Freshwater Carbonate Research Articles

Hydrogeochemical study of the Terme and Karakurt thermal and mineralized waters from Kirşehir Area, central Turkey

The Terme and Karakurt thermal resorts are located in the center of Kirsehir city in central Anatolia. Thermal waters with temperatures of 44–60°C are used for central heating and balneologic purposes. Paleozoic rocks of the Kirsehir Massif are the oldest units in the study area. The basement of the Massif comprises Paleozoic metamorphic schist and marbles which partly contain white quartzite layers of a few tens of cm thickness. The metamorphic schists which are cut by granites of Paleocene age are overlain by horizontally bedded conglomerate, sandstone, claystone, and limestone of upper Paleocene-Eocene age. Among the thermal and cold waters collected from the areas of Terme and Karakurt, those from thermal waters are enriched with Ca–HCO3 and cold waters are of Ca–Mg–HCO3 type waters. The pH values of samples are 6.31–7.04 for the thermal well waters, 6.41 for thermal spring, 7.25 and 7.29 for the cold waters, and 7.52 for the Hirla lake water. EC values are 917–2,295 μS/cm for the thermal well waters, 2,078 μS/cm for thermal spring, and 471 and 820 μS/cm for the cold springs. The lowest TDS content is from water of T10 thermal well in the Terme area (740.6 mg/l). The hot and cold waters of Terme show very similar ion contents while the Karakurt hot waters at western most parts are characterized by distinct chemical compositions. There is ion exchange in thermal waters from the T5 (5), T6 (6), T12 (7), and T1 (8) wells in the Terme area. The thermal waters show low concentrations of Fe, Mn, Ni, Al, As, Pb, Zn and Cu. Waters in the study area are of meteoric origin, and rainwater percolated downwards through faults and fractures, and are heated by the geothermal gradient, later rising to the surface along permeable zones. δ13CVPDB values measured on dissolved inorganic carbon in samples range from −1.65 to +5.61‰ for thermal waters and from −11.81 to −10.15‰ for cold waters. Carbon in thermal waters is derived from marine carbonates or CO2 of metamorphic origin while carbon in cold waters originates from freshwater carbonates.

Stratigraphic setting, facies types, depositional environments and mechanism of formation of Ash Shumaysi ironstones, Wadi Ash Shumaysi, Jeddah district, West Central Saudi Arabia

The ironstones of Wadi Ash Shumaysi are mainly enclosed within the middle part of the fluvio-lacustrine siliciclastic succession of Ash Shumaysi Formation (Oligo-Miocene). This succession is composed mainly of conglomerates, sandstones, siltstones, muddy, sandy and glauconitic ironstones, fresh water carbonates, tuffaceous mudstone and basalts in descending order. The study aims to determining the stratigraphic setting, facies types, depositional environments, and mechanism of formation of the succession and the associated ironstones, through field and petrographic investigations. The succession comprises the following four main facies associations arranged according to their predominance from the lower to the upper parts of the succession of Ash Shumaysi Formation: I: Proximal (Conglomerate Facies) Braided Streams, II: Distal (Sandstone Facies) Braided Streams, III: Floodplain facies association, including ironstones, IV: Lacustrine delta, including ironstones. The ironstones of facies associations III and IV represent the following types: 1. Sandy ironstones (ironstone type 1 (IST 1)): which formed by the deposition of water laid detrital quartz grains and Fe-bearing clays under different water depth and currents. 2. Glauconitic muddy ironstones (ironstone type 2 (IST 2)): which formed by the deposition of Fe-bearing clays within slightly agitated starved lakes. During periods of low clastic input, the deposited Fe-bearing clays become converted into green glauconitic clays as a result of the high Fe, Mg, K, Al, and Si activities. During ultimate stages of oxidized diagenetic alterations, the green glauconitic clays become hematitized into glauconitic ironstones. 3. Oolitic muddy ironstones-oolitic ferruginous sandstone (IST 3): which is represented by lacustrine successive shallowing-upward cycles. Each of these cycles begins by friable ferruginous in situ oolitized muddy ironstone which grades upward into oolitic sandy ironstone/oolitic ferruginous sandstone. The second interval is recorded overlying the fining-upward cycle of facies F5. 4. Flaser laminated silty ironstone/ferruginous siltstone (IST 4): which consists of small-scale shallowing-upward cycles of parallel laminated hematitic siltstone and flaser laminated silty ironstone/ferruginous siltstone. In this ironstone type, the inputted depositional components are composed mainly from silt-sized detrital quartz grains embedded within Fe-bearing clays. These components are vertically and laterally distributed within the depositional environments by the impact of the currents and wave actions. The diagenetic processes play an important role in the petrographic and mineralogic evolution of ironstone type IST 1. These processes includes: (a) the diagenetic recrystallization and dehydration of the precursor amorphous Fe-bearing clays and the intimately associated Fe-oxyhydroxies and formation of ferrihydrites, goethite. and hematite; (b) corrosion and embayment of the quartz grains by the enclosing different Fe mineral phases; and (c) in situ pellitization and oolitization throughout the tangential growth of the Fe-oxhyhdroxides during their dehydration and recrystallization and formation of pseudo-ooids. The diagenetic processes in ironstone type IST 2 includes glauconitization of the remained unglauconitized clays during the depositional stages, hematitization and the related oolitization, and pelletization processes. The main diagenetic processes involved in the formation of ironstone type IST 3 are: (a) hematitization of the amorphous Fe-clays and formation of goethite and hematite cement, (b) in situ pelletization and oolitization. The most important diagenetic process in ironstone type IST 4 is the dehydration and recrystallization of the amorphous Fe-clays and formation of goethite and hematite cement.

Preliminary palaeomagnetic investigation of riverine tufa: is riverine tufa a reliable recorder of the geomagnetic field direction?

SUMMARY Riverine tufas are freshwater carbonate deposits that precipitate in springs and streams distributed in limestone areas. They are noteworthy for their annual laminations that contain both dense and porous laminae and their fast precipitation rate. These features indicate that riverine tufa may be an excellent material for recording the secular variation of the geomagnetic field. Part of recently deposited tufas in Nagaya in Okayama Prefecture, Japan have a stable natural remanent magnetization (NRM), which is a reliable record of the average geomagnetic field direction. However, only the dense laminae in the recent tufa, however, carry a faithful record of the geomagnetic field direction; the porous laminae do not. The NRM of the porous laminae seems to be affected considerably by the dip direction of the slope on which the tufa is deposited. Based on the results of isothermal remanent magnetization acquisition experiments and microscopic observations, the dominant magnetic mineral in the recent tufa was determined to be magnetite. A Holocene palaeotufa, which was formed 2000–2800 years ago, was found to have no stable remanent magnetization. This is possibly due to the absence of dense layers where magnetic particles are tightly fixed. Since it is very important in palaeomagnetic secular variation studies to be able to reconstruct much older, continuous records of the variation of the geomagnetic field, tufa deposits are not ideal materials and their applicability for this purpose is rather low, although the further investigation is needed.

Malargüe Group (Maastrichtian–Danian) deposits in the Neuquén Andes, Argentina: Implications for the onset of the first Atlantic transgression related to Western Gondwana break-up

The discovery of marine to brackish and fresh-water carbonates in the inner Agrio fold-and-thrust belt at Pichaihue, Neuquén, Argentina, located to the west of the Andean orogenic front, imposes important constraints on the paleogeography of the first Atlantic transgression in the Neuquén Basin related to the break-up of Western Gondwana. The constraints on the timing and areal extent of these deposits shed light on the early uplift history of the southern Andes. These limestones are part of the Maastrichtian–Danian Malargüe Group, which was previously only known from its exposures in the extra-Andean area, representing foreland basin deposits. The presence of stromatolites, oncoids, serpulids, bivalves and gastropods as well as silicified stems of macrophytes indicates a shallow marine, partially brackish environment associated with non-marine deposits. These strata are interfingered with and overlie distal tuffs and proximal pyroclastic flows, whose geochemical characteristics point to a magmatic arc source. SHRIMP U–Pb dating of volcanic zircons of these tuffs yielded an age of 64.3 ± 0.9 Ma that confirms the correlation to the Maastrichtian–Paleocene marine transgression from the Atlantic Ocean. The change in the paleoslope of the basin from Pacific Ocean transgressions to this Atlantic transgression is related to the uplift and deformation of the Agrio fold-and-thrust belt. The Pichaihue Limestone is unconformably deposited on volcanic agglomerates which in turn unconformably overlie Early Cretaceous deposits. Based on these data, it is confirmed that the Cretaceous uplift of the Andes was episodic at these latitudes, with a first pulse in the Cenomanian and a second one in pre-Maastrichtian times. The episodic uplift is also related to an eastward migration of the thrust front and the volcanic arc, related to a previously proposed shallowing of the subduction zone. These episodes were controlled by the Western Gondwana break-up and the beginning of absolute motion of South America toward the west.

Hydrogeochemistry, environmental isotopes and the origin of the Hamamayagi-Ladik thermal spring (Samsun, Turkey)

Hamamayagi thermal spring (HTS) is located along the North Anatolian Fault Zone. The thermal spring has a temperature of 36°C, with total dissolved solids ranging from 485.6 to 508.5 mg/L. Hard, brittle, and gray limestones Permian aged are the reservoir rocks of the HTS. δ18O–δ2H isotope ratios clearly indicate a meteoric origin for the waters. The δ34S value of sulfate in the thermal water is nearly 4.1‰ and implies a diagenetic environment characterized by reduced sulfur compounds. The δ13C ratio for dissolved inorganic carbonate in the HTS lies between −1.78 and −1.62‰, showing that it originates from the dissolution of fresh-water carbonates. Quartz geothermometry suggests a reservoir temperature of 52–85°C for the Hamamayagi geothermal field, but chalcedony geothermometers suggest reservoir temperatures between 30 and 53°C.

Wet-dry seasonal variations of hydrochemistry and carbonate precipitation rates in a travertine-depositing canal at Baishuitai, Yunnan, SW China: Implications for the formation of biannual laminae in travertine and for climatic reconstruction

Biannually laminated freshwater carbonate deposits (tufas and travertines) are potential sources of high-resolution terrestrial paleoclimate data. In order to understand the processes of carbonate precipitation, the formation of the biannual laminae in endogenic (thermogene) travertine, and the characteristics of the climate information recorded in them, the seasonal variations of hydrochemistry and carbonate precipitation rates in a travertine-depositing canal at Baishuitai, Yunnan, SW China were investigated. Plexiglass substrates (5 cm × 5 cm × 0.5 cm) for carbonate collection were placed in the flowing water at ten sampling sites in the canal and replaced semimonthly for one hydrological year, to determine the rates of carbonate precipitation. Water temperature, pH and specific conductivity at these sampling sites were measured daily in situ throughout the year. From these data the concentrations of Ca2+, HCO3−, the calcite saturation index and CO2 partial pressure were derived. The hydrochemistry and rate of carbonate precipitation at all sites displayed the same clear seasonal pattern, lower in the warm rainy season and higher in the cold dry season. The lower solute chemical concentrations and carbonate precipitation rates in the rainy season were due to the dilution of ground water concentrations by overland flow after rainfall. The endogenic travertine had much clearer biannual lamination structure than is normal in the epigenic tufa in Japan. The lamina consisted of a thin dark porous warm-rainy-season layer alternating with a thick light dense cold-dry-season layer: in contrast, a thick, dark and dense summer–autumn layer alternates with a thin, light and porous winter–spring layer in any biannual lamina found in normal epigenic tufa that is controlled mainly by the change of temperature and CO2. The biannual lamination in the thermogene travertine was primarily controlled by changes in the dilution-dominated carbonate precipitation rate, the darker color of the warm-rainy-season layer being due to soil-derived clay and organic matter introduced by the overland flow. This study thus demonstrates that rates of carbonate precipitation and the formation of lamination in the travertine at Baishuitai were controlled chiefly by a climatic factor (rainfall), which has implications for using the fossil travertine at the site to reconstruct the climate in the past. Additionally, it shows that differences in rate-controlling mechanisms of carbonate precipitation must be considered when using epigenic tufa and endogenic travertine to reconstruct the paleoclimate and paleoenvironmental parameters (temperature + CO2 vs. rainfall).

About this title ‐ Tufas and Speleothems: Unravelling the Microbial and Physical Controls

Our understanding of calcium carbonate precipitation within freshwater carbonate systems is being revolutionized by new quantitative approaches at both field and laboratory scale. These systems cover a diverse range of topical research areas including tufas, speleothems, stromatolites and microbial processes. Progress by various international research groups has been impressive, with major contributions to such areas as climate change, absolute dating, carbon sequestration, and biofilm construction and precipitation. A diverse sample of interrelated research is presented that provides a tantalizing glimpse of the interplay between microbial, geochemical and physical processes that control the development of tufas and speleothems. This volume will provide a cross-disciplinary platform that will stimulate further exchanges about new concepts, methodologies and interpretations associated with freshwater carbonates. In particular, it will help reinforce the importance of cross-discipline research: the driving force behind the new field of Geobiology.

In vitro investigations of the impact of different temperature and flow velocity conditions on tufa microfabric

Abstract A series of experiments on freshwater carbonates (tufas) involving biofilm colonization in both fast-flow and slow-flow mesocosms was carried out in order to assess the changing nature of biofilm and associated precipitates under contrasting conditions. A thin biofilm developed over 14 weeks during the ‘summer’ experimental run contained a basal calcite layer overlain by small calcite crystals suspended within the Extracellular Polymeric Substances (EPS). The ‘autumn’ biofilm, however, showed the development of multi-laminated calcite precipitates within the EPS despite constant environmental conditions throughout the run. The experiments also showed that the largest volume of calcite precipitate developed in the fast-flow flumes regardless of temperature control. Development of an extensive calcite layer at the base of EPS in conditions of complete darkness within the sump was also observed. This study provides increased weight for the concepts: (1) that fresh- and saltwater stromatolites appear to be highly comparable multi-laminated systems with precipitation strongly influenced by both phototrophic and heterotrophic microbes; and (2) that microbial precipitation may be more common within aphotic (including cave, lake bottom and soil) systems than has previously been considered.

Trace element and strontium isotope constraints on sedimentary environment of Ediacaran carbonates in southern Anhui, South China

Two units of sedimentary carbonates in association with Neoproterozoic glaciations occur in the Lantian Formation of the Ediacaran system, equivalent to the Doushantuo Formation elsewhere in South China. Trace elements and strontium isotopes in the carbonates were analyzed by stepwise dissolutions with different strengths of acid. The results show that the carbonate component dissolved in 0.5 M acetic acid (Dissolution I) is principally composed of calcite and almost free from non-carbonate contamination. Thus its element and isotope compositions can be used to unravel the geochemical feature of water from which the carbonates precipitated. The strontium concentrations and isotope ratios, REE + Y concentrations and patterns of Dissolution I from the carbonates are similar to each other between the two units. They exhibit high strontium concentrations and high 87Sr/ 86Sr ratios, depletion of LREE relative to MREE and HREE, minor positive La anomalies, subtle negative Ce, Gd and Er anomalies, and near-chondritic Y/Ho ratios. These indicate that the Lantian carbonates were precipitated from the similar nature of basin water, which is devoid of trace element composition typical of normal marine carbonates but similar to those in freshwater carbonates. The negative Ce anomalies in the carbonates of the two units suggest oxygenation of the precipitation water. Therefore, the Lantian carbonates would form in a continental marginal basin, with predominant contributions from freshwater and high sedimentary rates. Since the Doushantuo–Lantian carbonate secessions may probably correspond to the chemical sediments of post-Marinoan and post-Gaskiers, it is hypothesized that the freshwater would originate from locally deglacial meltwater with significant inputs from continental weathering.

Depositional continuity of seasonally laminated tufas: Implications for δ 18O based palaeotemperatures

A ~ 100 ka old laminated tufa (freshwater carbonate) deposit from central Greece was studied for evidence of a seasonal origin of the laminae. Annual laminar couplets (6 mm thick) consist of dense β bands of spring–early summer calcified cyanobacterial bushes and porous α bands of autumn precipitated calcite on un-clustered cyanobacterial filaments. This is evidence of likely seasonal palaeoclimate during the last interglacial in Greece. δ 18O variability is explained by seasonal changes in stream water temperature; however, the total range in δ 18O translates to ~ 6 °C (± 1 °C) almost certainly underestimating the likely actual range of ~ 17 °C. As abrupt changes in δ 18O inferred temperatures (from warming to cooling and vice versa) occur exactly on sharp petrographic lamina boundaries, we infer that mid-summer and mid-winter precipitates are missing. We suggest that when stream conditions (summer drying and winter cold) are unsuited to cyanobacterial substrate development, no tufa build-up occurs. Laminated tufas are thus incomplete records of annual tufa formation, as are the δ 18O based palaeoclimate records they preserve. They require careful petrographic study linked to high resolution sampling for δ 18O to ensure that palaeotemperature ranges are not seriously underestimated.

Intra-annual perturbations of stable isotopes in tufas: Effects of hydrological processes

Tufas, which are freshwater carbonates, are potential archives of terrestrial paleoclimate. Time series of stable isotopic compositions commonly show regular seasonal patterns controlled by temperature-dependent processes, and some perturbation intrinsic to the locality. We examined three tufa-depositing sites in southwestern Japan with similar temperate climates, to understand the origin of local characteristics in the isotopic records. Seasonal change in the oxygen isotope is principally reflected by temperature-dependent fractionation between water and calcite but was perturbed after heavy rainfalls overwhelming the stability of the δ 18O value of the groundwater at one site. Isotopic mass balance indicates an undersaturated and relatively small aquifer at this locality. Water δ 18O values at the other two sites were stable, reflecting a regular seasonal change in the δ 18O value of tufa. Perturbation of the δ 13C values in tufa is largely due to CO 2 degassing from the stream, which significantly increases the δ 13C values of dissolved inorganic carbon (DIC). At a site with remarkably high pCO 2 in springwater and a sensitive response of flow rate to rainfall, the amount of CO 2 degassing changed distinctly with flow rate. In contrast, the other two sites having low pCO 2 springwater reflect a regular seasonal pattern of δ 13C in DIC and tufa specimens.

Freshwater calcite precipitates from in vitro mesocosm flume experiments: a case for biomediation of tufas

AbstractFreshwater carbonates (tufas) develop today from the Arctic to the tropics, many being localized about springs and upper water courses. Some Quaternary tufas, especially in the Mediterranean region, extend over tens of square kilometres and exceed 30 m in thickness. Radiometric dating of Holocene deposits shows that many have accumulated at an average rate of 1 mm year−1. However, local precipitation may be much faster and some Holocene deposits may even have outpaced their tropical marine carbonate counterparts. Recently, the study of active sites has attempted to quantify the precipitation mechanisms which lead to tufa deposition. However, field observation and sampling procedures suffer from the inherent disadvantages of uncontrolled fluctuations in environmental conditions during the study programme. These disadvantages compromise any interpretations, particularly where controls on spar versus micrite precipitation are concerned. Many of these problems have been overcome in the current study by the construction and operation of laboratory mesocosm flumes which simulate the natural conditions (e.g. pH, flow rate, ambient temperature and daylight) in which freshwater carbonate (tufa) is deposited. Three mesocosms were supplied with natural river water from tufa precipitating streams and two mesocosms were supplied with UV‐treated (sterile) river water from the same source. One of the untreated flume mesocosms was linked with a calcium reactor, which replaced calcium ions removed during the precipitation process in order to maintain tufa growth over extended experimental runs. Low‐magnesium calcite precipitates (both rhombic sparite grown from long‐crystallite dendrites and short‐crystallite dendrite triad precursors) and micrite peloids (grown from spherulitic precursors) were precipitated in intimate association with biofilm (extracellular polymeric substances) within the four mesocosms supplied with natural river water. Virtually, no tufa‐like precipitate was obtained from the flumes supplied with UV‐treated river water. A second extended run flume experiment was also carried out for comparison purposes using a calcium hydroxide solution in deionized water. Collectively, these experiments provide convincing evidence confirming that the presence of a microbial biofilm strongly influences the precipitation of carbonates in riverine freshwater settings. In particular, experimental results show that micro‐peloidal micrite and short‐crystallite calcite dendrites are only produced in the presence of microbial extracellular polymeric substances.

Terrestrial and freshwater carbonates in Hoxnian interglacial deposits, UK: micromorphology, stable isotopic composition and palaeoenvironmental significance

MIS 11 is often considered to be the best climatic analogue for the Holocene. Many studies have suggested, however, that it is a period of extreme climate warmth comparable in temperatures to the Middle and Late Pliocene. In Britain deposits of the Hoxnian interglacial are correlated to MIS 11 and multi-proxy techniques can be used to reconstruct the climate of this interglacial. Soil, groundwater and freshwater carbonates are common in Hoxnian deposits and the stable isotopic composition of these precipitates can be used to increase our understanding of MIS 11 environments in Britain. Carbonates from Marks Tey, Clacton, Swanscombe, Elveden and West Stow are studied, the stratigraphic context of which indicates that their formation is broadly synchronous (in the mid-Hoxnian, pollen zones Ho II to Ho III). The carbon isotopic composition of groundwater and pedogenic carbonates is typically depleted with respect to δ13C (ca −9 to −8‰ VPDB) reflecting uptake of plant respired CO2 during water migration/recharge. The carbon isotopic composition of lacustrine carbonate is more enriched with respect to δ13C (ca 0–1‰VPDB) reflecting the equilibration of lake waters with atmospheric CO2. The δ18O of groundwater and pedogenic carbonates is slightly more enriched than modern soil carbonates but not as enriched as soil carbonates formed under interglacials that were warmer than the Holocene (i.e. the Cromerian). The stable isotopic composition of Hoxnian carbonates does not, therefore, indicate that this interglacial was characterised by uniquely warm climates in the context of other Middle Pleistocene interglacials and the Holocene. This is contrary to many marine and littoral records from around the world but consistent with environmental records from Britain and Europe.

Tufas and travertines of the Mediterranean region: a testing ground for freshwater carbonate concepts and developments

AbstractActively forming travertine and tufa sites are widespread in temperate and warm climates, such as the Mediterranean region, but also develop in other climatic regimes from the Arctic to equatorial latitudes. The terms travertine and tufa are sometimes applied indiscriminately today; however, each has a specific definition. Most studied deposits are active or of Quaternary (especially Holocene) age. There are fewer Tertiary and even less late Mesozoic examples on record. In part, this is because of a recognition problem as these deposits commonly preserve as fragmentary erosional remnants. Physico‐chemical precipitation of calcium carbonate is commonly considered to be the main mechanism contributing to tufa and travertine deposition. Rapid cooling of waters further aids the process in thermal (travertine) sites. However, biomediation of calcium carbonate, associated with prokaryote–microphyte biofilms, is now recognized to be equally important in slow flowing and near static sites. Much of this precipitation occurs in close association with cyanobacterial, heterotrophic bacterial and diatoms. The Mediterranean region figures significantly in the development of travertine and tufa research. Consequently, this article employs key literature on Mediterranean examples to illustrate developments and concepts in freshwater carbonate research. Modern work started in the 1950s and defined many previously unknown outcrops. Many such sites became the targets for early radiocarbon dating studies. Since the late 1970s, a number of laboratory‐based and field‐based classification schemes have been proposed. Some schemes were based on associated macrovegetation types; others were based on petrological characteristics or on geometry and sedimentary facies characteristics. Of late, the sedimentological modelling has benefited greatly from the use of shallow geophysical techniques in order to better define the internal geometries of the deposits. Two distinct research strands were developed during the 1990s, both aimed at a better understanding of environmental change. The first considered biocomponents. The research was multifaceted and ranged from exploring the role of micro‐organisms in cement and lime mud precipitation to biostratigraphic analysis of contained pollen and invertebrate remains; these provided proxy environmental information which has helped to define past climate variability in the region. The second research strand focussed on geochemical signatures as a proxy tool for defining environmental change. This definition was made possible by new petrological studies which have greatly aided the recognition of primary depositional fabrics. This improved knowledge has reduced the sampling dangers of diagenetic contamination inherent in earlier geochemical studies. Lithofacies modelling has also brought to light important early post‐depositional neomorphic changes, which obscure primary fabrics, and complex diagenetic histories have been revealed. Stable isotope signatures of carbon and oxygen have also been used to define palaeo‐water temperature. Fine‐tuning of depositional time frames has also been achieved from absolute dating techniques employing radiocarbon and uranium/thorium.

New insights into biological influence on the geochemistry of freshwater carbonate deposits

Ambient temperature freshwater carbonates (“tufas”) have outstanding potential as palaeoclimate archives. However, it has proven difficult to constrain the behaviour of the geochemical parameters on which important proxy systems rely. Current thinking on the development of trace element ratios in tufa rests upon an assumption of precipitation driven by degassing of CO 2, with the final precipitate being in equilibrium with ambient river water. This study provides evidence that although supersaturation is largely regulated by degassing, precipitation in sterile flowing systems does not occur throughout the air–water interface, but only at lines of intersection between this interface and a solid surface. Precipitation at the bottom, as is normal in river sites, is not found to occur. However, when benthic microbial communities (“biofilms”) are present, extensive precipitation does occur within them in preference and/or addition to that at the air–water interface. Precipitation is therefore not purely a consequence of degassing, and some microbiological influence is necessary to generate precipitates analogous to nature. We further investigate whether these biofilms alter the behaviour of trace elements within tufa systems, in addition to modulating the sites and mechanisms of precipitation, via a series of experiments. Biofilms are shown to accumulate large inventories of calcium and other dissolved ions (Ba, Sr, and Mg), and we demonstrate that this process is chemoselective in favour of ions with low charge densities. These observations have consequences for interpretation of trace element records from tufas based on an assumption of equilibrium with river water, and also for other carbonate deposits associated with biofilm activity (travertines, marine stromatolites, lacustrine and soil carbonates, speleothems). Conversely, our study provides new insight into the reasons for discrepancy of tufa Mg/Ca ratios from expectation, and thus provides encouragement for the potential of Mg/Ca palaeothermometry in freshwater carbonate systems.

EXCEPTIONAL PRESERVATION OF PLANTS AND INVERTEBRATES BY PHOSPHATIZATION, RIVERSLEIGH, AUSTRALIA

Phosphatic concretions containing mineralized three-dimensional plant and invertebrate tissues occur in a mid-Cenozoic terrestrial freshwater carbonate deposit at the Riversleigh World Heritage area in northwestern Queensland, Australia. Mineralogical composition and microstructure of the concretions show that this fossil assemblage was preserved by rapid, early diagenetic phosphatization. Preserved material includes tissues typically considered to have a poor potential for phosphatization, such as internal plant structure and arthropod exoskeletal components. Phosphatization may have been mediated by the presence of microbial films. Study of this rich assemblage has the potential to make significant contributions to both the Australian Cenozoic terrestrial invertebrate fossil record and the Cenozoic plant megafossil record of northern Australia.

Cave Turbidites

Turbidites are uncommon in caves, but are more common as palaeokarst deposits. Marine carbonate turbidites, called cay­manites, are the most common cave and palaeokarst turbidites, but marine non-carbonate turbidites, freshwater carbonate turbidites and freshwater non-carbonate turbidites are also de­posited in caves and preserved in palaeokarst sequences. One of the most complex sequences of cave turbidites occurs in the Wellington Caves Phosphate Mine in Australia. Cave turbidites form in ponded water in caves and may be triggered by floods and highintensity rain events. While caymanites are most likely to form during marine transgressions, they can be emplaced by tsunami. Freshwater cave turbidites are most likely to form in flooded hypogene caves located in the seasonally wet tropics and in areas withirregular highintensity rainfall events.

Watershed reconstruction of a Paleocene-Eocene lake basin using Sr isotopes in carbonate rocks

Provenance studies have used Sr isotopes ( 87 Sr/ 86 Sr) of silicate source rocks as a link to their eroded basinal equivalents. This technique, however, cannot identify the proportional inputs from different watersheds or define more precisely sedimentation events due to tectonic or climatic change. Erosion of carbonate rocks dominates the Sr input within basin drainage and potentially can be used through 87 Sr/ 86 Sr ratios to reconstruct paleohydrology of the entire basin and trace watershed inputs and depositional patterns in continental basins. The Sr isotopic ratios from waters of the source area, allowing for the mixing of shallow groundwater and surface water along the transport path, are homogenized in the basinal carbonate sediments. Mineralogy and diagenesis of carbonate rocks generally do not affect the Sr isotopic signal in a near-surface system lacking external influence by volcanism, eolian dust, or deep geothermal waters. The 87 Sr/ 86 Sr ratios from the source area are directly comparable to those in the receiving continental basin. The Sr isotopic signal of the Paleocene–Eocene Flagstaff Formation (central Utah), a carbonate lake deposit in a foreland basin, is compared to that of projected source waters draining its thrust front, the Sevier fold-thrust belt. Freshwater carbonates compose a large portion of the lowermost Ferron Mountain and uppermost Musinia Peak Members of the formation, whereas gypsum and carbonates predominate in the middle Cove Mountain Member. Previous research had attributed gypsum deposition to the deposition of the middle Cove Mountain Member to either climatic change or unroofing of diapirs of Jurassic gypsiferous carbonates. To examine more closely the influence of climate versus tectonics on Flagstaff sedimentation as well as the efficacy of provenance studies using carbonates, we collected rock samples from the three members of the formation on the Wasatch Plateau of central Utah in addition to sampling stream water associated with Pennsylvanian–Permian and Jurassic carbonate terrains from the nearby thrust front. The 87 Sr/ 86 Sr ratios in carbonates and gyprock belonging to the Flagstaff Formation remained unchanged during deposition, the average Sr isotope composition of the Flagstaff rocks being identical to that of sampled waters draining the projected provenance area. There was little change in source rock weathering as the thrust front evolved. Deposition of gypsum occurred in the basinal lake only during the deposition of the middle Cove Mountain Member, despite its constant exposure in the drainage area, suggesting a changing balance of tectonic and climatic controls during lake sedimentation. The 87 Sr/ 86 Sr isotopic studies targeting carbonate rocks and their presumed source waters are a simple but accurate method for reconstructing the paleohydrology of lake basins.

Radium-226 chronology of Holocene travertine from Esanatoglia (Central Italy)

Here we investigate the feasibility of using alpha-spectrometric 226Ra methodology to date low U freshwater carbonate deposits (<0.1μgg−1) by analysing Holocene freshwater travertine deposits from Esanatoglia, Umbro-Marchean Apennines, Central Italy. Previously, such methods have been tested on hydrothermal deposits, which are characterised by high Ra and U. We calculate a 226Ra age estimate of 2.9±0.4ka (1σ) for Holocene travertine that is in reasonable agreement with archaeological evidence by assuming that the 226Ra initial activity of the Holocene travertine is comparable with that of present-day carbonate deposits in the same area. We also investigate the use of Ba to normalise 226Ra activities. Comparison of age estimates based on Ra methods with ages derived from alpha-spectrometric 230Th/234U isochron techniques for the same co-eval sub-samples indicates that, where only alpha-spectrometric methods are available, the former are likely to be much more useful for Holocene travertine material, especially if there is a significant detrital component.

Rapid cold water formation and recrystallization of relict bryophyte tufa at the Fall Creek cold springs, Alberta, Canada

The Fall Creek Tufa is a relict, mid- to late-Holocene freshwater carbonate spring deposit that is formed almost entirely of calcite-encased Cratoneuron commutatum gametophytes. The spring is presently active, emitting 13 °C meteorically derived waters, but it is not precipitating calcite. Wood embedded in the youngest part of the tufa yielded a radiocarbon (14C) date of 310 ± 50 years BP. The oldest parts of the deposit could not be dated by radiocarbon methods because of a lack of embedded organics. The Fall Creek Tufa, however, probably began forming ~5000 years BP in response to the same wet climatic conditions that initiated calcite precipitation at Miette Hot Springs in Jasper National Park and the Cave and Basin Hot Springs in Banff National Park. The relict deposit is composed largely of calcite cements that were precipitated around the bryophytes, with lesser amounts of internal sediments. The tufa formed through a repeated four-stage developmental process that involved (I) encrustation, (II) encapsulation, (III) cavity occlusion, and (IV) diagenetic alteration. These stages were temporally and spatially variable. Although a young deposit, much of it has undergone extensive aggrading recrystallization, whereby crystal size has increased as adjacent calcite crystals were sutured together. Epifluorescence micro s copy shows that primary depositional fabrics were masked, but not destroyed, during recrystallization. Narrow ranges of δ18O (VPDB, Vienna Pee-Dee Belemnite standard) values (–18.4‰ to –17.7‰) and δ13C (VPDB) values (–1.2‰ to 0.5‰) indicate that the calcite was in isotopic equilibrium with the water that it precipitated from, and that diagenesis did not reset the stable oxygen and carbon isotopic compositions.