Values Of Calcite Research Articles

Modelling the δ 18O value of cave drip water and speleothem calcite

Stable isotope signals recorded in speleothems have provided important insights about past climate variability in recent years. Quantitative reconstruction of mean annual temperature and the amount of precipitation, however, remains difficult because the stable isotope signals are influenced by various processes. Here we present a drip water model, which shows how these climate parameters affect the oxygen isotope signal of cave drip water. In the model the dependence of the δ 18O value of drip water on mean annual temperature is established by correlation to the amount of winter precipitation and winter temperature. Application of the model to two caves in western Germany reveals a strong influence of winter rainfall on the oxygen isotope composition of cave drip water in this region. Assuming equilibrium isotope fractionation between drip water and calcite, we provide a function relating the δ 18O value of speleothem calcite to mean annual surface temperature. This function shows a clear anticorrelation between temperature and the δ 18O value of speleothem calcite, which has been previously reported for several caves in central and northern Europe. By inverse application of this function, we tentatively reconstruct average temperatures for the period between 6 and 1.5 ka from the δ 18O signals of two stalagmites from Atta and Bunker Cave (western Germany). The resulting temperature curves are very sensitive to the value used for the correlation between the amount of winter precipitation and winter temperature. Since this correlation was probably not constant in the past, the reconstructed temperature curves are associated with substantial uncertainty.

CARBON AND OXYGEN ISOTOPIC SYSTEMATICS IN CALCITE AND DOLOMITE FROM THE SANGAN IRON SKARN DEPOSIT, NORTHEASTERN IRAN

A total of 40 samples from carbonates and 8 samples from silicate rocks were analyzed from the Sangan iron skarn deposit to examine behavior of the oxygen and carbon isotopes. The higher values of ?18O are from metamorphic calcite and dolomite and lower ones are from granitic rocks. Although the ?18O and ?13C values of calcite and dolomite in marble yield the highest values in the Sangan deposit, they are still lower than those of normal marine limestone and dolomite, i.e., reflecting coupled decarbonation and infiltrative metasomatic effects. The ?18O and ?13C values of calcite from the Sangan deposit show a progressive, gradual depletion from least-altered marbles into proximal skarn. The gradual decrease of the ?18O and ?13C values from metamorphic calcite to skarn calcite and dolomite can be ascribed to interaction of hydrothermal fluids with carbonate rocks in the area with an increase of water to rock ratio. The isotopic mass-balance calculations suggest the presence of magmatic water in the Sangan deposit.

Indications for the past redox environments in deep groundwaters from the isotopic composition of carbon and oxygen in fracture calcite, Olkiluoto, SW Finland

In paleohydrogeological studies, the geochemical and isotope geochemical composition of fracture calcites can be utilised to gain information about the evolution of the composition of deep groundwaters in crystalline bedrock. The aim of our study was to investigate the latest hydrogeochemical evolution of groundwaters in the crystalline bedrock at Olkiluoto, which is the planned site for deep geological disposal of spent nuclear fuel. Samples were collected from drill cores intercepting water-conducting fractures at the upper ∼ 500 m of the bedrock. The latest fracture calcite generations were identified using optical microscopy and electron microprobe. They occur as thin ∼ 10–200 μ m crusts or small euhedral crystals on open fracture surfaces. These latest calcite fillings were carefully sampled and analysed for the isotopic composition on carbon and oxygen. In addition, fluid inclusion homogenisation temperatures were determined on selected calcite samples. Fluid inclusion data indicated a low temperature of formation for the latest fracture calcite fillings. The δ18O values of calcite in these fracture fillings vary only slightly, from−7.3 to−11.5 ‰ (Vienna Pee Dee Belemnite, VPDB), whereas the δ13C values fluctuate widely, from−30 to+31 ‰ (VPDB). The δ13C values of latest calcite fillings show a systematic pattern with depth, with high and variable δ13C values below 50 m. The high δ13C values indicate active methanogenesis during the formation of the latest calcite fillings. In contrast, the present-day methanic redox environment is restricted to depths below 200–300 m. It is possible that the shift in the redox environment at Olkiluoto has occurred during infiltration of -rich marine waters, the latest of such events being the infiltration of brackish waters of the Littorina Sea stage of the Baltic Sea at ∼ 8000–3000 BP.

Centennial eolian cyclicity in the Great Plains, USA: a dominant climate pattern of wind transport over the past 4000 years?

Proxy evidence at decadal resolution from Late Holocene sediments from Pickerel Lake, northeastern South Dakota, shows distinct centennial cycles (400–700 years) in magnetic susceptibility; contents of carbonate, organic carbon, and major elements; abundance in ostracodes; and δ 18O and δ 13C values in calcite. Proxies indicate cyclic changes in eolian input, productivity, and temperature. Maxima in magnetic susceptibility are accompanied by maxima in aluminum and iron mass accumulation rates (MARs), and in abundances of the ostracode Fabaeformiscandona rawsoni. This indicates variable windy, and dry conditions with westerly wind dominance, including during the Medieval Climate Anomaly. Maxima in carbonates, organic carbon, phosphorous, and high δ 13C values of endogenic calcite indicate moister and less windy periods with increased lake productivity, including during the Little Ice Age, and alternate with maxima of eolian transport. Times of the Maunder, Spörer and Wolf sunspot minima are characterized by maxima in δ 18O values and aluminum MARs, and minima in δ 13C values and organic carbon content. We interpret these lake conditions during sunspot minima to indicate decreases in lake surface water temperatures of up to 4–5 °C associated with decreases in epilimnetic productivity during summer. We propose that the centennial cycles are triggered by solar activity, originate in the tropical Pacific, and their onset during the Late Holocene is associated with insolation conditions driven by precession. The cyclic pattern is transmitted from the tropical Pacific into the atmosphere and transported by westerly winds into the North Atlantic realm where they strengthen the Atlantic Meridional Overturning Circulation during periods of northern Great Plains wind maxima. This consequently leads to moister climates in Central and Northern Europe. Thus, Pickerel Lake provides evidence for mechanisms of teleconnections including an atmospheric link bridging between the different climate regimes from the tropical Pacific to the North Atlantic and onto the European continent.

Structural and host rock controls on the distribution, morphology and mineralogy of speleothems in the Castañar Cave (Spain)

AbstractThe Castañar Cave (central western Spain) formed in mixed carbonate–siliciclastic rocks of Neoproterozoic age. The host rock is finely bedded and shows a complex network of folds and fractures, with a prevalent N150E strike. This structure controlled the development and the maze pattern of the cave, as well as its main water routes. The cave formed more than 350 ka ago as the result of both the dissolution of interbedded carbonates and weathering of siliciclastic beds, which also promoted collapse of the overlying host rock. At present it is a totally vadose hypergenic cave, but its initial development could have been phreatic. The cave's speleothems vary widely in their morphology and mineralogy. In general, massive speleothems (stalactites, stalagmites, flowstones, etc.) are associated with the main fractures of the cave and bedding planes. These discontinuities offer a fairly continuous water supply. Other branching, fibrous, mostly aragonite speleothems, commonly occur in the steeper cave walls and were produced by capillary seepage or drip water. Detailed petrographical and isotope analyses indicate that both aragonite and calcite precipitated as primary minerals in the cave waters. Primary calcite precipitated in waters of low magnesium content, whereas aragonite precipitated from magnesium-rich waters. Differences in isotope values for calcite (−5.2 ‰ for δ18O and −9.6 ‰ for δ13C) and aragonite (δ18O of −4.5 ‰ and δ13C of −3.5 ‰) can be explained by the fact that the more unstable mineral (aragonite) tends to incorporate the heavier C isotope to stabilize its structure or that aragonite precipitates in heavier waters. Changes in the water supply and the chemistry and instability of aragonite caused: (1) inversion of aragonite to calcite, which led to the transformation of aragonite needles into coarse calcite mosaics, (2) micritization, which appears as films or crusts of powdery, opaque calcite, and (3) dissolution. Dolomite, huntite, magnesite and sepiolite were identified within moonmilk deposits and crusts. Moonmilk occurs as a soft, white powder deposit on different types of speleothems, but mostly on aragonite formations. Huntite and magnesite formed as primary minerals, whereas dolomite arose via the replacement of both huntite and aragonite. Owing to its variety of speleothems and location in an area of scarce karstic features, the Castañar Cave was declared a Natural Monument in 1997 and is presently the target of a protection and research programme. Although the main products formed in the cave and their processes are relatively well known, further radiometric data are needed to better constrain the timing of these processes. For example, it is difficult to understand why some aragonite speleothems around 350 ka old have not yet given way to calcite, which indicates that the environmental setting of the cave is still not fully understood.

Multistage dolomitization in the cretaceous carbonates of the east Shahmirzad area, north Semnan, central Alborz, Iran

Different types of Cretaceous limestone and dolostone are the host for lead–zinc deposits and occurrences in the northeast of Shahmirzad, central Alborz. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include (1) very fine-crystalline, non-planar, xenotopic-a (Rd1); (2) fine- to medium-crystalline, planar-e dolomite rhombs, idiotopic-e, scattered dolomite crystals in micrite matrix (Rd2); (3) fine- to medium-crystalline, planar-e (porphyrotopic) (Rd3); (4) medium-crystalline, planar-s, subhedral to euhedral mosaic dolomites, idiotopic-s (Rd4); they post-date early submarine cements. Two types of dolomite cements were identified: medium- to coarse-crystalline, planar-e, euhedral to subhedral dolomite cements, idiotopic-c (Cd1); coarse-crystalline, non-planar, saddle dolomite cements (Cd2). They post-date replacement dolomites and predate late-stage calcite cements that line mouldic vugs and fractures. The association of Rd1 with stromatolites and evaporites, combined with δ18O positive values, postdating early diagenetic features such as micrites and marine cements, could evidence the importance of evaporitic brines for the dolomitization in sabkha (salt flats) environment. The emersion of the carbonate successions at the end of Early Cretaceous caused an ingression of waters of mixed meteoric-marine composition. This caused a wide dissolution of the carbonates and the generation of a first karstic phase. Rd2 and Rd3 dolomites were formed in this environment, as well. A second generation of karstic dissolution happened during the Laramide orogeny in Late Cretaceous, when the carbonate successions were uplifted till emersion. This stage wsas followed by a shallow burial phase and by the hydrothermal formation of Rd4, Cd1 and Cd2 dolomites. These dolomite types are associated epigenetic base-metal mineralization (MVT). The δ18OPDB and δ13CPDB isotopic ratios of the dolomites range from −5.80 to 2.14‰ and from −0.03 to 2.94‰, respectively. The δ18OPDB values are suggesting that all the studied dolomites have been deposited in low-temperature conditions. The measured δ18O values of fine-grained dolomites are heavier than those occurring in the hydrothermal types. This is pointing to lower formation temperatures in respect to other dolomite types in the study area. The remaining porosity of vugs and veins was filled by banded calcites and/or macrocrystals of sparry calcite. The δ18OPDB values of these calcites vary between −12.46 and −7.45‰ and δ13CPDB is in a range between −3.28 and 1.56‰. The O-isotopic values of calcites are lower than those of dolomites and may therefore indicate higher temperatures of precipitation. The different generations of dolomite contain also variable amounts of calcium carbonate, ranging from 49.5 to 55.5%. Fe, Mn and Sr in the dolostones are also variably concentrated: 280–1,500, 75–101 and 71–178 ppm, respectively. Fe, Mn and Sr values can also reflect the complexity of the diagenetic processes and the hydrothermal activity, which was responsible for Pb–Zn mineralization throughout the area as well as for the late dolomite generations. The hydrothermal flow might have been caused by Eocene volcanism, which affected the Cretaceous succession. Late dolomite distribution suggests that the dolomitizing fluids were probably diffused from the tectonic network and were expanded laterally following the permeability conduits.

Arctic ocean shelf–basin interaction: An active continental shelf CO 2 pump and its impact on the degree of calcium carbonate solubility

The Arctic Ocean has wide shelf areas with extensive biological activity including a high primary productivity and an active microbial loop within the surface sediment. This in combination with brine production during sea ice formation result in the decay products exiting from the shelf into the deep basin typically at a depth of about 150 m and over a wide salinity range centered around S ∼33. We present data from the Beringia cruise in 2005 along a section in the Canada Basin from the continental margin north of Alaska towards the north and from the International Siberian Shelf Study in 2008 (ISSS-08) to illustrate the impact of these processes. The water rich in decay products, nutrients and dissolved inorganic carbon (DIC), exits the shelf not only from the Chukchi Sea, as has been shown earlier, but also from the East Siberian Sea. The excess of DIC found in the Canada Basin in a depth range of about 50–250 m amounts to 90±40 g C m −2. If this excess is integrated over the whole Canadian Basin the excess equals 320±140×10 12 g C. The high DIC concentration layer also has low pH and consequently a low degree of calcium carbonate saturation, with minimum aragonite values of 60% saturation and calcite values just below saturation. The mean age of the waters in the top 300 m was calculated using the transit time distribution method. By applying a future exponential increase of atmospheric CO 2 the invasion of anthropogenic carbon into these waters will result in an under-saturated surface water with respect to aragonite by the year 2050, even without any freshening caused by melting sea ice or increased river discharge.

Stable isotope evidence for involvement of deglacial meltwater in Ediacaran carbonates in South China

A carbon and oxygen isotope study was carried out on two units of sedimentary carbonates from the Lantian Formation in southern Anhui, which is equivalent to the Doushantuo Formation elsewhere in South China and Ediacaran sequences elsewhere in the world. The selective acid extraction method was used to liberate CO2 from coexisting calcite and dolomite, respectively, in the Lantian Formation carbonates. The results are used to infer the composition of sedimentary fluids in whose presence the carbonates precipitated and underwent diagenesis. The carbonate in the Lower Unit has δ18O values of − 14.8 to − 3.9‰, and δ13C values of − 8.6 to − 2.1‰; δ18O and δ13C values for calcite are generally lower than those for coexisting dolomite, indicating diagenetic alteration. In contrast, the carbonate in the Upper Unit has δ18O values of − 25.6 to − 18.6‰ and δ13C values of − 11.7 to − 7.8‰; the sedimentary isotopic records in this unit are considered near-primary because of a lack of correlation between the δ18O, δ13C values and the dolomite–calcite fractionation. The unusually low δ18O values of − 25.6 to − 18.6‰ for carbonate in the Upper Unit are comparable to those for modern Arctic carbonates, corresponding to water δ18O values as low as − 27 to − 20‰ during deposition. This carbonate is interpreted as having formed from the continental deglacial meltwater in a continental marginal basin. In contrast, the carbonate with the higher δ18O values of − 14.8 to − 3.9‰ for the carbonate in the Lower Unit is interpreted to have formed from the fluids mainly comprised water from global marine deglaciation. These interpretations are strengthened by previously presented terrigenous REE + Y patterns and high 87Sr/86Sr ratios for the both units of the Lantian Formation carbonates. Therefore, the unusually low δ18O meltwater is explained by regional continental deglaciation. This deglaciation process predates the appearance of mid-Ediacaran biota but postdates the Gaskiers/Moelv ice age. Decomposition of dissolved organic carbon in freshwater is considered as a likely cause for the low δ13C values in the Upper Unit, whereas the high δ13C for carbonate in the Lower Unit would be derived from the oxidation of methane hydrate in the post-Marinoan phase. The temporal correlation between the extreme 18O depletion and the paleobiological evolution suggests that continental deglaciation may have contributed to oxidation of the mid-Ediacaran seawater.

Modelling fractionation of stable isotopes in stalagmites

High resolution δ 13C and δ 18O profiles recorded in precisely dated speleothems are widely used proxies for the climate of the past. Both δ 13C and δ 18O depend on several climate related effects including meteorological processes, processes occurring in the soil zone above the cave and isotope fractionation processes occurring in the solution layer on the stalagmite surface. Here we model the latter using a stalagmite isotope and growth model and determine the relationship between the stable isotope values in speleothem calcite and cave parameters, such as temperature, drip interval, water p CO 2 and a mixing coefficient describing mixing processes between the solution layer and the impinging drop. The evolution of δ 13C values is modelled as a Rayleigh distillation process and shows a pronounced dependence on the residence time of the solution on the stalagmite surface and the drip interval, respectively. The evolution of δ 18O values, in contrast, is also influenced by buffering reactions between the bicarbonate in the solution and the drip water driving the δ 18O value of the bicarbonate towards the value expected for equilibrium isotope fractionation between drip water and calcite. This attenuates the dependence of the δ 18O values on drip interval. The temperature dependence of δ 18O, however, is more pronounced than for δ 13C and in a similar range as expected for fractionation under equilibrium conditions. We also investigate the isotopic enrichment of the δ 13C and δ 18O values along individual growth layers and, thus, the slopes expected for Hendy tests. The results show that a positive Hendy test is only possible if isotope fractionation occurred under disequilibrium conditions. However, a negative Hendy test does not exclude that isotope fractionation occurred under disequilibrium conditions. A more reliable indicator for disequilibrium fractionation is the enrichment of the δ 13C values along an individual growth layer.

Analysis of calcium carbonate for differentiating between pigments using terahertz spectroscopy

Calcium carbonate that is used as an art pigment exhibits strong absorption at approximately 3 THz. In this study, the authors investigated the relationship between the absorption and the condition of calcium carbonate crystals. By employing terahertz time-domain spectroscopy (0.5–4 THz), they verified that terahertz absorption energy depended on the crystal direction and crystal shape of the powder sample due to large birefringence. Further, the authors observed the difference in the crystal structure (calcite or aragonite) and the presence of impurities in natural calcium carbonate such as shells through terahertz absorbance spectra. The absorbance peak value of calcite at around 3 THz was four times as large as the peak value of aragonite. The absorbance spectral width increased because of the presence of these impurities. From the above observation, this study demonstrated that a certain kind of calcium carbonate crystal could be distinguished by terahertz spectroscopy.

Speleothem preservation and diagenesis in South African hominin sites implications for paleoenvironments and geochronology

AbstractPlio‐Pleistocene speleothems from australopithecine‐bearing caves of South Africa have the potential to yield paleoenvironmental and geochronological information using isotope geochemistry. Prior to such studies it is important to assess the preservation of geochemical signals within the calcitic and aragonitic speleothems, given the tendency of aragonitic speleothems to recrystallize to calcite. This study documents the geochemical suitability of speleothems from the principal hominin‐bearing deposits of South Africa. We use petrography, together with stable isotope and trace element analysis, to identify the occurrence of primary aragonite, primary calcite, and secondary calcite. This study highlights the presence of diagenetic alteration at many of the sites, often observed as interbedded primary and secondary fabrics. Trace element and stable isotopic values distinguish primary calcite from secondary calcite and offer insights into geochemical aspects of the past cave environment. δ13C values of the primary and secondary calcites range from +6 to −9‰ and δ18O values range from −4 to −6‰. The data are thus typical of meteoric calcites with highly variable δ13C and relatively invariant δ18O. High carbon isotope values in these deposits are associated with the effects of recrystallization and rapid outgassing of CO2 during precipitation. Mg/Ca and Sr/Ca ratios differ between primary and secondary calcite speleothems, aiding their identification. Carbon and oxygen isotope values in primary calcite reflect the proportion of C3 and C4 vegetation in the local environment and the oxygen isotope composition of rainfall. Primary calcite speleothems preserve the pristine geochemical signals vital for ongoing paleoenvironmental and geochronological research. © 2009 Wiley Periodicals, Inc.

Towards a better understanding of magnesium-isotope ratios from marine skeletal carbonates

This study presents magnesium stable-isotope compositions of various biogenic carbonates of several marine calcifying organisms and an algae species, seawater samples collected from the western Dutch Wadden Sea, and reference materials. The aim of this study is to explore the influence of mineralogy, taxonomy and environmental factors (e.g., seawater isotopic composition, temperature, salinity) on magnesium-isotopic (δ 26Mg) ratios of skeletal carbonates. Using high-precision multi-collector inductively coupled plasma mass spectrometry, we observed that the magnesium-isotopic composition of seawater from the semi-enclosed Dutch Wadden Sea is identical to that of open marine seawater. We further found that a considerable component of the observed variability in δ 26Mg values of marine skeletal carbonates can be attributed to differences in mineralogy. Furthermore, magnesium-isotope fractionation is species-dependent, with all skeletal carbonates being isotopically lighter than seawater. While δ 26Mg values of skeletal aragonite and high-magnesium calcite of coralline red algae indicate the absence or negligibility of metabolic influences, the δ 26Mg values of echinoids, brachiopods and bivalves likely result from a taxon-specific level of control on Mg-isotope incorporation during biocalcification. Moreover, no resolvable salinity and temperature effect were observed for coralline red algae and echinoids. In contrast, Mg-isotope data of bivalves yield ambiguous results, which require further validation. The data presented here, point to a limited use of Mg isotopes as temperature proxy, but highlight the method’s potential as tracer of seawater chemistry through Earth’s history.

A biogeochemical study of sediments from the eutrophic Lake Lugano and the oligotrophic Lake Brienz, Switzerland

The biomarker composition and stable isotope (C, O) ratio values of organic matter (OM) and carbonate from sediment cores from the oligotrophic Lake Brienz and the eutrophic Lake Lugano (both in Switzerland) are compared, in order to obtain information about OM sources and transformation processes. Eutrophic conditions at Lake Lugano are reflected in elevated total organic carbon (TOC) content and hydrogen index (HI) values, as well as higher lipid concentrations. Parallel down core trends in δ13C values of TOC and calcite in the Lake Lugano sediments reflect bioproductivity cycles. Variations in δ18O values of calcite are consistent with changes in mean summer temperature over the time interval covered by the core. In contrast, such a correlation does not exist for Lake Brienz and there the stable isotope composition of calcite reflects its allochthonous origin. In the sediments of both lakes, fatty acid (FA) distributions and the composition of n-alkanols and n-alkanes indicate highly variable proportions of autochthonous OM sources (algae, zooplankton, bacteria) and OM from land plants. Enhanced in situ microbial synthesis during sediment deposition in Lake Lugano is suggested by the higher TOC-normalised concentrations of branched chain FAs (C15–C17), hopanoic acids and triterpenoid alcohols (i.e. tetrahymanol, diplopterol). Variations in the concentrations of cholesterol are related to contributions from zooplankton and/or green algae, while sitosterol concentrations reflect the input of vascular plants. Periods of increased input of OM from diatoms are evidenced by high 24-methylcholesta-5,22-dien-3β-ol (either epibrassicasterol or brassicasterol) and/or highly branched isoprenoid (HBI) alkenes concentrations. High relative concentrations of diplopterol in Lake Lugano sediments are consistent with the predominance of cyanobacteria commonly observed in eutrophic lakes. The presence of archeol and hydroxyarcheol in very low concentrations in the Lugano sediments argues for the activity of methanogens and/or anaerobic methanotrophs.Differences in OM degradation processes are reflected in higher chlorin index values in the Brienz sediments but higher saturated vs. unsaturated n-FAs in the core from Lugano. Higher concentrations of branched chain FAs and 16:1ω7 n-FA, as well as enhanced 18:1ω7/18:1ω9 n-FA, are consistent with enhanced bacterial biomass in the Lugano water column or sediments. The preservation of phytol seems to be enhanced in sediments with a high relative contribution of land plant OM. Major factors affecting OM accumulation in the lakes are differences in OM sources (i.e. terrestrial OM vs. autochthonous production), extent of bacterial activity and most likely oxygen availability in the water column.

The influence of CO2 injection on the carbonate chemistry and scaling in multiple-effect distillers

The injection of carbon dioxide (CO2) into natural waters and aqueous solutions is used in several industrial applications. If pure water is saturated with 1600 ppm CO2, its pH would reach a value of 4.0. In this work, the influence of CO2 injection on the carbonate chemistry and scaling in multipleeffect distillers is studied. The approach is applied to a reference 5-stage multiple-effect distiller as a case with a distillate production of 380 t/h at a top brine temperature of 64°C. The pH of the feed seawater can be reduced of a factor of 2.15 when 150 ppm CO2 is injected. CO2- 3 is considered limiting to scale formation because Ca2+in seawater is much higher and is not limiting to CaCO3 precipitation except under at unusual circumstances. Increasing the concentration of CO2- in the feed water would lead to a decrease in the saturation ratios. The Langelier Saturation Index (LSI) values of calcite changed from 1.3 when zero amount is injected to reach a value of -0.81 when 150-ppm are injected. The calc...

Late Pleistocene paleohydrology near the boundary of the Sonoran and Chihuahuan Deserts, southeastern Arizona, USA

Ground-water discharge (GWD) deposits form in arid environments as water tables rise and approach or breach the ground surface during periods of enhanced effective precipitation. Where preserved, these deposits contain information on the timing and elevation of past ground-water fluctuations. Here we report on the investigation of a series of GWD deposits that are exposed in discontinuous outcrops along a ∼150-km stretch of the San Pedro Valley in southeastern Arizona, near the boundary of the Sonoran and Chihuahuan Deserts. Chronologic, isotopic, geochemical, faunal assemblage (ostracodes and gastropods), and sedimentological evidence collectively suggest that the elevation of the regional water table in the valley rose in response to a change in climate ∼50 ka ago and remained relatively high for the next ∼35 ka before falling during the Bølling–Allerød warm period, rebounding briefly during the Younger Dryas cold event, and falling again at the onset of the Holocene. The timing of these hydrologic changes coincides closely with variations in δ 18O values of calcite from a nearby speleothem to the west and changes in lake levels at pluvial Lake Cochise to the east. Thus, in southeastern Arizona, the assumption that changes in climate are reflected in all aspects of the hydrologic cycle of a region simultaneously is validated. The timing of these changes also broadly coincides with variations in the GISP2 δ 18O record, which supports the hypothesis that atmospheric teleconnections existed between the North Atlantic and the deserts of the American Southwest during the late Pleistocene.

Geochemical evidence and hydrodynamic modeling of two fluid systems involved in sandstone-hosted uranium mineralization in the northeast of the Ordos basin, China

The Ordos basin is a Meso-Cenozoic intracontinental basin developed upon Paleozoic sedimentary rocks, which in turn are underlain by Precambrian basement belonging to the North China Craton.With an area of about 2.5×10 km, the Ordos basin represents one of the top oil, gas and coal producers in China and is increasingly recognized as an important uranium mineralization province. Major uranium deposits have been found in the Dongsheng area in the north-eastern margin of the Ordos basin, where uranium mineralization occurs as dissemination in sandstones of the Jurassic Zhiluo Formation. Geologic, petrographic and fluid inclusion studies suggest that the uranium mineralization took place at temperatures from 100 to 200 °C, which indicates either deep-burial or hydrothermal rather than early diagenetic environments. δCVPDB and δOVSMOW values of calcite associated with uranium mineralization range from −2.7‰ to −14.0‰ and 18.4‰ to 20.0‰, respectively, and δOVSMOW and δDVSMOW values of hydrothermal kaolinite are 12.6‰ to13.8‰ and −116‰ to −133‰, respectively. These data suggest that oxidation of organic matter was involved in the formation of the uranium deposits. Based on zoning ofmineral assemblages indicating upward increase in oxidation, it is inferred that organic matter was derived from an

Combined textural and stable isotopic data as proxies for the mid-Cretaceous paleoclimate: A case study of lacustrine stromatolites in the Gyeongsang Basin, SE Korea

Microbial interference of the prevailing sedimentary dynamics during stromatolite growth, records vital biogeochemical information, which reflects the prevailing paleoclimatic conditions. This study documents the simultaneous geochemical and textural changes in mid-Cretaceous lacustrine stromatolites (Gyeongsang Basin, SE Korea), deposited in a semi arid setting. Fibrous calcite/micrite couplets in the stromatolites occurring in the Sinyangdong, Hwasan and Banyawol formations constitute the basic growth cycles and can be divided into first-, second and third-order cycles. Second-order cycles show couplets with gradual textural changes from fibrous calcite to micrite laminae, and three different types of micrite are present. Sometimes, one type of micrite evolves into another, within the second-order cycle in the Sinyangdong stromatolites. Consistent enrichments in δ 18O and depletion in δ 13C towards micrites within the Sinyangdong and Hwasan stromatolite couplets and a concomitant depletion of both isotopes from fibrous calcite to micrite in the Banyawol sample can be observed. These shifts reflect paleohydrological changes in the lake water, related to paleoproductivity and paleosalinity fluctuations. Relatively higher paleosalinity values within the Sinyangdong lake water system is registered by higher δ 18O values of fibrous calcite and micrite in the SY-3, than in the SY-1 and SY-2 stromatolites. The consistent shifts in δ 18O and δ 13C isotopes in different textural growth fabrics indicate that paleohydrological factors could play a significant role in the formation of these primary lacustrine carbonates. Textural changes shown by the frequency of the growth cycles also corresponds with oxygen and carbon isotopic values. These results appear to have been influenced by climate-related paleohydrological changes.

Climate deterioration in the Eastern Mediterranean as revealed by ion microprobe analysis of a speleothem that grew from 2.2 to 0.9 ka in Soreq Cave, Israel

Analysis of oxygen isotope ratios (δ 18O) by ion microprobe resolves a sub-annual climate record for the Eastern Mediterranean from a Soreq Cave stalagmite that grew between 2.2 and 0.9 ka. In contrast to conventional drill-sampling methods that yield a total variation of 1.0‰ in δ 18O calcite values across our sample, the methods described here reveal up to 2.15‰ variation within single annual growth bands. Values of δ 18O measured by ion microprobe vary in a regular saw-tooth pattern that correlates with annual, fluorescent growth banding where calcite grades from light to dark fluorescence. Modern records of precipitation and of cave dripwater indicate that variable δ 18O calcite values record regular seasonal differences in δ 18O rainfall modified by mixing in the vadose zone. Large differences in δ 18O values measured across a single band (i.e., between the dark and light fluorescent calcite, or Δ 18O dark-light) are interpreted to indicate wetter years, while smaller differences represent drier years. Oxygen isotopes record: 1) month-scale growth increments, 2) changes in Δ 18O dark-light that represent seasonality, 3) a systematic, long-term decrease in maximum Δ 18O dark-light values, and 4) an overall increase in average δ 18O calcite values through time. These results suggest a drying of regional climate that coincides with the decline of the Roman and Byzantine Empires in the Levant region.

Mechanisms of oxygen isotopic exchange and isotopic evolution of 18O/16O-depleted periclase zone marbles in the Alta aureole, Utah: insights from ion microprobe analysis of calcite

Oxygen isotope ratios have been measured by ion microprobe and millimeter-scale dental drill along detailed sampling traverses across the boundary between periclase-bearing (δ18O = 11.8‰) and periclase-free (δ18O = 17.2‰) marble layers in the periclase (Per) zone of the Alta Stock aureole, Utah. These data define a steep, coherent gradient in δ18O that is displaced a short distance (~4 cm) into the periclase-free (Cal + Fo) layer. SEM and ion microprobe analyses show two isotopically and texturally distinct types of calcite at the grain scale. Clear (well polished) calcite grains are isotopically homogeneous (within analytical uncertainty; ±0.27‰, 2SD). More poorly polished (pitted), texturally retrograde ‘turbid’-looking calcite has lower and more variable δ18O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries. Despite significant lowering of the δ18O values in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that individual clear calcite grains are now isotopically homogeneous across the entire gradient in δ18O. Diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by volume diffusion, 30,000–62,000 years at 575–600°C, exceed significantly the timescale (~1,250 years) estimated for the prograde development of the δ18O gradient at the boundary between these two marble layers. The ion microprobe data and these diffusion calculations suggest instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity of these calcite grains. Thus, the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange equilibrium with infiltrating fluid during prograde reaction and recrystallization.

Assessing pedogenic calcite stable-isotope values: Can positive linear covariant trends be used to quantify palaeo-evaporation rates?

Palaeoclimate models suggest enhanced evaporation rates in subtropical regions (15–30° latitude) during greenhouse-world conditions, however, there are no empirical data to support these estimates. Laboratory evaporation experiments have shown that calcites precipitated from variably saturated solutions yield a positive linear covariant trend (PLCT) in δ 18O calcite versus δ 13C calcite values. The goal of the present controlled laboratory experiments is to develop a method to quantify regional palaeo-evaporation rates from palaeosol calcrete PLCTs. Samples of powdered CaCO 3 were dissolved in de-ionized water in pressure sealed containers with a range of elevated atmospheric pCO 2 concentrations for 24 h. The solution was then allowed to evaporate completely from an open container within an incubator with the time of calcite crystallization noted, and aliquots removed for analysis every 24 h. The precipitated calcite produced an array of δ 18O calcite versus δ 13C calcite values that fall upon a PLCT projected from a theoretical meteoric calcite line (MCL). Water analyses yielded δ 18O water enrichments from an initial value of − 4.75‰ VSMOW ranging up to between + 10.0‰ and + 14.8‰. The experimental results show that solutions formed under higher pCO 2 conditions precipitated calcite very early on during evaporation, and thus have δ 18O calcite and δ 13C calcite values that are slightly enriched relative to the MCL. The solutions that formed under low pCO 2 conditions precipitated calcite after much of the fluid had evaporated, and thus yield more enriched δ 18O calcite and δ 13C calcite values. Repeat trials with varying temperature and relative humidity show that the PLCT is steeper under both higher temperature and low relative humidity. The wide range of pCO 2, temperature and relative humidity conditions used simulate meteoric phreatic and meteoric vadose conditions that may affect a calcrete horizon over time during numerous dissolution/precipitation reactions. The results of these experiments show that a dominant factor in the precipitation of vadose calcite is the saturation state of the fluid with respect to CaCO 3, while the primary factors affecting the magnitude and steepness of the PLCT are vadose zone temperature, relative humidity, the starting δ 18O water value and saturation state of the fluid with respect to CaCO 3. Since the pCO 2 of the rooting zone is cross-controlled by the local soil and surface ecology, the magnitude of the PLCT enrichment may not be a direct proxy for palaeo-evaporation rates.