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1.
Seagrass beds form an important part of the coastal ecosystem in many parts of the world but are very sensitive to anthropogenic nutrient increases. In the last decades, stable isotopes have been used as tracers of anthropogenic nutrient sources and to distinguish these impacts from natural environmental change, as well as in the identification of food sources in isotopic food web reconstruction. Thus, it is important to establish the extent of natural variations on the stable isotope composition of seagrass, validating their ability to act as both tracers of nutrients and food sources. Around the world, depending on the seagrass species and ecosystem, values of seagrass N normally vary from 0 to 8?‰ δ15N. In this study, highly unusual seagrass N isotope values were observed on the east coast of Qatar, with significant spatial variation over a scale of a few metres, and with δ15N values ranging from +2.95 to ?12.39?‰ within a single bay during March 2012. This pattern of variation was consistent over a period of a year although there was a seasonal effect on the seagrass δ15N values. Seagrass, water column and sediment nutrient profiles were not correlated with seagrass δ15N values and neither were longer-term indicators of nutrient limitation such as seagrass biomass and height. Sediment δ15N values were correlated with Halodule uninervis δ15N values and this, together with the small spatial scale of variation, suggest that localised sediment processes may be responsible for the extreme isotopic values. Consistent differences in sediment to plant 15N discrimination between seagrass species also suggest that species-specific nutrient uptake mechanisms contribute to the observed δ15N values. This study reports some of the most extreme, negative δ15N values ever noted for seagrass (as low as ?12.4?‰) and some of the most highly spatially variable (values varied over 15.4?‰ in a relatively small area of only 655 ha). These results are widely relevant, as they demonstrate the need for adequate spatial and temporal sampling when working with N stable isotopes to identify food sources in food web studies or as tracers of anthropogenic nutrients.  相似文献   

2.
The concentrations and isotopic compositions of the various forms of nitrogen in silty clay sediments from the Bay of Quinte (Lake Ontario) have been determined. The total organic-N content is high throughout the sediment profiles and generally decreases with depth. On the contrary, exchangeable NH+4-N concentration is quite low and tends to increase with depth in two out of three sediment cores examined. The concentration of non-exchangeable NH+4-N and the 6 N HCl hydrolyzable NH+4-N are relatively constant with depth. Among the N fractions analyzed, the exchangeable NH+4-is most enriched in 15N. In most cases, the δ 15 N values of the N fractions remain relatively constant with sediment depth. There is no apparent correlation of δ 15 N values with the N concentration for any of the individual N fractions. The observed ranges in the δ 15 N values are: exchangeable NH+4, + 5–+10‰; 6 N HCl hydrolyzable total N and 6 N HCl hydrolyzable NH+4-N, + 3.5–+5.5‰.  相似文献   

3.
Narragansett Bay has been heavily influenced by human activities for more than 200 years. In recent decades, it has been one of the more intensively fertilized estuaries in the USA, with most of the anthropogenic nutrient load originating from sewage treatment plants (STP). This will soon change as tertiary treatment upgrades reduce nitrogen (N) loads by about one third or more during the summer. Before these reductions take place, we sought to characterize the sewage N signature in primary (macroalgae) and secondary (the hard clam, Mercenaria mercenaria) producers in the bay using stable isotopes of N (δ15N) and carbon (δ13C). The δ15N signatures of the macroalgae show a clear gradient of approximately 4‰ from north to south, i.e., high to low point source loading. There is also evidence of a west to east gradient of heavy to light values of δ15N in the bay consistent with circulation patterns and residual flows. The Providence River Estuary, just north of Narragansett Bay proper, receives 85% of STP inputs to Narragansett Bay, and lower δ15N values in macroalgae there reflected preferential uptake of 14N in this heavily fertilized area. Differences in pH from N stimulated photosynthesis and related shifts in predominance of dissolved C species may control the observed δ13C signatures. Unlike the macroalgae, the clams were remarkably uniform in both δ15N (13.2 ± 0.54‰ SD) and δ13C (−16.76 ± 0.61‰ SD) throughout the bay, and the δ15N values were 2–5‰ heavier than in clams collected outside the bay. We suggest that this remarkable uniformity reflects a food source of anthropogenically heavy phytoplankton formed in the upper bay and supported by sewage derived N. We estimate that approximately half of the N in the clams throughout Narragansett Bay may be from anthropogenic sources.  相似文献   

4.
Over the past decade, nitrogen (N) loads to Narragansett Bay have decreased by more than 50%. These reductions were, in large part, the direct result of multiple wastewater treatment facility upgrades to tertiary treatment, a process which employs N removal. Here, we document ecosystem response to the N reductions and assess how the distribution of sewage N in Narragansett Bay has changed from before, during, and shortly after the upgrades. While others have observed clear responses when data were considered annually, our seasonal and regional comparisons of pre- and post-tertiary treatment dissolved inorganic nitrogen (DIN) concentrations and Secchi depth data, from bay-wide surveys conducted periodically from the early 1970s through 2016, resulted in only a few subtle differences. Thus, we sought to use stable isotope data to assess how sewage N is incorporated into the ecology of the Bay and how its distribution may have changed after the upgrades. The nitrogen (δ15N) and carbon (δ13C) stable isotope measurements of particulate matter served as a proxy for phytoplankton, while macroalgae served as short-term integrators of water column bio-available N, and hard clams (Mercenaria mercenaria) as integrators of water column production. In contrast to other estuarine stable isotope studies that have observed an increased influence of isotopically lower marine N when sewage N is reduced, the opposite has occurred in Narragansett Bay. The tertiary treatment upgrades have increased the effluent δ15N values by at least 2‰. The plants and animals throughout Narragansett Bay have similarly increased by 1–2‰, on average. In contrast, the δ13C values measured in particulate matter and hard clams have declined by about the same amount. The δ15N results indicated that, even after the N reductions, sewage N still plays an important role in supporting primary and secondary production throughout the bay. However, the δ13C suggests that overall net production in Narragansett Bay has decreased. In the 5 years after the major wastewater treatment facilities came on-line for nutrient removal, oligotrophication has begun but sewage remains the dominant source of N to Narragansett Bay.  相似文献   

5.
Quarterly field sampling was conducted to characterize variations in water column and sediment nutrients in a eutrophic southern California estuary with a history of frequent macroalgal blooms. Water column and sediment nutrient measures demonstrated that Upper Newport Bay (UNB) is a highly enriched estuary. High nitrate (NO3 −) loads from the river entered the estuary at all sampling times with a rainy season (winter) maximum estimated at 2,419 mol h−1. This resulted in water NO3 − concentration in the estuary near the river mouth at least one order of magnitude above all other sampling locations during every seasons; maximum mean water NO3 − concentration was 800 μM during springer 1997. Phosphorus (P)-loading was high year round (5.7–90.4 mol h−1) with no seasonal pattern. Sediment nitrogen (N)-content showed a seasonal pattern with a spring maximum declining through fall. sediment and water nutrients, as well as percent cover of three dominant macroalgae, varied between the main channel and tidal creeks. During all seasons, water column NO3 − concentrations were higher in the main channel than in tidal creeks while tidal creeks had higher levels of sediment total Kjeldhal nitrogen (TKN) and P. During each of the four sampling periods, percent cover ofEntermorpha intestinalis andCeramium spp. was higher in tidal creeks than in the main channel, while percent cover ofUlva expansa was always higher in the main channel. Decreases in sediment N in both creek and channel habitats were concurrent with increases in macroalgal cover, possibly reflecting use of stored sediment TKN by macroalgae. Our data suggest a shift in primary nutrient sources for macroalgae in UNB from riverine input during winter and spring to recycling from sediments duirng summer and fall.  相似文献   

6.
《Applied Geochemistry》2004,19(5):709-719
The potential for exploitation of urban aquifers is partly dependent on understanding the distribution and fate of urban N sources, such as sewage and fertilisers, that can limit the use of groundwater for public supplies. To investigate the application of the dual-isotope approach to understanding the N hydrochemistry of urban groundwater, this paper presents δ15N–NO3− and δ18O–NO3− data collected from two multi-level piezometers in the Sherwood sandstone aquifer beneath Nottingham in the English Midlands, UK. At one multi-level piezometer (Old Basford), depth sample measurements of δ15N–NO3− in the range +9.2 to +11.4 ‰ and δ18O–NO3− in the range +8.2 to +10.9‰, together with NO3− nitrate concentrations from 31.7 to 66.7 mg/l, are evidence for nitrification of sewage-derived inputs. In contrast, at the other multi-level piezometer (the Meadows), isotopically enriched samples (δ15N–NO3− in the range +24.3 to +42.2 ‰ and δ18O–NO3− in the range +20.5 to +29.4‰) are evidence for denitrification, although the compositional range of δ15N–NO3− does not identify the N source without corroborating data. For the Meadows location, a cross-plot of δ15N–NO3− versus δ18O–NO3− gave an enrichment of the 15N isotope relative to the 18O isotope by a factor of 1.9, within the range of 1.3–2.1 reported for denitrification in other studies. This study has shown that the dual-isotope approach provides improved understanding of N sources and fate in the urban environment but further work is required to identify nitrification pathways to provide more confidence in the application and interpretation of δ18O–NO3− measurements.  相似文献   

7.
Geochemical evidence of microbial activity within ooids   总被引：1，自引：0，他引：1       下载免费PDF全文
Ooid formation remains elusive despite their importance as palaeoclimatic indicators and important contributors to global carbonate budget. Based on stable isotopes, nutrient and elemental analyses on solid components and ooidal leachates, this study supports the notion of microbial involvement in the development of ooids from Great Bahama Bank. Carbon and nitrogen isotopic analyses on organic fractions identified geochemical signatures of microbial activity. The δ13C values for organic carbon in the bulk (?11·9 to ?16·9‰); intercrystalline/intracrystalline (?11·9 to 16·7‰); and intracrystalline phases (?12·4 to ?17·7‰) were similar and, except for the more enriched values of ooids from Butterfly Beach, were within the range of photosynthesisers. The δ15N values for the bulk (+0·5 to ?0·2‰); intercrystalline/intracrystalline (?0·3‰ to ?0·7‰) and intracrystalline organic matter (?0·3 to ?1·7‰) showed a narrow range consistent with nitrogen fixation. While positive δ15N and δ18O values of the leached from the ooids provided evidence of denitrification, the carbonate associated sulphate δ34SCAS of the bulk sediments (+19·2 to +19·6‰) and δ34S of the leachates (+16·6 to +18·3‰) provided weak indication of sulphate reduction, suggesting either that high concentrations of isotopically enriched S are overriding bio‐signatures of sulphate reduction or that microbes are preferentially using as an electron acceptor. In contrast, the elevated sulphate concentrations of the leachates suggest the occurrence of microbial sulphide oxidation within ooids. The high Mg/Ca of the leachates and scanning electron microscope analyses provide putative evidence of amorphous calcium carbonate and a formative role in CaCO3 precipitation. Together, these findings indicate that a redox dependent microbial consortium may influence CaCO3 precipitation in the form of ooid accretion, cementation and micritization. It is also inferred that ooid deposits are not suitable indicators of palaeoclimate because ooids are affected throughout their life by a complex chain of abiotic and biological processes which can lead to large geochemical offsets.  相似文献   

8.
A CO2-rich (>` 1 atm PCO2) travertine-depositing spring was studied to determine how it equilibrated, both chemically and isotopically, with conditions at the earth's surface. The water degassed rapidly along a 47 m section of its channel. Small quantities of siderite- and aragonite-rich precipitates were present along the first few meters and larger quantities of aragonite-rich precipitates were present in the later sections of the channel. CO2 degassing increased the δ13C of dissolved C from −9.7‰ near the spring orifice to +0.7‰ 47 m downstream. Along the first portion of the stream, the isotopic effect of degassing could be explained by a Rayleigh distillation process. Along the latter portion of the stream, photosynthesis may have enriched the dissolved C in 13C. The carbonate minerals appear to have formed in approximate isotopic equilibrium with the dissolved C. Older travertine samples from the terrace differ chemically, mineralogically, and isotopically from the more recently deposited minerals suggesting a possible alteration of the originally deposited material.  相似文献   

9.
We examined the spatial and temporal variability in drift macroalgal abundance in two seagrass dominated estuarine systems on the Texas coast: Redfish Bay (in the Copano-Aransas Estuary) and Lower Laguna Madre. Measurements of benthic macroalgal variability were made in conjunction with a suite of biotic (seagrass biomass, percent cover, blade width and length, shoot density, epiphyte biomass, seagrass blade C:N ratios, and drift macroalgal abundance and composition) and abiotic (inorganic nitrogen and phosphorus concentrations, chlorophylla, total suspended solids, light attenuation, salinity, temperature, total organic carbon and porewater NH4 +) indicators. All parameters were measured at 30 sites within each estuary semiannually from July 2002 to February 2004. Principal components analysis (PCA) was used to examine relationships between drift macroalgal abundance and biotic and abiotic parameters. In both Redfish Bay and Lower Laguna Madre, drift macroalgal distribution was widespread, and during three of four sampling periods, abundance was equal to abovegro und biomass ofThalassia testudinum, the dominant seagrass. Drift macro algal abundance was highly variable within sites, between sites, and between seasons in both estuaries. No significant differences in drift macroalgal abundance were found between Redfish Bay and Lower Laguna Madre. In Redfish Bay, drift macroalgae (90.1±10.2 gm−2) tended to accumulate in bare patches within seagrass beds. In Lower Laguna Madre, drift macroalgae (72.7±10.7 gm−2) tended to accumulate in areas of dense seagrass cover rather than in bare areas. We found no relationship between drift macroalgal abundance and low (<2μM) water column nutrient concentrations, and although several of our measured parameters were related to drift macroalgal abundance, none alone sufficiently explained the variability in abundance noted between the two estuarine systems. The contrasting patterns of macroalgal accumulation between Redrish Bay and Lower Laguna Madre likely reflect differences in water circulation characteristics between the two regions as dictated by local physiography, in cluding the shape and orientation of the lagoons, with seasonal variations in macroalgal abundance related to changes in freshwater inflow and nutrient loading.  相似文献   

10.
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11.
《Applied Geochemistry》1997,12(3):305-319
An analysis of the S and O isotopic compositions and concentrations of dissolved S04 in river-and lake-water from 7 major catchments of the North and South Islands, New Zealand, allows the distinction between natural (geological, geothermal and volcanic) and anthropogenic S sources.The Buller and the Wairau, relatively pristine rivers in the South Island, show two end-member mixing between34S- and18O-rich rain-water S04 (relatively enriched isotope values) and relatively depleted S04 from oxidation of bedrock sulfide. Tertiary sediments contribute the isotopically most depleted S (down to δ34SCDT−15‰) to the river-water S04, whereas Mesozoic greywacke contributes S with slightly positive δ34S values. River-water S04δ18OSMOW values range from 0 to + 5‰ most probably depending on the micro-environment of the oxidising zone. South Island rivers with S04δ34S> + 5‰ have low S04 concentrations (< 3 mgl−1) and are dominantly composed of rain-water S04 which is principally sea-water derived. In the North Island, the Hutt River S04 samples also lie on an isotopic mixing trend from “greywacke bedrock” to rain-water S04, the latter with δ34S and δ18O values up to + 16 and + 6‰ respectively and a So4/SO4 + Cl fraction of 0.15 (sea-water is 0.12. Although dominated by greywacke, some samples in the Wairarapa area have relatively enriched δ18Sand δ34S values and elevated S04 concentrations (up to 16 mgl−), together with higher SO4/SO4 + Cl fraction ratios. This suggests input of fertilizer S04 (δ34S+ 17.2‰andδ18O+ 12.7‰) in the rivers of this agricultural area. The fertilizer loading of the Ruamahanga river can be estimated by its graphical offset from a deduced baseline for bedrockrainfall derived S04 on a S versus O isotope plot. The fertilizer loading represents about 20% of the S04 in the river. Extrapolation of this figure to the annual river discharge indicates that approximately 18% of the amount applied within the catchment is lost to the river.The source of the Whangaehu river is the Ruapehu crater lake (active volcano) with high S04 concentrations and very enriched S04 isotopic signatures (δ34S> + 17‰andδ18O> + 12‰). Downstream this water is diluted by tributaries with lower S04 concentration and isotope signatures of Tertiary sediments similar to the rivers in the South Island. Both geothermal and rain-water S04 inputs to the streams flowing into Lakes Taupo and Rotorua were identified isotopically; in particular waters flowing out from Lake Rotorua have a higher geothermal derived S04 content than the inflows, indicating that there must be a considerable underwater geothermal input to the lake.  相似文献   

12.
Fourteen stratiform, stratabound and vein-type sulphide occurrences in the Upper Allochthon of the Central–North Norwegian Caledonides have been studied for their sulphur, oxygen and hydrogen isotope composition. Depositional ages of host rocks to the stratabound and stratiform sulphide occurrences range from 590 to 640?Ma. The sulphides and their host rocks have been affected by polyphase deformation and metamorphism with a peak temperature of 650?°C dated to 432?Ma. A total of 104 sulphide and 2 barite samples were analysed for δ34S, 16 whole-rock and quartz samples for δ18O and 12 samples of muscovite for δD. The overall δ34S values range from ?14 to +31‰ with the majority of sampled sulphides lying within a range of +4 to +15‰. In most cases δ34S within each hand specimen behaves in accordance with the equilibrium fractionation sequence, δ34Sgn<δ34Scp<δ34Ssph<δ34Spy. A systematic increase in δ34S from the vein sulphides (?8‰) through schist/amphibolite-hosted (+6‰) and schist-hosted (+7 to +12‰) to dolomite-hosted (+12 to +31‰) occurrences is documented. The δ34S averages of the stratiform schist-hosted sulphides are 17 to 22‰ lower than in the penecontemporaneous seawater sulphate. The Bjørkåsen (+4 to +6‰) occurrence is a volcanogenic massive sulphide (VMS) transitional to sedimentary massive sulphide (SMS), exhalative, massive, pyritic deposit of Cu–Zn–Pb sulphides formed by fluids which obtained H2S via high-temperature reduction of seawater sulphate by oxidation of Fe2+ during the convective circulation of seawater through underlying rock sequences. The Raudvatn, volcanic-hosted, disseminated Cu sulphides (+6 to +8‰) obtained sulphur via a similar process. The Balsnes, stratiform, ‘black schist’-hosted, pyrite–pyrrhotite occurrence (?6 to ?14‰) is represented by typical diagenetic sulphides precipitated via bacteriogenic reduction of coeval (ca. 600?Ma) seawater sulphate (+25 to +35‰) in a system open to sulphate supply. The δ34S values of the Djupvik–Skårnesdalen (+7 to +12‰), Hammerfjell (+5 to 11‰), Kaldådalen (+10 to +12‰) and Njallavarre (+7 to +8‰) stratiform, schist-hosted, massive and disseminated Zn–Pb (±Cu) sulphide occurrences, as well as the stratabound, quartzite-hosted, Au-bearing arsenopyrite occurrence at Langvatnet (+7 to +11‰), suggest that thermochemically reduced connate seawater sulphate was a principal sulphur source. The Sinklien and Tårstad, stratabound, dolomite- and dolomite collapse breccia-hosted, Zn (±Cu–Pb) sulphides are marked by the highest enrichment in 34S (+20 to +31‰). The occurrences ?are?assigned to the Mississippi-Valley-type deposits.?High δ34S values require reduction/replacement of contemporaneous (ca. 590?Ma) evaporitic sulphate (+23 to +34‰) with Corg-rich fluids in a closed system. The Melkedalen (+12 to +15‰), stratabound, fault-controlled, Cu–Zn sulphide deposit is hosted by the ca. 595?Ma dolomitised Melkedalen marble. The deposit is composed of several generations of ore minerals which formed by replacement of host dolomite. Polyphase hydrothermal fluids were introduced during several reactivation episodes of the fault zone. The positive δ34S values with a very limited fractionation (<3‰) are indicative of the sulphide-sulphur generated through abiological, thermochemical reduction of seawater sulphate by organic material. The vein-type Cu (±Au–W) occurrences at Baugefjell, Bugtedalen and Baugevatn (?8 to ?4‰) are of hydrothermal origin and obtained their sulphur from igneous sources with a possible incorporation of sedimentary/diagenetic sulphides. In a broad sense, all the stratiform/stratabound, sediment-hosted, sulphide occurrences studied formed by epigenetic fluids within two probable scenarios which may be applicable separately or interactively: (1) expulsion of hot metal-bearing connate waters from deeper parts of sedimentary basins prior to nappe translation (late diagenetic/catagenetic/epigenetic fluids) or (2) tectonically driven expulsion in the course of nappe translation (early metamorphic fluids). A combination of (1) and (2) is favoured for the stratabound, fault-controlled, Melkedalen and Langvatnet occurrences, whereas the rest are considered to have formed within option (1). The sulphides and their host rocks were transported from unknown distances and thrust on to the Fennoscandian Shield during the course of the Caledonian orogeny. The displaced/allochthonous nature of the Ofoten Cu–Pb–Zn ‘metallogenetic province’ would explain the enigmatically high concentration of small-scale Cu–Pb–Zn deposits that occur only in this particular area of the Norwegian Caledonides.  相似文献   

13.
The Han-Xing iron mineralization in the central North China Craton is a typical Fe skarn deposit associated with altered diorites. Here we report the Fe isotopic compositions of whole rocks and mineral separates from this deposit with a view to evaluate the Fe isotope fractionation during the formation of Fe skarn deposit, and to constrain the metal source. The Fe isotopes show a large variation both in whole rocks and mineral separates. Altered diorites show a wide range in δ56Fe values (− 0.07‰ to + 0.21‰ relative to the Fe isotope standard IRMM-014) which positively correlate with their TFe2O3/TiO2 ratios (Fe2O3 and FeO calculated as TFe2O3). The positive correlation indicates that heavy Fe isotopes were preferentially leached from diorites during the skarn-type alteration. Among the metallic minerals, pyrite and pyrrhotite are isotopically heavier (+ 0.12‰ to + 0.48‰) than the magnetite (+ 0.07‰ to + 0.21‰). Fe isotope fractionation between mineral pairs demonstrates that magnetite did not attain Fe isotopic equilibrium with pyrite and pyrrhotite, whereas pyrite and pyrrhotite might have attained isotopic equilibrium. Petrological observations and major element data also suggest that iron was leached from the diorites during the skarn-type alteration. If the leached iron provides the main Fe budget of the Han-Xing Fe skarn deposit, magnetite in ores would be isotopically heavier than the unaltered diorite. However, our results are in contrast with the magnetite being isotopically lighter than the unaltered diorite. This suggests that the major Fe source of the Han-Xing Fe skarn deposit is not from the leaching of diorites, and might be from magmatic fluid which is isotopically lighter than the silicate melt. Our data demonstrate that Fe isotopes can be used as important tracers in deciphering the metal source of Fe skarn deposits.  相似文献   

14.
《Applied Geochemistry》2003,18(5):765-779
Stable isotope systematics of C, N and S were studied in soils of 5 European forest ecosystems. The sites were located along a North–South transect from Sweden to Italy (mean annual temperatures from +1.0 to +8.5 °C, atmospheric deposition from 2 to 19 kg N ha−1 a−1, and from 6 to 42 kg S ha−1 a−1). In Picea stands, the behavior of C, N and S isotopes was similar in 3 aspects: (1) assimilation favored the lighter isotopes 12C, 14N and 32S; (2) mineralization in the soil profile left in situ residues enriched in the heavier isotopes 13C, 15N and 34S; and (3) NO3–N as well as SO4–S in soil solution was isotopically lighter compared to the same species in the atmospheric input. In this study, emphasis was placed on S isotope profiles which so far have been investigated to a much lesser extent than those of C and N. Sulfate in monthly samples of atmospheric input had systematically higher δ34S ratios than total soil S at the 0–5 cm depth, on average by 4.0‰. Sulfate in the atmospheric input had higher δ34S ratios than in deep (>50 cm) lysimeter water, on average by 3.2‰. Organic S constituted more than 50% of total soil S throughout most of the profiles (0–20 cm below surface). There was a tendency to isotopically heavier organic S and lighter inorganic SO4–S, with ester SO4–S heavier than C-bonded S at 3 of the 5 sites. With an increasing depth (0 to 20 cm below surface), δ13C, δ15N and δ34S ratios of bulk soil increased on average by 0.9, 4.2 and 1.6‰, respectively, reflecting an increasing degree of mineralization of organic matter. The isotope effects of C, N and S mineralization were robust enough to exist at a variety of climate conditions and pollution levels. In the case of S, the difference between isotope composition of the upper organic-rich soil horizon (lower δ34S) and the deeper sesquioxide-rich soil horizons (higher δ34S) can be used to determine the source of SO4 in streams draining forests. This application of δ34S as a tracer of S origin was developed in the Jezeřı́ catchment, Czech Republic, a highly polluted site suffering from spruce die-back. In 1996–1997, the magnitude and δ34S of atmospheric input (20 kg S ha−1 a−1, 5.8‰) and stream discharge (56 kg S ha−1 a−1, 3.5‰) was monitored. Export of S from the catchment was 3 times higher than contemporary atmospheric input. More than 50% of S in the discharge was represented by release of previously stored pollutant S from the soil. Stable isotope systematics of Jezeřı́ soil S (mean of 2.5‰ in the O+A horizon, 4.8‰ in the B horizon, and 5.8‰ in the bedrock) suggests that most of the soil-derived S in discharge must come from the isotopically light organic S present in the upper soil horizon, and that mineralized organically-cycled S is mainly flushed out during the spring snowmelt. The fact that a considerable proportion of incoming S is organically cycled should be considered when predicting the time-scale of acidification reversal in spruce die-back affected areas.  相似文献   

15.

This study contributes to identifying and spatializing the different types of nitrate sources by combining hydrogeochemical and isotopic data with principal component analysis (PCA) and t-distributed stochastic neighbor embedding (t-SNE) multicriteria statistical methods. The methodology is applied to the strategic Mons Basin chalk aquifer (Belgium). The results are based on a whole dataset containing 72 water samples with analyses of the hydrogeochemical parameters (temperature, pH, electrical conductivity (EC), redox potential, dissolved O2), alkalinity, total organic carbon (TOC), silica (SiO2), major and minor ions (NO3–, NH4+, Ca2+, dissolved Fe and Mn, K+, Mg2+, Na+, Sr2+, Cl–, F–, SO4–, B) and multiple stable isotope ratios (δ11B, δ15N–NO3–, δ18O–NO3–). Compared to classical PCA, the recently developed t-SNE method, which considers nonlinear relationships between variables and preserves local-scale similarities in a low-dimensional space, showed much better performance in discriminating different groups of samples and related zones in the aquifer. t-SNE results combined with isotope ratios highlighted four zones in the aquifer (grouped as A–D) and the presence of denitrification fronts. Group A presents a manure signature (δ15N–NO3– – mean (μ) +12.78‰, standard deviation (σ) 6.48‰; δ11B – μ 29.96‰, σ 6.91‰). Group B exhibits both manure and inorganic fertilizer signatures (δ15N–NO3– – μ 6.27‰, σ 2.55‰; δ11B – μ 15.86‰, σ 9.69‰). Group C shows a contamination by sewage (δ15N–NO3– – μ 12.67‰, σ 5.60‰; δ11B – μ 9.97‰, σ 7.08‰). Group D presents a mixed signature (δ15N–NO3– – μ 9.25‰, σ 2.94‰; δ11B – μ 20.00‰, σ 6.70‰).

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16.
The objective of this study was to investigate natural abundance and the distribution of nitrogen isotopic compositions to assess denitrification in two ~30 m thick vadose zones beneath the different land uses in the wastewater-irrigated area located in southern Shijiazhuang, China. Sediment samples were collected from cores of boreholes drilled in the vegetable growth plot and the wastewater-irrigated farmland for analyses of nitrogen isotopes, physical and chemical properties, respectively. The profile of borehole A drilled in the vegetable growth plot only applied animal wastes had lower δ15N values of mean +7.5 ‰ in the upper vadose zone, but higher values of mean +10.9 ‰ in the lower vadose zone. δ15N values in each part varied little with depth, indicating no or little denitrification occurred in the deep vadose zone below the soil zone. The profile of borehole B drilled in the wastewater-irrigated farmland had low δ15N values of mean +5.7 ‰ below the soil zone and little variations of δ15N values with depth, indicating no or little denitrification occurred in the deep vadose zone below the soil zone. This was also verified by consistent variations of NO3 ? and SO4 2? contents with Cl? contents. Our results suggested most of leachable nitrate from the soil zone was hardly subjected to biological attenuation into groundwater.  相似文献   

17.
Complex links between the top-down and bottom-up forces that structure communities can be disrupted by anthropogenic alterations of natural habitats. We used relative abundance and stable isotopes to examine changes in epifaunal food webs in seagrass (Thalassia testudinum) beds following 6 months of experimental nutrient addition at two sites in Florida Bay (USA) with different ambient fertility. At a eutrophic site, nutrient addition did not strongly affect food web structure, but at a nutrient-poor site, enrichment increased the abundances of crustacean epiphyte grazers, and the diets of these grazers became more varied. Benthic grazers did not change in abundance but shifted their diet away from green macroalgae + associated epiphytes and towards an opportunistic seagrass (Halodule wrightii) that occurred only in nutrient addition treatments. Benthic predators did not change in abundance, but their diets were more varied in enriched plots. Food chain length was short and unaffected by site or nutrient treatment, but increased food web complexity in enriched plots was suggested by increasingly mixed diets. Strong bottom-up modifications of food web structure in the nutrient-limited site and the limited top-down influences of grazers on seagrass epiphyte biomass suggest that, in this system, the bottom-up role of nutrient enrichment can have substantial impacts on community structure, trophic relationships, and, ultimately, the productivity values of the ecosystem.  相似文献   

18.
Submarine groundwater discharge (SGD) was quantified at select sites in San Francisco Bay (SFB) from radium (223Ra and 224Ra) and radon (222Rn) activities measured in groundwater and surface water using simple mass balance box models. Based on these models, discharge rates in South and Central Bays were 0.3?C7.4?m3?day?1?m?1. Although SGD fluxes at the two regions (Central and South Bays) of SFB were of the same order of magnitude, the dissolved inorganic nitrogen (DIN) species associated with SGD were different. In the South Bay, ammonium (NH 4 + ) concentrations in groundwater were three-fold higher than in open bay waters, and NH 4 + was the primary DIN form discharged by SGD. At the Central Bay site, the primary DIN form in groundwater and associated discharge was nitrate (NO 3 ? ). The stable isotope signatures (??15NNO3 and ??18ONO3) of NO 3 ? in the South Bay groundwater and surface waters were both consistent with NO 3 ? derived from NH 4 + that was isotopically enriched in 15N by NH 4 + volatilization. Based on the calculated SGD fluxes and groundwater nutrient concentrations, nutrient fluxes associated with SGD can account for up to 16?% of DIN and 22?% of DIP in South and Central Bays. The form of DIN contributed to surface waters from SGD may impact the ratio of NO 3 ? to NH 4 + available to phytoplankton with implications to bay productivity, phytoplankton species distribution, and nutrient uptake rates. This assessment of nutrient delivery via groundwater discharge in SFB may provide vital information for future bay ecological wellbeing and sensitivity to future environmental stressors.  相似文献   

19.
Neoproterozoic to Cambrian ultrapotassic (K2O up to 13 wt%) peralkalic alkali-feldspar-rich syenitic plutons were emplaced along the boundary between the Cachoeirinha-Salgueiro and Alto Pajeu tectonostratigraphic terranes of the Borborema structural province, northeastern Brazil. Syenite and alkalic pyroxenitic magmas coexisted in these plutons, which locally carry mica pyroxenite xenoliths. In the Triunfo batholith, the largest peralkalic pluton in the region, syenites and alkalic pyroxenites have high pyroxene-corrected δ18O values (+8.1 to + 8.5‰SMOW in the syenite and +7.6 to + 7.7‰ in the alkalic pyroxenite), high δ34S (+12.3‰CDT in syenite and + 11.2‰CDT in alkalic pyroxenite), high initial 87Sr/86Sr ratios (0.7098, syenite and alkalic pyroxenite data lying on the same Rb-Sr isochron), and low εNd (?15.3 to ?17.2 in syenite and ?16.1 in pyroxenite). Whole-rock δ18OSMOW for mica pyroxenite xenoliths varies from +7.5 to +8.0‰SMOW. Syenite, alkalic pyroxenite, and xenoliths all are enriched in large-ion lithophile elements (LILE). These geochemical and isotopic signatures suggest that the magmas were derived from an incompatible-element-enriched mantle source; this protolith probably resulted from hybridization by addition of crustal material via subduction at ～2.4 Ga, as estimated from Nd model ages. Partial melting of metasomatized material and magma emplacement at a late stage of the Brasiliano Orogeny (566 Ma) were controlled by mantle-deep shear zones during the amalgamation of the Cachoeirinha-Salgueiro and Alto Pajeu terranes.  相似文献   

20.
Quantifying Sediment Nitrogen Releases Associated with Estuarine Dredging   总被引：1，自引：0，他引：1  
Experimental studies of sediment pore water NH4 + chemistry, adsorbed NH4 + concentrations, sediment?Cwater NH4 + exchange and N2?CN flux were carried out to quantify the mass of labile N that can be released during large-scale dredging activities. Pore water NH4 + concentrations below 0.5-m sediment depth averaged 5 ± 2 mmol L?1 with average adsorbed NH4 + concentrations of 11 ??mol g?1. Elevated NH4 + concentrations found in rapidly accreting dredge channels are partly a result of the rapid advective burial of both reactive organic matter and pore water. Elutriate tests, a dilution of sediment with site water, yielded adsorbed NH4 + concentrations very similar to those using the more typical KCl extraction. Intact deep sediment sections exposed to overlying water, used to simulate postdredging conditions, showed high initial fluxes of ammonium and no development of coupled nitrification?Cdenitrification under the cold incubation conditions. Despite high concentrations and effluxes of NH4 + during dredging, the amount of NH4 + release during dredging was <0.5% of northern Chesapeake Bay sediment fluxes. The likelihood of large environmental effects of nitrogen release during the dredging of navigational channels in the Chesapeake Bay is low.  相似文献   

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