δ18O Values Of Samples Research Articles

Elemental and C, O and Mg isotope geochemistry of middle-late Miocene carbonates from the Tuz Gölü Basin (Central Anatolia, Turkey): Evidence for Mediterranean incursions

The Tuz Gölü basin, the largest of the Central Anatolian Neogene basins, is a NW-SE trending fault-controlled depression and includes many subbasins, such as the Ereğli, Ulukışla and Bor subbasins. In the basin, economically important evaporite deposits consisting mostly of halite, gypsum/anhydrite, glauberite and carbonates formed in the middle-late Miocene. The mineralogical and chemical properties and C, O and Mg isotopic compositions of the carbonate minerals that precipitated simultaneously with the other evaporite minerals were investigated using samples from boreholes to determine the precipitation conditions and paleoenvironment of the deposits. The lack of covariance between the δ13C and δ18O values of the samples and the wide range of isotopic compositional variations indicate a basin that was hydrologically open to some degree. In addition, the lack of correlations between δ18O values and MgO/(MgO + CaO), Mg/Ca and Sr/Ca ratios and the wide range of variations in both δ13C and δ18O values indicate that the carbonate minerals precipitated in a lagoonal environment where seawater incursions occurred through transgressions. The δ18O values of the dolomite samples are higher than those of Messinian normal-salinity seawater dolomites and therefore require contributions from evaporative fluids. Additionally, the isotopic data indicate extreme fluctuations in temperature and the inflow-evaporation balance during carbonate precipitation and indicate that the evaporite minerals may have precipitated from the evaporation of seawater trapped within the basin during and after uplift of the region. The Mg isotopic compositions of magnesium-rich carbonates are partially heterogeneous, and the δ26Mg values are higher than the δ25Mg values. Some of the δ26Mg values are close to the δ26Mg values of global seawater (–0.82‰) but higher than the global mean δ26Mg value of river water (–1.09‰). The δ26Mg values are partially within the range of marine sediments (–3.65 to +0.52‰), suggesting seawater contributions to carbonate precipitation.

Manganese nodules NOD-A-1 and NOD-P-1: Implications of pre-treatment on oxygen isotopes and mineralogy

Manganese nodules are a type of ferromanganese chemical precipitate containing a plethora of economically important metals increasingly being used in the high- and green-technology sectors. By measuring oxygen isotope ratios in these materials, we can increase our understanding of how they form, and constrain which metals are likely to be present in them. There is a lack of suitable reference materials for use in oxygen isotopic studies of samples dominated by Mn oxide minerals (i.e., Mn nodules and crusts) and, therefore, a need for their development. The samples NOD-A-1 and NOD-P-1 (United States Geological Survey), Mn nodules from the Atlantic and Pacific oceans, respectively, provide the best opportunity for such a reference material but their application has been hampered by their incomplete mineralogical characterization and lack of consensus on how to pre-treat (if at all) these type of samples for δ18O analysis. We have performed a detailed mineralogical study using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), powder X-ray diffraction (PXRD), and Raman spectroscopy on these reference materials. In sample NOD-A-1 todorokite, goethite and/or hematite, Mg-calcite, phillipsite or chabazite, quartz and barite were identified. The minerals todorokite, birnessite, asbolane, vernadite, goethite, pyrochroite, quartz, phillipsite or chabazite, anorthite, kaolinite and barite were identified in NOD-P-1. We conducted experiments to assess the effect on δ18O and mineralogy resulting from acidification with hydrochloric acid (HCl), pre-fluorination with bromine pentafluoride (BrF5) at ambient temperature, or both acidification and pre-fluorination treatments. Results from samples given both treatments were deemed most meaningful to the study of Mn crusts and nodules, and therefore nodule research in general, by providing δ18O values representative of the relatively pure oxide-silicate fraction only. We report a δ18O value of 11.0 ± 1.0‰ for the oxide-silicate fraction of NOD-A-1, and a value of 11.1 ± 0.7‰ for the same fraction in NOD-P-1, with results showing a δ18O shift of up to 10.3‰, as a function of nodule and the pre-treatment used. The δ18O values of samples given both treatments converge and are substantially 18O-depleted relative to that of corresponding untreated material, which was found to contain various 18O-enriched phases such as carbonate minerals and non-stoichiometric water. Based on our results, we recommend that for all future analyses of Mn nodules and crusts, the samples receive both treatments to ensure accurate and consistent results.

Testing the early Late Ordovician cool-water hypothesis with oxygen isotopes from conodont apatite

AbstractLatest Sandbian to early Katian sequences across Laurentia's epicontinental sea exhibit a transition from lithologies characterized as ‘warm-water’ carbonates to those characterized as ‘cool-water'carbonates. This shift occurs across the regionally recognized M4/M5 sequence stratigraphic boundary and has been attributed to climatic cooling and glaciation, basin reorganization and upwelling of open ocean water, and/or increased water turbidity and terrigenous input associated with the Taconic tectophase. Documentation of oxygen isotopic trends across the M4/M5 and through bracketing strata provides a potential means of distinguishing among these alternative scenarios; however, oxygen isotopic records generated to date have failed to settle the debate. This lack of resolution is because δ18O records are open to multiple interpretations and potentially confounding factors related to local environmental conditions have not been tested by examining the critical interval in multiple areas and different depositional settings. To begin to address this shortcoming, we present new species-specific and mixed assemblage conodont δ18O values in samples spanning the M4/M5 boundary from the Upper Mississippi Valley, Alabama, and Virginia. The new results are combined with previous studies, providing a record of δ18O variability across SE Laurentia. The combined dataset allows us to test for regional trends at a resolution not previously available. Our results document a ~1.5‰ decrease in values across Laurentia instead of increasing δ18O values across the M4/M5 as predicted in various ‘cool-water’ scenarios. In short, these results do not support a shift to ‘cool-water’ conditions as an explanation for changes in early Katian carbonates across the M4/M5.

地球化学的手法と数値解析を用いた下川鉱山坑内水の起源推定

The understanding of source and flow path of the groundwater provides important strategy for the environmental management of mines. Thus, groundwater samples from the shaft and level in the Shimokawa mine and the surrounding river water samples were taken and the stable isotopes of hydrogen and oxygen and water quality of the samples were analyzed. The results indicate that shallow groundwater starts mainly from mountain-sides and passes through rocks above ore bodies. The simulation of groundwater flow was also conducted. The distribution of velocity vector of the simulated result showed that down streamlines which flows more than 2×10-3 m per day from mountain-sides to the ore bodies were observed. By considering the altitudes of mountain-sides range from 300 to 550 m, these results correspond well with the altitudes estimated from δD and δ18O values of samples.

Correlative Relationships Between Carbon- and Oxygen-Isotope Records in Two Cool-Temperate Brachiopod Species off Otsuchi Bay, Northeastern Japan

We present high-resolution, three-dimensional carbon (δ13C)- and oxygen (δO)-isotope compositions from calcite shells of two modern brachiopod species (Terebratulina crossei and Terebratalia coreanica) and their correlative relationships. δ13C and δ18O values from the secondary shell layer, which constitutes the main body of a brachiopod shell, are in and/or out of the range of δ13C and δ18O values of calcite precipitated in isotopic equilibrium with ambient seawater (equilibrium calcite). The δC and δ18O values of samples from the outermost part of the secondary shell layer show positive correlations. The values of high-growth-rate portions are less than those of low-growth-rate portions; these results are ascribed to a kinetic isotope fractionation effect. Metabolic influences are identified in the isotopic compositions of the low-growth-rate portions for T. coreanica, resulting in decreases in δ13C values compared with those of equilibrium calcite. We illustrate the effects of kinetic isotope fractionation and metabolism on the isotopic compositions of brachiopod shell calcite, which vary among shell portions within a single shell, as well as between the two species. However, appropriate selection of brachiopod taxa and shell portions that reflect the isotopic composition of ambient seawater enables their use as a reliable paleoenvironmental proxy.

An online method combining a Gasbench II with continuous flow isotope ratio mass spectrometry to determine the content and isotopic compositions of minor amounts of carbonate in silicate rocks

An online method using continuous flow isotope ratio mass spectrometry (CF-IRMS) interfaced with a Gasbench II device was established to analyze carbon and oxygen isotopic compositions and to estimate the content of minor amounts of carbonate in silicate rocks. The mixtures of standard materials and high-purity quartz are firstly used to calibrate different quantities of carbonate in silicates. The results suggest that the accuracy and precision of the online analysis are both better than those obtained using an offline method. There is a positive correlation between the carbonate weight and the Mass44 ion beam intensity (or peak area). When the weight of carbonate in the mixtures is greater than 70 microg (equal to approximately 1800 mV Mass44 ion beam intensity), the delta(13)C and delta(18)O values of samples usually have accuracy and precision of +/-0.1 per thousand and +/-0.2 per thousand (1sigma), respectively. If the weight is less than 70 microg, some limitations (e.g., not perfectly linear) are encountered that significantly reduce the accuracy and precision. The measured delta(18)O values are systematically lower than the true values by -0.3 to -0.7 per thousand; the lower the carbonate content, the lower the measured delta(18)O value. For samples with lower carbonate content, the required phosphoric acid doses are higher and more oxygen isotope exchanges with the water in the phosphoric acid. To guarantee accurate results with high precision, multiple analyses of in-house standards and an artificial MERCK sample with delta(13)C values from -35.58 to 1.61 per thousand and delta(18)O from 6.04 to 18.96 per thousand were analyzed simultaneously with the unknown sample. This enables correction of the measured raw data for the natural sample based on multiple-point normalization. The results indicate that the method can be successfully applied to a range of natural rocks.

Ulster White Limestone Formation (Upper Cretaceous) of Northern Ireland: effects of basalt loading on chalk diagenesis

ABSTRACTThe Ulster White Limestone Formation is an unusual chalk because it underwent a period of post‐depositional emergence and erosion, followed by burial under 1·5–2·0 km of Tertiary basalts. A high degree of pressure solution and cementation produced a well‐lithified limestone with low porosities (2·3–10·4%). The Ulster White Limestone shows no evidence of thermally induced textural alteration, except for thin (<0·5 m) pseudospar contact recrystallization zones adjoining basalt dykes. Whole‐rock δ18O values of samples not associated with basalt dykes range from ‐ 3·26%o to ‐ 6·50%o (PDB). The δ18O values of macropore cements range from ‐ 4·96%o to ‐ 11·52%o (mean=‐8·27%o). Modelling of the diagenesis of the Ulster White Limestone using trace element concentrations and carbon, oxygen and strontium isotopic ratios of whole rock and cement samples suggests a low water‐rock ratio and either marine or mixed marine‐meteoric pore water environment during the main episode of recrystallization. The maximum possible burial temperature was modelled to be ≃ 105°C.The diagenetic history of the Ulster White Limestone is similar to that of North Sea chalks that are at comparable burial depths as the Ulster White Limestone after basalt deposition. The geochemical data show no indication of hydrothermal alteration associated with the overlying basalts. The degree of alteration of fine‐grained limestones composed predominantly of low‐magnesium calcite, such as the Cretaceous/Tertiary chalks, appears to be controlled largely by the burial (effective stress) history of the limestone.

Late Permian and Early Triassic evolution of the Northern Indian margin: carbon isotope and sequence stratigraphy

The Northern part of Great-India underwent an early rifting phase in the late Paleozoic, just at the end of the large scale Gondwanian glaciation. The beginning of the rifting processes is marked by large hiatus and discontinuities (para- conformities) between the early or middle Paleozoic sedimentary succession and the discontinuous middle-late Permian Traps and transgressive sediments. The Northern Indian passive margin consists of the present High and Lower Himalaya and a small part of the Indian craton and their sedimentary cover. The Permian rift shoulder is located in the Higher Himalaya, with part being in the underthrusted Lower Himalaya. The rim basin (landward of the shoulder) is well developed in the Pottawar- Salt Range area. From the rifting to the beginning of the drifting stages (early late Permian to late early Triassic time), the sedimentary evolution is characterised by three transgressive- regressive (T-R) second order cycles, two in the late Permian and one in the early Triassic. The break-up of the rift occurred during the second cycle (late Dzhulfian).In the Salt Range area, these three T-R cycles have been subdivided in eight third order sequences, five sequences for the upper Permian and three for the lower Triassic.At the end of Permian, hiatuses, gaps and local erosion of part of the margin are direct consequences of a first order relative sea-level fall; this is also the time of the largest extinction event of the Phanerozoic that deeply affected the carbonate productivity and the stratal patterns. With the following worldwide sea-level rise, a rapid and large scale transgression occurred in the early Triassic, well dated and recorded on the whole margin. High rate thermal subsidence gave way to generalized pelagic deposits about 2 My after the transgression.Profiles of whole rock inorganic carbon and oxygen isotopes from Guryul Ravine and Palgham sections in Kashmir, Nammal Gorge and Landu sections in Trans Indus Ranges (Pakistan), Thini Chu section in Kali Gandaki Valley, Central Nepal are presented in connection with the sequence stratigraphic analysis. The upper Permian record of high positive δ13C values are closely correlated with the second order T-R cycles and the third order sequences. The results presented in this study confirm the drastic drop of δ13C from the high positive values that characterised the upper Permian to lower values in the lower Triassic time. Stratigraphic correlation problems in the lower Triassic using carbon isotope geochemistry are briefly discussed. A positive δ13C excursion of 4–5% near the Smithian - Spathian substages boundary is observed for the first time. The δ18O values of samples from all the sections display major variations suggesting that the oxygen isotope record has been significantly affected by meteoric diagenesis, deep burial diagenesis or/and monsoon signature.

Relation of Na+ concentration and δ18O in winter precipitation with weather conditions

A chemical study of the winter precipitation was conducted in Tohkamachi, central Japan. The ratios of Cl−/Na+ and Mg2+/Na+ found in the winter precipitation samples are nearly equal to those of sea water. This result suggests that Na+, Mg2+, and Cl− found in the winter precipitation are originated from sea water. A relationship was determined between the Na+ concentration in the winter precipitation sample and the height of the convective mixing layer. Cloud droplets containing sea salt particles are believed to be captured by snow crystals through the rain‐out process. The results indicate that the higher the convective mixing layer heights, the larger the amount of sea salt particles captured by the snow crystals. A relationship between the δ18O values in the winter precipitation and the air temperature at the top of convective mixing layer was determined for several synoptic weather conditions that produced winter precipitation. The δ18O values in samples collected under conditions of winter monsoon are larger than those under other weather conditions at the same temperature. A good relationship is observed between the temperatures at the top of convective mixing layer and the δ18O of samples of winter precipitation under conditions of the winter monsoon.

Temporal Variations of Isotopic Composition of Glacier-River Water During Summer: Observations at Austre Okstindbreen, Okstindan, Norway

AbstractThe isotopic composition of river water discharging from the Norwegian glacier, Austre Okstindbreen, in summer varies on both daily and longer-term scales. Most δ18o values of samples from the principal river are within the range −12.5 to −14.0‰). Because new snow tends to be relatively depleted of 8180, water leaving the glacier early in the summer has low δ18O values. Subsequently, values rise as contributions of old snow, glacier ice, and their melt waters, which are isotopically heavier (median δ18O values generally above −12.0‰) dilute the δ18O depleted base-flow component of discharge, a mixture of waters with different histories of formation, storage, and transit. Accumulation-area melting contributes significantly to river discharge. Towards the end of the summer, as surface melting declines, δ18O values tend to fall. Between-year differences of within-summer trends reflect differences of development of the glacier’s drainage systems. The drainage systems are affected by outbursts from a glacier-dammed lake. During fine weather, δ18o variations follow the diurnal cycle of surface melting: they are strongly correlated with, but lag behind, air temperatures. Perturbations during rainfall cannot be explained simply in terms of the isotopic composition of the precipitation, since low values may be associated with isotopically heavy rainfall. Displacement of water previously stored within or below the glacier may account for the anomaly. Contrasts of composition characterize different rivers leaving the glacier, because the relative contributions of various water sources differ.

Temporal Variations of Isotopic Composition of Glacier-River Water During Summer: Observations at Austre Okstindbreen, Okstindan, Norway

AbstractThe isotopic composition of river water discharging from the Norwegian glacier, Austre Okstindbreen, in summer varies on both daily and longer-term scales. Most δ18o values of samples from the principal river are within the range −12.5 to −14.0‰). Because new snow tends to be relatively depleted of 8180, water leaving the glacier early in the summer has low δ18O values. Subsequently, values rise as contributions of old snow, glacier ice, and their melt waters, which are isotopically heavier (median δ18O values generally above −12.0‰) dilute the δ18O depleted base-flow component of discharge, a mixture of waters with different histories of formation, storage, and transit. Accumulation-area melting contributes significantly to river discharge. Towards the end of the summer, as surface melting declines, δ18O values tend to fall. Between-year differences of within-summer trends reflect differences of development of the glacier’s drainage systems. The drainage systems are affected by outbursts from a glacier-dammed lake. During fine weather, δ18ovariations follow the diurnal cycle of surface melting: they are strongly correlated with, but lag behind, air temperatures. Perturbations during rainfall cannot be explained simply in terms of the isotopic composition of the precipitation, since low values may be associated with isotopically heavy rainfall. Displacement of water previously stored within or below the glacier may account for the anomaly. Contrasts of composition characterize different rivers leaving the glacier, because the relative contributions of various water sources differ.