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1.
For centuries, Bermuda has been challenged with wastewater management for the protection of human and environmental health. By quantifying the δ15N of the common sea fan Gorgonia ventalina sampled from 30 sites throughout Bermuda we show that sewage-derived nitrogen is detectable on nearshore coral reefs and declines across the lagoon to the outer rim. We also sampled gorgonians from two museum collections representing a 50y time-series (1958–2008). These samples revealed an increase in δ15N of > 4.0‰ until the mid-1970s, after which δ15N values slowly declined by ~ 2.0‰. A δ15N chronology from a gorgonian skeleton exhibited a similar decline over the last 30–40 years of approximately 0.6‰. We conclude that policies have been effective in reducing sewage impacts to Bermudian reefs. However, significant sources of sewage pollution persist and are likely have a strong impact on harbor and nearshore coral communities and human health.  相似文献   

2.
Mercury (Hg) can reach the environment through natural and human-related sources, threatening ecosystems all over the planet due to its well known deleterious effects. Therefore, Antarctic trophic webs, despite being relatively isolated, are not exempt of its influence. To evaluate Hg concentrations in an Antarctic ecosystem, different tissues from 2 species of invertebrates, 2 of fish, 8 of birds, 4 of pinnipeds and at least 5 of vegetation were investigated (n = 176). For animals, values ranged from 0.018 to 48.7 μg g? 1 dw (whole Antarctic krill and Antarctic Fur Seal liver). They were generally correlated to trophic position (assessed by δ15N and δ13C) but also to cephalopods and myctophids consumption. For vegetation, values ranged from 0.014 to 0.227 μg g? 1 dw (Colobanthus quitensis and an unidentified lichen), with lichens presenting significantly higher values than mosses, likely due to year-round exposure and absorption of animal derived organic matter, as hypothesized by literature.  相似文献   

3.
While specialized species are linked to a particular resource, omnivorous species may switch between food items according to the availability and the quality of resources. Here we use larvae of the omnivorous caddisfly genus Hydropsyche (Trichoptera) to analyse changes in diet composition across an elevational gradient. Periphyton and Hydropsyche larvae were sampled from 22 populations at stream orders from 2 to 5 on the German part of the Bohemian Forest. Elevations of sampling sites ranged between 300 m and 900 m a.s.l.. Diet composition was estimated by the analyses of the gut content of larvae as well as by stable nitrogen isotopes (δ15N). The δ15N values of the periphyton decreased and the C/N ratio of periphyton increased with increasing environmental harshness (decreasing water pH, temperature and conductivity with increasing elevation) indicating a decrease of periphyton food quality. Across individuals, the proportion of animals in the gut of Hydropsyche larvae was positively related to the difference of δ15N values between larvae and periphyton. The proportion of animals within the gut and (baseline corrected) δ15N values of Hydropsyche populations increased with increasing environmental harshness. We suggest that the (i) low primary production caused by shading, low temperatures and low nutrient levels, (ii) the low nutrient quality of periphyton and (iii) the availability of animal prey due to the input of allochthonous resources in headwaters caused the shift in the diet of Hydropsyche larvae along the river continuum.  相似文献   

4.
《Marine pollution bulletin》2012,64(5-12):528-534
The Salt-water River watershed is one of the major river watersheds in the Kaohsiung City, Taiwan. Water quality and sediment investigation results show that the river water contained high concentrations of organics and ammonia–nitrogen, and sediments contained high concentrations of heavy metals and organic contaminants. The main pollution sources were municipal and industrial wastewaters. Results from the enrichment factor (EF) and geo-accumulation index (Igeo) analyses imply that the sediments can be characterized as heavily polluted in regard to Cd, Cr, Pb, Zn, and Cu. The water quality analysis simulation program (WASP) model was applied for water quality evaluation and carrying capacity calculation. Modeling results show that the daily pollutant inputs were much higher than the calculated carrying capacity (1050 kg day−1 for biochemical oxygen demand and 420 kg day−1 for ammonia–nitrogen). The proposed watershed management strategies included river water dilution, intercepting sewer system construction and sediment dredging.  相似文献   

5.
《Marine pollution bulletin》2014,78(1-2):227-236
Total organic carbon (TOC), total nitrogen (TN), δ13C and δ15N were measured in sediment cores at three sites in Sishili Bay, China, to track the impacts of anthropogenic activities on the coastal environment over the last 100 years. The increased TOC and TN in the upper section of sediment cores indicated a eutrophic process since 1975. In comparison, the TOC and TN in the sediment core near to a scallop aquaculture area displayed a much slower increase, indicating the contribution of scallop aquaculture in mitigating eutrophication. Combined information from δ13C, δ15N and TOC:TN indicated an increased terrestrial signal, although organic matter sources in Sishili Bay featured a mixture of terrestrial and marine sources, with phytoplankton being dominant. Increased fertilizer use since 1970s contributed to the eutrophic process in Sishili Bay since 1975, and increased sewage discharge from 1990s has added to this process.  相似文献   

6.
《Marine pollution bulletin》2012,64(5-12):77-85
Environmental fate of fish farm wastes (FFW) released from an open-sea-cage farm at Kat O, Hong Kong was examined by measuring carbon and nitrogen stable isotope (SI) ratios in selected benthic organisms collected along a 2000 m transect from the farm. Our results showed that FFW significantly influenced the energy utilization profile of consumers near the fish farm. Although nitrogen enrichment effect on δ15N was anticipated in biota near the farm, the predicted patterns did not consistently occur in all feeding guilds. Two species of suspension-feeders, which relied on naturally δ15N-depleted sources, were δ15N-enriched near the fish farm. In contrast, both species of benthic grazer and deposit-feeder, which relied on naturally δ15N-enriched algal sources, were δ15N-depleted under the influence of FFW. The SI signatures of biota can, therefore, serve as feasible biomarkers for FFW discharges only when the trophic structure of the receiving environment is fully elucidated.  相似文献   

7.
We tested the capacity of Ulva lactuca to mark N sources across large marine areas by measuring variation in its δ15N at several sites in the Gulf of Gaeta. Comparisons were made with the macroalga Cystoseira amentacea. Variation of δ15N values was assessed also in the coastal waters off the Circeo Natural Park, where U. lactuca and C. amentacea were harvested, as these waters are barely influenced by human activities and were used as reference site. A small fragment from each frond was preserved before deployment in order to characterize the initial isotopic values. After 48 h of submersion, U. lactuca was more responsive than C. amentacea to environmental variation and δ15N enrichment in the Gulf of Gaeta was observed. The spatial distribution of δ15N enrichment indicated that different macro-areas in the Gulf were affected by N inputs from different origins. Comparison of the δ15N values of fragments taken from the same transplanted frond avoided bias arising from natural isotopic variability.  相似文献   

8.
We investigated the provenance of organic matter in the inner fjord area of northern Patagonia, Chile (～44–47°S), by studying the elemental (organic carbon, total nitrogen), isotopic (δ13C, δ15N), and biomarker (n-alkanoic acids from vascular plant waxes) composition of surface sediments as well as local marine and terrestrial organic matter. Average end-member values of N/C, δ13C, and δ15N from organic matter were 0.127±0.010, ?19.8±0.3‰, and 9.9±0.5‰ for autochthonous (marine) sources and 0.040±0.018, ?29.3±2.1‰, and 0.2±3.0‰ for allochthonous (terrestrial) sources. Using a mixing equation based on these two end-members, we calculated the relative contribution of marine and terrestrial organic carbon from the open ocean to the heads of fjords close to river outlets. The input of marine-derived organic carbon varied widely and accounted for 13–96% (average 61%) of the organic carbon pool of surface sediments. Integrated regional calculations for the inner fjord system of northern Patagonia covered in this study, which encompasses an area of ～4280 km2, suggest that carbon accumulation may account for between 2.3 and 7.8×104 ton C yr?1. This represents a storage capacity of marine-derived carbon between 1.8 and 6.2×104 ton yr?1, which corresponds to an assimilation rate of CO2 by marine photosynthesis between 0.06 and 0.23×106 ton yr?1. This rate suggests that the entire fjord system of Patagonia, which covers an area of ～240,000 km2, may represent a potentially important region for the global burial of marine organic matter and the sequestration of atmospheric CO2.  相似文献   

9.
Macroalgae blooms of Gracilaria vermiculophylla, Hypnea spinella and Spyridia filamentosa have been found in coastal lagoons in the SE Gulf of California. Agriculture, livestock, shrimp and poultry farms and sewage contribute anthropogenic nitrogen to the systems. The δ15N of these sources, water column and macroalgae were studied in order to identify the N supply for macroalgae blooms. δ15N of three species of macroalgae (4.3-13.6‰) were enriched compared to the water column ( 3.7-6.8‰), probably because of fractioning from the macroalgae. δ15N of POM (1.4-10.3‰) was similar to the water column but the relationship was unclear. Depending on the site, macroalgae showed different δ15N values since some sites receive more or less influence from one given source of the associated watershed, which is reflected in the different δ15N values of the macroalgae of the same system and in the relative contributions of the sources.  相似文献   

10.
To investigate the interaction between cyanobacteria and heterotrophic bacteria under gradients of glucose and nitrate, a cyanobacterial strain of Microcystis aeruginosa was grown in microcosms with and without a freshwater bacterial mixture, which was collected from Lake Taihu. Concentrations of glucose (1350, 975, 600, 300, 150, and 37.5 μmol C L−1) and nitrate (150, 300, and 9000 μmol N L−1) were used in a range of combinations giving 9 different treatments of glucose:nitrate. In the microcosm without the bacterial mixture, M. aeruginosa abundance gradually increased with days in all treatments. However, M. aeruginosa had much lower density in some treatments with the bacterial mixture. The difference in M. aeruginosa growth could be explained by competition with bacteria in the cultures in which these were added. The abundance of M. aeruginosa and bacteria when grown together was nearly equal and the number of the bacterial species was highest in the treatment with 300 μmol C L−1 and 150 μmol N L−1. Our results suggest that at this glucose:nitrate ratio M. aeruginosa and the bacterial mixture maintain a balance, and bacteria maintain diversity. In conclusion, we propose that dissolved organic carbon and nitrate availability fundamentally affects the structure as well as stoichiometry of pelagic associations.  相似文献   

11.
The incidence and severity of extraordinary macroalgae blooms (green tides) are increasing. Here, climate change (ocean warming and acidification) impacts on life history and biochemical responses of a causative green tide species, Ulva rigida, were investigated under combinations of pH (7.95, 7.55, corresponding to lower and higher pCO2), temperature (14, 18 °C) and nitrate availability (6 and 150 μmol L? 1). The higher temperature accelerated the onset and magnitude of gamete settlement. Any two factor combination promoted germination and accelerated growth in young plants. The higher temperature increased reproduction, which increased further in combination with elevated pCO2 or nitrate. Reproductive success was highest (64.4 ± 5.1%) when the upper limits of all three variables were combined. Biochemically, more protein and lipid but less carbohydrate were synthesized under higher temperature and nitrate conditions. These results suggest that climate change may cause more severe green tides, particularly when eutrophication cannot be effectively controlled.  相似文献   

12.
《Marine pollution bulletin》2012,65(12):2650-2655
In the present work, fat, skin, liver and muscle samples from Leptonychotes weddellii (Weddell seal, n = 2 individuals), Lobodon carcinophagus (crabeater seal, n = 2), Arctocephalus gazella (Antarctic fur seal, n = 3) and Mirounga leonina (southern elephant seal, n = 1) were collected from King George Island, Antarctica, and analysed for POPs (PCBs, organochlorine pesticides and PBDEs) and stable isotopes (δ13C and δ15N in all tissues but fat). PBDEs could be found in only one sample (L. weddellii fat). Generally, PCBs (from 74 to 523 ng g−1 lw), DDTs (from 14 to 168 ng g−1 lw) and chlordanes (from 9 to 78 ng g−1 lw) were the prevailing compounds. Results showed a clear stratification in accordance with ecological data. Nonetheless, stable isotope analyses provide a deeper insight into fluctuations due to migrations and nutritional stress. Correlation between δ15N and pollutants suggests, to some degree, a considerable ability to metabolize and/or excrete the majority of them.  相似文献   

13.
In snowmelt-driven mountain watersheds, the hydrologic connectivity between meteoric waters and stream flow generation varies strongly with the season, reflecting variable connection to soil and groundwater storage within the watershed. This variable connectivity regulates how streamflow generation mechanisms transform the seasonal and elevational variation in oxygen and hydrogen isotopic composition (δ18O and δD) of meteoric precipitation. Thus, water isotopes in stream flow can signal immediate connectivity or more prolonged mixing, especially in high-relief mountainous catchments. We characterized δ18O and δD values in stream water along an elevational gradient in a mountain headwater catchment in southwestern Montana. Stream water isotopic compositions related most strongly to elevation between February and March, exhibiting higher δ18O and δD values with decreasing elevation. These elevational isotopic lapse rates likely reflect increased connection between stream flow and proximal snow-derived water sources heavily subject to elevational isotopic effects. These patterns disappeared during summer sampling, when consistently lower δ18O and δD values of stream water reflected contributions from snowmelt or colder rainfall, despite much higher δ18O and δD values expected in warmer seasonal rainfall. The consistently low isotopic values and absence of a trend with elevation during summer suggest lower connectivity between summer precipitation and stream flow generation as a consequence of drier soils and greater transpiration. As further evidence of intermittent seasonal connectivity between the stream and adjacent groundwaters, we observed a late-winter flush of nitrate into the stream at higher elevations, consistent with increased connection to accumulating mineralized nitrogen in riparian wetlands. This pattern was distinct from mid-summer patterns of nitrate loading at lower elevations that suggested heightened human recreational activity along the stream corridor. These observations provide insights linking stream flow generation and seasonal water storage in high elevation mountainous watersheds. Greater understanding of the connections between surface water, soil water and groundwater in these environments will help predict how the quality and quantity of mountain runoff will respond to changing climate and allow better informed water management decisions.  相似文献   

14.
The objective of this study was to explore the slope position and land use change effects on the variability in magnetic susceptibility and 137Cs inventory as the soil redistribution indicators in a hilly semiarid calcareous area in Iran. The selected study area is located in a hilly region with pasture and cultivation land use of Fereydunshahr, Isfahan Province in west-central Iran. In the two mentioned dominant ecosystems, four slope positions including summit, shoulder, backslope and footslope were identified and in each land use and slope position, three cores were selected to collect 72 soil samples from three depths (0–10, 10–20, 20–30 cm) in an area of 15 × 15 cm. Additional 28 soil samples were collected from the reference site for soil loss and deposition calculations by using the Cs-137 measurement. The results of the study with the use of the Cs-137 technique showed that the average soil loss in the pasture land (46.4 t ha− 1 yr− 1) was significantly (p < 0.05) lower than the average soil loss in the cultivated land (80.4 t ha− 1 yr− 1). The highest soil loss in both land uses was obtained in the shoulder position, 60.1 and 84.4 t ha− 1 yr− 1, respectively, for the pasture and cultivated lands. Moreover, the highest rates of soil deposition was observed in a footslope position in both land uses and they were 34 and 32.4 t ha− 1 yr− 1 for the pasture and cultivated lands, respectively. Magnetic susceptibility was significantly (p < 0.05) greater in pasture (χlf = 41.51 × 10− 8 m3/kg) than in the cultivated land (χlf = 34.90 × 10− 8 m3/kg). The pasture land with a lower soil loss rate, indicated significantly higher magnetic susceptibility in all landform positions as compared to that in the cultivated land. The results of the correlation analysis showed that among the studied soil physico-chemical properties, χlf (r = 0.83, p < 0.01) in the pasture land had the highest correlation with the Cs-137 inventory. Throughout the non-linear regression analysis, χlf was introduced for relating soil parameters and the cesium inventory explained 68% and 79% of the total variability of 137Cs in the pasture and cultivated lands, respectively. The results implied that the variability in the magnetic susceptibility within the hillslope is consistent with the variation of the Cs-inventory; and the results thus demonstrate the slope and land use effects on soil redistribution.  相似文献   

15.
To investigate the behavior of Li during low-grade metamorphism and fluid flux in an accretionary prism we measured the Li concentrations ([Li]) and isotopic compositions (δ7Li) of sub-greenschist and greenschist-facies Otago Schist composites, as well as cross-cutting quartz veins, which are interpreted to have precipitated from slab-derived fluids. The average [Li] of sub-greenschist facies composites (41 ± 13 μg/g, 2σ) is statistically distinct (97% confidence level, student t test) to that of greenschist facies composites (34 ± 9 μg/g, 2σ), which have experienced mass addition of silica in the form of quartz veins having [Li] between 0.4–2.3 μg/g. A linear regression of the correlation between [Li] and calculated mass additions suggests that the depletion of [Li] in greenschist facies composites is due to both dilution from the addition of the quartz veins, as well as metamorphic dehydration. The [Li] of both groups of composites correlates with their CIA (Chemical Index of Alteration) values (50–58), which are low, consistent with the inferred graywacke protolith of the Otago Schist. The δ7Li of sub-greenschist and greenschist facies composites are remarkably constant, with an average δ7Li of 0.2 ± 1.7 (2σ) and ?0.5 ± 1.9 (2σ), respectively, and comparable to that of the average upper continental crust. Thus, metamorphism has had no discernable effect on δ7Li in these samples. The Li isotopic signature of the schists is similar to that seen in pelitic sedimentary rocks and likely reflects the δ7Li of the protoliths. The surprisingly light δ7Li of the quartz veins (?2.8 to ?1.4) likely records kinetic fractionation associated with Li ingress into the veins from surrounding wallrock.An isotopic equilibrium fluid flow model indicates that: 1) if the [Li] of slab-derived fluids is less than a few μg/g, the δ7Li of the overlying lithologies (i.e., the schists) is not significantly influenced by the fluid flux, regardless of the δ7Li of the fluids, 2) the slab-derived fluids will have heavy δ7Li of > + 10 after reacting with the prism sediments during their ascent, and 3) the [Li] of the slab-derived fluids is likely in the range of 0 < [Li] ≤ 41(μg/g). Thus, isotopically heavy slab-derived fluids that traverse sediments in accretionary prisms may leave little trace in the rocks and their surface compositional characteristics will reflect the net result of their interaction with the sediments of the prism.  相似文献   

16.
The tropical riparian zone has a high diversity of plant species that produce a wide variety of chemical compounds, which may be released into streams. However, in recent decades there has been an extensive replacement of tropical native vegetation by Eucalyptus monocultures. Our objective was to compare fungal colonization of Eucalyptus camaldulensis leaves with fungal colonization of native plant species from riparian zones in Brazilian Cerrado (savannah) streams. The fungal colonization and enzymatic activity significantly influenced leaf litter decomposition. Fungal sporulation rates from leaf litter varied significantly with leaf species, with E. camaldulensis showing the highest sporulation rate (1226 conidia mg−1AFDM day−1) and leaf mass loss (23.2 ± 0.9%). This species has the lowest lignin content and highest N concentration among the studied species. Among the studied native species, we observed the highest sporulation rate for Protium spruceanum (271 conidia mg−1AFDM day−1), Maprounea guianensis (268 conidia mg−1AFDM day−1) and Copaifera langsdorffii (196 conidia mg−1AFDM day−1). Overall, native plant species of the Brazilian Cerrado exhibited recalcitrant characteristics and a higher lignin:N ratio. Therefore, variations in the physical and chemical characteristics of the leaf litter could explain the higher decay rate and reproductive activity observed for E. camaldulensis. However, the detritus of this species were colonized almost exclusively by Anguillospora filiformis (99.6 ± 0.4%) and exhibited a reduction in aquatic hyphomycetes species diversity. Our results suggest that the disturbance in the composition of riparian vegetation and consequently, in the diversity of leaf litter input into streams, could change the patterns and rates of leaf litter utilization by microbial decomposers. These changes may have important consequences in the processing of organic matter and, consequently, in the functioning of freshwater ecosystems.  相似文献   

17.
In watersheds impacted by nitrate from agricultural fertilizers, nitrification and denitrification may be decoupled as denitrification in the hyporheic zone is not limited to naturally produced nitrate. While most hyporheic research focuses on the 1–2 m of sediment beneath the stream bed, there are a limited number of studies that quantify nitrogen (N) cycling at larger hyporheic scales (10s of metres to kms). We conducted an investigation to quantify N cycling through a single meander of a low gradient, meandering stream, draining an agricultural watershed. Chemistry (major ions and N species) and hydrologic data were collected from the stream and groundwater beneath the meander. Evidence indicates that nearly all the shallow groundwater flowing beneath the meander originates as stream water on the upgradient side of the meander, and returns to the stream on the downgradient side. We quantified the flux of water beneath the meander using a numerical model. The flux of N into and out of the meander was quantified by multiplying the concentration of the important N species (nitrate, ammonium, dissolved organic nitrogen (DON)) by the modelled water fluxes. The flux of N into the meander is dominated by nitrate, and the flux of N out of the meander is dominated by ammonium and DON. While stream nitrate varied seasonally, ammonium and DON beneath the meander were relatively constant throughout the year. When stream nitrate concentrations are high (>2 mg litre?1), flow beneath the meander is a net sink for N as more N from nitrate in stream water is consumed than is produced as ammonium and DON. When stream nitrate concentrations are low (<2 mg litre?1), the flux of N entering is less than exiting the meander. On an annual basis, the meander hyporheic flow serves as a net sink for N. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

18.
Watershed mean transit times (MTTs) are used to characterize the hydrology of watersheds. Watershed MTTs could have important implications for water quality, as relatively long MTTs imply lengthier water retention, thereby allowing more time for pollutant transformation and more moderate release of pollutants into streams. Although estimates of MTTs are common for undisturbed watersheds, only a few studies to date have applied MTT models to urbanized watersheds. In the present study, we use δ18O to compare estimates of MTTs for paired suburban‐industrial and agricultural watersheds in Toronto, Canada. Although differences in precipitation δ18O between the two watersheds were negligible, there were significant differences in stream δ18O, suggesting differences in water transport pathways. Less damping between input precipitation δ18O and output stream δ18O in the suburban‐industrial watershed indicated a larger streamflow contribution from quick‐flow transport pathways. We applied three transit time models to quantify MTTs. Considering overall model fit, root mean square error, and uncertainty in model parameters, the exponential model performed the best of the three models. Optimized MTTs using this distribution across the suburban‐industrial subwatersheds ranged from 2.1 to 2.9 months, whereas those in the agricultural subwatersheds ranged from 2.7 to 7.5 months. The relatively small difference between urban and agricultural MTTs coincides with observations elsewhere. Model efficiencies could potentially be improved, and MTTs estimated more reliably, with a higher sampling frequency that captures a greater volume of overall discharge. Overall, this work provides a distinct first glimpse into the separation of MTTs between paired watersheds with such a large contrast in their land use.  相似文献   

19.
Soda lakes and pans represent saline ecosystems with unique chemical composition, occurring on all continents. The purpose of this study was to identify and characterise the main environmental gradients and trophic state that prevail in the soda pans (n = 84) of the Carpathian Basin in Central Europe. Underwater light conditions, dissolved organic matter, phosphorus and chlorophyll a were investigated in 84 pans during 2009–2010. Besides, water temperature was measured hourly with an automatic sensor throughout one year in a selected pan. The pans were very shallow (median depth: 15 cm), and their extremely high turbidity (Secchi depth median: 3 cm, min: 0.5 cm) was caused by high concentrations of inorganic suspended solids (median: 0.4 g L−1, max: 16 g L−1), which was the dominant (>50%) contributing factor to the vertical attenuation coefficient in 67 pans (80%). All pans were polyhumic (median DOC: 47 mg L−1), and total phosphorus concentration was also extremely high (median: 2 mg L−1, max: 32 mg L−1). The daily water temperature maximum (44 °C) and fluctuation maximum (28 °C) were extremely high during summertime. The combination of environmental boundaries: shallowness, daily water temperature fluctuation, intermittent hydroperiod, high turbidity, polyhumic organic carbon concentration, high alkalinity and hypertrophy represent a unique extreme aquatic ecosystem.  相似文献   

20.
To date, studies of the stability of subsurface ice in the McMurdo Dry Valleys of Antarctica have been mainly based on climate-based vapor diffusion models. In University Valley (1800 m), a small glacier is found at the base of the head of the valley, and adjacent to the glacier, a buried body of massive ice was uncovered beneath 20–40 cm of loose cryotic sediments and sandstone boulders. This study assesses the origin and stability of the buried body of massive ice by measuring the geochemistry and stable O–H isotope composition of the ice and applies a sublimation and molecular diffusion model that accounts for the observed trends. The results indicate that the buried massive ice body represents an extension of the adjacent glacier that was buried by a rock avalanche during a cold climate period. The contrasting δ18O profiles and regression slope values between the uppermost 6 cm of the buried massive ice (upward convex δ18O profile and SD-18O = 5.1) and that below it (progressive increase in δ18O and SD-18O = 6.4) suggest independent post-depositional processes affected the isotope composition of the ice. The upward convex δ18O profile in the uppermost 6 cm is consistent with the ice undergoing sublimation. Using a sublimation and molecular diffusion model, and assuming that diffusion occurred through solid ice, the sublimation rate needed to fit the measured δ18O profile is 0.2 ? 10? 3 mm yr? 1, a value that is more similar to net ice removal rates derived from 3He data from cobbles in Beacon Valley till (7.0 ? 10? 3 mm yr? 1) than sublimation rates computed based on current climate (0.1–0.2 mm yr?1). We suggest that the climate-based sublimation rates are offset due to potential ice recharge mechanisms or to missing parameters, particularly the nature and thermo-physical properties of the overlying sediments (i.e., temperature, humidity, pore structure and ice content, grain size).  相似文献   

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