Stable isotope characterisation of co-existing carbonates from the Holocene Gallocanta lake (NE Spain): palaeolimnological implications

The sedimentary succession of Gallocanta lake, a closed saline lake located in the Iberian Range (NE Spain), documents two successive lacustrine stages: (1) brackish lake stage and (2) shallow saline lake stage. The saline stage corresponds to the present-day situation in which the lake water properties are mainly controlled by a strongly negative annual water balance. The carbonates of the brackish lake stage have relatively constant δ¹⁸O values, however, they are rather high (δ¹⁸O_Do = 2.4‰ and δ¹⁸O_Cc = 4.5‰ mean values) suggesting a hydrologically closed lake with a long residence time of the waters. δ¹⁸O values of carbonates from the saline stage vary greatly, and are lighter than in the previous stage (δ¹⁸O_Do = 0.5‰, δ¹⁸O_Cc = −0.7‰, δ¹⁸O_Mgs = −2.3‰ mean values). These carbonates also precipitated in a hydrologically closed lake, but in equilibrium with a lake water of more variable isotopic composition. The δ¹³C values for carbonates of both stages reflect a mixing of different pools of carbon, but during saline stage δ¹³C values have been more controlled by the equilibrium of the lake waters with atmospheric CO₂. During the current stage, calcite and dolomite precipitate in Gallocanta lake mainly during spring and summer, although dolomite precipitation is more favoured towards the summer. Magnesite precipitates at the beginning of autumn, when the first rainfall re-dissolves the saline surface crust, producing saline waters with a high Mg²⁺ content. The isotopic composition of lake waters sampled in 2005 are far higher than those calculated from the carbonates. It is considered that this could be due to two factors: either because there have not been many extremely dry years (like the year 2005) during the development of the lake, or because the physical and chemical characteristics of the lake waters in such conditions are not appropriate for the development of these minerals.     

Determining the seasonality of groundwater recharge using water isotopes: a case study from the upper North Han River basin,Korea

The stable isotopes of oxygen and hydrogen were used to determine the seasonal contributions of precipitation to groundwater recharge at a forested catchment area in the upper North Han River basin, Korea. A comparison of the stable isotopic signatures of groundwater and precipitation indicates that the precipitations which occurred during both the dry and rainy seasons are the important source of groundwater recharge in this region. A stable isotopic signature shown in the stream waters at the upstream reaches is similar to that of groundwaters, indicating that stream waters are mostly fed by groundwater discharge. Reservoir waters in the downstream flood control dams have lower deuterium excess values or d-values compared with those of the upstream waters, indicating a secondary evaporative enrichment. These results can provide a basis for the effective management of groundwater and stream water resources in the North Han River basin.     

Glacial-interglacial changes in the surface water characteristics of the Andaman Sea: Evidence from stable isotopic ratios of planktonic foraminifera

Stable carbon and oxygen isotopic analyses of the planktonic foraminifera (Globigerinoides ruber) from a deep sea sediment core (GC-1) in the Andaman Sea show high glacial-to-Holocene δ¹⁸O amplitude of 2.1%o which is consistent with previously published records from this marginal basin and suggest increased salinity and/or decreased temperature in the glacial surface waters of this region. A pulse of¹⁸O enrichment during the last deglaciation can be attributed to a Younger Dryas cooling event and/or to a sudden decrease of fresh water influx from the Irrawady and Salween rivers into the Andaman Sea. High δ¹³C values observed during the isotopic stages 2 and 4 are probably due to the enhanced productivity during glacial times in the Andaman Sea.     

Investigation of groundwater mineralization in the Hammamet–Nabeul unconfined aquifer,north-eastern Tunisia: geochemical and isotopic approach

Detailed hydrogeochemical and isotopic data of groundwaters from the Hammamet–Nabeul unconfined aquifer are used to provide a better understanding of the natural and anthropogenic processes that control the groundwater mineralization as well as the sources of different groundwater bodies. It has been demonstrated that groundwaters, which show Na–Cl and Ca–SO₄–Cl water facies, are mainly influenced by the dissolution of evaporates, the dedolomitization and the cation-exchange process; and supplementary by anthropogenic process in relation with return flow of irrigation waters. The isotopic signatures permit to classify the studied groundwaters into two different groups. Non-evaporated groundwaters that are characterized by depleted δ ¹⁸O and δ ²H contents highlighting the importance of modern recharge at higher altitude. Evaporated groundwaters with enriched contents reflecting the significance infiltration of return flow irrigation waters. Tritium data in the studied groundwaters lend support to the existence of pre-1950 and post-1960 recharge. Carbon-14 activities in shallow wells that provide evidence to the large contamination by organic ¹⁴C corroborate the recent origin of the groundwaters in the study area.     

Origin of salinity in produced waters from the Palm Valley gas field,Northern Territory,Australia

The chemical composition and evolution of produced waters associated with gas production in the Palm Valley gas field, Northern Territory, has important implications for issues such as gas reserve calculations, reservoir management and saline water disposal. The occurrence of saline formation water in the Palm Valley field has been the subject of considerable debate. There were no occurrences of mobile water early in the development of the field and only after gas production had reduced the reservoir pressure, was saline formation water produced. Initially this was in small quantities but has increased dramatically with time, particularly after the initiation of compression in November 1996.The produced waters range from highly saline (up to 300,000 mg/L TDS), with unusual enrichments in Ca, Ba and Sr, to low salinity fluids that may represent condensate waters. The Sr isotopic compositions of the waters (⁸⁷Sr/⁸⁶Sr = 0.7041–0.7172) are also variable but do not correlate closely with major and trace element abundances. Although the extreme salinity suggests possible involvement of evaporite deposits lower in the stratigraphic sequence, the Sr isotopic composition of the high salinity waters suggests a more complex evolutionary history.The formation waters are chemically and isotopically heterogeneous and are not well mixed. The high salinity brines have Sr isotopic compositions and other geochemical characteristics more consistent with long-term residence within the reservoir rocks than with present-day derivation from a more distal pool of brines associated with evaporites. If the high salinity brines entered the reservoir during the Devonian uplift and were displaced by the reservoir gas into a stagnant pool, which has remained near the reservoir for the last 300–400 Ma, then the size of the brine pool is limited. At a minimum, it might be equivalent to the volume displaced by the reservoired gas.     

Isotopic and chemical intra-shell variations in low-Mg calcite of rudist bivalves (Mollusca-Hippuritacea): disequilibrium fractionations and late Cretaceous seasonality

 Isotopic (δ¹³C, δ¹⁸O) and elemental (Mg, Sr, Mn, Fe) compositions were analysed in sclerochronological profiles of several shells of late Cretaceous rudist bivalves from Greece, Turkey, Somalia and the Arabian Peninsula. The preservation of original compositions of low-Mg calcite of outer shell layers is indicated by constant and high Sr, generally low Fe and Mn, and the preservation of fibrous-prismatic ultrastructures. Cyclic variations in δ¹⁸O and Mg are interpreted to reflect seasonal temperature/salinity cycles and, thus, annual growth increments. In shells of Torreites, amplitudes of correlated δ¹³C and δ¹⁸O cycles cannot be related to reasonable palaeotemperatures or salinity. This isotopic pattern reflects vital fractionations of an extent which is unknown from modern bivalves. In contrast, almost identical ranges and amplitudes of δ¹⁸O cycles are observed in 13 shells of five species from Santonian-Campanian localities in central Greece and northern Turkey, suggesting that seasonal variations in environmental conditions were recorded without significant vital fractionations. The effect of seasonal salinity changes on δ¹⁸O of the shells is evaluated, and mean palaeotemperatures are constrained within the range of 30–32.5  °C. The annual range of temperature was estimated to be 7  °C, assuming a constant salinity. This agrees with other isotopic proxies of Late Cretaceous palaeotemperatures, and with global circulation models which predict higher low-latitude sea-surface temperatures than the present ones. Received: 12 February 1998 / Accepted: 24 May 1999     

Study of shallow groundwater quality evolution under saline intrusion with environmental isotopes and geochemistry

Evolution of the shallow groundwater quality under saline intrusion in porous aquifer system has been studied with environmental isotopes and geochemistry in the Laizhou Bay area, China. Two campaigns of water sampling from various sources were carried out in spring and winter for environmental isotopic and chemical analyses. The origin of groundwater salinity from intrusion of both modern seawater and deep brine water was identified by analysing the correlations between ¹⁸O, D, T, Cl⁻, SO₄²⁻ and electrical conductivity. The results indicate that the brine is originated from evaporating and concentrating of intruded seawater and its δD and δ¹⁸O are different from modern seawater but similar to those of mixture of seawater with fresh groundwater. It is hard to distinguish the salinity origin in this area by the δD–δ¹⁸O relationship alone. The relations between δ¹⁸O and conductivity, Cl⁻ and SO₄²⁻ have been used to identify the salinity origin due to the distinct difference in salinity between the brine and seawater, conjunctively with use of T. A threshold of T = 12 TU was adopted to identify the origin of saline groundwater.     

Salt Waters of the Northern Apennine Foredeep Basin (Italy): Origin and Evolution

The salt waters from the Emilia-Romagna sector of the Northern Apennine Foredeep have been investigated using major and trace element and stable isotope (δ²H, δ¹⁸O, δ³⁷Cl, δ⁸¹Br and ⁸⁷Sr/⁸⁶Sr ratio). Ca, Mg, Na, K, Sr, Li, B, I, Br and SO₄ vs. Cl diagrams suggest the subaerial evaporation of seawater beyond gypsum and before halite precipitation as primary process to explain the brine’s salinity, whereas saline to brackish waters were formed by mixing of evaporated seawater and water of meteoric origin. A diagenetic end-member may be a third component for mud volcanoes and some brackish waters. Salinization by dissolution of (Triassic) evaporites has been detected only in samples from the Tuscan side of the Apennines and/or interacting with the Tuscan Nappe. In comparison with the seawater evaporation path, Ca–Sr enrichment and Na–K–Mg depletion of the foredeep waters reveal the presence of secondary processes such as dolomitization–chloritization, zeolitization–albitization and illitization. Sulfate concentration, formerly buffered by gypsum-anhydrite deposition, is heavily lowered by bacterial and locally by thermochemical reduction during burial diagenesis. From an isotopic point of view, data of the water molecule confirm mixing between seawater and meteoric end-members. Local ¹⁸O-shift up to +11‰ at Salsomaggiore is related to water–rock interaction at high temperature (≈150°C) as confirmed by chemical (Mg, Li, Ca distribution) and isotopic (SO₄–H₂O) geothermometers. ³⁷Cl/³⁵Cl and ⁸¹Br/⁷⁹Br ratios corroborate the marine origin of the brines and evidence the diffusion of halogens from the deepest and most saline aquifers toward the surface. The ⁸⁷Sr/⁸⁶Sr ratio suggests a Miocene origin of Sr and rule out the hypothesis of a Triassic provenance of the dissolved components for the analyzed waters issuing from the Emilia-Romagna sector of the foredeep. Waters issuing from the Tuscan side of the Apennines and from the Marche sector of the foredeep show higher ⁸⁷Sr/⁸⁶Sr ratios because of the interaction with siliciclastic rocks.     

Climatic Signals in the Last 200 Years from Stable Isotope Record in the Shells of Freshwater Snails in Lake Xingcuo,Eastern Tibet Plateau,China

Lake Xingcuo is a small closed,hard-water lake ,situated on eastern Tibet Plateau.Stable isotope data(δ^18O and δ^13C) from the freshwater snail Gyraulus sibirica(Dunker)in a34 cm long,radioactive isotope-dated sediment core represent the last 200 years of Lake Xingcuo environmental history.Carbon and oxygen isotope ratios in the shells of the freshwater snail bear information on the isotopic composition of the water in which the shells were formed ,which in turn characterizes the climatic conditions prevailing during the snail‘s life span.Whole-shell and incremental growth data were collected from modern and fossil shells from Lake Xingcuo.The δ18^O values of modern shells from Lake Xingcuo are in equilibrium with high δ^13CTDIC.By calibrating δ^18O and δ^13C in the shell Gyraulus sibirica(Dunker)with in-strument-measured data for the period 1954-1992,we found that the δ^18O of the snail shells is an efficient indicator to reveal air temperature in the warmer half year instead of that around the whole year,and that there is a certain positive correlation between index δ^18O and the run-ning average temperature in the warmer half-yiar period.Climatic variability on eastern Tibet Plateau,for the last two centuries,has been successfully inferred from the δ^18O record in freshwater snails in the sediments of Lake Xingcuo.As such,the last 200 years of palaeocli-matic record for this region can be separated into three periods representing oscillations between warming and cooling,which are confirmed by the Guliya ice record on the Tibet Plateau.     

Carbon and oxygen isotope constraints on fluid sources and fluid–wallrock interaction in regional alteration and iron-oxide–copper–gold mineralisation, eastern Mt Isa Block, Australia

The source of metasomatic fluids in iron-oxide–copper–gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, δ ¹⁸O and δ ¹³C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5‰ δ ¹⁸O and 0.5‰ δ ¹³C calcite) and graphitic meta-sedimentary rocks (ca. 14‰ δ ¹⁸O and −18‰ δ ¹³C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na–(Ca) alteration systems strongly cluster around 11‰ δ ¹⁸O and −7‰ δ ¹³C, with shifts towards higher δ ¹⁸O values and higher and lower δ ¹³C values, reflecting interaction with different hostrocks. Na–(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the δ ¹³C signatures of CO₂-rich metasomatic fluids associated with Na–(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO₂-rich fluids and represent the most probable fluid source for Na–(Ca) alteration. Intrusion-proximal, skarn-like Cu–Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11‰ δ ¹⁸O and −7‰ δ ¹³C (calcite), indicating an isotopically similar fluid source as for Na–(Ca) alteration and that significant fluid–wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in δ ¹⁸O and δ ¹³C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid–wallrock equilibration before mineralisation. Low temperature, low salinity, low δ ¹⁸O (<10‰ calcite) and CO₂-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu–Au mineralisation.     

Environmental Evolution of the Water Body of Qinghai Lake since the Postglacial Age

Based on the data developed from various s natural waters in the Qinghai Lake area and ostracode shells present in drill core QH-16A of recent lake-floor sediments ,this paper discusses the distribution of stable isotopes in the modern water body of Qinghai Lake,and the initial isotopic composition of the lake water has been deduced ,Studies of δ^18O,δ^13C,Mg/Ca and Sr/Ca in ostracode shells provide the basis for the establishment of the model of climatic fluctuation in the Qinghai Lake area since the postaglacial age,as well as for the elucidation of the environmental evolution of the water body of Qinghai Lake since the postglacial age.     

Stable isotopes and heavy metal distribution in Dreissena polymorpha (Zebra Mussels) from western basin of Lake Erie, Canada

 Dreissena polymorpha is an exotic freshwater bivalve species which was introduced into the Great Lakes system in the fall of 1985 through the release of ballast water from European freighters. Utilizing individual growth rings of the shells, the stable isotope distribution (δ¹⁸O and δ¹³C) was determined for the life history of selected samples which were collected from the western basin of Lake Erie. These bivalves deposit their shell in near equilibrium with the ambient water and thus reflect any annual variation of the system in the isotopic records held within their shells. Observed values for δ¹⁸O range from -6.64 to –9.46‰ with an average value of –7.69‰ PDB, while carbon values ranged from –0.80 to –4.67‰ with an average value of –1.76‰ PDB. Dreissena polymorpha shells incorporate metals into their shells during growth. Individual shell growth increments were analyzed for Pb, Fe, Mg, Mn, Cd, Cu, and V concentrations. The shells show increased uptake of certain metals during periods of isotopic enrichment which correspond with warmer water temperatures. Since metals are incorporated into the shells, the organism may be useful as a biomonitor of metal pollution within aquatic environments. Received: 31 October 1996 · Accepted: 21 May 1997     

Fouling community of the Loxahatchee River estuary,Florida, 1980–81

Monthly growth of the fouling community at eight test panel sites in the Loxahatchee River Estuary was related to salinity and temperature. Growth was lowest in January 1981 (averaging 23 g per m², dry weight), and increased during spring and early summer with increasing water temperature. Maximum growth occurred during early or midsummer at upstream locations, before river or canal discharge substantially reduced salinity, and in late summer at downstream locations. Growth was greatest at salinities slightly less than that of seawater and decreased at salinities less than about 10‰. Growth was suppressed throughout the estuary in August 1981, probably because of the sudden decrease in temperature and salinity, and perhaps the increase in physical scouring, caused by runoff from Tropical Storm Dennis. Large loads of nutrients transported to the estuary from storm runoff, however, may have subsequently stimulated growth, which increased in September 1981 to the maximum for the year (averaging 683 g per m², dry weight).     

Die Isotopenzusammensetzung von Dolomiten und Kalken aus dem süddeutschen Malm

On dolomites and calcites from the Malm (Upper Jurassic) formation of Southern Germany ¹³C and ¹⁸O-determinations have been done. The most striking feature of our results is that the dolomites are isotopically lighter in oxygen isotope composition than the coexisting calcites. The isotopic composition of the formation waters during dolomitization and diagenetic recrystallization seem to be more effective in influencing the isotope composition of dolomites than a possible fractionation under isotopic equilibrium conditions. Although it is often assumed that dolomite is more stable against diagenetic isotope exchange than calcite, it is demonstrated that the ¹⁸O-content of dolomites seems to decrease too with increasing geologic age.     

Isotopic Ratios Reveal Mixed Seasonal Variation Among Fishes from Two Subtropical Estuarine Systems

Characterizing dietary resources and species interactions in estuaries is challenging, particularly when considering the dynamic nature of these ecosystems, the ranges in body sizes of species, and the potential for trophic roles to vary with ontogeny. We examined the influence of season and location on relationships between body size and δ¹⁵N, δ¹³C, and δ³⁴S values across a range of fishes from two subtropical estuaries. The results suggest that isotopic values of estuarine fishes are independent of body size. However, seasonal variation propagated throughout the assemblage as the majority of fishes integrated different δ¹⁵N, δ¹³C, or δ³⁴S values. The absence of δ¹⁵N–, δ¹³C–, and δ³⁴S–body size relationships suggests that either (1) dietary preference of these fishes do not shift within the range of body sizes sampled, (2) these fishes shift to an alternate diet that is not isotopically distinct, or (3) that spatial and temporal variation in isotopic signatures of prey negate any size-based relationships. Seasonal variability in the isotopic values of these fishes suggests either movement to an alternative habitat or a shift in organic matter source associated with the transition of dry to wet seasons. Moreover, variance distributions of the best-fit models indicate that seasonal dietary preferences of conspecifics do not vary over moderate spatial scales. Seasonal variability among fishes in these estuaries suggests plasticity in feeding strategies that may afford greater adaptive flexibility to these species in response to changes in food availability resulting from variable environmental conditions.     

Groundwater origins and mixing pattern in the multilayer aquifer system of the Gafsa-south mining district: a chemical and isotopic approach

Major ion geochemistry and environmental isotopes were used to identify the origins and the mineralisation processes of groundwater flowing within the three aquifer levels of the multilayer system of the Gafsa-south mining district (Southwestern Tunisia). It has been demonstrated that groundwaters are characterised by a Ca–Mg–SO₄ water type. Geochemical pattern is mainly controlled by the dissolution of halite, gypsum and/or anhydrite as well as by the incongruent dissolution of dolomite. δ¹⁸O and δ²H values are much lower than the isotopic signature of regional precipitation and fall close to the meteoric water lines, indicating that groundwaters have not been significantly affected by evaporation or mineral–water reactions. The distribution of stable and radiogenic isotopes (δ¹⁸O, δ²H, δ¹³C and ¹⁴C) within the aquifer levels suggests that the deep confined aquifer receives a significant modern recharge at higher altitudes, while, the shallow unconfined aquifer has been mainly recharged under cooler paleoclimatic condition, likely during Late Pleistocene and Early Holocene humid periods. However, waters from the intermediate confined/unconfined aquifer have composite isotopic signatures, highlighting that they are derived from a mixture of the two first end-members.     

The trophic structure of the pipefish community (Pisces: Syngnathidae) from a western Mediterranean seagrass meadow based on stable isotope analysis

Syngnathus abaster andSyngnathus typhle (Pisces: Syngnathidae) from a dense Cymodocea nodesa meadow in the western Mediterranean Stagnone di Marsala (Italy) were studied using δ¹³N and δ¹⁵N analysis. Because of the presence of these two species in the same habitat and the specialized parental care by the male, the effect of species and sex on the isotopic composition was also studied to investigate the different feeding strategies between and within species.S. abaster andS. typhle exhibited enriched¹³C and¹⁵N values throughout the sampling period (mean ±SE, δ¹³C =−10.5±1.8‰ and−10.8±2.0‰, δ¹⁵N=11.9±0.7‰ and 10.6±1.0%., respectively), suggesting that these species receive their energy mainly from mixed sources, particularly from sedimentary and particulate organic matter and the seagrassC. nodosa as ultimate organic matter sources. ANOVA results demonstrated that the interaction between season, species, and sex significantly affected the isotopic composition of the pipefish (p<0.001 for both δ¹³C and δ¹⁵N). Differences between species and sex, although significant, were smaller than the values generally reported for trophic level differences (≈1‰ and ≈3.5‰ for δ¹⁵N, respectively) and change in foraging habitat, Slight isotopic differences may mirror small differences in resource exploitation and resource partitioning. Evidence from stomach content analysis from the literature coupled with stable isotope measurements, while disagreeing somewhat, provide additional knowledge of pipefish feeding strategies.     

Salinization properties of a shallow groundwater in a coastal reclaimed area,Yeonggwang, Korea

The need for more agricultural or residential land has encouraged reclamation at the coastal areas of Korea since 1200 ad (approximately). The groundwaters of these reclaimed areas could be expected to reveal hydrogeochemical properties different from those of areas directly affected by seawater intrusion. The purpose of this study, therefore, was to examine the salinization of shallow groundwater in a coastal reclaimed area and to identify the effect of land reclamation on groundwater quality. Major cations and anions, iodide, total organic carbon, δD, δ ¹⁸O and δ ¹³C were measured to assist the hydrogeochemical analysis. Chloride, δD and δ ¹⁸O data clearly show that the Na–Cl type water results from mixing of groundwater with seawater. In particular, the δD and δ ¹⁸O of Ca+Mg–Cl+NO₃ type groundwaters are close to the meteoric water line, but Na–Cl type waters enriched in chloride are ¹⁸O-enriched with respect to the meteoric water line. Meanwhile, carbon isotopic data and I/Cl ratios strongly suggest that there are various sources of salinity. The δ ¹³C values of Na–Cl type groundwaters are generally similar to those of Ca+Mg–Cl+NO₃ type waters, which are depleted in ¹³C with respect to seawater. I/Cl ratios of Na–Cl type groundwater are 10–100 times higher than that of seawater. Because the reclamation has incorporated a large amount of organic matter, it provides optimum conditions for the occurrence of redox processes in the groundwater system. Therefore, the salinization of groundwater in the study area seems to be controlled not only by saltwater intrusion but also by other effects, such as those caused by residual salts and organic matter in the reclaimed sediments.     

Plankton and dissolved inorganic carbon isotopic composition in a river-dominated estuary: Apalachicola Bay, Florida

To characterize the isotopic composition of organisms at the base of the food web and the controls on their variability, the concentration and δ¹³C isotopic composition of dissolved inorganic carbon (DIC) and plankton δ¹³C, δ¹⁵N, and δ³⁴S were measured. The measurements were made during periods of high and low river flow in Apalachicola Bay, Florida, United States, over 3 yr. DIC concentration and δ¹³C values were related to salinity, indicating that conservative mixing of riverine and marine waters was responsible for the overall distributions. The usefulness of DIC δ^13C data for characterizing the trophic processes within the estuary was dependent upon the residence time of water within the season. Plankton δ¹³C values varied from −22‰ to −30‰ and were directly related to estuarine DIC δ¹³C, offset by a factor of roughly −20‰. This offset factor varied with salinity. Values of δ³⁴S in estuarine plankton (station means ranged from 11.4‰ to 13.1‰) were depleted relative to marine plankton (17.7±0.4‰) possibly due to the admixture of³⁴S-depleted sedimentary sulfide with estuarine samples. Values of δ³⁴S in plankton were not related to δ¹³C values of plankton and were only weakly correlated to the salinity of the water from which the plankton were collected, indicating that marine sulfate was the primary source of planktonic sulfur. Values of δ¹⁵N in plankton varied from 5.5‰ to 10.7‰ and appeared related to dominance of the sample by phytoplankton or zooplankton. Estuarine plankton was¹⁵N enriched relative to offshore plankton and estuarine sediment.     

Hydrochemical and stable isotopic investigation of groundwater quality and its sustainability for irrigation in the Hammamet-Nabeul basin,northeastern Tunisia

The major ion hydrochemistry, sodium absorption ratio (SAR), sodium percentage, and isotopic signatures of Hammamet-Nabeul groundwaters were used to identify the processes that control the mineralization, irrigation suitability, and origin of different water bodies. This investigation highlights that groundwater mineralization is mainly influenced by water-rock interaction and pollution by the return flow of irrigation water. The comparison of groundwater quality with irrigation suitability standards proves that most parts of groundwater are unacceptable for irrigation and this long-term practice may result in a significant increase of the salinity and alkalinity in the soils. Based on isotopic signatures, the shallow aquifer groundwater samples were classified into (i) waters with depleted δ¹⁸O and δ²H contents, highlighting recharge by modern precipitation, and (ii) waters with enriched stable isotope contents, reflecting the significance of recharge by contaminated water derived from the return flow of evaporated irrigation waters. The deep-aquifer groundwater samples were also classified into (i) waters with relatively enriched isotope contents derived from modern recharge and mixed with shallow-aquifer groundwater and (ii) waters with depleted stable isotope contents reflecting a paleoclimatic origin. Tritium data permit to identify three origins of recharge, i.e., contemporaneous, post-nuclear, and pre-nuclear. Carbon-14 activities demonstrate the existence of old paleoclimatic recharge related to the Holocene and Late Pleistocene humid periods.