Tufa Samples Research Articles

Early-Middle Holocene high lake levels of Rinqen Shubtso on the southern Tibetan Plateau and the formation mechanisms

The lake-level highstands on the southern Tibetan Plateau (TP) during the Early-Middle Holocene have traditionally been attributed to increased monsoonal precipitation. However, there has been limited discussion and evaluation regarding how the elevated shoreline indicates the formation of mega-paleolakes and the effects of glacial meltwater on rising lake levels. In this study, we conducted an investigation into the well-preserved paleoshorelines of Rinqen Shubtso, a closed-basin lake system located on the southern TP. By utilizing 14C dating and analyzing shoreline elevations, the Holocene lake-level fluctuation history of Rinqen Shubtso was reconstructed. Through examining strontium (87Sr/86Sr) and oxygen isotopes (δ18O), as well as Rb/Sr ratios in tufa samples from the shoreline, we evaluated the relative contribution of glacial meltwater and East Asian Monsoon precipitation to the lake-level expansion throughout this period. Our findings indicate that prior to 8.5 cal ka BP, the lake level reached its highest elevation before experiencing a rapid drop by approximately 44 m within a short timeframe. Subsequently, maintaining a stable highstand between 8.5 and 5.8 cal ka BP before gradually declining to its present elevation thereafter. We argue that the glacial meltwater induced by rising temperature due to solar insolation likely played a significant role in contributing to these large amplitude high lake levels prior to 8.5 cal ka BP, whereas the maximum East Asian Monsoon precipitation was responsible for sustaining high water levels during 8.5–5.8 cal ka BP when the mean latitudinal position of the summer Intertropical Convergence Zone shifted northward until reached its northernmost point at 8.5 cal ka BP. Following 5.8 cal ka BP, with the weakening of summer monsoon precipitation observed, gradually decreased lake level occurred accordingly. Our results provide valuable insights into understanding past changes in lake level, which are of great importance to predicting future lake variations on the TP.

Humid phases on the southwestern Arabian Peninsula are consistent with the last two interglacials

Past environmental and climatic conditions within the Arabian Peninsula are key to understanding the setting for hominin dispersal across the Saharo-Arabian dryland belt. The tufa deposits within the volcanic harrats on the southwest coast of Saudi Arabia fill a significant spatial gap in the distribution of palaeoenvironmental records on the west coast of the Arabian Peninsula adjacent to the Red Sea. In the catchment of Wadi Dabsa in the Harrat Al Birk, there are widespread fossil palustrine to shallow-lacustrine tufa deposits with fluvial elements. Several phases of tufa accumulation, separated by fluvial downcutting, are observable within these powerful palaeoenvironmental proxies. U–Th dating of targeted dense, banded tufa facies, yield ages that are stratigraphically consistent at the landscape scale, and indicate that tufa accumulation occurred during distinct humid phases broadly coeval with the last two warm interglacial Marine Isotope Stages (MIS 7 and MIS 5). For the first time this shows humid intervals in southwest Arabia coincident with the southern coast. There is a simlar pattern emerging further north in the Arabian Peninsula, The Sinai and Levant and further on into continental Europe. Furthermore, tufa δ18O ranges from −14.6 to −1.9‰, covering a range similar to those reported for tufa from north African oasis sites and speleothems elsewhere on the Arabian Peninsula and The Levant. The lowest δ18O values are derived from MIS 5e samples, a pattern in agreement with speleothems in Yemen and Oman, and consistent with an isotopic-enabled climate model simulation for this time slice. The δ13C and Sr isotopic compositions of dated tufa samples indicate deposition from shallow-circulating meteoric water, with no geothermal influence. This, along with the δ18O values, suggest a freshwater supply that was a potable water source in this landscape. The δ13C signatures at Wadi Dabsa are more negative than for parts of north Africa, suggesting Wadi Dabsa may have experienced comparatively higher biomass, thicker soils and wetter conditions with lower evaporative losses. This new record of tufa deposition during the middle and late Pleistocene, suggests for the first time that the west coast of Arabia experienced a similar history of humid phases over the past 250 ka as southern Arabia and the Nefud in the northern interior. These regional changes in hydroclimatic regime occur at timescales coincident with hominin dispersals.

Facies and Origin of Tufa Deposits from the Gostilje River Basin and the Sopotnica River Basin (SW Serbia)

Tufa accumulations from the Gostilje River Basin and the Sopotnica River Basin in SW Serbia are represented by both active and fossil tufa precipitates. The aim of this study is to distinguish and describe different tufa facies and to determine the environmental conditions, based on stable isotope data. We also compare our analysis with other tufa deposits in Europe. Four facies are distinguished: moss tufa, algal tufa, stromatolitic laminated tufa, and phytoclastic tufa. The dominant constituent of all tufa samples is low Mg-calcite, whereas the presence of sylvite is noted in two samples from the Gostilje River Basin. The δ18O values range from −9.07‰ to −10.79‰ (mean value: −9.81‰), while the δ13C values range from −6.50‰ to −10.34‰ (mean values −9.01‰). The stable isotope values (δ13C and δ18O) indicate that these tufa deposits were precipitated from cold, ambient water supported by CO2 of an atmospheric origin. We emphasize that this is the first data about stable isotope analyses of tufa deposits from Serbia.

Microbial influences on tufa deposition in a tropical climate

Tufas are freshwater carbonate deposits that attracted attention as paleoclimatic archives and analogs of microbial carbonates. Recent geomicrobiological studies revealed that cyanobacterial photosynthesis significantly contributed to the precipitation of CaCO3 (photosynthesis-induced CaCO3 precipitation; PICP), and that the chemical properties of extracellular polymeric substances (EPS) secreted by cyanobacteria significantly influenced on the depositional fabrics. These depositional processes were revealed from tufas formed in cool and temperate climates; however, their applicability to tufa deposits in other climate conditions remains unclear. This study, therefore, investigated tufa deposits that formed in a tropical climate in Brazil and applied geomicrobiological techniques, including microelectrode measurements and lectin-binding analysis, to clarify their depositional processes. The surfaces of eight investigated tufa samples were dominated by cyanobacteria. Microelectrode measurements indicated the occurrence of PICP in all the samples. However, five samples additionally exhibited a Ca2+ concentration decrease at the tufa surface, even in dark conditions, which was difficult to explain by abiotic CaCO3 precipitation and light-independent microbial metabolisms. Based on the fluxes calculated from the results of microelectrode measurements, the contribution of PICP to the formation of CaCO3 precipitates was estimated as 100% for three samples, similar to tufa deposits in cool and temperate climates. For the remaining five deposits, the average contributions of PICP, Ca2+ adsorption to organic matter, and other processes were estimated as 36%, 3%, and 61%, respectively. Among the phototrophs living the tufa surface, Phormidium and Leptolyngbya secreted EPS abundantly containing carboxyl groups, and were heavily calcified. In contrast, Phormidesmis, Calothrix, Pleurocapsa, and diatoms secreted EPS without detectable amount of carboxyl groups, and they were largely not calcified and formed pore spaces. Such influence of microbial EPS on the depositional fabrics were common to cool and temperate tufas. Nonetheless, some features would be specific to tropical tufas, including the contribution of unidentified light-independent processes, conical and columnar surface morphologies, and unclear annual lamination.

The Potential of Tufa as a Tool for Paleoenvironmental Research—A Study of Tufa from the Zrmanja River Canyon, Croatia

Tufa is a fresh-water surface calcium carbonate deposit precipitated at or near ambient temperature, and commonly contains the remains of macro- and microphytes. Many Holocene tufas are found along the Zrmanja River, Dalmatian karst, Croatia. In this work we present radiocarbon dating results of older tufa that was found for the first time at the Zrmanja River near the Village of Sanaderi. Tufa outcrops were observed at different levels, between the river bed and up to 26 m above its present level. Radiocarbon dating of the carbonate fraction revealed ages from modern, at the river bed, up to 40 kBP ~20 m above its present level. These ages fit well with the hypothesis that the Zrmanja River had a previous surface connection with the Krka River, and changed its flow direction toward the Novigrad Sea approximately 40 kBP (Marine Isotope Stage 3). Radiocarbon AMS dating of tufa organic residue yielded a maximum conventional age of 17 kBP for the highest outcrop position indicating probable penetration of younger organic material to hollow tufa structures, as confirmed by radiocarbon analyses of humin extracted from the samples. Stable carbon isotope composition (δ13C) of the carbonate fraction of (−10.4 ± 0.6)‰ and (−9.7 ± 0.8)‰ for the Holocene and the older samples, respectively, indicate the autochthonous origin of the carbonate. The δ13C values of (−30.5 ± 0.3)‰ and (−29.6 ± 0.6)‰ for organic residue, having ages <500 BP and >5000 BP, respectively, suggest a unique carbon source for photosynthesis, mainly atmospheric CO2, with an indication of the Suess effect in δ13C during last centuries. The oxygen isotopic composition (δ18O) agrees well with deposition of tufa samples in two stages, the Holocene (−8.02 ± 0.72‰) and “old” (mainly MIS 3 and the beginning of MIS 2) (−6.89 ± 0.34‰), suggesting a ~4 °C lower temperature in MIS 3 compared to the current one.

Tools for uranium characterization in carbonate samples: case studies of natural U–Pb geochronology reference materials

Abstract. Laser ablation U–Pb analyses of carbonate (LAcarb) samples has greatly expanded the potential for U–Pb dating to a variety of carbonate-producing settings. Carbonates that were previously considered impossible to date using isotope dilution methods may preserve radiogenic domains that can be dated using spatially resolved laser ablation geochronology techniques. Work is ongoing to identify reference materials and to consider best practices for LAcarb. In this study we apply standard and emerging characterization tool sets on three natural samples with the dual goal of enhancing the study of carbonates and establishing a new set of well-characterized natural reference materials for LAcarb studies. We start with the existing carbonate reference material WC-1 from the Permian Reef Complex of Texas, building on the published description to offer a deeper look at U and associated trace elements. We consider a tufa sample from the Miocene Barstow Formation of the Mojave Block, California, as a possible secondary calcite reference material due to its well-behaved U–Pb systematics. There are currently no natural dolomite standards. We present an unusual dolomite sample with very well-behaved U–Pb systematics from the Miocene of the Turkana Basin of Kenya as a possible dolomite reference material for LAcarb dating. In addition to using X-ray fluorescence (XRF) mapping and spectroscopy to better understand U in these natural samples, we have analyzed multiple aliquots of each of them for 87Sr/86Sr by thermal ionization mass spectrometry (TIMS). The Sr isotope compositions are analytically homogeneous within petrographically homogeneous regions of all three samples, and thus these materials could be used as Sr isotope standards as well. While not part of the current contribution, this combination could streamline simultaneous LA analyses of 87Sr/86Sr and U–Pb geochronology.

Depositional and environmental controlling factors on the genesis of Quaternary tufa deposits from Bonito region, Central-West Brazil

The Pleistocene and Holocene tufas of the Serra da Bodoquena (Bonito region, Brazilian Midwest) consist of several deposits related to a karstic system developed in the carbonatic rocks of Corumbá Group. Tufas currently forming in the Serra da Bodoquena area (Mimoso and Formoso rivers) were investigated for a better understanding of which environmental changes control the deposition of different types of tufa facies. The Mimoso River displays waterfalls, cascades, barriers and large pools as morphological elements. The topographic breaks on the bedrock of this river control the distribution of these main morphological elements. Five facies were identified: (i) stromatolitic boundstones, characterized by an alternation of dense and porous laminae; (ii) phytohermal boundstones, corresponding to bryophyte cushions; (iii) phytoclastic rudstones, formed by the accumulation of leaves and plant fragments; (iv) oncoidal rudstones, composed by coated grains with concentric laminae (oncoids); and (v) unconsolidated detrital sediment, formed by bioclasts and tufa fragments. SEM images indicated the presence of microorganisms such as Oocardium stratum Nägeli 1849 and Phormidium incrustatum Gomont ex Gomont 1892, as well as diatoms and fungi filaments colonizing tufa samples. On the other hand, the Formoso River is more sinuous, lacking significative topographic breaks and presents flooded sub-environments, such as inactive barriers and other shallow fluvial areas, containing unconsolidated detrital sediment, lime mud and oncoidal rudstones. No microbial remains have been observed. The Bonito region has a humid warm climate with a rainy summer season and a dry winter season, and temperatures barely varying through year. The seasonality of rainy and dry seasons influences both the physicochemical changes of water and, indirectly, the microbial communities of tufa. However, the very different facies displayed by each river, Mimoso and Formoso, are not directly linked to climate. The stepped profile of the Mimoso River allows fast flowing waters producing fast CO2 outgassing that leads to high calcite precipitation rates. The gentle slope of the Formoso River and its slow flowing waters leads to slow passive CO2 outgassing and photosynthesis controlling calcite precipitation at lower rates than in the Mimoso River.

Tortuosity of the predominant pore directional partitions and of their effective pore network: case study for a calcareous tufa sample

Tortuosity of the predominant pore directional partitions and of their effective pore network: case study for a calcareous tufa sample

Slope retreat rates estimated from chronology of tufa deposits sheltered by inland notches on Mt. Carmel, Israel

Inland notches are elongated half tube indentations that develop on the carbonate rocky cliffs of mountainous zones. Occasionally tufa accumulates within notches as stalactites, drapes and crusts, covering the cavity backwall or floors. In attempt to determine the age of the latest stage of notch formation and calculate the relative rate of slope retreat, UTh dating was used on 16 tufa samples from 2 notches, providing 28 ages. Tufa grow during the time interval between the backwall erosion and visor collapse, and therefore tufa age is an approximation of the timing of the formation of the surface it developed on, i.e. the age of the notch.The oldest known tufa sample is 39.0 ± 10.4 ky (2σ), and the ages of other tufa samples range from 2.1 to 23.3 ky. At the Nahal Oren notch, the present-day backwall formed by the end of the last glacial period (14 to18 ky), whereas the visor in its present form is from the early Holocene (7 to10 ky). The oldest sample was found ~1 m from the backwall, suggesting that backwall retreat rate ranges from 20 to 35 mm/ky. Other samples were deposited 50 to 200 cm from the visor edge, suggesting that slope retreat order of magnitude ranges from 101 to 102 mm/ky. The backwall of the Nesher notch formed between 10 and 14 ky. Our results correspond with the rates of tens of meters per million years, similar to the magnitude of denudation found by previous studies in the Mediterranean zone of Israel.Dating tufa deposits is a good proxy to estimate the rate of erosion in carbonate slope retreat; however, a precautionary approach should be taken, as rock particles from the eroded bedrock might embed within the tufa, affecting the age results. Questionable samples can be identified using Cathodoluminescence (CL) microscopy.

Measurement of beryllium-10 in terrestrial carbonate deposits from South China: A pilot study

Cosmogenic nuclides contained in the annual layers of natural materials record temporal variations in the galactic cosmic ray flux. 14C in tree rings and 10Be in ice cores from polar regions are often used to retrieve accurate histories of galactic cosmic rays. However, the variation in 14C produced in the atmosphere is strongly attenuated in the carbon cycle and the amplitude of decadal scale variations in tree rings is approximately 0.2‰. In the case of 10Be in ice cores, the variation remains at a relatively large amplitude; however, the records are accompanied by dating uncertainties. Another material that contains 10Be is sediment in continental areas. Terrestrial carbonate deposits sometimes form clear annual layers; however, detailed analyses on such deposits have not yet been widely conducted. Therefore, we evaluate the possibility of retrieving information concerning the galactic cosmic ray flux from the terrestrial carbonate deposits. In this paper, we report our preliminary results obtained from the measurements of 10Be in travertine and tufa samples from South China. We find that the 10Be flux obtained from the travertines is comparable to the level produced by galactic cosmic rays and that travertines might have potential as a tool for investigating past cosmic-ray variations with annual resolution.

Interannual changes in radiocesium concentrations in annually laminated tufa following the Fukushima Daiichi Nuclear Power Plant accident

Chemical and radiocesium concentrations in water and riverbed tufa of the Nigori River, flowing out of the southern foot of Mt. Asama volcano in central Japan, were investigated following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. This tufa is characterized by porous and dense laminae. Its annual lamination (varve) is formed by seasonal changes in dissolved CO2 concentration in water with temperature, and the tufa calcite is mostly precipitated during summer. We analyzed radiocesium concentrations in ∼4-mm-thick tufa samples that included four or five varves collected after the fall in 2011–2014. The activity ratio of 134Cs/137Cs in tufa collected in 2011 and 2014 was 0.96 ± 0.01 and 0.99 ± 0.01, respectively, which implies radiocesium generated as a result of the FDNPP. Sequential extractions indicate that 137Cs in tufa collected in 2011 is occupied in exchangeable and carbonate phases (55.2% and 44.8%, respectively). The 137Cs concentrations equivalent to tufa calcite that were precipitated after 2011 showed variations similar to the amount of summer rainfall during 2011–2014, which may be attributed to increases in dissolved 137Cs concentrations in river water associated with intensified rainfall and microbial action over the summer. In contrast, the 137Cs sorbed to tufa calcite was not retained in the riverbed, and its migration may be attributed to the porous texture of tufa and the high flow rate of the river. Therefore, tufa calcite is easier to dissolve even though it adsorbs compared with other minerals (mica, andosol etc.).

Seasonal temperature changes obtained from carbonate clumped isotopes of annually laminated tufas from Japan: Discrepancy between natural and synthetic calcites

Carbonate clumped isotopes provide a novel paleo-thermometer that does not require knowledge of the isotopic compositions of environmental water. Despite considerable effort, there are substantial inter-laboratory disagreements in empirical calibrations between the abundance anomaly of the clumped isotopes (Δ47) and absolute temperature, and discrepancies in paleo-temperature estimates from natural samples. In this study, clumped isotopes using two types of 17O corrections were analyzed for two sample sets of synthetic calcites and tufas collected monthly from two sites in southwest Japan with known δ18O values and temperatures of parent water. These measurements form the basis for two new temperature calibrations of carbonate clumped isotopes, the first from our Kyushu University laboratory in Japan. The calcites synthesized at four different temperatures (2.9–61.0 °C) yield a temperature-Δ47 relationship of Δ47=0.0354±0.0013×106/T2+0.290±0.015 (R2 = 0.986), which is indistinguishable from the calibrations of other recent studies based on acid digestion experiments at high temperature (70–100 °C).The Δ47 thermometry for the natural tufa samples display the seasonal pattern of water temperature better than the δ18O thermometry. However, the Δ47 values of the tufa samples deposited at temperatures ranging from 5.6 to 16.0 °C were lower than the values expected from the calibration using synthetic calcites, thus providing another temperature calibration of Δ47=0.0336±0.0036×106/T2+0.301±0.048 (R2 = 0.735). Therefore, our results clearly identify a detectable, but fairly uniform, discrepancy in Δ47-temperature calibration between synthetic calcite and natural samples (tufas). The amplitude of Δ47 offset between the tufa and synthetic samples appears to be independent of the seasonal change in water temperature and the chemical properties of water, including pH, pCO2, and precipitation rate of calcite. The lower Δ47 values have been previously reported from other tufas and travertines, and therefore appear to be associated with processes inherent to tufas and travertines, such as CO2 degassing. We also find that our results are insignificantly influenced by types of 17O corrections and almost independent with bulk isotopic composition.

Depositional and geochemical characteristics of geomorphologically controlled recent tufa deposits on the Göksü River in Yerköprü (Konya, southern Turkey)

Recent tufa deposition has been taking place on a 500 m-long natural bridge on the Goksu River in the Yerkopru region, southern Konya, Turkey. The Karasu spring arises from the Karasu normal fault, flows over this natural bridge and drops down from the end of the bridge, forming a spectacular waterfall. There is a dense CO2 degassing from this water and the H2CO 3 −1 content is 506,3 mg/L at the orifice and 549 mg/L on the waterfall, but it drops almost to half at the bottom of the waterfall. The field observation and chemical data showed that the deposition of the studied tufa was related only to the water derived from the Karasu spring. At the active waterfall area, tufa deposition takes place on the cascade, in small and large ponds and in channels. People living in the area build canals for irrigation and milling purposes, which cause the spreading of tufa deposits to other than the main course of the Karasu spring water. The most common types of facies are the different phytotherm facies, but stromatolitic, micritic and phytoclastic facies are also present to a lesser amount. The δ13C content of the tufa deposits ranges from 1,6 to − 3,3 and δ18O contents from − 9,6 to − 11,5. The higher δ13C values of the Yerkopru tufa deposits indicate that the carbonate-rich water of the Karasu spring originated from a carbonate aquifer. The isotopic values of the studied tufa samples are similar to those of travertine, but the Karasu water is interpreted as cold water. Therefore, it may be considered as “travitufa”.

Factors Affecting Tufa Degradation in Jiuzhaigou National Nature Reserve, Sichuan, China

Water and tufa samples were collected from Arrow Bamboo Lake, the stream from Panda Lake to Five-Color Lake, Pearl Shoal and Shuzheng Lakes in Jiuzhaigou National Nature Reserve, China, between October 2013 and September 2014, to investigate tufa growth rate and water environment (water temperature, pH, electric conductivity, major ions and nutrients), and analyzed to explore the main causes of tufa degradation. The mean annual rate of tufa growth was low and varied within lakes, with the maximum deposit thickness of 332 μm/y. The calcite saturation index ranged from 0.65 to 0.83. Scanning electron microscope images showed that the tufa deposits had non-isopachous structures, and diatoms were the dominant microorganisms that participated in tufa deposition. Porous and crystalline structures of deposits were linked with a high tufa growth and small amounts of diatoms. Conversely, tufa deposits with amorphous and loose structures showed a low crystal growth rate and a high number of diatoms. A one-way analysis of variance and a least significant difference test were applied to identify site differences in water chemistry. Linear correlations indicated that nitrate, phosphate and sulfate inhibit tufa growth (p < 0.05). Increased nitrogen and phosphorus concentrations that originate mainly from atmospheric pollution and tourist activities at scenic attractions could trigger excessive diatom growth, which inhibits tufa precipitation. A series of measures should be implemented (e.g., the visitor number and vehicles should be regulated and controlled) to minimize tufa degradation in the Jiuzhaigou National Nature Reserve.

Stable isotopic and elemental characteristics of recent tufa from a karstic Krka River (south‐east Slovenia): useful environmental proxies?

AbstractTufa samples from 16 consecutive barrages along a 13 km section of the groundwater‐fed Krka River (Slovenia) were analysed for their petrographical, mineralogical, elemental and stable carbon (δ13C) and oxygen (δ18O) isotope composition, to establish their relation to current climatic and hydrological conditions. Waters constantly oversaturated with calcite and the steep morphology of the Krka riverbed stimulate rapid CO2 degassing and subsequent tufa precipitation. The carbon isotope fractionation (Δ13C) between dissolved inorganic carbon and tufa in the Krka River evolves towards isotopic equilibrium being controlled by continuous CO2 degassing and tufa precipitation rate downstream. The Δ13C increased from 1·9 to 2·5‰ (VPDB); however, since tufa precipitation rates remain similar downstream, the major controlling factor of carbon isotope exchange is most probably related to the continuous 12CO2 degassing downstream leaving the carbon pool enriched in 13C. In the case of oxygen, the isotope fractionation (Δ18O) was found to be from 1·0 to 2·3‰ (VSMOW) smaller than reported in the literature. The observed discrepancies are due to different precipitation rates of calcite deposits because Krka tufas on cascades grow relatively faster compared to slowly precipitated calcite deposits in cave or stream pools. Due to non‐equilibrium oxygen isotope exchange between Krka tufa and water, the δ18O proxy showed from 1·2 to 8·2°C higher calculated water temperatures compared to measured water temperatures, demonstrating that δ18O proxy‐based temperature equations are not reliable for water temperature calculations of fast‐growing tufa on cascades. Because Mg is bound to the terrigenous dolomite fraction in the Krka tufa samples, the Mg/Ca was also found to be an unreliable temperature proxy yielding over up to 20°C higher calculated water temperatures.

Celestine in a sulfidic spring barite deposit - A potential biomarker?

We have documented the presence of celestine (SrSO4) within sediment accumulating at an artesian sulfidic spring (Zodletone Spring, Oklahoma) dominated by barite (BaSO4) precipitation associated with microbiological activity. The distribution and speciation of Sr in solid phases was determined by synchrotron-based micro-X-ray fluorescence spectroscopy and micro-X-ray diffraction, and particle morphology and texture was determined using electron microscopy. In all the natural sulfidic spring samples and lithified tufa sample, celestine was detected in fine-grained micron-scale Sr-rich phases but not in euhedral, Sr-poor grains. In parallel laboratory-based precipitation experiments, celestine was not observed even when solutions contained high Sr/Ba. Thermodynamic predictions alone do not account for the presence of micron-scale celestine in the sulfidic spring, and they do not account for the differences in Sr presence and distribution in naturally-occurring versus synthetic grains. While the mechanism is unclear, based on this evidence we hypothesize that the combination of bacterial surfaces and microenvironments within the crusts and microbial mats creates a synergistic effect where Sr is preferentially exchanged over Ba between the overlying stream water and the pore water within the mats allowing celestine to precipitate. Ultimately, our results point to an important role of biological activity for preferential Sr uptake. The presence of micron-scale celestine in ancient barite deposits can therefore potentially be used as a biomarker for conditions similar to modern sulfidic springs.

Temperature dependence of oxygen- and clumped isotope fractionation in carbonates: A study of travertines and tufas in the 6–95 °C temperature range

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 δ18O- and Δ47-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 Δ47 data from the travertines show a strong relationship with temperature and define the regression Δ47=(0.044±0.005×106)/T2+(0.205±0.047). The pH of the parent solution, mineralogy and precipitation rate do not appear to significantly affect the Δ47-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.

Sedimentological characteristics of Quaternary Aydıncık tufa (Mersin-Türkiye)

The Aydincik tufa crops out in an area of approximately 1 km2 along the Mediterranean coast of Aydincik town (Mersin-Turkey) located at the southern flank of the central Taurus mountain belt comprising clastics and carbonates ranging from Late Proterozoic to middle Eocene and several NE-SW aligned faults. It comprises seven texturally different facies: phytoherm framestone facies, phytoherm boundstone facies, micritic tufa facies, crossbedded carbonate sandstone facies, carbonate-coated conglomerate facies, pebbly carbonate sandstone facies, and tufa breccias facies. Of these, the phytoherm framestone facies and the phytoherm boundstone facies were deposited at the peripheral part of lake, whereas the micritic tufa facies toward the lake depocenter. Tufa breccias facies and crossbedded carbonate sandstone facies were resulted from erosion and redeposition of fragments from previously formed units such as phytoherm framestone and boundstone by fluvial activities developed within the environment. The tufa samples are composed of low-Mg calcite, and the terrigenous material in all tufa facies is generally very low. C and O isotop data show that the deposition of the Aydincik tufa took place under cold water condition. The sedimentological and mineralogical characteristics of the facies points that the Aydincik tufa was deposited in a fluvio-lacustrine environment in which the terrigen input was very low and was feeded by water from springs developed along the faults close to the Aydincik tufa.

OSL dating of Brazilian fluvial carbonates (tufas) using detrital quartz grains

Robust chronologies for Quaternary fluvial carbonate sediments (tufas) from Brazil are still challenging. We obtain Optically Stimulated Luminescence (OSL) ages of Brazilian tufa deposits using the Single Aliquot Regeneration (SAR) dose protocol applied to single grains and multigrain aliquots of quartz extracted from tufa matrix. Additionally, radiocarbon (14C) ages were obtained for the carbonate matrix and organic matter from tufas. The methodology was applied in samples from tufa deposits in southwestern and southeastern regions with Quaternary carbonate deposition. Equivalent doses distributions have overdispersion between 19% and 81%, indicating that the studied samples comprise well bleached as well as poorly bleached sediments. The quartz grains extracted from the studied tufas show an OSL signal dominated by the fast component and reliable dose response curves, with saturation doses (2D0) ranging from 35 to 210 Gy. Most of the samples has low radiation dose rates (0.40 ± 0.02 to 0.78 ± 0.05 Gy/ka), with variation in the water content due to compaction and matrix recrystallization, being the major factors that could induce dose rate changes through time. Hard tufa samples with intense carbonate cementation show quartz OSL ages from 51.9 ± 4.8 to 150.3 ± 35.9 years. These ages are much older than the radiocarbon ages of the carbonate matrix and organic matter. The OSL ages of soft tufa samples related with the active fluvial system ranged from 0.8 ± 0.1 to 5.5 ± 0.4 years and are in the same interval of radiocarbon ages presented in the previous works. We interpret that carbonate recrystallization or uptake of organic matter after tufa formation are responsible for the younger radiocarbon ages compared to quartz OSL ages in the older tufa samples. For these samples, age differences vary up to tens of thousands years between deposition of quartz grains and the last episode of carbonate recrystallization or organic matter uptake. Consequently we believe that the obtained OSL ages are the best estimates for the starting deposition time of the studied tufa samples.

The extraction of past environmental information using 137Cs and some elements in terrestrial carbonate deposits

Abstract Terrestrial carbonate deposits with a banded layer structure can be good tools for the extraction of past environmental information on global and local scales. The absolute age dating is most important for the reconstruction of an environmental chronicle. The terrestrial carbonate deposits(tufa) was collected at Hirao-dai Plateau in Fukuoka, Japan. Tufa has wide banded layers in which the growth rates are about a few mm a year. The activity of 137Cs that was a global or local fallout radionuclide was measured in each layer of the tufa sample. The absolute age was determined by the numbering of a banded layer and 137Cs activity profiles. The trace of the atomic bombing at Nagasaki was detected. The growth rate of the tufa is related to the environment of the year. The variations of a depositional condition of tufa sample were checked by some metal concentrations determined by neutron activation analysis. The relationship between the variation of environmental condition and the historical event at Hirao-dai was discussed using these metal concentration profiles.