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ABSTRACT: 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.
Journal of Oceanography 04/2012; 64(2):303-310. · 1.23 Impact Factor
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ABSTRACT: Precise measurement of the carbon isotopic compositions of dissolved inorganic carbon (DIC) in water samples is very important for earth and environmental sciences, and the H(3) PO(4) acidification method has long been applied for this purpose. Due to the large variation in DIC concentration in various types of water samples, however, it is difficult to sample the optimized amount of water for high-precision mass spectrometry analysis and in many cases this has led to poor data quality. In addition, when the amount of water sample is very limited or when the water has a complicated composition such as seawater and saline marine pore waters, it is often difficult to obtain high-quality data using the conventional method.
Here we report a new method of obtaining CO(2) -water equilibrium for high-precision carbon isotope analysis of DIC in water samples. The instrument used was a Delta Plus XP stable isotope mass spectrometer coupled with an on-line a Gas Bench II sample preparation device. In general, 0.5 mL of water is sampled and equilibrated with CO(2) in headspace. The CO(2) is then transferred into the mass spectrometer and δ(13) C values are obtained.
The δ(13) C values for four marine pore water samples determined by this new method are only slightly different (<0.15‰) from those determined using the conventional H(3) PO(4) method. The new method of obtaining CO(2) -water equilibrium also allows a constant amount of CO(2) for every sample to finally enter the mass spectrometer and hence better precision can be achieved.
This new method is suitable for the measurement of both oxygen isotopic composition in water and carbon isotopic composition of DIC for seawater and pore water samples. Another benefit of this new method is that the water sample can be re-used for other further analyses and this is critical when the amount of sample is limited.
Rapid Communications in Mass Spectrometry 04/2012; 26(7):805-10. · 2.79 Impact Factor
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ABSTRACT: The cause of the most marked changes in the evolution of life, which define the first-order stratigraphic boundary between the Precambrian and the Phanerozoic eon, remains enigmatic and a highly topical subject of debate. A global ocean anoxic event, triggered by large-scale hydrogen sulphide (H(2)S) release to surface waters, has been suggested by Wille et al., on the basis of two data sets from South China and Oman, to explain the fundamental biological changes across the Precambrian/Cambrian (PC/C) boundary. Here we report a new precise SHRIMP U-Pb zircon age of 532.3 +/- 0.7 million years (Myr) ago (Fig. 1) for a volcanic ash bed in the critical unit that reflects the ocean anoxic event, the lowermost black shale sequence of the Niutitang Formation in the Guizhou Province, South China. This age is significantly younger than the precise PC/C boundary age of 542.0 +/- 0.3 Myr ago, approximately 10 Myr younger than the extinction of the Ediacaran fauna, and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the PC/C boundary.
Nature 07/2009; 459(7248):E5-6; discussion E6. · 36.28 Impact Factor
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ABSTRACT: This paper reports all available geochemical data on sediments and pore waters from the Xisha Trough on the northern continental
margin of the South China Sea. The methane concentrations in marine sediments display a downhole increasing trend and their
carbon isotopic compositions (δ
13C = −25 to −51‰) indicate a thermogenic origin. Pore water Cl− concentrations show a range from 537 to 730 mM, and the high Cl− samples also have higher concentrations of Br−, Na+, K+, and Mg2+, implying mixing between normal seawater and brine in the basin. The SO4
2− concentrations of pore waters vary from 19.9 to 36.8 mM, and show a downhole decreasing trend. Calculated SMI (sulfate-methane
interfaces) depths and sulfate gradients are between 21 and 47 mbsf, and between −0.7 and −1.7 mM/m, respectively, which are
similar to values in gas hydrate locations worldwide and suggest a high methane flux in the basin. Overall, the geochemical
data, together with geological and geophysical evidence, such as the high sedimentation rates, high organic carbon contents,
thick sediment piles, salt and mud diapirs, active faulting, abundant thermogenic gases, and occurrence of huge bottom simulating
reflector (BSR), are suggestive of a favorable condition for occurrence of gas hydrates in this region.
Journal of Oceanography 05/2008; 64(3):459-470. · 1.23 Impact Factor
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ABSTRACT: Except for direct drilling and sampling of marine gas hydrates, the occurrence of gas hydrates has been identified generally by inference from indirect evidence, derived from geological, geophysical, and geochemical data. In this paper, we intend to discuss the geochemical anomalies of pore waters and their implications for gas hydrate occurrence in the northern continental slope of the South China Sea. The molecular concentration and isotopic composition of methane in sediments can provide clues to gas sources, whereas ionic and isotopic compositions of pore waters, such as steep SO42- gradients, shallow SMI (sulfate-methane interface) depths; decreasing pore water chlorinity, and heavy oxygen isotopic compositions, are used to identify gas hydrate occurrence and the distribution and thickness of sediment layers containing gas hydrates. Other good geochemical indicators include anions and cations concentrations such as Br-, I-, PO43-, NH4+, Ca2+, Mg2+, Sr2+, B3+, Li+, and Ba2+ in pore waters. We also found that the very negative carbon isotopic compositions of dissolve inorganic carbon (DIC) in pore waters can serve as good indicators for gas hydrate occurrence. In the South China Sea, three most promising target areas for gas hydrates include the Dongsha, Shenhu, and Xisha Trough. Other Other
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ABSTRACT: The northern margin of South China Sea contains several favorable areas for occurrence of gas hydrate. In this study, we collected pore water samples in two piston cores (X-01 and D-01) from Xisha Trough and Dongsha area, respectively, and the concentrations of sulfate and carbon isotopic compositions of dissolved inorganic carbon (DIC) were measured. The results showed different geochemical characteristics in these two sites. The X-01 core shows relatively constant δ13C-DIC values and sulfate concentrations, which suggest that anaerobic methane oxidation (AMO) processes did not occur in this site. In contrast, very large variation in δ13C-DIC values and sulfate concentrations are revealed in D-01 core, and good linear correlations for sulfate gradients and δ13C-DIC values are observed. The calculated sulfate-methane interface (SMI) depth is 9.6 mbsf. These data indicate that an AMO process occurred in sediments with large methane flux from depth in the Dongsha area, which are comparable to other gas hydrate locations in the world oceans such as the Blake Ridge. We suggest that the Dongsha area is one of the most favorable targets for future gas hydrate exploration. Other Other
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ABSTRACT: Trace element and rare earth element (REE) concentrations in cold seep carbonates from the northern continental slope of the South China Sea are used in this study to indicate source fluid characteristics and redox conditions. Carbonate samples from the two study areas (Shenhu and Dongsha) all show low total REE concentrations (mostly 10–20 ppm). In the Shenhu area, the shale-normalized REE patterns of the chimney carbonates display slight light REE enrichment, positive Ce anomaly, and a consistently positive Gd anomaly. These carbonate chimneys show a two-stage formation history, with a slight increase in the positive Ce anomaly from the rim to core. Trace and rare earth element data suggest that the core may have formed in a more anoxic condition than the rim. In the Dongsha area, seep carbonate samples show a slight heavy REE enrichment, with both negative and positive Ce anomalies, and more positive Gd anomalies, which are consistent with precipitation in an anoxic environment. Redox sensitive trace elements, such as Mo, U, Ni, V and Co, also indicate anoxic conditions for this cold seep carbonate precipitation.
Marine Geology. 277:21-30.
