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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]