Diagenesis or dire genesis? The origin of columnar spar in tufa stromatolites of central Greece and the role of chironomid larvae

Two Greek Pleistocene tufa stromatolites were examined petrographically and with stable isotope geochemistry to determine whether calcite spar is of primary or diagenetic origin. The younger (ca 100 ka) tufa from Zemeno primarily is micritic, with primary columnar calcite spar restricted to areas immediately above chironomid larval tubes. This relationship suggests that chironomid larval feeding behaviour is responsible for the development of Zemeno tufa columnar calcite, probably involving biological substances smeared onto the tufa surface. Most micritic crystals are not suitably oriented to allow later post‐depositional growth resulting in columnar fabrics. The older (ca 1 Ma) predominantly sparry tufa from Nemea contains some chironomid tubes and organic cyanobacterial filaments preserved in crystal fans but also contains many fabrics found in primary speleothem spar. Columnar spar here is unlikely to be the result of post‐depositional crystal growth. A comparison of stable isotopic trends between the two tufa deposits suggests that both contain interpretable seasonal trends and implies little or minor post‐depositional alteration of either tufa. Consequently, there is no basis for the common assumption that sparry tufa fabrics must be of diagenetic origin.     

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

Septarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ¹³C and δ¹⁸O values in most concretion bodies range from ?10·0‰ to ?17·3‰ and +0·3‰ to ?0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ¹³C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ¹³C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ¹⁸O range for petrographically identical cement (?1·3‰ to ?15·6‰) is difficult to explain.     

Microbe–mineral interactions: early carbonate precipitation in a hypersaline lake (Eleuthera Island, Bahamas)

Microbialites (benthic microbial carbonate deposits) were discovered in a hypersaline alkaline lake on Eleuthera Island (Bahamas). From the edge towards the centre of the lake, four main zones of precipitation could be distinguished: (1) millimetre‐sized clumps of Mg‐calcite on a thin microbial mat; (2) thicker and continuous carbonate crusts with columnar morphologies; (3) isolated patches of carbonate crust separated by a dark non‐calcified gelatinous mat; and (4) a dark microbial mat without precipitation. In thin section, the precipitate displayed a micropeloidal structure characterized by micritic micropeloids (strong autofluorescence) surrounded by microspar and spar cement (no fluorescence). Observations using scanning electron microscopy (SEM) equipped with a cryotransfer system indicate that micrite nucleation is initiated within a polymer biofilm that embeds microbial communities. These extracellular polymeric substances (EPS) are progressively replaced with high‐Mg calcite. Discontinuous EPS calcification generates a micropeloidal structure of the micrite, possibly resulting from the presence of clusters of coccoid or remnants of filamentous bacteria. At high magnification, the microstructure of the initial precipitate consists of 200–500 nm spheres. No precipitation is observed in or on the sheaths of cyanobacteria, and only a negligible amount of precipitation is directly associated with the well‐organized and active filamentous cyanobacteria (in deeper layers of the mat), indicating that carbonate precipitation is not associated with CO₂ uptake during photosynthesis. Instead, the precipitation occurs at the uppermost layer of the mat, which is composed of EPS, empty filamentous bacteria and coccoids (Gloeocapsa spp.). Two‐dimensional mapping of sulphate reduction shows high activity in close association with the carbonate precipitate at the top of the microbial mat. In combination, these findings suggest that net precipitation of calcium carbonate results from a temporal and spatial decoupling of the various microbial metabolic processes responsible for CaCO₃ precipitation and dissolution. Theoretically, partial degradation of EPS by aerobic heterotrophs or UV fuels sulphate‐reducing activity, which increases alkalinity in microdomains, inducing CaCO₃ precipitation. This degradation could also be responsible for EPS decarboxylation, which eliminates Ca²⁺‐binding capacity of the EPS and releases Ca²⁺ ions that were originally bound by carboxyl groups. At the end of these processes, the EPS biofilm is calcified and exhibits a micritic micropeloidal structure. The EPS‐free precipitate subsequently serves as a substrate for physico‐chemical precipitation of spar cement from the alkaline water of the lake. The micropeloidal structure has an intimate mixture of micrite and microspar comparable to microstructures of some fossil microbialites.     

Seasonal temperatures from δ18O in recent Spanish tufa stromatolites: Equilibrium redux!

This study focuses on recent debate over the value of stable isotope‐based environmental proxies recorded in riverine tufa stromatolites. A twelve‐year record (1999 to 2012) of river‐bed tufa stromatolites in the River Piedra (north‐east Spain) was recovered in this study, along with a partly overlapping fifteen‐year record (1994 to 2009) of accumulations in a drainage pipe: both deposits formed in water with near identical physico/chemical parameters. Measured water temperature data and near‐constant δ ¹⁸O_water composition allowed selection of an ‘equilibrium’ palaeotemperature equation that best replicated actual temperatures. This study, as in some previous studies, found that just two published formulas for water temperature calculation from equilibrium calcite δ ¹⁸O compositions were appropriate for the River Piedra, where tufa deposition rates are high, with means between 5·6 mm and 10·8 mm in six months. The δ ¹⁸O_calcite in both the river and the pipe deposits essentially records the full actual seasonal water temperature range. Only the coldest times (water temperature <10°C), when calcite precipitation mass decreased to minimum, are likely to be unrepresented, an effect most noticeable in the pipe where depositional masses are smaller and below sample resolution. While kinetic effects on δ ¹⁸O_calcite‐based calculated water temperature cannot be ruled out, the good fit between measured water temperature and δ ¹⁸O_calcite‐calculated water temperature indicates that temperature is the principal control. Textural and deposition rate variability between the river and pipe settings are caused by differences in flow velocity and illumination. In the river, calcification of growing cyanobacterial mat occurred throughout the year, producing composite dense and porous laminae, whereas in the pipe, discontinuous cyanobacterial growth in winter promoted more abiogenic calcification. High‐resolution δ ¹⁸O_calcite data from synchronous pipe and river laminae show that reversals in water temperature occur within laminae, not at lamina boundaries, a pattern consistent with progressive increase in calcite precipitation rate as cyanobacterial growth re‐established in spring.     

Carbon and Oxygen Isotopic Signatures in Albian-Danian Limestones of Cauvery Basin, Southeastern India

The Albian-Danian limestones of Cauvery Basin show a wide range of d¹³C and d¹⁸O values (–13.2 to +1.1% and –9.0 to –2.5%, respectively). The cement samples show negative carbon and oxygen isotope values (–18.9 to –3.9% and –9.0 to –4.3%, respectively). The petrographic study reveals the presence of algae, molluscs, bryozoans, foraminifers and ostracods as major framework constituents. The limestones have microspar and equant sparry calcite cements. The pore spaces and vugs are filled with sparry calcite cement. The bivariate plot of d¹³C and d¹⁸O suggests that most of the samples fall in the freshwater limestone and meteoric field, while few samples fall in the marine limestone and soil calcite fields. The presence of sparry calcite cement, together with negative carbon and oxygen isotope values, indicates that these limestones have undergone meteoric diagenesis.     

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

Tufa 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.     

Sedimentology and geochemistry of fluvio-lacustrine tufa deposits controlled by evaporite solution subsidence in the central Ebro Depression, NE Spain

The Urrea de Jalón tufa deposits constitute the 20‐ to 50‐m‐thick caprock (0·3 km²) of an isolated mesa. They disconformably overlie horizontal strata of the Tertiary Ebro Basin (NE Spain), which contains a thick succession of lacustrine gypsum and marls, followed by limestones, marls and, locally, fluvial sandstones and mudstones. The tufa deposits show a complex, large‐scale framework of basin‐like structures with centripetal dips that decrease progressively from the base to the top of the tufa succession, and beds that thicken towards the centre of the structure (cumulative wedge‐out systems). These geometries reveal that the tufa deposits were affected by differential synsedimentary subsidence. Distinct onlapping depressions reflect time migration of the subsiding areas. The studied carbonates are composed mostly of low‐Mg calcite, with minor quartz. Some samples have anomalously high contents of Fe, Mn and Ba that may exceed 1% (goethite, haematite and barite are present). Carbonate facies are: (a) macrophyte encrustation deposits; (b) bryophyte build‐ups; (c) oncolite and coated grain rudstones; (d) non‐concentric stromatolite‐like structures; (e) massive or bioturbated biomicrites; and (f) green and grey marls. Facies a and c show a great variety of microbial‐related forms. These facies can be arranged in dm‐ to 2‐m‐thick vertical associations representing: (i) fluvial–paludal sequences with bryophyte growths; (ii) pond‐influenced fluvial sequences; and (iii) lacustrine–palustrine sequences. The Urrea de Jalón tufa deposits formed in a fluvio‐lacustrine environment that received little alluvial sediment supply. Isotope compositions (δ¹³C and δ¹⁸O) reveal meteoric signatures and accord with such a hydrologically open system of fresh waters. The Fe, Mn and Ba contents suggest an additional supply of mineralized waters that could be related to springs. These would have been discharge points in the Ebro Depression of a regional aquifer of the Iberian Ranges. Rising groundwater caused the solution of the underlying evaporites and the synsedimentary subsidence of the tufa deposits.     

Electron backscatter diffraction documents randomly orientated c-axes in moonmilk calcite fibres: evidence for biologically induced precipitation

Moonmilk represents a conspicuous but controversially discussed precipitate of cave settings. Here, new electron backscatter diffraction microscopic and petrographic evidence on the origin of moonmilk calcite is presented. Calcite fibres in a moonmilk mat from the walls of an active cave (Tunnel‐Cave) in Devonian massive limestones in the northern part of the Rhenish Massif (Germany) show orientations of the crystallographic c‐axis independent of the morphological fibre orientation. This observation and the morphology of the fibres are in agreement with microbially induced, as opposed to abiogenic, calcite precipitation. Carbon‐isotope data are higher (1·9 to 3·3‰) than those commonly measured regionally in speleothem calcite (stalagmites, stalactites and flowstones), an observation attributed to kinetic effects. In combination, these findings add independent evidence to the complex interplay of inorganic and bio‐induced carbonate precipitation in cave environments.     

Selective blackening of bioclasts via mixing‐zone aragonite neomorphism in Late Triassic limestone,Zlatibor Mountains,Serbia

A peculiar facies of the Norian–Rhaetian Dachstein‐type platform carbonates, which contains large amounts of blackened bioclasts and dissolutional cavities filled by cements and internal sediments, occurs in the Zlatibor Mountains, Serbia. Microfacies investigations revealed that the blackened bioclasts are predominantly Solenoporaceae, with a finely crystalline, originally aragonite skeleton of fine cellular structure. Blackening of other bioclasts also occurs subordinately. Solenoporacean‐dominated reefs, developed behind the platform margin patch‐reef tract, were the main source of sand‐sized detritus. The blackened and other non‐blackened bioclasts are incorporated in automicrite cement. Radiaxial fibrous calcite cements in the dissolutional cavities are also black, dark grey or white. Reworked black pebbles were reported from many occurrences of peritidal deposits; in those cases, the blackening took place under pedogenic, meteoric diagenetic conditions. In contrast, in the inner platform deposits of the Ilid?a Limestone, the blackening of bioclasts occurred in a marine–meteoric mixing‐zone, as indicated by petrographic features and geochemical data of the skeleton‐replacing calcite crystals. Attributes of mixing‐zone pore waters were controlled by mixing corrosion, different solubility of carbonate minerals and microbial decomposition of organic matter. In the moderate‐energy inner platform environment, large amounts of microbial organic tissue were accumulated and subsequently decomposed, triggering selective blackening in the course of early, shallow burial diagenesis. The δ¹⁸O and δ¹³C values of the mixing‐zone precipitates and replacive calcite do not produce a linear mixing trend. Variation mainly resulted from microbial decomposition of organic matter that occurred under mixing‐zone conditions. The paragenetic sequence implies cyclic diagenetic conditions that were determined by marine, meteoric and mixing‐zone pore fluids. The diagenetic cycles were controlled by sea‐level fluctuations of moderate amplitude under a semi‐arid to semi‐humid climate.     

Abiotic versus biotic controls on the development of the Fairmont Hot Springs carbonate deposit, British Columbia, Canada

The relict Fairmont Hot Springs deposit, formed largely of carbonates, covers an area of 0·5 km², and is up to 16 m thick. The triangle‐shaped discharge apron, which broadens down‐valley, is divided into a proximal part with beds dipping at <10° and a distal part with beds dipping at 10° to 15°. The deposit is formed of the: (1) Basal Macrophyte; (2) Lower Carbonate; (3) Middle Clastic; (4) Upper Carbonate; and (5) Upper Clastic Sequences. Two charcoal samples embedded in the Lower Carbonate Sequence yielded dates of 8690 ± 90 and 8270 ± 70 cal yr bp , indicating that much of the deposit formed post‐glacially during the Early to Mid‐Holocene. Deposit aggradation ceased in the Mid to Late Holocene when the Fairmont Creek valley was incised. The Lower and Upper Carbonate Sequences, which are the thickest sequences, are composed of nearly equal parts of travertine (abiotic) and tufa (biotic), with feather dendrite travertine, radiating dendrite travertine and stromatolite tufa dominating. Competition between calcite precipitation rates and biotic growth rates controlled the distribution of tufa and travertine across the discharge apron. Calcite and biotic growth rates were controlled largely by flow velocity across the apron which, in turn, was controlled by topography and regular fluctuations in spring water discharge volume. During times of high spring discharge, slow sheet flow over the proximal part of the apron promoted stromatolite growth, whereas fast, turbulent flow on the distal part of the apron induced rapid feather dendrite formation. During times of low spring discharge, quiescent, shallow evaporative pools, conducive to radiating dendrite formation, formed on the proximal part of the apron, whereas slow flow on the distal part promoted stromatolite growth. Facies with high calcite supersaturation experienced rapid abiotic dendrite growth that precluded most biotic growth.     

Quartz and sheet-silicate preferred orientations of low symmetry, Pikikiruna Schist, New Zealand

The Pikikiruna Schist of Nelson, New Zealand, displays a fabric in which the patterns of quartz c-axes, the poles to planes of inequidimensional quartz grains, and the statistical maxima of poles to sheet-silicate cleavages are oblique to each other. The quartz c-axes patterns consist of type-1 and type-2 crossed-girdles. The triclinic fabric can be explained in terms of one complex rotational deformation of an essentially plane strain nature. Rotation of approximately 90° about the intermediate strain-axis was combined at a late stage with subsidiary rotations about the extension axis. The quartz c-axes patterns can be related to the kinematic framework rather than the finite strain-axes. On the other hand, the dimensional quartz preferred orientation may be closely related to the finite strain-axes, though the quantity of strain can not be measured because of recrystallisation.     

The concretions of the Bearreraig Sandstone Formation: geometry and geochemistry

The sandbodies of the Bearreraig Sandstone Formation (Inner Hebrides, UK) are cemented by two generations of calcite. The first generation, an inhomogeneous ferroan calcite (0.05?3.28 mol% FeCo₃) formed during sulphate reduction (δ¹³C =?24 to ?32%o PDB) in marine porewaters (δ¹⁸O of cement from ?1 to ?4%o PDB) at very shallow burial depths (a few centimetres). These cements are rare but form millimetre-scale clusters of crystals which acted as nuclei to the later, concretionary cements. The second generation of cements are more homogeneous ferroan calcites (mean 1?58% mol% FeCo₃) which evolve to progressively higher Fe/Mg ratios. They are sourced by shell dissolution (δ¹³C of cement from +1 to ?3%o PDB) into meteoric (δ¹⁸O of cement from ?6 to ?10%o PDB) or mixed marine meteoric waters (δ¹⁸O of cement from ?4 to ?6%o SMOW). These were introduced into the formation either during Bathonian times as a freshwater lens, or, subsequent to partial inversion, by confined aquifer flow. Corroded feldspars within the concretions suggest that an interval of at least 8 Ma separated the deposition of the sediments from the onset of concretion growth. Abundant concretions are preferentially developed at certain horizons within the sandbodies, where the early generation of ferroan calcite cements provided nuclei. The latter formed close to the sediment-water interface, the concentration of cement within the sediment being related to sedimentation rate. The relatively high concentrations of the first generation of cement, upon which the concretionary horizons are nucleated, formed during periods of minimal sedimentation.     

Low-Mg calcite marine cement in Cretaceous turbidites: origin, spatial distribution and relationship to seawater chemistry

Lower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low-Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable-isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole-rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ¹⁸O value of+0·5 to -1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ¹³C=0 to -2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral-controlled transformation. Electron probe microanalyses demonstrate that early calcite cement contains <2 mol% MgCO₃, despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg²⁺/Ca²⁺ ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg²⁺/Ca²⁺ ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late-stage ferroan, ¹⁸O-depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial. Our results emphasize that bioclast dissolution and low-Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis.     

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

Freshwater 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.     

Pleistocene speleothems of Mallorca: implications for palaeoclimate and carbonate diagenesis in mixing zones

The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ¹³C and δ¹⁸O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO₂ at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ¹³C and δ¹⁸O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ¹⁸O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ¹⁸O and highly variable δ¹³C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.     

Shallow-marine carbonate cementation in Holocene segments of the calcifying green alga Halimeda

Early-diagenetic cementation of tropical carbonates results from the combination of numerous physico-chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, diagenetic cements that developed early in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. When lithified sediment samples are studied, this complicates the recognition of processes involved in early cementation, and their geological implications. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of polished samples, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant High-Mg calcite (3.9 to 7.2 Mol% Mg), bladed Low-Mg calcite (0.4 to 1.0 Mol% Mg) and mini-micritic Low-Mg calcite (3.2 to 3.3 Mol% Mg). The co-existence and consecutive development of fibrous aragonite and equant High-Mg calcite results initially from the flow of oversaturated seawater along the aragonite template of the Halimeda skeleton, followed by an adjustment of cement mineralogy towards High-Mg calcite as a result of reduced permeability and fluid flow rates in the pores. Growth of bladed Low-Mg calcite cements on top of etched substrates of equant High-Mg calcite is explained by shifts in pore water pH and alkalinity through microbial sulphate reduction. Microbial activity appears to be the main trigger for the precipitation of mini-micritic Low-Mg calcite as well, based on the presumable detection of an extracellular polymeric matrix during an early stage of mini-micrite Low-Mg calcite cement precipitation. Radiocarbon analyses of five Halimeda segments furthermore indicate that virtually complete intragranular cementation in the marine phreatic environment with thermodynamically/kinetically controlled aragonite and High-Mg calcite takes place in about 100 years. Collectively, this study shows that early-diagenetic cements are highly diverse and provides new quantitative constraints on the rate of diagenetic cementation in tropical carbonate factories.     

Influence of microbial photosynthesis on tufa stromatolite formation and ambient water chemistry, SW Japan

Photosynthetic influences on tufa stromatolite formation and ambient water chemistry were investigated at two well-studied streams depositing tufa in Southwestern Japan (Shirokawa and Shimokuraida). The tufa stromatolites in both streams are composed of fine-grained calcite crystals showing annual lamination, and colonized by a number of filamentous cyanobacteria as well as non-phototrophic bacteria. Microelectrode measurements of pH, O₂, and Ca²⁺ near the stromatolite surface (the diffusive boundary layer; DBL) revealed that the investigated tufa stromatolites are formed by photosynthesis-induced CaCO₃ precipitation (PCP): cyanobacterial photosynthesis induces calcite precipitation under light conditions, while respiration of cyanobacteria and non-phototrophic bacteria inhibits precipitation in the dark. The bulk water chemistry at the lower sites of the investigated streams showed the daytime decreases of Ca²⁺ concentration and alkalinity that was expected for significant influence of PCP, while the other expected change, increased pH, was not observed. In order to examine this discrepancy, a novel approach using semi-in situ microelectrode measurements was applied to perform precise quantitative calculations. The calculation results demonstrated that the observed Ca²⁺ concentration and alkalinity decreases were caused by PCP, and that the concomitant pH increase was expected to be under the detection level of a conventional pH meter. Although the amount of PCP is supposed to be significantly affected by light intensity, observations in Shimokuraida revealed that the amount of PCP on cloudy day nonetheless accounted for about 80% of that on sunny day. Despite the significant role of PCP for tufa stromatolite formation, PCP accounted for only about 10% of the precipitated calcite in the investigated streams, which indicates that tufa stromatolites, the characteristic deposits in the streams, are responsible for only a small portion of calcite precipitation, and the rest is considered to precipitate inorganically at biofilm-free substrates.     

Changes in carbon and oxygen isotope composition during limestone diagenesis

The calcite fossils of the Derbyhaven Beds, Isle of Man, have δ¹³C values (+ 1·8 PDB) similar to modern, shallow-water marine skeletons, but the δ¹⁸O values (?6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re-equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ¹⁸O of ?6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3-8 to + 1-2‰. and oxygen from ? 2-6 to ? 12-4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault-controlled dolomite, and is terminated by vein-filling calcite and dolomite. The younger calcite, interpreted as a near-surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ¹³C ? 6-8‰.     

Fabrication of synthetic calcite–muscovite rocks with variable texture — An analogue to cataclasite fabrics?

A series of large diameter calcite–muscovite aggregates has been prepared from calcite and muscovite powders, in order to gain a better understanding of how texture develops in impure carbonate rocks. The development of the microstructure and the crystallographic preferred orientation (CPO, texture) during the preparation process is described. The synthetic rocks have been fabricated from powders of calcite and muscovite by uniaxial cold-pressing at loads up to 400 MPa and subsequent hot isostatic pressing (HIPping) at pressures of 150 to 170 MPa and a temperature of 670 °C. The resulting textures and microstructures are homogeneous throughout the samples. The calcite CPO is generated by rigid body rotation and twinning during cold-pressing and is not significantly altered by recrystallization during HIPping. Grain growth during HIPping is observed in pure calcite samples, but is inhibited through high porosity and the presence of muscovite in the mixed aggregates. The preferred orientation of the calcite c-axes is found to increase with increasing uniaxial cold pressure, and to be independent of the muscovite content. The magnetic bulk susceptibility of the starting material has been changed by the formation of ferromagnetic impurities during fabrication. Comparison of the samples to natural calcite fabrics from fault zones show the potential of the experiments and fabric analyses presented to analyze and to better understand the deformation mechanisms of fault zones.     

Climatic change recorded by stable isotopes and trace elements in a British Holocene tufa

Combined stable isotope (δ¹⁸O and δ¹³C) and trace element (Mg, Sr) geochemistry from bulk tufa calcite and ostracod shell calcite from an early Holocene British tufa reveal clear records of Holocene palaeoclimatic change. Variation in δ¹⁸O is caused principally by change in the isotopic composition of Holocene rainfall (recharge), itself caused mainly by change in air temperature. The δ¹³C variability through much of the deposit reflects increasing influence of soil‐zone CO₂, owing to progressive woodland soil development. Bulk tufa Mg/Ca and Sr/Ca are controlled by their concentrations in the spring water. Importantly, Mg/Ca ratios are not related to δ¹⁸O values and thus show no temperature dependence. First‐order sympathetic relationships between δ¹³C values and Mg/Ca and Sr/Ca are controlled by aquifer processes (residence times, CO₂ degassing and calcite dissolution/reprecipitation) and probably record intensity of palaeorainfall (recharge) effects. Stable isotope records from ostracod shells show evidence of vital effects relative to bulk tufa data. The ostracod isotopic records are markedly ‘spiky’ because the ostracods record ‘snapshots’ of relatively short duration (years), whereas the bulk tufa samples record averages of longer time periods, probably decades. The δ¹⁸O record appears to show early Holocene warming, a thermal maximum at ca. 8900 cal. yr BP and the global 8200 yr BP cold event. Combined δ¹³C, Mg/Ca and Sr/Ca data suggest that early Holocene warming was accompanied by decreasing rainfall intensity. The Mg/Ca data suggest that the 8200 yr BP cold event was also dry. Warmer and wetter conditions were re‐established after the 8200 yr BP cold event until the top of the preserved tufa sequence at ca. 7100 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.