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Triple oxygen isotope (Δ′17O) analysis of CO 2 or carbonate is increasingly applied in diverse fields in earth and environmental sciences. One of the widely-used analytical methods for Δ′17O of CO 2 is through Pt-catalyzed oxygen isotope exchange between CO 2 and O 2. However, large inter-laboratory inconsistencies in oxygen isotope fractionation between the two gases have been reported, potentially hindering data comparison across different laboratories. By examining the oxygen isotope fractionations in thermal diffusion experiments for pure O 2 and CO 2 gases as well as CO 2 -O 2 mixtures, we demonstrate that thermal-gradient-induced (TG-induced) isotope fractionation is the main reason for the inconsistencies. The degree of TG-induced oxygen isotope fractionation is affected by temperature contrasts and volume ratios between the hot and cold parts of the extraction line as well as gas composition. Larger thermal contrasts generated larger oxygen isotope fractionations. CO 2 fractionated less than O 2 in the same thermal gradient conditions. For CO 2 -O 2 mixtures, TG-induced oxygen isotope fractionation of O 2 was reduced while that of CO 2 was enhanced, compared with the pure gases. Based on the results of thermal diffusion experiments, we found that the anomalous mass fractionation exponents (0.2 ∼ 0.9) observed in the Pt-catalyzed CO 2 -O 2 isotope exchange experiments can be rationalized quantitatively by a combination of isotopic exchange equilibrium and TG-induced oxygen isotope fractionation. Our model suggests that by weakening TG-induced oxygen isotope fractionation and by increasing the volume fraction of hot zone of a reactor, we can achieve a high precision (<10 per meg) for Δ′17O CO2 through CO 2 -O 2 exchange. Considering the widely existing thermal-gradient conditions in nature, the findings in this study may shed light on the origin of non-mass-dependent isotope fractionation. [ABSTRACT FROM AUTHOR]

Unlike modern Mn(IV)-dominated manganese nodules, ancient manganese deposits primarily contain Mn(II) (manganous) mineral phases. The general assumption is that these manganous minerals formed post-depositionally from a Mn(IV) precursor phase during diagenesis and metamorphism. However, recent studies have shown that aqueous Mn(II) may combine with either CO 3 2– (aq) or H 4 SiO 4(aq) to nucleate and precipitate as primary manganous phases within anoxic waters. These findings thus make it difficult to assess what comprised the primary mineral assemblage of ancient Mn deposits, which in turn, has implications for understanding the redox fabric of the past marine water column. Here, we investigate the paragenesis and oxidation state of various Mn minerals (oxides, carbonates, and silicates) from the late Devonian Xialei and middle to late Permian Zunyi manganese deposits in South China. Through characterization of the micro-scale distribution, composition, and speciation of different Mn-bearing mineral phases, we provide evidence to support an oxidative mechanism during Mn cycling at the time of deposition. We document the presence of a residual oxide phase (hausmannite; Mn 3 O 4) that represents a primary to early formed Mn(III)-bearing oxide preserved in Mn(II)-carbonate nodules. Additionally, we observe spherulitic textures comprised of mixed-valence Mn(II,III,IV)-silicates enclosed in diagenetic Mn(II)-carbonate laminations. Our results allow for the construction of a paragenetic model for these deposits beginning with the reduction of a primary Mn(IV) phase and the subsequent precipitation of intermediate Mn(III)-oxide or -silicate mineral phases. Following continued exposure to reducing conditions during early diagenesis, Mn(III)-oxides were then replaced by Mn(II)-carbonates. Similarly, Mn(III)-silicates were transformed into various Mn(II)-silicates during late-stage diagenesis and metamorphism. This model suggests that the accumulation of various divalent Mn mineral phases in ancient sediments may, therefore, be indicative of deposition under a water column with a sufficient oxidizing potential to induce Mn(II) oxidation. [ABSTRACT FROM AUTHOR]

Several Late Devonian sedimentary successions host pyrite with highly positive sulfur isotope values (δ34S pyrite). These anomalous values have been linked to marine anoxia, low sulfate concentrations in seawater, or aerobic re-oxidation of dissolved sulfide within well-oxygenated bottom waters in a local depositional environment. Implicit to these previous models is the assumption of a biogenic genesis from microbial sulfate reduction (MSR) of these pyrites, which, in turn, can be used to understand the biogeochemical sulfur cycle and reconstruct paleoenvironmental conditions. In South China, the Late-Devonian bulk-rock sulfur isotope excursion occurs in the lower Fammenian Wuzhishan Formation of the Youjiang Basin, in both the sedimentary limestone that corresponds to the Late triangularis conodont zone at the Fuhe section and the sediment-hosted Xialei Mn deposit that lies between the Late triangularis and crepida conodont zones. To further constrain the significance of these 34S-enriched pyrites, we carried out detailed textural and in-situ sulfur isotope examinations of pyrites in the Xialei Mn deposit. We found one type of pyrite in siliceous limestone referred to as the host rocks and three distinct pyrite types in Mn ores: Type I – pyrites with Mn-carbonate inclusion-rich cores; Type II – pyrite overgrowths surrounding an "inclusion-rich" core; and Type III – relatively inclusion-poor and mainly subhedral disseminated pyrite grains. Type I pyrites in Mn ores have negative δ34S pyrite values from −7.8‰ to −2.0‰. Types II and III pyrites, and pyrites in the host rock are characterized by highly positive δ34S pyrite values (+4.2‰ to +37.6‰), with some values being higher than that of contemporaneous seawater sulfate. Type I pyrites with negative δ34S values likely formed via MSR in a closed porewater system, driven by increased sedimentation rates during a sea level lowstand. The other pyrites with highly positive δ34S values appear to have formed during secondary hydrothermal alteration, with their sulfur sourced through thermochemical sulfate reduction (TSR) of sulfate from dissolved barites in the underlying Liujiang Formation. This implies that the positive δ34S values of the Late Devonian pyrites from the Wuzhishan Formation represent a later alteration event caused by post-depositional hydrothermal fluids, and may lack a direct biogeochemical connection to the surface sulfur cycle. These results provide an important perspective on the origin of variability in sulfur isotope records and suggest the need for careful petrographic screening and micrometer-scale analysis of sedimentary units used to reconstruct paleoenvironmental conditions. [ABSTRACT FROM AUTHOR]

Understanding the process of CO 2 degassing during precipitation of calcite from a Ca2+–HCO 3 − solution is crucial for interpreting isotope compositions in the calcite precipitates. Unlike diffusion-controlled outgassing, i.e., dissolved CO 2 escaping from the solution via diffusion, degassing caused by precipitation of calcite is accompanied by a large carbon isotope fractionation between CO 2 (g) and HCO 3 − due to breaking of C O bond, with an equilibrium fractionation of ca. −9‰ at ambient temperature. Such a magnitude of fractionation has a great influence on carbon isotope compositions (δ13C) of DIC (dissolved inorganic carbon) reservoir in the solution and thus on δ13C of calcite precipitated from it. However, knowledge on isotope fractionation is limited during precipitation-associated degassing of CO 2 in a supersaturated solution where rapid calcite precipitation drives CO 2 degassing out of isotopic equilibrium. Here we show the data of water chemistry and carbon isotope compositions of DIC and carbonate precipitates in a mountain stream at Baishuitai, China. Results from numerical models show there exist large degrees of carbon isotope disequilibrium during precipitation-associated degassing of CO 2 via HCO 3 − dehydration and dehydroxylation. Average carbon isotope fractionation between CO 2 (g) and HCO 3 − ( ε C O 2 (g) - HC O 3 - ) calculated from our dataset is about −20‰ which is much lower than the equilibrium value. Moreover, an inverse correlation between ε C O 2 (g) - HC O 3 - and precipitation rate was observed, indicating higher precipitation rates cause ε C O 2 (g) - HC O 3 - farther from equilibrium value. By compiling the data from this study and literatures, we infer that disequilibrium isotope fractionation of carbon between CO 2 (g) and HCO 3 − may be common during the growth of speleothem and travertine from a solution supersaturated with respect to calcite. The rate dependence of ε C O 2 (g) - HC O 3 - has special implications for speleothem archives from ventilated caves. As partial pressure of CO 2 in cave atmosphere evolves with ventilation, variable precipitation rates of calcite will cause inconstant degrees of disequilibrium isotope fractionation between CO 2 and DIC and thus perturb the time-series of speleothem's δ13C records for paleo-environmental reconstruction. [ABSTRACT FROM AUTHOR]

Travertines are potential archives of continental paleoclimate. Records of stable carbon and oxygen isotopic composition (δ 13 C and δ 18 O) in laminated travertine deposits from endogene spring waters show regular cyclic patterns which may be due to seasonal change in climate determinants such as temperature and rainfall. In this study, δ 13 C and δ 18 O measurements of three travertine specimens that grew naturally over the eight years, 2004–2011, at upstream, middle and downstream sites in a canal at Baishuitai, SW China, are presented. They exhibit clear seasonal variations that generally correlate with biannual laminations. Specifically, δ 13 C and δ 18 O values show significant positive correlation with each other for the three travertine specimens, with the correlation coefficients increasing downstream along the canal. To reveal the factors governing the seasonal and spatial variations in δ 13 C and δ 18 O values, newly formed travertines precipitated on Plexiglas substrates are also examined. Both δ 13 C and δ 18 O of the substrate travertines are low in the summer/rainy season and high in the winter/dry season, showing a great consistency with the patterns in the natural travertines. Spatially, isotope values increase downstream in both seasons, with higher increase rates in winter that are related to removal of larger fractions of dissolved inorganic carbon (DIC) from the solution and stronger kinetic isotopic fractionation in winter. Due to in-stream physicochemical processes, including CaCO 3 precipitation and the associated degassing of CO 2 , seasonal changes in δ 13 C and δ 18 O in the travertines are amplified by two times between the upstream and downstream sites: this is opposite to trends for epigene (meteogene) tufas whose seasonal changes in stable isotope compositions are reduced downstream. We suggest in-stream physicochemical processes are a potential reason for underestimation of annual temperature ranges that are inferred from epigene tufa δ 18 O data. [ABSTRACT FROM AUTHOR]

Abstract: Water samples and modern endogenic (thermogene) travertine calcite deposited on plexiglass substrates in travertine pools and a ramp stream were collected along the Huanglong Ravine, Sichuan, SW China at regular ∼10day intervals from early May to early November in 2010, including both wet and dry conditions. Temporal and spatial variations in the δ13C and δ18O values of the modern travertine were examined to understand their potential for paleoclimatic and paleoenvironmental interpretations. It was found that δ13C and δ18O of travertine formed in the ramp stream were low in the warm rainy season and high in the cold dry season. Their positive correlation was mainly due to dilution and rainfall seasonal effects on δ13C and δ18O values, respectively, i.e., low δ13C values were caused by dilution by overland flow with depleted δ13C values and reduced CO2-degassing in the warm rainy season while low δ18O values of travertine were because of low δ18O values of water induced by seasonal variation in oxygen isotopic ratios of rainwater. Meanwhile, kinetic effect on oxygen isotopic fractionation during ramp travertine deposition existed and reduced this positive correlation. In contrast, the δ13C and δ18O values of the pool travertines displayed a converse behavior which was caused mainly by the temperature effect. Low δ18O values and high δ13C values in the warm rainy season were correlated chiefly with the higher water temperatures. Therefore, the δ13C and δ18O values of the travertine may be used for paleo-rainfall or paleotemperature reconstruction respectively. This study demonstrates that endogenic travertine, like epigenic (meteogene) tufa, may be a suitable candidate for high-resolution paleoclimatic and paleoenvironmental reconstructions. However, since travertines deposited under differing hydrodynamic conditions (e.g., pools with still water contrasted to fast flow streams) have different climatic responses, it is necessary to check the depositional facies of fossil travertine samples before they can be used for palaeoclimate (temperature and/or rainfall) reconstruction. [Copyright &y& Elsevier]

Abstract: In order to understand the behavior of oxygen isotopic fractionation and obtain more reliable paleoclimatic data from surficial travertine deposits, the hydrochemistry, carbonate precipitation rates, and the temporal and spatial variations in δ18O values of the modern travertine in two low-temperature travertine-depositing systems (canal and pool) with differing hydrodynamic conditions at Baishuitai, Yunnan, SW China, were investigated during the warm rainy season (May 24 to November 2) in 2010. It was found that the oxygen isotopic composition of travertine in the two systems showed distinct controlling mechanisms mainly due to different carbonate precipitation rates and water residence times. For the canal system with fast flow, large deviations between the calculated and measured temperatures indicated that travertine precipitated out of isotopic equilibrium. This was due to rapid carbonate precipitation with intense CO2 degassing. δ18Otravertine values and water temperature increased downstream (∼1‰ and 4°C, respectively) while δ18Owater values remained relatively stable. It was shown that the δ18O values of travertine in the canal system recorded the δ18O values of dissolved carbonates (HCO3 −) which also increased along the canal, controlled by Rayleigh-distillation effects. In contrast, in the pools system with slow flow, oxygen isotopic equilibrium between dissolved carbonates (HCO3 −) and H2O was achieved and the δ18Otravertine values did not display spatial variations. Calculated water temperatures using the equilibrium fractionation factors of Coplen (2007) were identical to the measured ones, which indicates that travertine in such pools system is suitable for the reconstruction of paleo-precipitating temperatures. Finally, by comparing these two systems we have obtained a simple criterion to determine the suitability of travertine to estimate temperatures – the δ18O value of BaCO3 precipitated from water samples is compared with the theoretical δ18O value of dissolved carbonate calculated using water temperature and δ18Owater, to check if the oxygen isotopes of the dissolved carbonate are in equilibrium with the water. [Copyright &y& Elsevier]

Abstract: Travertine δ18O values can be used to reconstruct paleo-temperatures if the oxygen isotope fractionation factors between travertine and water are accurately understood. For this purpose, the δ18O values of pool travertine and its parent water, and the deposition rates of the calcite were investigated at Baishuitai (Yunnan, SW China) over the course of the full hydrological year, April 23 2006–April 25 2007. The results show that the travertine–water isotope fractionation factors are close to the commonly accepted equilibrium line of Kim and O’Neil (1997). This differs from the results obtained by Yan et al. (2012) who found that the oxygen isotope fractionation factors in the travertine-depositing pools were close to the line suggested as equilibrium relationship by Coplen (2007). The average calcite deposition rate (2.30mgcm−2 d−1) in the present study is six times larger than that (0.38mgcm−2 d−1) in Yan et al. (2012). If slower calcite precipitation leads to equilibrium oxygen isotopic fractionation, then the results of this study support the results of Coplen (2007) that indicate that the equilibrium fractionation factor may be greater than the commonly accepted one derived by Kim and O’Neil (1997). The relationship between oxygen isotope fractionation factor and calcite deposition rate in our study also agrees with the results of Dietzel et al. (2009) who found that the kinetic-isotope effect favors preferential incorporation of 16O in solid calcite as the calcite deposition rate increases. There was a threshold for calcite precipitation rate control on oxygen isotopic equilibrium. In our case of travertine-depositing pools, when the calcite deposition rate was lower than 0.38mgcm−2 d−1, oxygen isotopic equilibrium between calcite and water was attained. Therefore, calcite deposition rate is a potentially important consideration when using δ18O in natural carbonates as a proxy for terrestrial and ocean temperature. [Copyright &y& Elsevier]

Conventional carbonate–water oxygen isotope thermometry and the more recently developed clumped isotope thermometer have been widely used for the reconstruction of paleotemperatures from a variety of carbonate materials. In spite of a large number of studies, however, there are still large uncertainties in both δ 18 O- and Δ 47 -based temperature calibrations. For this reason there is a need to better understand the controls on isotope fractionation especially on natural carbonates. In this study we analyzed oxygen, carbon and clumped isotopes of a unique set of modern calcitic and aragonitic travertines, tufa and cave deposits from natural springs and wells. Together these samples cover a temperature range from 6 to 95 °C. Travertine samples were collected close to the vents of the springs and from pools, and tufa samples were collected from karstic creeks and a cave. The majority of our vent and pool travertines and tufa samples show a carbonate–water oxygen isotope fractionation comparable to the one of Tremaine et al. (2011) with some samples showing higher fractionations. No significant difference between the calcite–water and aragonite–water oxygen isotope fractionation could be observed. The Δ 47 data from the travertines show a strong relationship with temperature and define the regression Δ 47 = (0.044 ± 0.005 × 10 6 )/ T 2 + (0.205 ± 0.047). The pH of the parent solution, mineralogy and precipitation rate do not appear to significantly affect the Δ 47 -signature of carbonates, compared to the temperature effect and the analytical error. The tufa samples and three biogenic calcites show an excellent fit with the travertine calibration, indicating that this regression can be used for other carbonates as well. This work extends the calibration range of the clumped isotope thermometer to travertine and tufa deposits in the temperature range from 6 °C to 95 °C. [ABSTRACT FROM AUTHOR]

Monthly hydrochemical data and δ 13 C of dissolved inorganic carbon (DIC) in karst water samples from September 2007 to October 2012 were obtained to reveal the controlling mechanisms on DIC geochemistry and δ 13 C DIC under different conditions of climate and land cover in three karst catchments: Banzhai, Dengzhanhe and Chenqi, in Guizhou Province, SW China. DIC of karst water at the Banzhai site comes mainly from carbonate dissolution under open system conditions with soil CO 2 produced by root respiration and organic carbon decomposition with lowest δ 13 C values under its dense virgin forest coverage. Weaker carbonate bedrock dissolution due to sparse and thin soil cover results in lower δ 13 C DIC , pCO 2 , DIC and EC, and lower cation and anion concentrations. At the Chenqi site, larger soil CO 2 input from a thick layer of soil results in high pCO 2 and DIC, and low pH, SI c and δ 13 C DIC in the karst water. At the Dengzhanhe site, a lesser soil CO 2 input due to stronger karst rock desertification and strong gypsum dissolution contribute to higher δ 13 C DIC , high EC and high cation and anion concentrations. Soil CO 2 inputs, controlled by biological activity and available soil moisture, carbonate bedrock dissolution, dilution and degassing effects, vary seasonally following rainfall and temperature changes. Consequently, there are seasonal cycles in hydrochemistry and δ 13 C DIC of the karst water, with high pCO 2 and low pH, EC, SI c , and δ 13 C DIC values in the warm and rainy seasons, and vice versa during the cold and dry seasons. A strongly positive shift (>3‰) in δ 13 C DIC occurred in the drought year, 2011, indicating that δ 13 C DIC in groundwater systems can be an effective indicator of environmental and/or climate changes. [ABSTRACT FROM AUTHOR]