Negative Isotope Values Research Articles

Geochemistry of reduced fluids from shallow cold vents hosting chemosynthetic communities (Comau Fjord, Chilean Patagonia, ∼42°S)

Reduced fluids from shallow-marine vents sustain chemosynthetic bacterial mats located at the base of the volcano Barranco Colorado in Comau Fjord (X-Huinay; 42°23.279′S, 72°27.635′W). We characterized the chemical environment in which these bacteria thrive. To this end, we analyzed CH4, ∑H2S, O2, DIC, and stable isotopes (δ13C, δ2H and δ18O) and compared them with readings taken at a control station (which lacks evidence of fluid venting and mat-forming bacteria). In addition, CTD measurements, chlorophyll-a, and nutrient analyses were performed. At depths of approximately 5–7m, the water column exhibited a pycnocline that formed under the influence of fresh water discharges, especially during the summer season, which also affected the surface temperature. Bacterial mats and elemental sulfur flocs were observed in the vicinity of the vents (X-Huinay station), and higher concentrations of reduced compounds (CH4 and ∑H2S) were found in the vent fluids, in particular at a depth of 36m. No significant differences in the temperatures of vent and ambient waters were detected. The bottom water close to the vents possessed notably low concentrations of reduced compounds, indicating a rapid and large-quantity dilution of the chemicals from vents. The surface water featured light isotopic values of δ2H and 18O due to the freshwater input from melting ice and precipitation. A linear mixing trend was observed between the freshwater (negative isotopic values) and the fjord bottom water (positive values) that was not influenced by vent fluid. This trend suggests that the venting water corresponds to the mixing among local meteoric water, spring water, and seawater. This result is relevant to understanding how freshwater and meteoric water influence the chemical composition of seawater and how this mixing could impact the marine biota in the vicinities of the vents.

Paleocene–Eocene palaeoenvironmental conditions of the main phosphorite deposits (Chouabine Formation) in the Gafsa Basin, Tunisia

A detailed sedimentary section of the marine Chouabine Formation in the palaeogeographic Gafsa Basin, south-western Tunisia, was investigated in order to characterize environmental and depositional conditions focusing on the interval that spans the Paleocene–Eocene transition. We did stable isotope analyses of bulk sediments. Both phosphorite and carbonate yielded relatively similar isotopic compositions; while marls show observable trends, with negative shifts in both δ13Cinorg and δ18O values at the Paleocene–Eocene transition.The diversity of the calcareous microfossils is low. The presence of few environmentally tolerant small benthic foraminifera and the absence of planktic forms indicate a restricted palaeoenvironment subject to variation in salinity and temperature. The ostracod fauna is more diverse and is rather comparable to Paleocene and PETM (Paleocene–Eocene thermal maximum) assemblages in northwest Tunisia. While all these microfossils demonstrate various diagenetic features (i.e., phosphatization, secondary mineral overgrowths and infills), the different groups retain distinct stable isotopic compositions, suggesting partial preservation of signatures derived from their respective ecological niches.Few organic-rich layers below the negative δ13C shift yielded dinoflagellate assemblages with the common forms of Adnatosphaeridium sp., Operculodinium sp., Spiniferites spp. and Thalassiphora sp. The rare presences of Apectodinium-complex agree with depositional age prior to the PETM.The combined data emphasize that most of the Chouabine Formation in the Gafsa Basin was deposited during the late Paleocene. The most negative isotopic values from marls are believed to represent the onset of PETM or pre-PETM record, and the related perturbation of the carbon cycle in shallow water deposits in Tunisia.

Genesis and Exploration Implications of Epithermal Gold Mineralization and Porphyry-Style Alteration at the Endeavour 41 Prospect, Cowal District, New South Wales, Australia

Endeavour 41 is a deep-level, structurally controlled epithermal gold deposit hosted by Early Ordovician subaqueous volcanosedimentary rocks. Calc-alkalic to shoshonitic, mafic to intermediate sills, dikes, and stocks intruded the volcanosedimentary units in the Middle Ordovician. The most felsic intrusions, together with pyroxene-bearing dikes, are temporally related to gold mineralization. Postmineralization intrusions are exclusively of mafic character. Endeavour 41 evolved from early, high-temperature porphyry-style veins and alteration to lower-temperature epithermal-style gold mineralization. Early magnetite and garnet-bearing veins (stage 1 and 2, respectively) associated with actinolite, magnetite, and biotite-bearing alteration assemblages have been cut by gold-bearing veins and associated alteration assemblages. There were two main epithermal-style gold mineralizing events: (1) quartz-pyrite ± calcite ± adularia ± chlorite veins (stage 3) and (2) carbonate-base metal sulfide veins (calcite, ankerite, quartz, pyrite, sphalerite, galena, chalcopyrite, Ag tellurides, arsenopyrite, apatite, hematite, illite-muscovite, and chlorite [stage 4]). Gold occurs principally as a refractory phase in pyrite. It also occurs as grains of Au-Ag tellurides and as inclusions of free gold in pyrite, sphalerite, and chalcopyrite. Hydrothermal alteration associated with gold-mineralized veins produced early epidote and K-feldspar-epidote–bearing alteration halos and later-stage illite-muscovite-K-feldspar and calcite-rich alteration halos. The highest gold grades are associated with muscovite and illite alteration. Stable isotope analyses and fluid inclusion data provide evidence of a magmatic-hydrothermal component to the mineralizing fluids. Fluid inclusion data suggest that gold precipitated from boiling saline waters (~9.0 wt % NaCl) at temperatures of about 310°C. Stage 3 veins are estimated to have formed approximately 1 km below the paleosurface at hydrostatic pressure (~90 bars). Stage 4 illite formed at temperatures below ~280°C. Stage 3 calcite has δ 13 C calcite and δ 18 O calcite values that range from −5.2 to −4.6 and from 11.6 to 12.1‰, respectively. Calculated fluids for these mineral values at 300°C (δ 13 C fluid = −3‰; δ 18 O fluid = 6‰) are consistent with a magmatic-hydrothermal source of carbon and oxygen during stage 3. A component of meteoric waters is inferred for stage 4, because δ 13 C carbonate and δ 18 O carbonate values range from −6.9 to −0.5 and from 10.9 to 30.1‰, respectively, corresponding to δ 13 C fluid and δ 18 O fluid values of −5 and −2‰ at 200° to 250°C. The δ 34 S sulfide values for early vein stages range between −4.9 and −0.5‰. Stage 3 has δ 34 S sulfide values ranging from −5.2 to +0.8‰ with the most 34 S enriched values deposited away from the mineralized center. Stage 4 sulfides have isotopic compositions from −7.5 to +2.5‰. The negative isotopic values are consistent with oxidized (sulfate-predominant) magmatic-hydrothermal fluids. Sulfur isotopic zonation patterns show that the most negative δ 34 S values correlate with gold-enriched domains and also with areas that contain high-temperature, porphyry-style alteration facies. The negative sulfur isotope values define zones of upflow for the mineralizing magmatic-hydrothermal fluids. The paragenetic history of Endeavour 41 records a transition from deep-level to shallow-level magmatic-hydrothermal activity. This transition implies erosion and unroofing of the system synchronous with mineralization. High-temperature assemblages (e.g., actinolite-magnetite, biotite, and K-feldspar-epidote) indicate that epithermal mineralization occurred proximal to a magmatic-hydrothermal center and that there is potential for the discovery of porphyry copper-gold mineralization below the current level of diamond drilling.

Multiple sources of selenium in ancient seafloor hydrothermal systems: Compositional and Se, S, and Pb isotopic evidence from volcanic-hosted and volcanic-sediment-hosted massive sulfide deposits of the Finlayson Lake District, Yukon, Canada

Volcanic-hosted massive sulfide (VHMS) and volcanic-sediment-hosted massive sulfide (VSHMS; i.e., hosted by both volcanic and sedimentary rocks) deposits in the Finlayson Lake District, Yukon, Canada, provide a unique opportunity to study the influence of seafloor and sub-seafloor hydrothermal processes on the formation of Se-poor (GP4F VHMS deposit; 7ppm Se average), intermediate (Kudz Ze Kayah—KZK VHMS deposit; 200ppm Se average), and Se-enriched (Wolverine VSHMS deposit; 1100ppm Se average) mineralization. All three deposits are hosted by mid-Paleozoic (∼360–346Ma) felsic volcanic rocks, but only the Wolverine deposit has voluminous coeval carbonaceous argillites (black shales) in the host rock package. Here we report the first application of Se isotope analyses to ancient seafloor mineralization and use these data, in conjunction with Pb and S isotope analyses, to better understand the source(s) and depositional process(es) of Se within VHMS and VSHMS systems. The wide range of δ82Se (−10.2‰ to 1.3‰, relative to NIST 3149), δ34S (+2.0‰ to +12.8‰ CDT), and elevated Se contents (up to 5865ppm) within the Wolverine deposit contrast with the narrower range of δ82Se (−3.8‰ to −0.5‰), δ34S (9.8‰ to 13.0‰), and lower Se contents (200ppm average) of the KZK deposit. The Wolverine and KZK deposits have similar sulfide depositional histories (i.e., deposition at the seafloor, with concomitant zone refining). The Se in the KZK deposit is magmatic (leaching or degassing) in origin, whereas the Wolverine deposit requires an additional large isotopically negative Se source (i.e. ∼−15‰ δ82Se). The negative δ82Se values for the Wolverine deposit are at the extreme light end for measured terrestrial samples, and the lightest observed for hypogene sulfide minerals, but are within calculated equilibrium values of δ82Se relative to NIST 3149 (∼30‰ at 25°C between SeO4 and Se2−). We propose that the most negative Se isotope values at the Wolverine deposit record the δ82Se of the Se-source, and that the wide range in δ82Se values results from the combined effects of thermal and chemical degradation and Se-loss from the carbonaceous argillite source to a hydrothermal fluid (including magmatic Se i.e., leached and/or magmatic-hydrothermal) with deposition at or near the paleoseafloor. Pristine unaltered black shales show little variation in δ82Se relative to bulk earth; Se accumulation and fractionation to more negative isotopic values is interpreted to have been produced by post-sediment deposition, but pre-ore stage, reduction of seawater Se within the black shales.

Rock pool tufa stromatolites on a modern South African wave‐cut platform: partial analogues for Archaean stromatolites?

Terra Nova, 23, 375–381, 2011AbstractCalcifying tufa stromatolites are forming on a high energy coast of the South African Indian Ocean. The tufa stromatolites form in upper intertidal to supra tidal rock pools, encrust bedrock and are linked both laterally and vertically to physico‐chemical properties of their pool water. Calcification only occurs in pools where dissolved carbonate‐rich, spring‐fed groundwater is routed, and δ18O values are consistent with calcification in a mixture of freshwater and seawater. Increasing pH and less negative isotope values away from the spring are consistent with CO2 degassing and in‐stream calcification, albeit with some in‐mixing of seawater. The rocky shore setting of these active tufa stromatolites is new and invites serious comparison with some Archaean rocky substrate peritidal stromatolites (e.g. the c. 3.45 Ga Strelley Pool sequences, Pilbara Craton). This new association shows that initial encrustation of rocky intertidal substrates can begin with freshwater influence in the intertidal zone, a facies detail that should be sought in ancient peritidal stromatolites.

Diagenesis of the Cretaceous-Tertiary Willow Creek sandstones, southwestern region of Alberta

Abstract Seventy sandstone samples of the Late Maastrichtian? Early Paleocene strata from the Willow Creek Formation of the southwestern region of Alberta were selected in order to characterize factors that control the sandstone’s properties and to investigate diagenetic processes related to porosity evolution. The sandstones are fine- to medium-grained, moderate- to well-sorted litharenite and have undergone a complicated diagenetic history. The diagenetic regimes of the sandstones combine several processes related to three stages of diagenesis: eodiagenesis after deposition, mesodiagenesis during burial, and telodiagenesis after uplift. The diagenetic processes recognized during eogenesis include compaction, cementation (calcite, clay minerals), and the dissolution of feldspar grains. Extensive occurrences of early calcite cement suggest that calcite was a main control on the porosity. Abundant calcite cements precipitated from pore water with a significant meteoric component, as shown by the strong negative oxygen isotopic values (−19.546 to −3.019). However, the calcite cements inhibited the later mechanical compaction of the sandstones, thus preserving an unfilled part of the primary porosity. The most important mesogenesis processes that controlled reservoir quality were the illitization and chloritization of smectite followed by dickite formation. The pore-filling illite and chlorite resulted in a considerable loss of porosity, whereas the pore-lining chlorite may have helped in retaining the porosity by preventing the precipitation of quartz overgrowths. During telogenesis dissolution of calcite cement created secondary porosity, whereas the precipitation of kaolinite caused a reduction in porosity. This study may enhance the understanding of the role of diagenesis on reservoir quality in similar aged fluvial sandstone deposits in the subsurface such as the Scollard Formation in the central Alberta plains and the Coalspur Formation in the southwestern Alberta foothills.

Marine osmium isotope record across the Triassic-Jurassic boundary from a Pacific pelagic site

The Triassic-Jurassic (T-J) boundary ca. 200 Ma represents one of the major mass extinction events of the Phanerozoic; however, the cause of this event remains controversial because of a paucity of geological evidence. In this study we present an isotopic record of osmium extracted from a bedded chert succession across the T-J boundary in the Kurusu section of Japan, deposited within a Paleo-Pacific (Panthalassa) deep basin. The data show a gradual decrease in seawater 187 Os/ 188 Os values during the Rhaetian, followed by a sharp increase in the latest Rhaetian, and a subsequent stable phase across the T-J boundary. The decreasing trend of 187 Os/ 188 Os values during the Rhaetian indicates a gradual increase in the relative supply rate of unradiogenic Os from the mantle associated with emplacement of the Central Atlantic Magmatic Province. The subsequent shift toward radiogenic values reflects an increased supply of radiogenic Os due to enhanced continental weathering. This interval marks more negative isotopic values of organic carbon, the onset of radiolarian faunal turnover, and conodont extinctions, indicating that the rapid increase in continental weathering rate was closely linked to the perturbation of the carbon cycle and the T-J biotic crisis.

Correlation between δ 13C and δ 15N in C 4 and C 3 plants of natural and artificial sand-binding microhabitats in the Tengger Desert of China

Carbon isotope ratio (δ 13C) and nitrogen isotope ratio (δ 15N) of leaf in shrubs and overground matter in herbage were measured on plant species occurring in different aged artificial sand-binding microhabitats, as well as in natural habitat at the south-eastern margin of the Tengger Desert, China. Both δ 13C and δ 15N of C 3 and C 4 plants varied widely (− 28.12 ≤ δ 13C (‰) ≤ − 23.77 and − 4.45 ≤ δ 15N (‰) ≤ 3.66 for C 3 species, respectively; and − 15.79 ≤ δ 13C (‰) ≤ − 12.63 and − 7.56 ≤ δ 15N (‰) ≤ 1.08 for C 4 species, respectively), representing the different photosynthetic pathway (C 3/C 4) environmental controls. The relative abundance of C 4 species increased over the development of the sand-fixing vegetation, and the significant differences among sites in δ 15N of C 4 plants were found. Among the microhabitats, the isotopic pattern of artificial sand-fixing community is a cluster compared to that of natural vegetation and transitional zones in C 4 plants; however, this pattern of C 3 plants is different significantly. The biological soil crust had significant effect on δ 13C of C 3 plants, not for δ 13C of C 4 plants and δ 15N of both C 3 and C 4 species. Our results also describe a distinct pattern of dual-isotopic signatures, which indicated that a different water-use source and soil nitrogen compartment occurred and may promote the coexistence of different life forms in extreme poor water and nutrients ecosystems. With respect to C 4 plants, the gramineous plants based on family — level exhibit obviously negative isotopic values (both δ 15N and δ 13C) than those of chenopodiaceous plants, indicating a high water-use efficiency of gramineous C 4 plants and more 15N depleted, and these differences may be a result of different functional groups of C 4 species. For C 3 plants, the chenopodiaceous species also show more 15N enrichment than those of plants from other families (e.g. composite, leguminous plants and others). Contrast to previous studies, the δ 15N values of leguminous plants were not closer to the δ 15N of atmospheric nitrogen gas, indicating that leguminous plants in research region used nitrogen mainly from other N sources such as soil, rain/dust, not mainly from biological fixed nitrogen. The correlation between δ 13C and δ 15N of C 3 plants (except leguminous plants) were negative significantly ( P = 0.001), while there were positive relationship between δ 13C and δ 15N in C 3 leguminous plants ( P = 0.003) and C 4 plants ( P < 0.0001), indicating that in the water and nutrient limiting region the carbon and nitrogen discriminations are dependent and linked intimately, and vary with the plant species and photosynthetic pathway.

Pleistocene variations in dust input and marine productivity in the northern Benguela Current: Evidence of evolution of global glacial–interglacial cycles

We infer variations in paleoproductivity and eolian input at ODP Site 1082 in the Walvis Basin from stable oxygen isotope compositions of the planktonic foraminifera Globorotalia inflata, total organic carbon mass accumulation rates (TOC MAR), and X-ray fluorescence analyses of Fe content. The most pronounced paleoclimatic changes correspond to the time at about 0.9 Ma, when glacial conditions in the northern hemisphere (NH) led to the onset of pronounced 100-kyr glacial–interglacial cycles. We used Fe intensity as a proxy for eolian terrigenous input, and TOC MAR as a paleoproductivity indicator. Paleoproductivity and eolian input show generally higher-amplitude variations of glacial–interglacial cyclicity from 1.5 to 0.58 Ma, indicating pronounced variations in upwelling-favorable winds in this area. At 0.58 Ma, paleoproductivity and eolian input shifted abruptly to lower-amplitude variations with a periodicity of 100 kyr while δ 18O values show a trend toward more negative isotope values for the past 0.65 Myr. Especially during glacial periods, oxygen isotope values indicate increasingly warmer sea-surface temperatures toward the end of the Pleistocene. To evaluate the relative influences of NH glaciation and southern hemisphere (SH) insolation as potential forcing mechanisms for variations of eolian input and productivity in the northern Benguela system, we filtered our proxy records at orbital frequencies. The filtered records of Fe intensity and TOC MAR indicate a strong influence of the 100-kyr and 41-kyr frequency bands, supporting our assumption that strong ice buildup in the NH is the dominant trigger for climate changes on the continent and probably in trade-wind intensity. SH insolation and low-latitude precession-related insolation changes were important for paleoproductivity variations in the northern Benguela system, modifying the nutrient supply by southern ocean intermediate waters and the zonal direction of upwelling-inducing trades by the African monsoon system, respectively.

DOLOMITIZATION OF THE EARLY EOCENE JIRANI DOLOMITE FORMATION, GABES‐TRIPOLI BASIN, WESTERN OFFSHORE, LIBYA

Dolomitization in the early Eocene Jirani Formation in the Gabes‐Tripoli Basin (offshore western Libya) occurred in two stages. Stage I dolomites are composed of two types, one associated with anhydrite (Type I) the other anhydrite free (Type II,). The stratigraphic and sedimentological settings together with petrographic and geochemical criteria suggest that dolomitization was effected by refluxed evaporative seawater. Stable isotope and trace element analyses suggest dolomitization of both Types from a fluid of near‐surface seawater composition under oxidising conditions modified by evaporation. Non‐luminescence and lack ofzonation of all the dolomite indicate that the dolomitizing fluids maintained a relatively constant composition. The geologic setting during the early Eocene, interpreted as hypersaline lagoon, supports an evaporative reflux origin for the anhydritic dolomite Type I. Type II developed under less saline conditions in the transition zone between lagoon and open marine shelf.Stage II dolomitization is recorded by negative isotope values in both Types I and II indicating their dissolution and recrystallization (neomorphism) by dilute solutions. A period of exposure of the overlying Jdeir Formation following a relative sea‐level fall allowed ingress of meteoric waters into both the Jdeir and the underlying Jirani Formations. Flushing by meteoric waters also resulted in development of excellent secondaly porosity and caused major dissolution of anhydrite to form the anhydritic‐free dolomite facies typical of Type II. Following, and possibly during, both Stages I and II, low temperature dolomites (Type IIIa) precipitated in pore spaces from residual jluids at shallow burial depths, partially occluding porosity. In the late stage of basin evolution, medium clystalline, pore‐filling saddle dolomite precipitated, causing some filling of mouldic and vuggy porosity (Type IIIb). Very light oxygen isotopic signatures confirm that it developed from high temperature fluids during deep burial diagenesis. Calculation of temperatures and timings of the dolomitization and cement phases show that the main dolomitization phases and Type IIIa cements occurred in the early Eocene, and that the saddle dolomite precipitated in the Miocene; these results are consistent with age relationships established from stratigraphic, petrographic and geochemical signatures. The most common porosity includes intercrystal, vuggy and mouldic types. Porosity is both pre‐dolomitization and syn‐dolomitization in origin, but the latter is the most dominant. Hence, reservoir quality is largely controlled by fluid dynamics.

From accumulation to discharge: modification of stable isotopes during glacial and post-glacial processes

The isotopes of the water molecule (tritium, deuterium and oxygen-18) are considered to be ideal tracers of the water cycle, an isobalance of accumulation and discharge from glaciated areas is therefore challenging. In this article we review data from the Alps concerning both the isotopic information of accumulated snow and ice and the relevant information of glacial discharge on a local to regional scale. In the Alps the seasonal distribution of accumulated snow is governed by small-scale meteorology and exposition and distribution of cold and temperate glaciers. From comparison with direct precipitation measurements it follows that uncontrolled erosive loss of parts of the seasonal (mainly winter) snow cover is more important for input-output considerations of glacial discharge, reconstruction of mean accumulation rates and palaeotemperatures by means of isotopes, than occasional melting or even rainfall events. The order of magnitude of the latter is controlled by laboratory and field experiments. Sublimation and evaporation turn out to be of minor influence if only the absolute values and not the deuterium excess (the relationship of both stable isotopes, deuterium and oxygen-18) is considered. Nevertheless, during warming periods a pronounced increase of deuterium excess caused by intensive melting and evaporation may be used as an additional climate indicator. The different isotopic signature of the seasonal snow cover, old glacier ice and groundwater stored in the glacier bed enables a quantitative separation of the individual water components discharging at a given time of the day or the season. Although several detailed isotope studies are reported, a complete isobalance from glaciated areas based on an interdisciplinary approach, including hydrology and glaciology, is still missing. The hydroclimatic imprint of the discharge from glaciated areas remains even on a regional scale. Snowmelt shifts the more negative isotope values from winter precipitation to the summer months. Interannual climatic variability and changes in seasonal distribution of precipitation (for instance reduced winter precipitation during the late 1980s) are recorded by the stable isotopes in precipitation and discharge as well indicating that the modification of the isotopic information during glacial and post-glacial processes does not override climatic trends.

Life associated with a 2.76 Ga ephemeral pond?: evidence from Mount Roe #2 paleosol.

Dark sericitic material at and near the top of the 2.765 +/- 0.01 Ga Mount Roe #2 paleosol in Western Australia contains 0.05-0.10 wt% organic carbon with delta 13C values between -33% and -51% PDB (Peedee belemnite). Such negative isotopic values strongly indicate that methanotrophs once inhabited this material. The textures and the chemical composition of the dark sericitic material indicate that the methanotrophs lived in or at the edges of ephemeral ponds, that these ponds became desiccated, and that heavy rains transported the material to its present sites. The discovery of methanotrophs associated with the Mount Roe #2 paleosol may extend their geologic record on land by at least 1.5 b.y. Methanotrophy in this setting is consistent with the notion that atmospheric methane levels were > or = 20 (mu)atm during the Late Archean. The radiative forcing due to such high atmospheric methane levels could have compensated for the faint younger sun and helped to prevent massive glaciation during the Late Archean.

Abstracts

The Barranco (creek) León section is located in the NE marginal area of the lacustrine Plio-Pleistocene Baza Basin (Southern Spain). The Barranco León BL-5D mammal and archeological site has delivered abundant lithic tools and a hominin tooth; the age ascribed to this site is comprised between the Olduvai and Jaramillo subchrons, close to 1.4 Ma, based on magnetostratigraphy, biostratigraphy and ERS dating. The studied BL-5 section consists from base to top, of dark sandy mudstones and lutites with interbedded sandy marls. The lower part of the dark sandy mudstones contains a brackish fauna of foraminifera, ostracods, and mollusks. Sandy carbonate mudstones and sandy lutites with freshwater-oligosaline ostracods and mollusks form the upper part of the archeological excavation sequence. Stable isotope (δ13C, δ18O) and trace element analyses on ostracod and mollusk shells allow depicting the paleoenvironmental conditions recorded in the lacustrine sequence. Moreover, indications on provenance for waters have been obtained from 87Sr/86Sr ratios from shells and opercula from mollusks. The isotopic composition of the biogenic carbonates shows a strong correlation between δ13C and δ18O, which is noticeable for some gastropod shells. This correlation is a characteristic feature of closed lake settings. In our case, the more negative isotopic values reflect the input of meteoric waters and the more positive ones the skeletal carbonates formed in more isotopically concentrated waters. Some of the more negative isotopic values correspond to biogenic carbonates from the BL-5D level. They correspond to isotopically diluted waters, although the trace element data and the invertebrate fauna indicate oligosaline to mesosaline waters. Moreover, the presence in BL-5D of shells from Melanopsis tuberculata, a thermophilous gastropod, together with their isotopic signatures suggest the input of oligosaline waters with a thermal influence. The available geochemical data indicate a first stage of varying influence of several water types under changing water level in a closed lake recorded in the base of the studied sequence (brackish fauna level). The overlying beds record a through flowing open lacustrine environment mainly fed by meteoric surface and ground waters with significant inputs of stream waters.

A stable isotope study of cave seepage waters

An isotope study (δ 18O, δ 2-H) of 62 samples of waters from both the interior of the Nerja cave (Málaga, Southern Spain) and the exterior sinkholes and springs at the site was carried out. Rainfall water in the study area was also studied. The mean value of seepage waters can be considered as the mean isotopic value of the precipitation waters of the area. The origin and aquifers of these different types of waters has also been studied, obtaining a clear meteoric origin both for the seepage waters and the waters from the wells and springs, but the aquifer of the latter is different, showing more negative isotopic values. This suggests that they could have mixed with waters from other aquifers which are either deeper or originate at other altitudes.