共查询到20条相似文献,搜索用时 718 毫秒
1.
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
2.
Keiji Horikawa Masao Minagawa Yoshihisa Kato Masafumi Murayama Seiya Nagao 《Journal of Oceanography》2006,62(4):427-439
N2 fixation is an important biological process that adds new nitrogen to oceans and plays a key role in modulating the oceanic
nitrate inventory. However, it is not known how, when, and where N2 fixation rates have varied in response to past climate changes. This study presents a new record of nitrogen isotopic composition
(δ15N) over the last 83 kyr from a sediment core (KH02-4 SUP8) taken in the Sulu Sea in the western equatorial Pacific region;
data allow the N2 fixation variability in the sea to be reconstructed. Sediments, sinking, and suspended particulate organic matter (POM) all
have lighter isotopic values compared to the δ15N values of substrate nitrate (av. 5.8‰) in North Pacific Intermediate Water. These lighter δ15N values are regarded as reflecting N2 fixation in the Sulu Sea surface water. A δ15N mass balance model shows that N2 fixation rates were significantly enhanced during 54–34 kyr in MIS-3 and MIS-2. It has been speculated that higher interglacial
denitrification rates in the Arabian Sea and the eastern tropical Pacific would have markedly decreased the global oceanic
N inventory and contributed to the increase in N2 fixation in oligotrophic regions, but such a model was not revealed by our study. It is possible that changes in N2 fixation rates in the Sulu Sea were regional response, and accumulation of phosphate in the surface waters due to enhanced
monsoon-driven mixing is thought to have stimulated enhancements of N2 fixation during MIS-3 and MIS-2. 相似文献
3.
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
4.
Assessment of nitrogen loading from the Kiso-Sansen Rivers into Ise Bay using stable isotopes 总被引:1,自引:0,他引:1
Ryo Sugimoto Akihide Kasai Kouichi Fujita Kenichi Sakaguchi Tomomi Mizuno 《Journal of Oceanography》2011,67(2):231-240
Concentrations of particulate organic nitrogen (PN), dissolved inorganic nitrogen (DIN), and their nitrogen isotope ratios
(δ
15N) in the Kiso-Sansen Rivers were determined from monthly observations over the course of a year to assess variations in the
form and sources of riverine nitrogen discharged into Ise Bay. The δ
15N values of NO3
− observed in the Kiso-Sansen Rivers showed a logarithmic decreasing trend from 8 to 0‰, which varied with the river discharge,
indicating mixing between point sources with high δ
15N and non-point sources with low δ
15N. The influence of isotope fractionation of in situ biogeochemical processes (mainly DIN assimilation by phytoplankton) on
δ
15N of NO3
− was negligible, because sufficient concentrations of NH4
+ for phytoplankton demand would inhibit the assimilation of NO3
−. A simple relationship between river discharge and δ
15N of NO3
− showed that the fraction of total NO3
− flux arising from point sources increased from 4.0–6.3% (1.1–1.8 tN day−1) during higher discharge (>600 m3 s−1) to 30.2–48.3% (2.6–4.1 tN day−1) during lower discharge (<300 m3 s−1). Riverine NO3
− discharge from the Kiso-Sansen Rivers can explain 75% of the variations in surface NO3
− at the head of Ise Bay over the year. 相似文献
5.
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
6.
Modified Method for the Analysis of Nitrogen Isotopic Composition of Oceanic Nitrate at Low Concentration 总被引:1,自引:0,他引:1
A method for the determination of the δ15N of nitrate in seawater described by Cline and Kaplan (1975) has been modified for application to low-level nitrate samples.
We have minimized the reagent blank problem by replacing the Devarda's alloy with an aluminum reagent, and have also established
a procedure that yields quantitative (93 ± 2%) extraction of nitrogen even at low nitrate levels. Though the amounts and the
δ15N of the blank N varied from one reagent set to another, with these modifications, an overall N blank was reduced to approximately
0.80 ± 0.33 μmole N having an estimated δ15N value of −1.8‰. After blank and yield corrections, the measured isotopic composition of nitrate differed by approximately
0.1‰ from the actual value while the precision was within ±0.2‰ at the 1.25 μM level. The modified procedure was applied to
seawater samples collected from the equatorial Pacific in order to compare the N blanks in field samples with those derived
from laboratory experiments. The results support the suitability of the modified approach for isotopic analysis of oceanic
nitrate in shallow water.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
A method has been developed for determination of15N isotope ratio in nitrate nitrogen, which is a major analytical step in tracer experiments for studies of nitrate metabolism
in the marine environment. The method is based on diazotization of nitrite with sulfanilic acid following reduction of nitrate
to nitrite by a cadmium-copper column. The diazonium compound is then subject to the azo coupling reaction with 2-naphthol,
and the azo dye formed is extracted by a solid phase extraction column. The dye eluted from the column is collected, and total
nitrogen and15N content of the dye are determined by mass spectrometry. Sulfanilic acid can also remove preexisting nitrite by heating the
sample under acidic conditions before passing through the cadmium-copper reduction column. The average recovery of nitrate
nitrogen was 86%. A procedure for reducing the background nitrogen that derives from the analytical operations has been developed;
background nitrogen was limited to about 0.25 μg-atomN. The variation in the background nitrogen levels reflects the range of error in15N determination of nitrate nitrogen by this method. Application of the present method to a15NO3
− isotope dilution experiment for determination of nitrification rate in sea water is demonstrated. 相似文献
8.
海洋中的氮循环是海洋生物地球化学研究的热点领域之一,而硝化过程是氮循环的关键一环,准确获取硝化速率对于丰富海洋氮循环的认识至关重要。15N标记同位素技术是目前国际上最为广泛使用的硝化速率测定方法,该方法的核心在于准确测定15N加富样品产生的15NO2-和15NO3-的含量,但目前的方法普遍存在测试时间较长、测试成本较高、所需样品体积较大或者检测限较高等问题。研究以低成本的膜进样质谱作为15N加富样品测试设备,建立了基于镉柱与氨基磺酸双还原体系测定15N加富样品中15NO3-含量的方法。经条件优化实验确定的具体方法:采用1 mol/L HCl配制15 mmol/L的氨基磺酸(SA)作为反应试剂除去样品原有的NO2-,然后利用镉柱将15NO 相似文献
9.
The mechanisms regulating N2O production in an estuarine sediment (Tama Estuary, Japan) were studied by comparing the change in N2O production with those in nitrification and denitrification using an experimental continuous-flow sediment–water system with15N tracer (15N-NO−3 addition). From Feburary to May, both nitrification and denitrification in the sediment increased (246 to 716 μmol N m−2 h−1and 214 to 1260 μmol N m−2 h−1, respectively), while benthic N2O evolution decreased slightly (1560 to 1250 nmol N m−2 h−1). Apparent diffusion coefficients of inorganic nitrogen compounds and O2at the sediment–water interface, calculated from the respective concentration gradients and benthic fluxes, were close to the molecular diffusion coefficients (0·68–2·0 times) in February. However, they increased to 8·8–52 times in May except for that of NO−2, suggesting that the enhanced NO−3 and O2supply from the overlying water by benthic irrigation likely stimulated nitrification and denitrification. Since the progress of anoxic condition by the rise of temperature from February to May (9 to 16 °C) presumably accelerated N2O production through nitrification, the observed decrease in sedimentary N2O production seems to be attributed to the decrease in N2O production/occurrence of its consumption by denitrification. In addition to the activities of both nitrification and denitrification, the change in N2O metabolism during denitrification by the balance between total demand of the electron acceptor and supply of NO−3+NO−2 can be an important factor regulating N2O production in nearshore sediments. 相似文献
10.
T. Yoshinari M. A. Altabet S. W. A. Naqvi L. Codispoti A. Jayakumar M. Kuhland A. Devol 《Marine Chemistry》1997,56(3-4)
Nitrous oxide (N2O) is a trace gas that is increasing in the atmosphere. It contributes to the greenhouse effect and influences the global ozone distribution. Recent reports suggest that regions such as the Arabian Sea may be significant sources of atmospheric N2O.In the ocean, N2O is formed as a by-product of nitrification and as an intermediary of denitrification. In the latter process, N2O can be further reduced to N2. These processes, which operate on different source pools and have different magnitudes of isotopic fractionation, make separate contributions to the 15N and18O isotopic composition of N2O. In the case of nitrification in oxic waters, the isotopic composition of N2O appears to depend mainly on the 15N/14N ratio of NH+4 and the 18O/16O ratio of O2 and H2O. In suboxic waters, denitrification causes progressive 15N and 18O enrichment of N2O as a function of degree of depletion of nitrate and dissolved oxygen. Thus the isotopic signature of N2O should be a useful tool for studying the sources and sinks for N2O in the ocean and its impact on the atmosphere.We have made observations of N2O concentrations and of the dual stable isotopic composition of N2O in the eastern tropical North Pacific (ETNP) and the Arabian Sea. The stable isotopic composition of N2O was determined by a new method that required only 80–100 nmol of N2O per sample analysis. Our observations include determinations across the oxic/suboxic boundaries that occur in the water columns of the ETNP and Arabian Sea. In these suboxic waters, the values of δ15N and δ18O increased linearly with one another and with decreasing N2O concentrations, presumably reflecting the effects of denitrification. Our results suggest that the ocean could be an important source of isotopically enriched N2O to the atmosphere. 相似文献
11.
An Ecosystem Model Including Nitrogen Isotopes: Perspectives on a Study of the Marine Nitrogen Cycle
We have developed an ecosystem model including two nitrogen isotopes (14N and 15N), and validated this model using an actual data set. A study of nitrogen isotopic ratios (δ15N) using a marine ecosystem model is thought to be most helpful in quantitatively understanding the marine nitrogen cycle.
Moreover, the model study may indicate a new potential of δ15N as a tracer. This model has six compartments: phytoplankton, zooplankton, particulate organic nitrogen, dissolved organic
nitrogen, nitrate and ammonium in a two-box model, and has biological processes with/without isotopic fractionation. We have
applied this model to the Sea of Okhotsk and successfully reproduced the δ15N of nitrate measured in seawater and the seasonal variations in δ15N of sinking particles obtained from sediment trap experiments. Simulated δ15N of phytoplankton are determined by δ 15N of nitrate and ammonium, and the nitrogen f-ratio, defined as the ratio of nitrate assimilation by phytoplankton to total nitrogenous nutrient assimilation. Detailed
considerations of biological processes in the spring and autumn blooms have demonstrated that there is a significant difference
between simulated δ15N values of phytoplankton, which assimilates only nitrate, and only ammonium, respectively. We suggest that observations of
δ 15N values of phytoplankton, nitrate and ammonium in the spring and autumn blooms may indicate the ratios of nutrient selectivity
by phytoplankton. In winter, most of the simulated biogeochemical fluxes decrease rapidly, but nitrification flux decreases
much more slowly than the other biogeochemical fluxes. Therefore, simulated δ15N values and concentrations of ammonium reflect almost only nitrification. We suggest that the nitrification rate can be parameterized
with observations of δ15N of ammonium in winter and a sensitive study varying the parameter of nitrification rate. 相似文献
12.
Masao Minagawa Mami Ohashi Toshikatsu Kuramoto Norihiro Noda 《Journal of Oceanography》2001,57(3):285-300
Nitrogen isotope compositions of particulate organic matter and nitrate were analyzed for seawater sampled at five stations at the Alaskan Gyre, Western Subarctic Gyre and East China Sea, focusing on the samples from the surface to 5000 m water to characterize the nitrogen cycling in the subarctic North Pacific Ocean and its marginal sea. The 15N of particulate organic matter showed little agreement with a conceptual closed model that interprets isotopic variation as being caused by isotope discrimination on nitrate utilization. The 15N and 13C of particulate organic matter varied with the water depth. A correlation between isotope compositions and C/N elemental ratio was found generally at all stations, although some irregular data were also found in deep layers. We developed a hypothetical nitrogen balance model based on N2 fixation and denitrification in seawater and attempted to apply it to distinguish nutrient cycling using both 15N-NO3
– and N* variation in seawater. This model was applied to the observed data set of 15N-NO3
– and N* in the North Pacific water and estimated the 15N-NO3
– of primordial nitrate in the North Pacific deep water as 4.8. The North Pacific intermediate water for all stations showed similar 15N-NO3
– and N* values of 6 and –3 µmol/kg, respectively, suggesting a similar nitrogen biogeochemistry. In the East China Sea, analysis showed evidence of water exchange with the North Pacific intermediate water but a significant influence of nitrogen from the river runoff was found in depths shallower than 400 m. 相似文献
13.
Natural marine gas hydrate was discovered in Korean territorial waters during a 2007 KIGAM cruise to the central/southwestern
Ulleung Basin, East Sea. The first data on the geochemical characterization of hydrate-bound water and gas are presented here
for cold seep site 07GHP-10 in the central basin sector, together with analogous data for four sites (07GHP-01, 07GHP-02,
07GHP-03, and 07GHP-14) where no hydrates were detected in other cores from the central/southwestern sectors. Hydrate-bound
water displayed very low concentrations of major ions (Cl−, SO42−, Na+, Mg2+, K+, and Ca2+), and more positive δD (15.5‰) and δ18O (2.3‰) signatures compared to seawater. Cl– freshening and more positive isotopic values were also observed in the pore water at gas hydrate site 07GHP-10. The inferred
sulfate–methane interface (SMI) was very shallow (<5 mbsf) at least at four sites, suggesting the widespread occurrence of
anaerobic oxidation of methane (AOM) at shallow sediment depths, and possibly high methane flux. Around the SMI, pore water
alkalinity was very high (>40 mM), but the carbon isotopic ratios of dissolved inorganic carbon (δ13CDIC) did not show minimum values typical of AOM. Moreover, macroscopic authigenic carbonates were not observed at any of the
core sites. This can plausibly be explained by carbon with high δ13C values diffusing upward from below the SMI, increasing alkalinity via deep methanogenesis and eventually escaping as alkalinity
into the water column, with minor precipitation as solid phase. This contrasts, but is not inconsistent with recent reports
of methane-fuelled carbonate formation at other sites in the southwestern basin sector. Methane was the main hydrocarbon component
(>99.85%) of headspace, void, and hydrate-bound gases, C1/C2+ ratios were at least 1,000, and δ13CCH4 and δDCH4 values were in the typical range of methane generated by microbial reduction of CO2. This is supported by the δ13CC2H6 signatures of void and hydrate-bound gases, and helps clarify some contradictory interpretations existing for the Ulleung
Basin as a whole. In combination, these findings suggest that deep biogenic gas and pore waters migrate upward through pathways
such as hydrofractures, and measurably influence the shallow carbon cycle. As a result, cation-adjusted alkalinity/removed
sulfate diagrams cannot always serve to estimate the degree of alkalinity produced by sulfate reduction and AOM in high methane
flux areas. 相似文献
14.
We have elucidated the seasonal and spatial variation in the potential denitrifying activity in estuarine and coastal lagoonal
sediments in Lakes Shinji and Nakaumi, Japan. The denitrifying activity increased from summer through autumn and was positively
correlated with the temperature of the overlying water at all sites except one, where the bottom was always more reductive
than at the other sites and there was no NO3
− as a substrate for denitrification from spring to autumn. Moreover, the relationship between the denitrifying activity and
the distance from the sea showed different trends in estuarine and lagoonal sediments. These spatial differences indicate
that different factors regulate the denitrification in estuarine and lagoonal sediments. Denitrifying activity in estuarine
sediment was regulated by the discharge of freshwater containing NO3
− or organic matter, while in the lagoonal sediments the occurrence of nitrification via the intrusion of oxic seawater into
the reductive sediment appears to be a key requirement for the process of denitrification. Therefore, the denitrifying activity
in the lagoonal sediment appears to be greater near the sea. Water intrusion is one of the key factors controlling denitrification
in coastal marine ecosystems by affecting the supply of substrate available for denitrification. 相似文献
15.
Perran L. M. Cook Bradley D. Eyre Rhys Leeming Edward C. V. Butler 《Estuarine, Coastal and Shelf Science》2004,59(4):675-685
Benthic fluxes of dissolved inorganic nitrogen (NO3− and NH4+), dissolved organic nitrogen (DON), N2 (denitrification), O2 and TCO2 were measured in the tidal reaches of the Bremer River, south east Queensland, Australia. Measurements were made at three sites during summer and winter. Fluxes of NO3− were generally directed into the sediments at rates of up to −225 μmol N m−2 h−1. NH4+ was mostly taken up by the sediments at rates of up to −52 μmol N m−2 h−1, its ultimate fate probably being denitrification. DON fluxes were not significant during winter. During summer, fluxes of DON were observed both into (−105 μmol m−2 h−1) and out of (39 μmol m−2 h−1) the sediments. Average N2 fluxes at all sampling sites were similar during summer (162 μmol N m−2 h−1) and winter (153 μmol N m−2 h−1). Denitrification was fed both by nitrification within the sediment and NO3− from the water column. Sediment respiration rates played an important role in the dynamics of nitrification and denitrification. NO3− fluxes were significantly related to TCO2 fluxes (p<0.01), with a release of NO3− from the sediment only occurring at respiration rates below 1000 μmol C m−2 h−1. Rates of denitrification increased with respiration up to TCO2 fluxes of 1000 μmol C m−2 h−1. At sediment respiration rates above 1000 μmol C m−2 h−1, denitrification rates increased less rapidly with respiration in winter and declined during summer. On a monthly basis denitrification removed about 9% of the total nitrogen and 16% of NO3− entering the Bremer River system from known point sources. This is a similar magnitude to that estimated in other tidal river systems and estuaries receiving similar nitrogen loads. During flood events the amount of NO3− denitrified dropped to about 6% of the total river NO3− load. 相似文献
16.
L.J. Hou M. Liu S.Y. Xu D.N. Ou J. Yu S.B. Cheng X. Lin Y. Yang 《Estuarine, Coastal and Shelf Science》2007,73(3-4):607-616
To explore the influences of semi-lunar spring and neap tidal changes on nitrogen cycling in intertidal sediments, a comparative study among waterlogged, desiccated and reflooded systems was carried out in August 2005 and February 2006 by analyzing nitrification, denitrification and N2O depth profiles in the intertidal flats of the Yangtze estuary. Laboratory experiments showed that alternating emersion and inundation resulted in the significant changes in nitrification and denitrification rates in the intertidal sediment systems. Due to the desiccation-related effects, lowest nitrification and denitrification rates were observed in the desiccated sediment cores. Highest nitrification and denitrification rates were however detected in the waterlogged and reflooded systems, respectively. It is hypothesized that the highest nitrification rates in the waterlogged sediments were mainly attributed to higher nitrifier numbers and NH4+ being more available, whereas the availability of NO3− might dominate denitrification in the reflooded sediments. In addition, the highest N2O concentrations were detected in the reflooded sediment cores, and the lowest found in the dried sediment cores. It was also shown that N2O in the intertidal sediments was mainly from nitrification under the desiccated condition. In contrast, N2O in the intertidal sediments was produced mainly via denitrification under the waterlogged and reflooded conditions. It is therefore concluded that the semi-lunar tidal cycle has a significant influence on nitrification, denitrification and N2O production in the intertidal sediment systems. 相似文献
17.
The stable nitrogen isotope ratio (δ
15N) in macroalgae is effectively used as a time-integrated bioindicator to record nitrogen sources for primary producers during
their growing periods in aquatic ecosystems. However, the utility of this tool is limited because the occurrence of these
organisms is often restricted in space and time. To investigate the potential of chemical composition in sedimentary organic
matter (SOM) as a proxy for time-integrated environmental conditions, nitrogen (N) and carbon (C) contents and their stable
isotope ratios (δ
15N and δ
13C) were determined, and systematically cross-checked against corresponding values in macroalgae at the Shiraho fringing reef
in Okinawa, Japan. Preliminary trials showed that δ
15N in SOM processed by the “wash-out method” for δ
13C analysis yielded similar δ
15N values to the bulk sediment, despite the loss of some SOM during the process. The amounts of organic matter and the ratio
of the HCl-insoluble portion were variable within the reef, probably reflecting local vegetation and subsequent decomposition.
The distribution of δ
15N and δ
13C in SOM showed similar trends to those of macroalgae, with mostly constant differences of 1.4‰ and −6.7‰, respectively. These
differences throughout the reef appeared to be explained in terms of mixed contributions from macrophyte and epibenthic microalgae
growing in different seasons and years, with their debris undergoing diagenetic alteration. Therefore, macroalgae and SOM
δ-values can be used in a complementary manner, over various time scales, as indicators of the integrated effect of dissolved
inorganic nitrogen (DIN) sources on coral reef ecosystems. 相似文献
18.
A carbon,nitrogen, and sulfur elemental and isotopic study in dated sediment cores from the Louisiana Shelf 总被引:2,自引:0,他引:2
Robert J. Rosenbauer Peter W. Swarzenski Carol Kendall William H. Orem Frances D. Hostettler Mark E. Rollog 《Geo-Marine Letters》2009,29(6):415-429
Three sediment cores were collected off the Mississippi River delta on the Louisiana Shelf at sites that are variably influenced
by recurring, summer-time water-column hypoxia and fluvial loadings. The cores, with established chronology, were analyzed
for their respective carbon, nitrogen, and sulfur elemental and isotopic composition to examine variable organic matter inputs,
and to assess the sediment record for possible evidence of hypoxic events. Sediment from site MRJ03-3, which is located close
to the Mississippi Canyon and generally not influenced by summer-time hypoxia, is typical of marine sediment in that it contains
mostly marine algae and fine-grained material from the erosion of terrestrial C4 plants. Sediment from site MRJ03-2, located
closer to the mouth of the Mississippi River and at the periphery of the hypoxic zone (annual recurrence of summer-time hypoxia
>50%), is similar in composition to core MRJ03-3, but exhibits more isotopic and elemental variability down-core, suggesting
that this site is more directly influenced by river discharge. Site MRJ03-5 is located in an area of recurring hypoxia (annual
recurrence >75%), and is isotopically and elementally distinct from the other two cores. The carbon and nitrogen isotopic
composition of this core prior to 1960 is similar to average particulate organic matter from the lower Mississippi River,
and approaches the composition of C3 plants. This site likely receives a greater input of local terrestrial organic matter
to the sediment. After 1960 and to the present, a gradual shift to higher values of δ13C and δ15N and lower C:N ratios suggests that algal input to these shelf sediments increased as a result of increased productivity
and hypoxia. The values of C:S and δ34S reflect site-specific processes that may be influenced by the higher likelihood of recurring seasonal hypoxia. In particular,
the temporal variations in the C:S and δ34S down-core are likely caused by changes in the rate of sulfate reduction, and hence the degree of hypoxia in the overlying
water column. Based principally on the down-core C:N and C:S ratios and δ13C and δ34S profiles, sites MRJ03-3 and MRJ03-2 generally reflect more marine organic matter inputs, while site MRJ03-5 appears to be
more influenced by terrestrial deposition. 相似文献
19.
Data on the NO2 content in the vertical column of the atmosphere obtained with the Ozone Monitoring Instrument (OMI) aboard the EOS Aura
satellite (United States) in the period from October 2004 to October 2007 are compared with the results of ground-based measurements
at the Zvenigorod Scientific Station (55.7° N, 36.8° E). The “unpolluted”; part of the total NO2 content in the atmospheric column, which mostly represents the stratosphere, and the NO2 contents in the vertical column of the troposphere, including the lower layer, which is subject to pollution, are included
in the comparison. The correlation coefficient between the results of ground-based and satellite measurements of the “unpolluted”
total NO2 content is ∼0.9. The content values measured with the OMI instrument are smaller than the results of ground-based measurements
(on average, by (0.30 ± 0.03) × 1015 cm−2 or by (11 ± 1)%). Therms discrepancy between the satellite and ground-based data is 0.6 × 1015 cm−2. The NO2 content in the vertical column of the troposphere from the results of satellite measurements is, on average, (1.4 ± 0.5)
× 1015 cm−2, (or about 35%) smaller than from the results of ground-based measurements, and the rms discrepancy between them is about
200%. The correlation coefficient between these data is ∼0.4. This considerable discrepancy is evidently caused by the strong
spatial (horizontal) inhomogeneity and the temporal variability of the NO2 field during episodes of pollution, which leads to different (and often uncorrelated) estimates of the NO2 content in the lower troposphere due to different spatial resolutions of ground-based and satellite measurements. 相似文献
20.
We present high-resolution isotopic records and cathodoluminescence studies of recently dead and live bivalve specimens from
cold seeps, in an attempt to reconstruct environmental conditions during organism growth, and thereby the possible variability
of fluid-venting activity at the seafloor. Shells of the burrowing lucinid Myrtea aff. amorpha were collected at three localities near actively venting methane seeps in the Eastern Mediterranean deep sea, using the Nautile
submersible during two French oceanographic cruises: from the Kazan mud volcano, in the vicinity of the Anaximander mounts
(MEDINAUT cruise, 1998), and from the central pockmark province and the Amon mud volcano of the Nile deep-sea fan (NAUTINIL
cruise, 2003). The oxygen and carbon isotope compositions of 18 shells from the various localities, and also from different
sites at the same locality show a rather strong scatter (1.8 < δ
18O‰ < 3.4; −10.2 < δ
13C‰ < 2.2), and values lower than those expected for carbonate precipitated at equilibrium with present-day bottom waters.
This means that warm methane-rich fluids were mixed with bottom seawater during precipitation of shell carbonates. We have
tried to determine ontogenetic age of two shells by using cathodoluminescence as a sclerochronological proxy, because the
direct counting of carbonate increments was not possible in these specimens. There is a relatively good correspondence between
cathodoluminescence trends and oxygen isotope profiles that might support the link between manganese incorporation during
growth and temperature. Eight specimens of lucinid shells were selected for high-resolution isotopic profiling. A few shells
exhibit decreasing δ
18O and δ
13C values from the umbo to the actively growing ventral shell margin, which can be attributed to the commonly observed physiologically
controlled deceleration of growth with increasing organism age, this metabolic effect corresponding to the increase of incorporation
of respiratory CO2. A few shells exhibit high-frequency δ
18O variations with an amplitude of about 1.5‰ that might be related to temperature variations controlled by fluid-venting activity.
One shell from the pockmark province of the Nile deep-sea fan records a strong, sharp δ
13C decrease of about 9‰, and extending over a 5-mm interval in the shell that can be related to a major methane release event.
Another shell from the Kazan mud volcano exhibits a progressive increase of δ
13C values from −10‰ to 0‰ with age, which might indicate decreasing methane flow throughout the organism’s life. This study
has demonstrated that bivalve shells from deep-sea cold seeps represent good indicators of variability in seepage activity
of methane-rich fluids, at various scales in both space and time. Although the precise chronology of the observed events was
not established, because shell growth rate is not known in this case, this remains a priority for future studies in such environments. 相似文献