shenhu sea area, Dongsha sea area, Southwest Taiwan Basin were separately 11m, 8m, 6m and delta C-13(DIC) of pore water in the sediments range from -10 parts per thousand to -27 parts per thousand, which were similar with geochemistry characteristics of pore water in the sediments at other international regions of being found gas hydrate, such as Blake Ridge and Mexico Gulf. These geochemistry characteristics may suggest that natural gas hydrate reservoir released methane in deep sediments and methane was mixture gas as main causes of pyrolysis. Summary a series anomaly indicators and identification methods that use these geochemical anomalies characteristics about pore water of shallow sediments to trace gas hydrate in deep stratum. (C) 2011 Published by Elsevier B. V. Selection and/or peer-review under responsibility of National University of Singapore

The indicative effect of structures of archaeal communities at deep-water sediment cores on natural gas hydrate: A case study from Station 973-4 in the Southwest Taiwan Basin, South China Sea

The formation and decomposition of submarine natural gas hydrate (hereinafter hydrate for short) are greatly influenced by methanogenesis and anaerobic methane oxidation of microorganisms, so it is necessary to conduct further analysis on the metabolic mechanism of archaeal communities and its indicative effect on hydrate reservoir formation. In this paper, the structures of archaeal communities at the cores of Station 973-4 in the Southwest Taiwan Basin of the South China Sea were studied by means of 16S rDNA molecular biological technologies, and then were compared with those at hydrate occurrence and non-occurrence zones in the Japan Sea and the Shenhu area of the South China Sea. It is shown that Methanosarcina is the predominant community at the cores from Station 973-4, with a percentage of 50% in the surface zone, 46.1% in the sulfate–methane transition zone (SMTZ) and 66.7% in the deep zone. Methanomicrobiales is the secondary predominant community, with a percentage of 28.3% in the surface zone, 30.7% in SMTZ and 11.1% in the deep zone. The percentage changing tendency of the above-mentioned communities are closely related with the variation of organic matter content and compositions during the early diagenetic process. It is concluded that the structures of archaeal communities are in accordance with the analysis results of bacteria, geochemistry and mineralogy, indicating the occurrence of hydrate at the bottom of the cores from Station 973-4. In addition, the structures of archaeal communities are characterized by diversity, so attention shall be paid to the synergy between geochemical parameters and geologic characteristics, so that the indicative effect on hydrate reservoir formation will be presented more accurately

Carbon and oxygen isotope composition of carbonate in bulk sediment in the southwest Taiwan Basin, South China Sea: Methane hydrate decomposition history and its link to mud volcano eruption

Anaerobic oxidation of methane (AOM) coupled with sulfate reduction (SR) generates authigenic carbonate which has the unique carbon and oxygen isotope composition in sulfate methane transition zone (SMTZ). The carbon and oxygen isotope composition of carbonate in sediments, which can be used to revel the variable methane flux, is affected by the percentage of AOM-driven carbonate in sediments. To investigate the methane hydrate decomposition history in the southwest Taiwan Basin, we analyzed a 13 m-long sediment core for grain size, carbonate and elemental carbon contents and stable carbon and oxygen isotopes composition of carbonate (δ13Ccarb and δ18Ocarb). The AMS 14C dating results and carbonate content reveal that a turbidity current occurred at around 14 ka B.P., which reversed sediment accumulation between 455 and 885 cm in depth. Combining the chronological framework and δ13Ccarb values, we identified that the methane hydrate decomposition scale began to decrease at 14 ka B.P. Moreover, we discovered that three transiently enhanced methane hydrate decomposition events occurred from then on. Although the δ13Ccarb and δ18Ocarb variability are as expected, the low δ18Ocarb values are inconsistent with the previous theory which favors 18O-enriched in AOM-driven authigenic carbonate. Thus, we suggest that a large amount of 18O-depleted water which originally migrated from southwest Taiwan island was injected into surface sediment along with the eruption of mud volcanos. As the terminal electron accepter of AOM-SR reaction, the exogenous 18O-depleted water leads its special oxygen isotope to be inherited by the authigenic AOM-driven carbonate. The proposed theory provides new insights on the authigenic carbonate in methane hydrate area and the association between methane hydrates and mud volcanoes

Effect of Sea-Level Change on Deep-Sea Sedimentary Records in the Northeastern South China Sea over the past 42 kyr

We integrated multiple geochemical analysis of a 13.75 m-long core 973-4 recovered from the northeastern South China Sea (SCS) to detect the response of deep-sea sediment archives to sea-level change spanning the last 42 kyr. The age-depth model based on AMS 14C dating, together with the sediment grain size, shows an occurrence of turbidity current at around 14 kyr, which was associated with submarine landslides caused by gas hydrate dissociation. A dominantly terrigenous sediment input was supplied from southwestern Taiwan rivers. By synthesizing environment-sensitive indexes, four distinct stages of paleoenvironmental evolutions were recognized throughout the studied interval. Well-oxygenated condition occurred during the stage I (42.4-31.8 kyr) with low sea-level stand below -80 m, accompanied by flat terrigenous input. The largest amounts of terrigenous sediment input occurred during the late phase of stage II (31.8-20.4 kyr) with the lowest sea-level stand below -120 m because of a short distance from paleo-Taiwan river estuaries to the core location. An occurrence of Ca-enriched turbidity current disturbed the original sediments during the stage III (20.4-13.9 kyr). The stepwise elevated sea-level stand resulted in an enclosed (semi-enclosed) system and contributed to a relatively low-oxygen environment in deep ocean during the stage IV (13.9 kyr—present). Temporal variations of TOC and CaCO3 display contrary pattern synchronously, indicating a decoupled relationship between organic carbon burial and carbonate productivity. Our results highlight that these sedimentary records as reflected in the paleoenvironmental changes in the northeastern SCS were mainly driven by sea-level fluctuations and later, since the mid-Holocene, the strengthening East Asian summer monsoon (EASM) overwhelmed the stable sea level in dominating the environmental changes

Lipid Distribution in Marine Sediments from the Northern South China Sea and Association with Gas Hydrate

National Major Fundamental Research and Development Project of China ( [2009CB219501]; National Natural Science Foundation of China [40976035, 41276046]The distributions of lipids in surface and subsurface sediments from the northern South China Sea were determined. The n-alkanes were in bimodal distribution that is characterized by a centre at n-C-16-n-C-20 with maximum at C-18 (or C-19) and n-C-27-n-C-31 as well as at C-29 (or C-31). The short-chain alkanes suffered from significant losses due to their slow deposition in the water column, and their presence with a slight even carbon predominance in shallow seafloor sediments was ascribed mainly to the direct input from the benthos. The long-chain alkanes with odd predominance indicate transportion of terrigenous organic matter. Immature hopanoid biomarkers reflect the intense microbial activity for bacteria-derived organic matter and the gradual increase of maturity with burial depth. Abundant n-fatty acid methyl esters (n-FAMEs) that are in distributions coincident with fatty acids were detected in all samples. We proposed that the observed FAMEs originated from the methyl esterification of fatty acids; methanol production by methanotrophs and methanogenic archaea related to the anaerobic oxidation of methane, and sulfate reduction provided an O-methyl donor for methylation of fatty acids. The CH4 released from hydrate dissociation at oxygen isotope stage II of Cores ZD3 and ZS5, which had been confirmed by the occurrence of negative C-13 excursion and spherical pyrite aggregates, could have accelerated the above process and thus maximized the relative content of FAMEs at ZD3-2 (400-420 cm depth) and ZS5-2 (241-291 cm depth)