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1.
Geochemistry of pore waters from the Xisha Trough, northern South China Sea and their implications for gas hydrates 总被引:2,自引:0,他引:2
Shao-Yong Jiang Tao Yang Lu Ge Jing-Hong Yang Hong-Fei Ling Neng-You Wu Jian Liu Dao-Hua Chen 《Journal of Oceanography》2008,64(3):459-470
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. 相似文献
2.
Dissolved inorganic carbon (DIC) and its carbon isotopic composition in sediment pore waters from the Shenhu area,northern South China Sea 总被引:2,自引:0,他引:2
Tao Yang Shao-Yong Jiang Jing-Hong Yang Ge Lu Neng-You Wu Jian Liu Dao-Hua Chen 《Journal of Oceanography》2008,64(2):303-310
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. 相似文献
3.
Variations of methane induced pyrite formation in the accretionary wedge sediments offshore southwestern Taiwan 总被引:4,自引:0,他引:4
Yee Cheng Lim Saulwood Lin Tsanyao Frank Yang Yue-Gau Chen Char-Shine Liu 《Marine and Petroleum Geology》2011,28(10):1829-1837
The accretionary wedge of offshore southwestern Taiwan contains abundant deposits of gas hydrate beneath the sea floor. High concentrations of methane in pore waters are observed at several locations with little data concerning historical methane venting available. To understand temporal variation of methane venting in sediments over geologic time, a 23-m-long Calypso piston core (MD05-2911) was collected on the flank of the Yung-An Ridge. Pore water sulfate, dissolved sulfide, dissolved iron, methane, sedimentary pyrite, acid volatile sulfide, reactive iron, organic carbon and nitrogen as well as carbonate δ13C were analyzed.Three zones with markedly different pyrite concentration were found at the study site. Unit I sediments (>20 mbsf) were characterized with a high amount of pyrite (251–380 μmol/g) and a δ13C-depleted carbonate, Unit II sediments (15–20 mbsf) with a low pyrite (15–43 μmol/g) and a high content of iron oxide mineral and Unit III sediments (<10 mbsf) by a present-day sulfate–methane interface (SMI) at 5 m with a high amount of pyrite (84–221 μmol/g) and a high concentration of dissolved sulfide.The oscillation records of pyrite concentrations are controlled by temporal variations of methane flux. With an abundant supply of methane to Unit I and III, anaerobic methane oxidation and associated sulfate reduction favor diagenetic conditions conducive for significant pyrite formation. No AOM signal was found in Unit II, characterized by typical organically-limited normal marine sediments with little pyrite formation. The AOM induced pyrite formation near the SMI generates a marked pyrite signature, rendering such formation of pyrite as a useful proxy in identifying methane flux oscillation in a methane flux fluctuate environment. 相似文献
4.
Stable Carbon and Nitrogen Isotopic Characterization of Organic Matter in a Mangrove Ecosystem on the Southwestern Coast of Thailand 总被引:1,自引:0,他引:1
Organic matter in a tropical mangrove ecosystem was characterized by stable carbon and nitrogen isotopic analyze, conducted
on various organic samples, including land and mangrove plants, soils, particulate organic matter (POM), and sea and river
sediments along the southwestern coast of Thailand. The δ13C values of land plants and POM in river water can be explained in terms of a greater influence of C3 plants than C4 plants in this area. The POM and sediments from the Trang River and Ko Talibong area showed systematically higher δ15N values than those from Ko Muk and other coastal areas. Organic matter in the Trang River might be influenced by nitrogen
released from agricultural or human waste, which could affect the isotopic composition of POM and sediments in the Trang River
estuary and along the coast near the river mouth. We used a stochastic method to estimate the contributions of four organic
end-members, identifiable by their δ13C and δ15N values. The results implied that seagrasses were a major source of sedimentary organic matter, contributing 42 ± 5% in the
Ko Muk area and 36 ± 5% in the Ko Talibong area. The contribution of coastal POM to sediments was estimated to be only 13%
in Ko Muk and 19% in Ko Talibong. Mangrove plants contributed approximately 23% in both areas. It was concluded that seagrasses
are an important source of sedimentary organic matter in this coastal region of southwestern Thailand.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
5.
Surface sediment from the coastal bays of Gwangyang and Masan in South Korea were analyzed for their contents and isotopic
values of organic carbon and total nitrogen. The sources and diagenetic alteration of organic matter were also assessed. Total
organic carbon varied from 0.22% to 3.48% (average = 1.40%, n = 75), and C/N ratios varied from 2.4 to 15.2 (average = 8.79,
n = 75). δ13Corg ranged from −19.92‰ to −25.86‰ (average = −21.21‰, n = 75), and δ15NTN ranged from 8.57‰ to 3.93‰ (average = 6.49‰, n = 75). Total organic carbon in both areas was associated with grain-size,
with higher contents in finer grained sediment. The high carbon content observed in Masan Bay sediment correlated with its
higher C/N ratio. δ13Corg and δ15NTN varied widely, attributable to various influences such as the input of terrestrial organic matter and diagenetic alteration.
The depleted δ13Corg and higher δ15NTN observed in the sediment of Gwangyang Bay reflected terrestrial supply, implying that biogeochemical processes, i.e. bacterial
degradation, were more active in Masan Bay sediment, which showed less depleted δ13Corg and higher δ15NTN than Gwangyang Bay sediment. δ15NTN was the more useful indicator of biogeochemical processes in the highly anoxic sediment. These results indicate that the
δ13Corg and δ15NTN of sedimentary organic matter in coastal bays can indicate the source and degree of diagenetic alteration of sedimentary
organic matter. 相似文献
6.
Organic carbon accumulation and sulfate reduction rates in slope and basin sediments of the Ulleung Basin,East/Japan Sea 总被引:1,自引:0,他引:1
This study investigated the organic carbon accumulation rates (OCARs) and sulfate reduction rates (SRRs) in slope and basin
sediments of the Ulleung Basin, East/Japan Sea. These sediments have high organic contents at depths greater than 2,000 m;
this is rare for deep-sea sediments, except for those of the Black Sea and Chilean upwelling regions. The mean organic carbon
to total nitrogen molar ratio was estimated to be 6.98 in the Ulleung Basin sediments, indicating that the organic matter
is predominantly of marine origin. Strong organic carbon enrichment in the Ulleung Basin appears to result from high export
production, and low dilution by inputs of terrestrial materials and calcium carbonate. Apparent sedimentation rates, calculated
primarily from excess 210Pb distribution below the zone of sediment mixing, varied from 0.033 to 0.116 cm year−1, agreeing well with previous results for the basin. OCARs fluctuated strongly in the range of 2.06–12.5 g C m−2 year−1, these rates being four times higher at the slope sites than at the basin sites. Within the top 15 cm of the sediment, the
integrated SRRs ranged from 0.72 to 1.89 mmol m−2 day−1, with rates approximately twice as high in the slope areas as in the basin areas. SRR values were consistently higher in
areas of high sedimentation and of high organic carbon accumulation, correlating well with apparent sedimentation rates and
OCARs. The sulfate reduction rates recorded in the basin and slope sediments of the Ulleung Basin are higher than those reported
for other parts of the world, with the exception of the Peruvian and Chilean upwelling regions. This is consistent with the
high organic carbon contents of surface sediments of the Ulleung Basin, suggesting enhanced organic matter fluxes. 相似文献
7.
Stable isotopes of carbon and nitrogen in suspended matter and sediments from the Godavari estuary 总被引:2,自引:0,他引:2
V. V. S. S. Sarma J. Arya Ch. V. Subbaiah S. A. Naidu L. Gawade P. Praveen Kumar N. P. C. Reddy 《Journal of Oceanography》2012,68(2):307-319
Spatial distribution of the carbon and nitrogen content and their isotopic enrichment in suspended matter and sediments were
measured in the Godavari estuary to identify the sources and transformation mechanism of organic matter. Significant variability
in isotopic distribution was found over the entire length of the Godavari estuary, suggesting multiple sources of organic
matter. The mean isotopic ratios (δ13Csed −25.1 ± 0.9, δ13Csus −24.9 ± 1, δ15Nsed 8.0 ± 2 and δ15Nsus 6.5 ± 0.9‰) and elemental concentrations (Csed 0.45 ± 0.2%, Csus 0.9 ± 0.7%, Nsed 0.07 ± 0.05% and Nsus 0.16 ± 0.1%) support a predominantly terrigenous source. Significant enrichment in the isotopic ratios of δ13C from the upper to lower estuary in both suspended (−27.5 and −24.3‰, respectively) and sedimentary (−26.2 and −24.9‰, respectively)
phases indicates a decrease in the influence of terrigeneous material toward the mouth of the estuary. A significant positive
relationship exists between the δ13C of suspended and sediment, which indicates that these two organic carbon pools are likely coupled in the form of a significant
exchange between the two phases. A positive relationship exists between chlorophyll a and suspended organic matter, which may mean that a significant source of organic carbon is the in situ produced phytoplankton.
But, applying a simple mixing model to our isotopes, data yielded about 46% as the contribution of the terrestrial source
to suspended matter, which may support the excessive heterotrophic activity in the Godavari estuary reported earlier. 相似文献
8.
A. S. Savvichev E. E. Zakharova E. F. Veslopolova I. I. Rusanov A. Yu. Lein M. V. Ivanov 《Oceanology》2010,50(6):893-908
The results of microbiological, biogeochemical, and isotope geochemical studies in the Kara Sea are described. The samples
for these studies were obtained during the 54th voyage of the research vessel Akademik Mstislav Keldysh in September 2007. The studied area covered the northern, central, and southwestern parts of the Kara Sea and the Gulf of
Ob. The quantitative characteristics of the total bacterial population and the activity of the microbial processes in the
water column and bottom sediments were obtained. The total population of the bacterioplankton (BP) varied from 250000 cells/ml
in the northern water area to 3000000 cells/ml in the Gulf of Ob. The BP population depended on the content of the water suspension.
The net BP production was minimal in the central water area, amounting to 0.15–0.2 μg C/(l day), and maximal (0.5–0.75 μg
C/(l day)) in the Gulf of Ob. The organic material at the majority of the stations in the Ob transect predominantly contained
light carbon isotopes (−28.0 to −30.18‰) of terrigenous origin. The methane content in the surface water layer varied from
0.18 to 2.0 μl CH4/l, and the methane oxidation rate changed in the range of 0.1–100 nl CH4/(l day). The methane concentration in the upper sediment layer varied from 30 to 300 μl CH4/dm3; the rate of the methanogenesis was 44 to 500 nl CH4/(dm3 day) and that of the methane oxidation, 30 to 2000 nl CH4/(dm3 day). The rate of the sulfate reduction varied from 4 to 184 μg S/(dm3 day). 相似文献
9.
Molecular and isotopic characteristics of gas hydrate-bound hydrocarbons in southern and central Lake Baikal 总被引:4,自引:4,他引:0
Akihiro Hachikubo Oleg Khlystov Alexey Krylov Hirotoshi Sakagami Hirotsugu Minami Yutaka Nunokawa Satoshi Yamashita Nobuo Takahashi Hitoshi Shoji Shin’ya Nishio Masato Kida Takao Ebinuma Gennadiy Kalmychkov Jeffrey Poort 《Geo-Marine Letters》2010,30(3-4):321-329
We investigated the molecular composition (methane, ethane, and propane) and stable isotope composition (methane and ethane) of hydrate-bound gas in sediments of Lake Baikal. Hydrate-bearing sediment cores were retrieved from eight gas seep sites, located in the southern and central Baikal basins. Empirical classification of the methane stable isotopes (δ13C and δD) for all the seep sites indicated the dominant microbial origin of methane via methyl-type fermentation; however, a mixture of thermogenic and microbial gases resulted in relatively high methane δ13C signatures at two sites where ethane δ13C indicated a typical thermogenic origin. At one of the sites in the southern Baikal basin, we found gas hydrates of enclathrated microbial ethane in which 13C and deuterium were both highly depleted (mean δ13C and δD of –61.6‰ V-PDB and –285.4‰ V-SMOW, respectively). To the best of our knowledge, this is the first report of C2 δ13C–δD classification for hydrate-bound gas in either freshwater or marine environments. 相似文献
10.
Ryo Sugimoto Akihide Kasai Toshihiro Miyajima Kouichi Fujita 《Journal of Oceanography》2008,64(1):39-48
Temporal changes in nitrogen isotopic composition (δ15N) of the NO3
− pool in the water column below the pycnocline in Ise Bay, Japan were investigated to evaluate the effect of nitrification
on the change in the δ15N in the water column. The δ15N of NO3
− in the lower layers varied from −8.5‰ in May to +8.4‰ in July in response to the development of seasonal hypoxia and conversion
from NH4
+ to NO3
−. The significantly 15N-depleted NO3
− in May most likely arose from nitrification in the water column. The calculated apparent isotopic discrimination for water
column nitrification (ɛnit = δ15Nsubstrate − δ15Nproduct) was 24.5‰, which lies within the range of previous laboratory-based estimates. Though prominent deficits of NO3
− from hypoxic bottom waters due to denitrification were revealed in July, the isotopic discrimination of denitrification in
the sediments was low (ɛdenit = ∼1‰). δ15NNO3 in the hypoxic lower layer mainly reflects the isotopic effect of water column nitrification, given that water column nitrification
is not directly linked with sedimentary denitrification and the effect of sedimentary denitrification on the change in δ15NNO3 is relatively small. 相似文献
11.
Microbially mediated methane and sulfur cycling in pockmark sediments of the Gdansk Basin, Baltic Sea 总被引:2,自引:2,他引:0
Nikolay V. Pimenov Marina O. Ulyanova Timur A. Kanapatsky Elena F. Veslopolova Pavel A. Sigalevich Vadim V. Sivkov 《Geo-Marine Letters》2010,30(3-4):439-448
In the Russian sector of the Gdansk Basin (Baltic Sea), high organic matter influx fuels microbial processes resulting in the formation of reduced sediments with elevated methane concentrations. Investigated areas of geoacoustic anomalies (~245 km2) were found to contain three distinct geomorphologic structures (pockmarks), with a total area of ~1 km2. Methane anomalies recorded in the water above one of these pockmarks were traced as high as 10 m above the bottom. In pockmark sediments, sulfate reduction and anaerobic oxidation of methane (AOM) occurred at high rates of 33 and 50 µmol dm?3 day?1, respectively. Integrated over 0–180 cm sediment depths, AOM exceeded methanogenesis almost tenfold. High AOM rates resulted from methane influx from deeper sediment layers. The δ13C signature of methane carbon (?78.1 to ?71.1‰) indicates the biogenic origin of pockmark methane. In pockmark sediments, up to 70% of reduced sulfur compounds was possibly produced via AOM. 相似文献
12.
Data from piston cores collected from Carolina Rise and Blake Ridge, and from many DSDP/ODP sites indicate that extreme 13C-depletion of methane and ΣCO2 occurs within the uppermost methanogenic zone of continental rise sediments. We infer that 13C-depleted methane is generated near the top of the methanogenic zone when carbon of 13C-depleted ΣCO2, produced by microbially-mediated anaerobic methane oxidation, is recycled back to methane through CO2 reduction. Interstitial water and gas samples were collected in 27 piston cores, 16 of which penetrated through the sulfate reduction zone into methane-bearing sediments of the Carolina Rise and Blake Ridge. Isotopic measurements (δ13CCH4, δ13CCO2, δDCH4, and δDH2O) indicate that this methane is microbial in origin, produced by microbially-mediated CO2 reduction. Methane samples form two distinct isotopic pools. (1) Methane from a seafloor seep site shows a mean δ13CCH4 value of − 69 ± 2%., mirroring values found at ≥ 160 mbsf from a nearby DSDP site. (2) Twenty, areally-separated sites (sample depth, 10 to 25 mbsf) have δ13CCH4 values ranging from −85 to −103%., and δ13CCO2 as negative as −48%.. The very low δ13C values from the methane and CO2 pools highlight the importance of carbon cycling within continental rise sediments at and near the sulfate-methane boundary. 相似文献
13.
14.
Bulk δ15N values in surface sediment samples off the southwestern coast of Africa were measured to investigate the biogeochemical
processes occurring in the water column. Nitrate concentrations and the degree of utilization of the nitrate pool are the
predominant controls on sedimentary δ15N in the Benguela Current region. Denitrification does not appear to have had an important effect on the δ15N signal of these sediments and, based on δ15N and δ13C, there is little terrestrial input.
Received: 9 May 1996 / Revision received: 27 May 1997 相似文献
15.
Katja U. Heeschen Hans Jürgen Hohnberg Matthias Haeckel Friedrich Abegg Manuela Drews Gerhard Bohrmann 《Marine Chemistry》2007,107(4):498-515
Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct–Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C1 through C5) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg− 1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH4. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH4 concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis. 相似文献
16.
Late Pleistocene-Holocene Paleoceanography and Ventilation of the Gulf of California 总被引:1,自引:0,他引:1
Lloyd D. Keigwin 《Journal of Oceanography》2002,58(2):421-432
Sediment cores collected in 1990 from the Gulf of California have been studied using stable isotope and radiocarbon techniques
to reconstruct the climate and ventilation histories since the last glacial maximum. Benthic foraminiferal δ18O from core tops in a water depth range of 145 to 1442 m increases by about 2% with increasing depth. This is consistent with
a composite temperature profile constructed from several hydrocasts in the various gulf basins. However, the δ18O water/salinity relationship is not sufficiently linear in gulf locations or in nearby open Pacific Geochemical Ocean Sections
Study (GEOSECS) stations to be useful in solving paleotemperature equations. Of the most common benthic foraminifera, only
Planulina ariminensis has δ13C that is consistent with the measured δ13C of ΣCO2. Several cores in the depth range 500 to 900 m have the laminated Holocene and Bolling/Allerod sediments, and the nonlaminated
glacial age and Younger Dryas sediments that are typical of the gulf and other locations such as Santa Barbara Basin. The
best of those, Jumbo Piston Core (JPC) 56 from 818 m water depth on the western margin of Guaymas Basin, was sampled for intensive
study. Oxygen isotope ratios in benthic and planktonic foraminifera show little evidence for deglacial temperature oscillations.
Carbon isotope ratios are generally lower during warm epochs, but the most striking result is strongly lowered benthic and
planktonic δ13C about 9500 years ago. This may reflect water column oxidation of locally released methane. Neither benthic δ13C in depth section nor paired benthic and planktonic 14C data in JPC56 are consistent with increased intermediate water ventilation during the glacial maximum and Younger Dryas.
Likewise, 14C data from 5 pairs of foraminifera from the Okhotsk Sea fail to support better ventilation in that basin during the last
glacial maximum.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
17.
Franck Lartaud Laurent Emmanuel Marc de Rafelis Stephane Pouvreau Maurice Renard 《Geo-Marine Letters》2010,30(1):23-34
Compared to oxygen isotopes, the carbon isotope composition of biogenic carbonates is less commonly used as proxy for palaeoenvironmental
reconstructions because shell δ13C is derived from both dissolved inorganic (seawater) and organic carbon sources (food), and interactions between these two
pools make it difficult to unambiguously identify any independent effect of either. The main purpose of this study was to
demonstrate any direct impact of variable food supply on bivalve shell δ13C signatures, using low/high rations of a 13C-light mixed algal diet fed to 14-month-old (adult) cultured Japanese Crassostrea gigas under otherwise essentially identical in vitro conditions during 3 summer months (May, June and July 2003, seawater temperature
means at 16, 18 and 20 °C respectively) in experimental tanks at the Argenton laboratory along the Brittany Atlantic coast
of France. At a daily ration of 12% (versus 4%) oyster dry weight, the newly grown part of the shells (hinge region) showed
significantly lower δ13C values, by 3.5‰ (high ration: mean of −5.8 ± 1.1‰, n = 10; low ration: mean of −2.3 ± 0.7‰, n = 6; ANOVA Scheffe’s test, p < 0.0001). This can be explained by an enhanced metabolic activity at higher food supply, raising 13C-depleted respiratory CO2 in the extrapallial cavity. Based on these δ13C values and data extracted from the literature, and assuming no carbon isotope fractionation between food and shell, the
proportion of shell metabolic carbon would be 26 ± 7 and 5 ± 5% for the high- and low-ration C. gigas shells respectively; with carbon isotope fractionation (arguably more realistic), the corresponding values would be 69 ± 14
and 24 ± 9%. Both groups of cultured shells exhibited lower δ13C values than did wild oysters from Marennes-Ol éron Bay in the study region, which is not inconsistent with an independent
influence of diet type. Although there was no significant difference between the two food regimes in terms of δ18O shell values (means of 0.1 ± 0.3 and 0.4 ± 0.2‰ at high and low rations respectively, non-significant Scheffe’s test),
a positive δ13C vs. δ18O relationship recorded at high rations supports the interpretation of a progressive temperature-mediated rise in metabolic
activity fuelled by higher food supply (in this case reflecting increased energy investment in reproduction), in terms not
only of δ13C (metabolic signal) but also of δ18O (seawater temperature signal). Overall, whole-shell δ18O trends faithfully recorded summer/winter variations in seawater temperature experienced by the 17-month-old cultured oysters. 相似文献
18.
To understand the processes transporting nitrate to the surface layer of the western and central equatorial Pacific, we measured
the nitrogen isotopic ratio of nitrate (δ
15NO
3
−
), which is a very useful tracer of the source of nitrate, above 200 m depth in this region in December 1999. δ
15NO
3
−
is higher (about 13.0‰) in the surface water than in the subsurface water (where it is about 6.5‰) due to isotopic fractionation
during nitrate uptake by phytoplankton. The δ
15NO
3
−
value has a roughly linear relationship with the natural logarithm of nitrate concentration (ln[NO
3
−
]). However, for values above 150 m depth, the intercept of this linear relationship varies with position from east to west.
On the other hand, the data at 200 m depth at all observation stations are concentrated around a single point (ln[NO
3
−
] = 2.5 and δ
15NO
3
−
= 6.5‰) and do not fit the linear relationships for the shallower values. To examine the meaning of the observed distributions
of δ
15NO
3
−
and nitrate concentration we developed a box model including nitrogen and nitrogen isotopic cycles. By reproducing the observed
relationship between δ
15NO
3
−
and nitrate concentration using this model we found that most nitrate is transported horizontally from the eastern equatorial
Pacific. We also conducted case studies and investigated the effects of differences in pathways of nitrate transport on the
distributions of δ
15NO
3
−
and nitrate concentration. From these studies we concluded that the observed linear relationships between δ
15NO
3
−
and ln[NO
3
−
], having a common slope around 6‰ but different intercepts at each station, are evidence of the significant horizontal transport
of nitrate to the surface water in this area. 相似文献
19.
Ikaite crystals (CaCO3×6H2O) have been found at 232- to 238-cm sediment depth in R/V Polarstern core PS2460-4 from the Laptev Sea continental margin in a water depth of 204 m. δ13C values of this phase average −36.3±0.4‰ PDB (N=2), which is significantly outside the range of normal marine carbonates. The CO2 involved in the precipitation of the ikaite is most probably derived from methane, which has extremely depleted 13C isotope values. Two possible sources of methane in these sediments are: (1) methanogenesis (decomposition of organic matter
under anaerobic conditions); and (2) gas hydrates, which are known to occur in the Siberian shelf regions.
Received: 20 March 1996 / Revision received: 22 July 1996 相似文献
20.
Isotopic composition of gas hydrates in subsurface sediments from offshore Sakhalin Island, Sea of Okhotsk 总被引:1,自引:1,他引:0
Akihiro Hachikubo Alexey Krylov Hirotoshi Sakagami Hirotsugu Minami Yutaka Nunokawa Hitoshi Shoji Tatiana Matveeva Young K. Jin Anatoly Obzhirov 《Geo-Marine Letters》2010,30(3-4):313-319
Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C1/C2+ from all the seepage sites were in the range of 1,500–50,000, while δ13C and δD values of methane ranged from ?66.0 to ?63.2‰ VPDB and ?204.6 to ?196.7‰ VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO2. δ13C values of ethane and propane (i.e., ?40.8 to ?27.4‰ VPDB and ?41.3 to ?30.6‰ VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the δD hydrate gas was smaller than that of dissolved gas was in the range 4.3–16.6‰, while there were no differences in δ13C values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites. 相似文献