validity of the environmental chloride method for recharge evaluation of coastal aquifers india the work presented in the paper was carried out with the objective of ascertaining the validity of environmental chloride method for evaluation of groundwater recharge to the coastal aquifers situated in semi arid tropical area under the influence of monsoonic type of hydrologic regime towards this end a comparative study using environmental chloride in conjunction with injected tritium method the more commonly used method was undertaken in the coastal aquifers of pondicherry situated about 160 km south of madras along the coast of bay of bengal five each of environmental chloride repeated twice and injected tritium profiles were studied in the alluvium and cuddalore aquifers of pondicherry recharge from the chloride study was estimated using 1 concentration approach which involves estimating the relative enrichment of soil chloride concentration in relation to the input chloride and 2 flux approach determining the turnover time of soil moisture by estimating the total chloride in the profile and yearly input recharge from the injected tritium profiles was estimated by studying the displacement of soil moisture during the time interval between sampling and injection of the tracer repeated chloride profiles show fairly good reproducibility within 15 30 of recharge rates determined by the two approaches for two different sampling times at the same site also the results compare well among the two approaches used for the chloride method mean recharge rates obtained by the injected tritium method for the cuddalores is about 26 cm yr 1 and by the chloride method about 22 cm yr 1 and for the alluvium the respective rates are 10 and 14 cm yr 1 this shows fairly good agreement between the two methods the study thus demonstrates conformity of recharge rates determined by the environmental chloride method and injected tritium technique and proves the validity of the chloride method for recharge measurements for tropical coastal aquifers situated in semi arid regions

monsoon fluctuations in two 20 000 yr b p oxygen isotope pollen records off southwest india abstract two marine pollen diagrams for sediments off the southwest coast of india provide a 20 000 yr history of the nearby continental vegetation determined by the monsoon climate within the framework of the isotopic stratigraphy two important phases of the evolution of the monsoon climate are a very arid period about 22 000 18 000 yr b p and a very humid period culminating at 11 000 yr b p the very arid period corresponds to the lowest pollen representation of mangrove vegetation which is used here as an indirect indicator of monsoonal runoff this aridity is due to a very weak southwest airflow a great reduction of summer monsoonal rainfall and reduced runoff of the western ghats rivers the extension of the monsoon over india after the last glacial maximum has been a gradual process following the northward progression of the intertropical convergene zone the very humid period corresponding to the highest pollen representation of mangrove vegetation and the heaviest summer rainfall is a climatic response to the maximum summer insolation of the northern hemisphere at 11 000 yr b p

stable isotope ratios of carbon in phytoliths as a quantitative method of monitoring vegetation and climate change the 13c 12c ratios of occluded carbon within opal phytoliths from the northern great plains show potential as a basis for paleoclimatic reconstruction a significant correlation exists between the carbon isotopic composition of a host plant and that of the organic matter in its phytoliths the 13c 12c ratios for phytoliths from surface layers of soils along climatic gradients reflect the current proportions of c3 and c4 plants variations in the 13c values of phytoliths with soil depth are caused by a variety of processes burial of soil surface by dust bioturbation and possible illuviation by percolating water also contributions of phytoliths by dust and roots have unknown isotopic effects the 13c values of phytoliths from soils increase with decreasing 14c age suggesting that the proportion of c4 plants in this region has increased during the holocene phytoliths of apparent mid holocene age suggest exclusive dominance by c4 plants which agrees with paleoclimatic interpretations of an arid middle holocene climate

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

carbonate replacement of lacustrine gypsum deposits in two neogene continental basins eastern spain abstract bedded nonmarine gypsum deposits in the miocene teruel and cabriel basins eastern spain are partly replaced by carbonate the libros gypsum teruel graben is associated with fossiliferous carbonate wackestones and finely laminated organic matter rich mudstones which accumulated under anoxic conditions in a meromictic permanent lake the gypsum is locally pseudomorphed by aragonite or less commonly replaced by calcite low 13c values indicate that sulphate replacement resulted from bacterial sulphate reduction processes that were favoured by anacrobic conditions and abundant labile organic matter in the sediments petrographic evidence and oxygen isotopic composition suggest that gypsum replacement by aragonite occurred soon after deposition a subsequent return to oxidising conditions caused some aragonite to be replaced by diagenetic gypsum native sulphur is associated with some of these secondary gypsum occurrences the los ruices sulphate deposits cabriel basin contain beds of clastic and selenitic gypsum which are associated with limestones and red beds indicating accumulation in a shallow lake calcite is the principal replacement mineral bacterial sulphate reduction was insignificant in this basin because of a scarcity of organic matter stable isotope composition of diagenetic carbonate indicates that gypsum replacement occurred at shallow burial depths due to contact with dilute groundwaters of meteoric origin depositional environment evidently has a major influence upon the diagenetic history of primary sulphate deposits the quantity of preserved organic matter degradable by sulphate reducing bacteria is of particular importance and along with groundwater composition is the main factor controlling the mechanism of gypsum replacement by carbonate

replacement of evaporites within the permian park city formation bighorn basin wyoming usa the permian park city formation consists of cyclically bedded subtidal to supratidal carbonates cherts and siltstones early diagenesis of park city formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resulted in evaporite emplacement syndepositional nodules and cements as well as dolomitization silicification and leaching of carbonate grains major differences are seen however in the diagenetic patterns of subsurface and surface sections of park city formation rocks subsurface samples are characterized by extensively preserved evaporite crystals and nodules and preserve evidence of significant silicification chert chalcedony and megaquartz and minor calcitization of evaporites in outcrop sections the evaporites are more poorly preserved and have been replaced by silica and calcite and also leached the resultant mouldic porosity is filled with widespread very coarse blocky calcite spar these replacements appear to be multistage phenomena field and petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of burial prior to hydrocarbon migration siliceous sponge spicules provided a major source of silica and the fluids involved in replacement were probably a mixture of marine and meteoric waters a second period of replacement and minor calcitization is inferred to have occurred during deep burial under the influence of thermochemical sulphate reduction although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval the major period of evaporite diagenesis however occurred during late stage uplift the late stage replacement and pore filling calcites have 13c values ranging from 0 5 to 25 3 and 18o values of 16 1 to 24 30 pdb reflecting extensive modification by meteoric water vigorous groundwater flow associated with mid tertiary block faulting led to migration of meteoric fluids through the porous carbonates to depths of several kilometres these waters reacted with the in situ hydrocarbon rich pore fluids and evaporite minerals and precipitated calcite cements the tosi chert appears to have been an even more open system to fluid migration during its burial and has undergone a much more complex diagenetic history as evidenced by multiple episodes of silicification calcitization ferroan and non ferroan and hydrocarbon emplacement the multistage replacement processes described here do not appear to be restricted to the permian of wyoming similarly complex patterns of alteration have been noted in the permian of west texas new mexico greenland and other areas as well as in strata of other ages thus multistage evaporite dissolution and replacement may well be the norm rather than the exception in the geological record

the timing of alluvial sedimentation and floodplain formation in the lowland humid tropics of ghana sierra leone and western kalimantan indonesian borneo abstract temporal patterns in floodplain genesis and alluvial sedimentation in lowlands tropical rain forest zones of ghana sierra leone and western kalimantan indonesian borneo based upon 14 c age determinations are described alluvial low terraces or buried sediments in west africa yielded ages of 36 21 ka in west kalimantan a widespread episode of alluviation has yielded dates of 54 51 ka the 20 13 ka interval was characterised by channel incision with valley floor erosion and neither region records sedimentation holocene alluvial sedimentation and floodplain construction in west africa occurred during two temporal intervals 10 7 ka and 4 ka to present and in western kalimantan in response to early holocene sea level rise followed by late holocene regression and coastal outgrowth the clustering of 14 c dates closely corresponds to regional lake level fluctuations and vegetational changes and to global indications of climatic change we propose that periods of more frequent episodes of accelerated floodplain erosion and reconstitution channel morpho sedimentary activity and alluvial accumulation 1 are responses to interstadial and interglacial periods of higher precipitation following intervening periods of cooler and drier conditions and 2 may be synchronous during the last 60 ka throughout the african and asian inner humid lowland tropics

late quaternary alluvial history of the northwestern deccan upland region continental and oceanic sediments from tropical africa australia and south america have recorded analogous changes in climate and stream behaviour during the late quaternary1 4 recently data from north central india have provided evidence for late quaternary changes in river regime in response to global changes in climate in the intertropical zone4 5 here we report evidence for late quaternary river metamorphosis6 from the northwestern deccan upland region of india radiocarbon dates combined with stratigraphic data have allowed the identification of major episodes of erosion and deposition7 8 supporting evidence from the arabian sea2 3 demonstrates that the successive phases of aggradation and incision are linked to late quaternary climatic changes7 8

methane related microbial gypsum calcitization in stromatolites of a marine evaporative setting munder formation upper jurassic hils syncline north germany fossil stromatolites may reveal information about their hydrochemical palaeoenvironment provided that assignment to a specific microbial community and a corresponding biogeochemical mechanism of formation can be made tithonian stromatolites of the munder formation at thuste north germany have traditionally been considered as formed by intertidal cyanobacterial communities however thin sections of the stromatolites show elongated angular traces of former gypsum crystals in a dense arrangement but no algal or cyanobacterial filament traces moreover high fe 2 and mn 2 contents oxygen isotope and sulphur isotope ratios of carbonate bound sulphates and sulphurized hydrocarbon biomarkers of the stromatolitic carbonate indicate that caco3 precipitation occurred near the oxic anoxic interface as a result of intensive bacterial sulphur cycling rather than photosynthetic activity furthermore anaerobic oxidation of methane by archaea may have driven caco3 precipitation in deeper parts of the biofilm community as reflected by high concentrations of squalane with a strongly negative d 13 c in conjunction with evaporite pseudomorphs showing extremely low d 13 ccarb ratios consequently the thuste stromatolites are now interpreted as having initially formed by gypsum impregnation of biofilms subsequently early mg calcitic calcitization within the biofilms occurred because of combined bacterial iron manganese and sulphate reduction with an increasing contribution of anaerobic oxidation of methane with depth this model plausibly explains the prominent preservation of signals derived from oxygen independent metabolic pathways whereas virtually no geochemical record exists for an aerobic community that may nevertheless have prevailed at the stromatolite surface photic zone stromatolites with a prominent signal of anaerobic oxidation of methane may be common in and indicative of oxygen depleted sulphate bearing environments with high rates of methane production conditions that possibly were fulfilled at the archaean to proterozoic transition

water level fluctuations of lake tanganyika in phase with oceanic changes during the last glaciation and deglaciation there has been considerable controversy about the magnitude of fluctuations of the levels of lake tanganyika the earth s second deepest lake 1 470 m following the discovery of submerged valleys extending down to 550 m below present lake levels1 these fluctuations register changes in the precipitation evaporation evapotranspiration ratio in a large equatorial tropical area of catchment south of the equator here we report new palaeohydrological data back to 40 kyr bp from carbon dating of the total organic matter in two diatomaceous cores the results constrain the vertical lake level fluctuations more than has hitherto been possible2 5 and show that the fluctuations from 26 kyr bp are correlated with changes in global sea level and ice volume surpris ingly fluctuations seem to be in phase with those of the african lakes north of the equator which are clearly linked to the milankovitch mechanisms

a δ 13 c record of late quaternary climate change from tropical peats in southern india stable isotope ratios of carbon in soils or lake sediments1 3 and of oxygen and hydrogen in peats4 5 have been found to reflect past moisture variations and hence to provide valuable palaeoclimate records previous applications of the technique to peat have been restricted to temperate regions largely because tropical climate variations are less pronounced making them harder to resolve here we present a 13c record spanning the past 20 kyr from peats in the nilgiri hills southern india because the site is at high altitude 2 000 m above sea level it is possible to resolve a clear climate signal we observe the key climate shifts that are already known to have occurred during the last glacial maximum 18 kyr ago and the subsequent deglaciation in addition we observe an arid phase from 6 to 3 5 kyr ago and a short wet phase about 600 years ago the latter appears to correspond to the mediaeval warm period which previously was believed to be confined to europe and north america6 7 our results therefore suggest that this event may have extended over the entire northern hemisphere

10be 14c and u th decay series nuclides and δ18o in a box core from the central north atlantic abstract the stratigraphy of several radioisotopes has been determined in an undisturbed core from the central north atlantic 14c 238u 230th 232th 226ra 210pb 10be carbon 14 indicates a sedimentation rate of 1 cm 1000 yrs 1 the core represents about 30 000 years the oxygen isotope signal agrees with this scale the 18 o range is about 1 5 from glacial to holocene the maximum change is near 11 000 years b p mixing effects are minor because of the low fertility of the overlying waters excess lead 210 shows a minimum between 1 and 2 cm depth and a maximum immediately below it does not yield a diffusion coefficient for the goldberg and koide 1962 mixing model the 10be concentration in the holocene sediment on a caco3 free basis is somewhat higher than that deposited during the previous cold period however both clay and 10be deposition rates are lower by a factor of over two during the holocene the source of the excess 10be in the glacial section could be a continental reservoir or increased cosmic ray production the u and th concentrations as well as the excess 230 th 232 th activity ratios of the sediment deposited during the holocene are higher by 30 100 than in glacial sediments again this is in contrast to the deposition rates of u and th which are lower in the holocene by a factor of about two the most reliable signals are the radiocarbon and the oxygen isotope stratigraphy

estimates of holocene precipitation for rajasthan india based on pollen and lake level data abstract a pollen profile obtained from lake sediments at lunkaransar rajasthan in northwest india was used along with a pollen climate calibration function to estimate past precipitation between 10 500 and 3500 yr b p the estimated precipitation was about 500 mm yr or about 200 mm yr above the modern value a model was used for the energy and hydrologic budget of a lake basin and lake at sambhar located 240 km se of lunkaransar to calculate that a 200 mm yr increase in rainfall above the modern amount would have raised the lake level about 20 m above the modern level topographic charts and satellite imagery provided some evidence in support of an enlarged paleolake of that elevation but field exploration would be required to confirm the size and date of a former lake after about 3500 yr b p the lunkaransar profile indicated a desiccated lake bed because no pollen was preserved the pollen climate calibration function was of no use for estimating the amount of the precipitation decline a reduction of precipitation of about 200 mm yr below the modern amount was estimated from the energy and hydrologic budget for paleolake sambhar by assuming that the lake was one tenth of its present size during the dry interval

geothermics of the williston basin in canada in relation to hydrodynamics and hydrocarbon occurrences over 8 400 bottom hole temperature bht values from the canadian part of the williston basin were analyzed and a temperature high was discovered in the weyburn area of southeastern saskatchewan geothermal gradients thermal conductivities and heat flow have been investigated for most of the mesozoic cenozoic clastic unit as well as the upper paleozoic carbonate evaporite unit regional heat flow variations with depth occur which are closely related to the hydrodynamics governed by the topography and geology the blanketing effect of low conductivity shaly formations may cause a temperature anomaly in the south where the thickest phanerozoic cover exists however the weyburn high can be explained only partially in this way hydrodynamics has also contributed to formation of the temperature anomaly there the process of forming the anomaly by the blanketing effect and hydrodynamics also contributed to oil deposition there is a correlation between mississippian oil occurrences in the southeastern part o

isotope evidence for pleistocene climatic changes in kashmir india stable carbon isotope ratios of organic carbon from palaeosols in loess and oxygen isotope ratios of pedogenic and lacustrine carbonates reveal significant pleistocene climatic changes in the kashmir basin the extreme 13c values of 16 and 25 reported here indicate a shift from arid to sub humid conditions the 18o values of pedogenic carbonates is 2 higher than that of the underlying lacustrine carbonates suggesting that they were deposited in different climatic conditions

reaction pathways and textural aspects of the replacement of anhydrite by calcite at 25 c the replacement of sulfate minerals by calcium carbonate polymorphs carbonation has important implications in a variety of geological processes occurring in earth surface environments in this paper we report the results of an experimental study of the interaction between anhydrite 100 010 and 001 surfaces and na2co3 aqueous solutions under ambient conditions carbonation progress was monitored by glancing incidence x ray diffraction gixrd and scanning electron microscopy sem we show that the reaction progresses through the dissolution of anhydrite and the simultaneous growth of calcite the growth of calcite occurs oriented on the three anhydrite cleavage surfaces and its formation is accompanied by minor vaterite the progress of the carbonation always occurs from the outer ward to the inner ward surfaces and its rate depends on the anhydrite surface considered with the 001 surface being much more reactive than the 010 and 100 surfaces the thickness of the formed carbonate layer grows linearly with time the original external shape of the anhydrite crystals and their surface details e g cleavage steps are preserved during the carbonation reaction textural characteristics of the transformed regions such as the gradation in the size of calcite crystals from 2 m in the outer region to 17 m at the calcite anhydrite interface the local preservation of calcite crystalographic orientation with respect to anhydrite and the distribution of the microporosity mainly within the carbonate layer without development of any significant gap between at the calcite anhydrite interface finally we compare these results on anhydrite carbonation with those on gypsum carbonation and can explain the differences on the basis of four parameters i the molar volume change involved in the replacement process in each case ii the lack existence of epitactic growth between parent and product phases iii the kinetics of dissolution of the different surfaces and iv the chemical composition amount of structural water of the parent phases

dolomite calcitization and cement zonation related to uplift of the raisby formation zechstein carbonate northeast england abstract the raisby formation is the basal zechstein carbonate of northeast england it has undergone a complex diagenetic history during burial including multiphase dolomitization and replacement by sulphates upon uplift meteoric derived fluids have dissolved this replacive sulphate and precipitated calcite cements in the resultant porosity study of these cements by cathodoluminescence and trace element geochemistry shows that they are zoned and different cathodoluminescence zones correlate well with iron and manganese geochemical changes demonstrating a gradual increase in oxygenation of the precipitating fluids through time major diagenetic processes during cement growth include precipitation of anhydrite fluorite and barite calcitization of dolomite and internal sedimentation relating these processes to cement zonation allows this complex diagenesis to be explained in terms of uplift under the influence of meteoric derived groundwaters of constantly changing geochemical and electrochemical characteristics

an isotopic investigation of groundwater in the central san juan basin new mexico carbon 14 dating as a basis for numerical flow modeling carbon 14 dating of groundwater can be used to help determine the transmissivity distributions of aquifers this method may offer significant advantages in certain respects over traditional aquifer pumping test techniques we have applied 14c dating to a hydraulic analysis of a multilayer aquifer system in the central san juan basin of new mexico after corrections for geochemical evolution of the solutes the influence of dispersive processes on the 14c distribution was investigated a model incorporating stochastic dispersion theory indicated that macroscopic dispersion exerted only a small influence on the measured 14c activities the 14c derived transmis sivity distribution was used to construct a numerical flow model which was applied to an analysis of interaquifer leakage the model showed that even though vertical flow between aquifers was significant in this case it did not cause the 14c distribution to differ significantly from that predicted by a simple piston flow model carbon 14 can be used as a basis for detailed hydraulic evaluations of groundwater flow in areas where traditional well hydraulics methods are not practical

origins of gypsum in deep carbonate reservoirs implications for hydrocarbon exploration and production gypsum can form penecontemporaneously in carbonate sediments and it may be altered to anhydrite during burial some workers have suggested that gypsum dehydration is complete at burial depths of 3500 ft 1067 m however gypsum has been documented at depths as great as 13 100 ft 3993 m two examples of deeply buried gypsum illustrate contrasting origins gypsum in the permian san andres formation in the permian basin of west texas at a depth of 6000 ft 1829 m occurs as anhydrite nodules with an outer rind of gypsum suggesting that anhydrite was rehydrated to gypsum by interaction with interstitial brines in contrast gypsum in the carbonate beds of the cretaceous ferry lake anhydrite of the mississippi interior salt basin at 13 100 ft 3993 m occurs as gypsum nodules with a rind of anhydrite suggesting that the gypsum is primary and in the process of dehydration to anhydrite these contrasting origins are likely a function of evaporite and carbonate stratigraphy in the permian basin a thick section of carbonate rocks is overlain by a relatively thin section of evaporites in the mississippi interior salt basin carbonates are interbedded with multiple evaporite beds which inhibit circulation of interstitial brines the burial temperature pressure and salinity of these formations suggest that gypsum could be preserved as primary or secondary gypsum the presence of gypsum in reservoir rocks can affect porosity calculations especially calculations using neutron porosity logs which measure bound water of hydration in gypsum as porosity identification of gypsum at the depth of 13 100 ft 3993 m indicates that log estimates of porosity in some deep carbonate reservoirs containing gypsum could have significant porosity and water saturation measurement errors

geochemical evidence for anoxic deep water in the arabian sea during the last glaciation various paleoceanographic studies have indicated that the deep ocean was probably depleted in dissolved oxygen during the last glacial period approximately 18 kyr b p delta sup 18 o stage 2 compared to present time however direct evidence of low oxygen content in the deep waters has been lacking here the authors report geochemical evidence of near anoxic conditions in the deep arabian sea during the entire last glacial cycle delta sup 18 o stages 2 3 and 4 anoxia is inferred from the concomitant enrichment of organic carbon and authigenic uranium in the glacial sections of a core from the deep eastern arabian sea the anoxic conditions during the last glacial period probably caused by a change in deep water circulation evidently enhanced preservation of organic matter and simultaneous removal of uranium from seawater 57 refs 3 figs 2 tabs

late diagenetic calcitization of anhydrite from the mississippian of saskatchewan western canada mississippian nodular anhydrites beneath an unconformity in the subsurface of southern saskatchewan are locally replaced by calcite pyrite and celestite triassic clastics above the unconformity are green rather than red and a usually developed subunconformity alteration zone where carbonates are dolomitized and porosity is filled with anhydrite is absent the unconformity lacks karstic features unlike in the usa and probably formed in a hyperarid climate mississippian anhydrites near the unconformity are not preferentially dissolved nor were they extensively hydrated anhydrite calcitization occurred only after the unconformity was shallowly buried by redbeds and it probably involved sulphate reducing bacteria hydrogen sulphide generated by bacteria reduced redbed pigments the replacement calcite contains pseudomorphs and relicts of anhydrite and pseudomorphs of secondary gypsum these indicate calcitization occurred only after original mississippian gypsum was altered to anhydrite and this in turn was partially converted back to secondary gypsum beneath the unconformity replacement occurred concurrently with the formation elsewhere of the dolomitized zone beneath the unconformity sulphur isotopic ratios of replacement pyrite are depleted relative to mississippian sulphate values consistent with the activities of sulphate reducing bacteria carbon isotopic ratios of replacive calcites however do not support this interpretation and are identical to those of mississippian limestones simple replacement of sulphate by pore water bicarbonate in equilibrium with host limestones is unlikely because protons generated during the reaction should have created acidic conditions in which calcite would have dissolved a full explanation of the calcitization remains elusive but may involve replacement occurring in an active groundwater system and or bacterial sulphate reduction occurring upstream of the site of calcitization

coastal sabkha dolomites and calcitised sulphates preserving the lomagundi jatuli carbon isotope signal palaeoproterozoic tulomozero formation carbonates of the onega basin were targeted by icdp far deep hole 10b the recovered rocks contain abundant evidence for palaeoproterozoic gypsum and anhydrite particularly in the form of pseudomorphs of inclusion containing nodules swallow tail twinned crystals and evaporite dissolution breccias similarly sand patch fabric indicates the former presence of surface covering salt crusts in the palaeoproterozoic atmospheric oxygen and seawater sulphate levels at that time must have been sufficient to allow such sulphates to form carbonate 13c values are considered sedimentary or early diagenetic predating greenschist facies metamorphism associated with the svecofennian orogeny these carbonate 13c values range from 7 7 to 15 7 with the highest and lowest values found in dolostones the dolostones as well as calcitised calcium sulphates record the lomagundi jatuli signal however evidence for activity of sulphate reducing and or methanogenic bacteria seems to be lacking dolomite precipitation or dolomitisation calcium sulphate calcitisation and production of the high 13c values processes which typically involve such bacteria in the phanerozoic are thus inferred to have proceeded without the direct influence of sulphate reducers and methanogens the most plausible explanation for the lomagundi jatuli excursion seems to remain significant sequestration of organic carbon in a location yet to be identified

early dolomitization and recrystallization in shallow marine carbonates mississippian alida beds williston basin canada evidence from petrography and isotope geochemistry abstract the mississippian lower carboniferous alida beds of the northern williston basin canada are carbonate rocks that contain a number of significant hydrocarbon reservoirs these shallow marine carbonate platform deposits consist of abundant very finely to finely crystalline dolomite that occurs mainly in mud dominated sediments dolomite crystals are 4 to 30 m in length subhedral to anhedral in shape and typically replace the matrix but also occur as thin lamellae within ooid cortices textural relations indicate that the dolomite predates all other diagenetic phases including coarse calcite cement anhydrite and bitumen dolomite 18o values 9 3 to 4 5 pdb are depleted relative to that expected for precipitation from osagean late tournaisian seawater 0 5 to 1 5 pdb while 13c values 2 3 to 3 4 pdb are within the range of osagean seawater values 1 8 to 4 8 pdb dolomite 87sr 86sr values 0 70816 to 0 70921 are somewhat elevated with respect to osagean seawater 0 70768 to 0 70817 a trend of increasing average dolomite crystal size with decreasing 18o is observed in matrix replacive dolomite based on petrographic relations 13c compositions and the pervasive occurrence of finely crystalline dolomite initial dolomitization is interpreted to have taken place in the presence of slightly evaporated seawater the absence of depositional evaporites in dolomitized intervals suggests that dolomitization was driven by reflux of slightly evaporated seawater that had not reached gypsum saturation depleted 18o values elevated 87sr 86sr ratios and correlation of dolomite crystal sizes and 18o values suggest that the early formed dolomite was variably recrystallized during burial

antecedent aeolian dune topographic control on carbonate and evaporite facies middle jurassic todilto member wanakah formation ghost ranch new mexico usa

late quaternary environments in north central india microfossil sediment and oxygen isotope studies of deep sea cores from the bay of bengal and northern arabian sea1 5 have revealed strong contrasts between high late pleistocene and low early holocene salinity values3 indicative of major changes in runoff from the large rivers of southern asia2 3 here we present the first radiocarbon dated continental evidence for late quaternary changes in river regime and loess accumulation in this region our data support earlier palaeoclimatic inferences based solely on oceanic evidence2 3 have important implications for fluvial palaeohydrology6 and provide clear environmental constraints against which to test existing regional models of prehistoric land use and early agriculture 7 9

the oxygen and carbon isotope composition of carboniferous fossil components sea water effects the aragonitic molluscs and lime mud of the pennsylvanian buckhorn asphalt deese group of southern oklahoma precipitated calcium carbonate in oxygen and carbon isotopic equilibrium with ambient sea water in addition 18o values indicate that the pelecypods precipitated their shells during the warmer months of the year the coiled nautiloids probably precipitated their shells in the warm surface water and throughout the year for the orthocone nautiloids the 18o values suggest that they precipitated their shells in deeper cooler water the low mg calcite brachiopods of the mississippian lake valley formation of new mexico precipitated shells in oxygen and carbon isotopic equilibrium with ambient sea water the 18o and 13c values of the buckhorn and lake valley faunas in conjunction with other published results suggest that carboniferous sea water was on a average depleted in 18o by 1 5 2 pdb relative to recent sea water however the 13c value of 2 6 2 pdb for average carboniferous sea water is similar to that of recent ocean water early diagenetic alteration of metastable carbonates probably occurs in a meteoric sea water mixing zone in this zone the oxygen and carbon isotopic compositions of these components are increased by about 2 4 pdb over their marine composition

constraining the fluid history of a co2 h2s reservoir insights from stable isotopes ree and fluid inclusion microthermometry reservoirs that host co2 h2s bearing gases provide a key insight into crustal redox reactions such as thermochemical sulphate reduction tsr despite this there remains a poor understanding of the extent duration and the factors limiting this process on a reservoir scale here we show how a combination of petrography fluid inclusion rare earth element ree and carbon 13c oxygen 18o and sulphur 34s stable isotope data can disentangle the fluid history of the world s largest co2 accumulation the labarge field in wyoming usa the carbonate hosted labarge field was charged with oil around 80 ma ago which together with nodular anhydrite represent the reactants for tsr the nodules exhibit two distinct trends of evolution in 13c with both 34s and 18o that may be coupled to two different processes the first trend was interpreted to reflect the coupled dissolution of anhydrite and reduction to elemental sulphur and the oxidation of organic compounds and associated precipitation of calcite during tsr in contrast the second trend was interpreted to be the result of the hydrothermal co2 influx after the cessation of tsr in addition mass balance calculations were performed to estimate an approximate tsr reaction duration of 80 ka and to identify the availability of organic compounds as the limiting factor of the tsr process such an approach provides a tool for the prediction of tsr occurrence elsewhere and advancing our understanding of crustal fluid interactions

palaeomagnetism of lower cambrian sediments from the olenek river section northern siberia palaeopoles and the problem of magnetic polarity in the early cambrian summary new palaeomagnetic data from the lower and middle cambrian sedimentary rocks of northern siberia are presented during stepwise thermal demagnetization the stable characteristic remanence chrm directions have been isolated for three cambrian formations both polarities have been observed and mean chrm directions for normal polarity are kessyusa formation lower cambrian d 145 i 4o n 12 ug5 12 8 pole position cd 38 s a 165 e erkeket formation lower cambrian stratigraphically highly d 152 i 47 n 23 ug5 6 8 pole position cd 45 s a 159 e yunkyulyabit yuryakh formation middle cambrian d 166 i 33 n 38 ctg5 4 6 pole position 0 36 s l 140 e these poles are in good agreement with the apparent polar wander path based on the bulk of existing cambrian palaeomagnetic data from the siberian platform in cambrian times the siberian platform probably occupied southerly latitudes stretching from about 35 to o and was oriented reversely with respect to its present position siberia moved northwards during the cambrian by about 10 of latitude this movement was accompanied by anticlockwise rotation of about 30 the magnetostratigraphic results show the predominance of reversed polarity in the early cambrian and an approximately equal occurrence of both polarities in the part of the middle cambrian studied these results are in good agreement with the palaeomagnetic polarity timescale for the cambrian of the siberian platform constructed previously by khramov et al 1987

evidence of past climatic change in the namib desert abstract a preliminary survey of some areas of the central and northern namib desert was undertaken in order to identify geomorphic and sedimentary features that could potentially provide information on past climatic change in the region wherever possible samples were collected for radiocarbon dating the visits to the skeleton coast area were especially revealing practically every river valley in this tract shows a sequence of terraces and deposits that would warrant a detailed study in all about 120 new radiocarbon dates were obtained for the region these have supplied a wealth of individual results and collectively have enabled the construction of a tentative chronological sequence of the changes in climate for the past 40 000 yr three periods of increased aridity in the hinterland but with decreasing intensity are postulated during the last glacial maximum the early holocene and between a d 1200 and a d 1600 the inter pleniglacial up to 23 000 yr b p the late glacial and the period between about 4000 yr b p and 1200 yr b p on the other hand were more humid than today also with decreasing intensity other indicators of past climate were also identified but since they lie beyond the range of radiocarbon dating they cannot as yet be placed in a chronological context of significance is the fact that convincing evidence was obtained for climatic change within the last millennium and especially the rainfall is found to have been increasing since the 16th century

isotopic fingerprints of paleoclimates during the last 30 000 years in deep confined groundwaters of southern india isotopic and geochemical evidence of paleoclimates especially for the last glaciation has been obtained from deep confined groundwaters of southern india the 13c 18o chloride and deuterium analyses of groundwaters show distinct excursions inferred to be related to climatic variations the arid climatic episode associated with the last glaciation 18 000 2000 yr b p is conspicuously identified by signatures of relatively enriched 13c 10 to 12 pdb and 18o 5 3 to 4 8 smow values and high chloride concentration 80 to 160 mg l the transition from an arid to humid period ca 12 000 8000 yr b p is shown by a decreasing trend in the 13c 9 5 to 17 and 18o 4 5 to 6 3 contents of groundwaters the late holocene since 4000 yr b p marked by a more humid but unstable climate is identified by further depletion of 13c 13 to 20 and 18o 5 2 to 6 3 similar variation between 18o and chloride values in confined groundwaters further demonstrates two distinct climatic excursions arid and humid governed by the amount effect this is the first time that isotopic and geochemical signatures related to changing paleoclimates have been identified in the confined groundwaters of the southern indian landmass

high resolution holocene monsoon record from the eastern arabian sea through stable oxygen and carbon analyses of rapidly accumulating sediment cores from the eastern arabian sea we show that the excess of evaporation over precipitation e p steadily appears to have decreased during the last 10 000 to 2000 years most probably due to an increasing trend in the summer monsoon rainfall contrary to the land based paleoclimatic data from this region which indicate onset of aridity around 4000 years ago our results are consistent with the hypothesis that significant spatial variability in the monsoon rainfall observed today was persistent during most of the holocene alternatively the trend can be seen as an adjustment between two phases one between 10 000 and 6000 years ago of increasing precipitation and another between 3500 and 2000 years ago after the arid episode we also report a significant 700 year periodicity similar to that reported recently from the south china sea indicating that the centennial millennial scale response of the indian and chinese monsoons to high latitude forcing may be alike

late quaternary environmental and climatic changes in central brazil paleoenvironmental changes dating back to 30 000 yr b p documented in a pollen record from central brazil lat 19 s permit the reconstruction of climatic changes related to shifts of the antarctic polar fronts the paleoclimatic inferences were obtained by a study of modern vegetation and pollen distribution taking into account present day climatic parameters at 30 000 yr b p the climate must have been warmer and moister than today judging from the high amount of tree pollen taxa characteristic of floodplain forest from 17 000 to 14 000 yr b p the climate was drier although tree pollen percentages were relatively high after 12 000 yr b p araucaria forest elements increased suggesting a moister and cooler climate the araucaria forest disappeared during a short interval between 11 000 and 10 000 yr b p this could be related to the younger dryas event at the beginning of the holocene the climate became cool and moist again as indicated by the reexpansion of the araucaria forest the latter was progressively replaced by a mesophytic semideciduous forest indicating warmer and drier climate after 8500 yr b p at 5000 yr b p an arid interval was followed by the expansion of mesophytic semideciduous forest elements

environmental and injected tracers methodology to estimate direct precipitation recharge to a confined aquifer abstract direct precipitation recharge to a confined aquifer is a vital parameter for groundwater budgeting management and modelling we describe the conjunctive use of environmental isotopes and geochemical tracer technique and injected tracer method to estimate direct precipitation recharge to an important coastal aquifer undergoing large withdrawal stress the methodology evolved delineates the intake area of the confined aquifer based on environmental isotope and geochemical data and then utilises the recharge rates of the intake area only as determined using injected tracer and geochemical data to estimate the amount of recharge by direct precipitation to the confined aquifer

holocene palaeoecology of the eastern sahara selima oasis buried lake sediments and tufa strandlines of holocene age provide evidence of a former lake in the selima oasis depression northwest sudan near the hyperarid core of the eastern sahara desert a 1 6 m section through laminated lake muds and precipitates has been analysed in detail and 30 radiocarbon dates geochemical sedimentological diatom and pollen analyses indicate that the unit dated from approximately 8 4 ka bp to 6 ka bp and was deposited in an environment that passed through one arid to humid to arid cycle the arid episodes from prior to 8 4 ka bp and subsequent to 7 ka bp are characterized by high caco3 deposition abundant diatom indicators of relatively high salinity and well developed periphytic communities and pollen spectra dominated by desert grassland indicators a humid period centered on 8 ka bp is indicated by low caco3 deposition high organic rich mud and al and si oxide deposition diatoms indicating a fresh deep 20 m lake and a pollen assemblage with maximum values for sahelo sudanian tree taxa suggesting a thorn tree and shrub savanna vegetational mosaic this reconstruction concordant with higher groundwater table and precipitation between 9 3 and 7 ka bp agrees with palaeoecological inferences from other sites in north africa

origin and history of nitrate in confined groundwater in the western kalahari abstract data are presented for nitrate dinitrogen and argon concentrations and 15 n 14 n ratios in groundwater with radiocarbon ages up to 40 000 yr for three confined sandstone aquifers in the western kalahari of south west africa namibia the nitrate is probably generated within the soil of the recharge areas and its production rate during the period 3000 40 000 b p has remained between 0 5 and 1 6 meq no 3 l 1 of recharge water with 15 n between 4 and 8 variations in the amount of nitrate and of excess air in groundwater recharge are found and can only reflect changes in the environmental conditions during recharge they must therefore be caused by the climatic changes that have taken place during the past 25 000 yr

understanding groundwater systems and their functioning through the study of stable water isotopes in a hard rock aquifer maheshwaram watershed india summary groundwater degradation through abstraction contamination etc shows a world wide increase and has been of growing concern for the past decades in this light the stable isotopes of the water molecule 18 o and 2 h from a hard rock aquifer in the maheshwaram watershed andhra pradesh india were studied this small watershed 53 km 2 underlain by granite is endorheic and representative of agricultural land use in india with more than 700 bore wells in use in such a watershed the effect of overpumping can be severe and the environmental effects of water abstraction and contamination are of vital importance a detailed and dynamic understanding of groundwater sources and flow paths in this watershed thus is a major issue for both researchers and water managers especially with regards to water quality as well as the delimitation of resources and long term sustainability to this end the input from monsoon precipitation was monitored over two cycles as well as measuring spatial and temporal variations in 18 o and 2 h in the groundwater and in precipitation individual recharge from the two monsoon periods was identified leading to identification of periods during which evaporation affects groundwater quality through a higher concentration of salts and stable isotopes in the return flow such evaporation is further affected by land use rice paddies having the strongest evapotranspiration