首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
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   

2.
To understand the origin and biogeochemistry of the organic matter in surface sediments of Lake Shihwa and Lake Hwaong, organic nitrogen, inorganic nitrogen, labile organic carbon, and residual organic carbon contents as well as stable isotope ratios for carbon and nitrogen were determined by KOBr-KOH treatment. Ratios of organic carbon to organic nitrogen (Corg/Norg) (mean = 24) were much higher than ratios of organic carbon to total nitrogen (Corg/Ntot) (mean=12), indicating the presence of significant amounts of inorganic nitrogen in the surface sediments of both lakes. Stable isotope ratios for organic nitrogen were, on average, 5.2‰ heavier than ratios of inorganic nitrogen in Lake Shihwa, but those same ratios were comparable in Lake Hwaong. This might be due to differences in the origin or the degree of degradation of sedimentary organic matter between the two lakes. In addition, stable isotope ratios for labile organic carbon were, on average, 1.4‰ heavier than those for residual organic carbon, reflecting the preferential oxidation of13C-enriched organic matter. The present study demonstrates that KOBr-KOH treatment of sedimentary organic matter can provide valuable information for understanding the origin and degradation state of organic matter in marine and brackish sediments. This also suggests that the ratio of Corg/Norg and stable isotope ratios for organic nitrogen can be used as indexes of the degree of degradation of organic matter.  相似文献   

3.
Relationships between organic carbon, total nitrogen and organic nitrogen concentrations and variations in δ13Corg and δ15Norg are examined in surface sediments from the eastern central Arctic Ocean and the Yermak Plateau. Removing the organic matter from samples with KOBr/KOH and determining residual as well as total N shows that there is a significant amount of bound inorganic N in the samples, which causes TOC/Ntotal ratios to be low (4–10 depending on the organic content). TOC/Norg ratios are significantly higher (8–16). This correction of organic TOC/N ratios for the presence of soil-derived bound ammonium is especially important in samples with high illite concentrations, the clay mineral mainly responsible for ammonium adsorption. The isotopic composition of the organic N fraction was estimated by determining the isotopic composition of the total and inorganic nitrogen fractions and assuming mass-balance. A strong correlation between δ15Norg values of the sediments and the nitrate concentration of surface waters indicates different relative nitrate utilization rates of the phytoplankton in various regions of the Arctic Ocean. On the Yermak Plateau, low δ15Norg values correspond to high nitrate concentrations, whereas in the central Arctic Ocean high δ15Norg values are found beneath low nitrate waters. Sediment δ13Corg values are close to −23.0‰ in the Yermak Plateau region and approximately −21.4‰ in the central Arctic Ocean. Particulate organic matter collected from meltwater ponds and ice-cores are relatively enriched in 13C (δ13Corg=−15.3 to −20.6‰) most likely due to low CO2(aq) concentrations in these environments. A maximum terrestrial contribution of 30% of the organic matter to sediments in the central Arctic Ocean is derived, based on the carbon isotope data and various assumptions about the isotopic composition of the potential endmembers.  相似文献   

4.
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   

5.
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   

6.
To examine the source and preservation of organic matter in the shelf sediments of the East China Sea (ECS), we measured bulk C/N and isotopes, organic biomarkers (n-alkanes and fatty acids) and compound-specific (fatty acids) stable carbon isotope ratios in three sediment cores collected from two sites near the Changjiang Estuary and one in the ECS shelf. Contrasting chemical and isotopic compositions of organic matter were observed between the estuarine and shelf sediments. The concentrations of total n-alkanes and fatty acids in the shelf surface sediments (0–2 cm) were 5–10 times higher than those in estuarine surface sediments but they all decreased rapidly to comparable levels below the surface layer. The compositions of n-alkanes in the estuarine sediments were dominated by C26-C33 long-chain n-alkanes with a strong odd-to-even carbon number predominance. In contrast, the composition of n-alkanes in the shelf sediment was dominated by nC15 to nC22 compounds. Long-chain (>C20) fatty acids (terrestrial biomarkers) accounted for a significantly higher fraction in the estuarine sediments compared to that in the shelf sediment, while short-chain (<C20) saturated and unsaturated fatty acids were more abundant in the shelf surface sediments than in the estuarine sediments. Stable carbon isotopic ratios of individual fatty acids showed a general positive shift from estuarine to shelf sediments, consistent with the variations in bulk δ 13CTOCTOC. These contrasts between the estuarine and shelf sediments indicate that terrestrial organic matter was mainly deposited within the Changjiang Estuary and inner shelf of ECS. Post-depositional diagenetic processes in the surface sediments rapidly altered the chemical compositions and control the preservation of organic matter in the region.  相似文献   

7.
This study reports on measurements of organic carbon (Corg) and total nitrogen (Ntot) in surface sediments originating from 6 transects along the northwest European continental margin. After elimination of carbonates by an acidification technique using sulphurous acid, both elements were analysed in the same sediment sample using an elemental analyser. Corg and Ntot in the sample were comparatively low, ranging between 1 and 10 mg C and 0.2 to 1 mg N g−1 dry sediment. In a second analysis, the samples were analysed without acid addition, resulting in Ntot concentrations of 0–50% higher compared to their acidified counterparts. As a consequence, molar C/N ratios derived from the analysis of Corg and Ntot in the acidified sample ranged between 6 to 11, while the Ntot separate analysis reduced C/N ratios to 6 to 8. It is suggested that the addition of sulphurous acid to eliminate inorganic carbon volatilises nitrogenous organic matter.  相似文献   

8.
We have measured inorganic nitrogen (IN) content and the isotope ratio of IN (δ15NIN) in a sediment core covering the last 145 kyr in the western subarctic Pacific (WSAP). IN content was generally high during glacial periods and shows positive correlations with both eolian dust content and the ratio of organic carbon (C) to organic nitrogen (ON) (C/ON) found in our previous studies. This means that IN was transported from continental areas to the WSAP together with eolian dust and that the IN was not contaminated by volcanic materials, because the eolian dust content was reconstructed using metal components to remove contaminating volcanic materials. Therefore, IN content in the WSAP sediments, the clay fraction of which is not greatly affected by drift deposits seen at the other sites in this region, may potentially be an effective proxy for eolian dust, without the need to consider contamination by volcanic materials. δ15NIN was generally low during glacial periods and shows negative correlations with IN, eolian dust, and C/ON. The possible causes of the observed variations in δ15NIN are as follows: (1) authigenic fixation of NH4 + in water-column and pore water of sea-floor sediments to clay minerals; (2) contamination of measured IN by highly resistant organic matter; or (3) variations in the continental source region of the eolian dust supplied to the WSAP and climatically induced changes in δ15N of soil organic matter there. The last mechanism shows the potential for δ15NIN to be used as a proxy for climate change on land, and is consistent with other published explanations of the spatial distribution of δ15NIN in modern sea-floor sediments.  相似文献   

9.
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   

10.
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   

11.
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   

12.
To establish the relative importance of terrigenous and marine organic matter in the southern Beaufort Sea, we measured the concentrations and the stable isotopic compositions of organic carbon and total nitrogen in sediments and in settling particles intercepted by sediment traps. The organic carbon content of surface sediment in the Chukchi and southern Beaufort Seas ranged from 0.6 to 1.6% dry wt., without a clear geographical pattern. The CORG:NTOT ratio ranged from 7.0 to 10.4 and did not vary significantly downcore at any one station. Values of δ13CORG and δ15NTOT in the sediment samples were strongly correlated, with the highest values, indicative of a more marine contribution, in the Amundsen Gulf. In contrast, the organic matter content, elemental (CORG:NTOT ratio) and isotopic (δ13CORG and δ15NTOT) composition of the settling particles was different from and much more variable than in the bottom sediments. The isotopic signature of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a labile marine component produced in situ by planktonic organisms, a refractory marine component, the end product of respiration and diagenesis, and a refractory terrigenous component. A three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates an apparent two-component mixing model of the organic matter in sediments. The suspended matter in the water column contains organic matter varying from essentially labile and marine to mostly refractory and terrigenous. As it settles through the water column, the labile marine organic matter is degraded, and its original stable isotope signature changes towards the signature of the marine refractory component. This process continues in the bottom sediment with the result that the sedimentary organic matter becomes dominated by the refractory terrigenous and marine components.  相似文献   

13.
 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   

14.
Carbon and nitrogen stable isotope ratios (13C and 15N) of surface sediments were measured within Osaka Bay, in the Seto Inland Sea in Japan, in order to better understand the sedimentation processes operating on both terrestrial and marine organic matter in the Bay. The 13C and 15N of surface sediments in the estuary of the Yodo River were less than –23 and 5 respectively, but increased in the area up to about 10 km from the river mouth. At greater distances they became constant (giving 13C of about –20 and 15N about 6). It can be concluded that large amounts of terrestrial organic matter exist near the mouth of the Yodo River. Stable isotope ratios in the estuary of the Yodo River within 10 km of the river mouth were useful indicators allowing study of the movement of terrestrial organic matter. Deposition rates for total organic carbon (TOC) and total nitrogen (TN) over the whole of the Bay were estimated to be 63,100 ton C/year and 7,590 ton N/year, respectively. The deposition rate of terrestrial organic carbon was estimated to be 13,200 (range 2,000–21,500) ton C/year for the whole of Osaka Bay, and terrestrial organic carbon was estimated to be about 21% (range 3–34) of the TOC deposition rate. The ratio of the deposition rate of terrestrial organic carbon to the rate inflow of riverine TOC and particulate organic carbon (POC) were estimated to be 19% (range 3–31) and 76% (range 12–100), respectively.  相似文献   

15.
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   

16.
Origin of sedimentary organic matter in the north-western Adriatic Sea   总被引:1,自引:0,他引:1  
In order to evaluate the origin and the transformation of organic matter on the shallow shelf of the NW Adriatic Sea, organic carbon, total nitrogen and stable isotope ratios of organic carbon were analysed in riverine suspended matter and sediments as well as in marine suspended and sedimentary organic matter, in marine phytoplankton and zooplankton.The deposition of organic matter is influenced by fine sediment concentration. Surface sediments were characterised by highly variable biogeochemical conditions on the sea floor, whereas sub-surface sediments showed a more homogeneous hypoxic/anoxic environment.Low Corg/N ratio and high organic carbon and nitrogen concentrations in riverine suspended organic matter indicate an important contribution of freshwater phytoplankton within rivers, particularly during low flow regimes, which adds to the marine phyto- and zooplankton at shelf locations.In order to evaluate the importance of terrestrial, riverine and marine sources of OM in shelf sediments, a three end-member mixing model was applied to shelf surface sediments using 13C/12C values for organic matter and N/C ratios. The model showed an elevated contribution of terrestrial organic substances at intermediate depths (10–15 m), mostly corresponding to an area of coarser grain-size, whereas the riverine and marine organic fractions were mainly accumulating near the coast and offshore, respectively.  相似文献   

17.
A time-series sediment trap was deployed at 1,034 m water depth in the eastern Bransfield Strait for a complete year from December 25, 1998 to December 24, 1999. About 99% of total mass flux was trapped during an austral summer, showing distinct seasonal variation. Biogenic particles (biogenic opal, particulate organic carbon, and calcium carbonate) account for about two thirds of annual total mass flux (49.2 g m-2), among which biogenic opal flux is the most dominant (42% of the total flux). A positive relationship (except January) between biogenic opal and total organic carbon fluxes suggests that these two variables were coupled, due to the surface-water production (mainly diatoms). The relatively low δ13C values of settling particles result from effects on C-fixation processes at low temperature and the high CO2 availability to phytoplankton. The correspondingly low δ15N values are due to intense and steady input of nitrates into surface waters, reflecting an unlikely nitrate isotope fractionation by degree of surface-water production. The δ15N and δ13C values of sinking particles increased from the beginning to the end of a presumed phytoplankton bloom, except for anomalous δ15N values. Krill and the zooplankton fecal pellets, the most important carriers of sinking particles, may have contributed gradually to the increasing δ13C values towards the unproductive period through the biomodification of the δ13C values in the food web, respiring preferentially and selectively12C atoms. Correspondingly, the increasing δ15N values in the intermediate-water trap are likely associated with a switch in source from diatom aggregates to some remains of zooplankton, because organic matter dominated by diatom may be more liable and prone to remineralization, leading to greater isotopic alteration. In particular, the tendency for abnormally high δ15N values in February seems to be enigmatic. A specific species dominancy during the production may be suggested as a possible and speculative reason.  相似文献   

18.
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   

19.
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   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号