Allan Pentecost

Stable carbon and oxygen isotope composition of calcites associated with modern freshwater cyanobacteria and algae

Abstract The composition of carbon and oxygen stable isotopes was determined for calcite associated with the freshwater cyanobacteria Homoeothrix Crustacea, Phormidium incrustatum, and Rivularia haematites and the green alga Gongrosira incrustans in a UK travertine‐depositing stream. The δ13C values of Rivularia calcites deposited in summer were significantly higher than those deposited during winter. This was interpreted as the result of photosynthetic activity within colonies. The δ13C values were similar for the three cyanobacterium species studied, and colonies of Homoeothrix sampled over a 531‐m length of stream showed progressive downstream enrichment of 13C (2.0‰). This resulted from CO2 degassing of the downstream water, augmented during summer by bryophyte photosynthesis. Subsequent deposition of calcite takes place irrespective of photosynthetic activity of the algae and may blur the isotopic characteristics in older colonies and fossil assemblages.

Geochemistry of carbon dioxide in six travertine-depositing waters of Italy

Abstract The chemical compositions of six travertine-depositing hot spring waters in Italy are described with emphasis on the carbon dioxide system. All springs contained high concentrations of CO 2 (>20 mM l −1 ) with equilibrium partial pressures well above those which could have been formed in contact with a soil atmosphere. After surfacing, the CO 2 is rapidly lost to the atmosphere, with evasion rates close to the springs ranging from 0.45–4.41 mM m −2 s −1 . Partial pressures of C02 showed an exponential decline with distance, which is consistent with the static film model where temperature and turbulence are constant. Downstream CO 2 transfer coefficients, which ranged from 66 to 360 cm h −1 were consistent with moderately turbulent flow, however, there was no correlation between turbulence, measured as the mean shear stress, and CO 2 evasion rate. The channels investigated had all been modified by man and most possessed even widths and gradients. All waters became increasingly supersaturated with aragonite and calcite downstream and both of these minerals were present in fresh travertine deposits. The supersaturation was driven almost exclusively by gas evasion. Comparison of daytime and nighttime evasion rates demonstrated that photosynthetic activity was an insignificant source of CO 2 flux in the reaches investigated. Carbon dioxide evasion is therefore primarily responsible for the supersaturation and probably also the deposition of travertine at these sites. The CaCO 3 content of the travertines ranged from 91.3 to 96.0 wt% with 1.7–4.1% CaSO 4 , traces of organic matter and acid-insoluble minerals.

The formation of travertine shrubs: Mammoth Hot Springs, Wyoming

The structure and microbiology of active travertines is described from Canary and Minerva springs, with emphasis on ‘shrubs’ growing in terracette pools. These dendritic growths of aragonite consist of intricately branched sprays containing thousands of radiating needles. Shrub microstructure could be explained by the principle of ‘Keimauslese” and the preferential elongation of sharp protuberances in a rapidly depositing environment. The shrubs, and other active travertines, contain unicellular and filamentous bacteria. Estimates of total bacteria numbers ranged from 0.6−1.7 × 10 5 mm −3 but biomass was low, and always less than 1% of the travertine by weight. No evidence was found to indicate that bacteria played a role in shrub growth or morphology, but crystal trapping on bacterial strings may influence travertine fabrics on cascades. The shrubs are considered to have developed by inorganic processes, in hot spring waters supersaturated with aragonite.

British travertines: a review

A review of British Quaternary travertines is provided, including details of 159 sites presented as a cross-referenced coded list with a full bibliography. Almost all of these travertines were formed by the degassing of calcium bicarbonate waters. The ‘carrier’ CO 2 originated from the soil and epigean atmospheres. At most sites, the travertine is composed of calcite as micrite or microspar, but aragonite occurs in the thermal waters of Bath Spa. The chemical composition of 20 samples gave an average CaCO 3 content of 94.2% for active sites and 91.6% for inactive sites. Levels of Fe, Mg, Mn, P and Sr are also reported. Stable carbon and oxygen isotope compositions gave values consistent with the degassing of limestone waters in contact with a soil atmosphere. Some evidence indicated that photo-synthetic activity could also influence travertine deposition. Bryophytes and algae in particular, can influence travertine fabrics through the trapping and binding of calcite. The median δ 13 C PDB was −11.5‰. The British travertines have been classified into seven fairly broad categories: (1) paludal deposits; (2) cascades; (3) barrages; (4) stream/spring crusts; (5) lake crusts; (6) cemented rudites; and (7) clastic deposits. Spring/stream crusts and paludal deposits are the most common types of active and inactive travertines, respectively. Further statistics on deposit size, relation to solid geology and age are presented. Most sites are Holocene in age but the available dates do not point to a specific period of high travertine deposition, even though the Atlantic period (6000–7000 bp ) is considered to represent such a time. Only a handful of travertines are known from the Pleistocene.

One day in the life of a stream—a diurnal inorganic carbon mass balance for a travertine‐depositing stream (waterfall beck, Yorkshire)

Abstract A carbon mass balance for photosynthesis, carbonate precipitation, out‐gassing, and respiration was calculated for a stream in Yorkshire, UK, from diurnal variations in total dissolved inorganic carbon (TDIC) and its isotopic composition. Complementary measurements of the photosynthetically available radiation, water temperature, pH, and dissolved calcium were taken at the springhead (site A) and downstream (site B). The difference in carbon isotope composition between the lower station and the springhead increases from a nighttime minimum of 3.4%oPDB to a daytime maximum of 7.1%o as a result of the biological and chemical processes affecting the dissolved inorganic carbon pool. The diurnal carbon balance input consisted of 194 mol carbon that entered via the spring (accounting for 94.6% of the budget) and 5.4% that was added by aquatic plant respiration. Of the output, 10.0% was fixed by photosynthesis, 12.2% was precipitated as CaCO3, and 9.4% was outgassed over 24 h. At midday 17.8% was fixed ...

Stable isotopes in charophyte incrustations: relationships with climate and water chemistry

Abstract Age gradients of stable C and O isotopes were found in modern Chara hispida calcite incrustations allowing seasonal changes in water chemistry to be observed. Gradients resulted from apical calcification in growing charophytes. In a regional study of 12 species and varieties of charophytes the carbonate δ13C differed significantly with ‘low value’ and ‘high value’ species such as Chara globularis and C. baltica, respectively. Samples from lakes and other stagnant waters had significantly higher δ13C values than those sampled from flowing water. The differences were related mainly to photosynthesis. Charophyte δ18O (carbonate) was not significantly different between species but water chemistry was important for brackish water Lamprothamnium papulosum. The overall δ18O (carbonate) was positively correlated with groundwater δ18O and it is suggested that more reliable environmental information is available from δ18O than δ13C in charophyte marls. The mean charophyte carbonate δ13C and δ18O was −5.7‰ and −6.4‰ VPDB, respectively. Environmental temperature estimates suggested that at some sites the carbonate was deposited close to isotopic equilibrium, but in others a slight to moderate disequilibrium was indicated.

Carbonate chemistry of surface waters in a temperate karst region: the southern Yorkshire Dales, UK

Abstract A detailed study of surface water chemistry is described from an important limestone region in northern England. Major ions and pH were determined for 485 sites (springs, seeps, streams, rivers and lakes) during summertime. The saturation state of the waters with respect to calcite was determined as the calcite saturation ratio (Ω). An unexpectedly large number of samples were found to be supersaturated (65.5% of the 268 km of watercourses surveyed). As a consequence, several streams entering major cave systems were incapable of further limestone solution, at least during periods of low flow. Many waters were supersaturated from their source and some deposited travertine. A significant negative correlation was found between spring discharge and both (Ω) and pH. Supersaturation was caused primarily by atmospheric degassing, with some contribution from aquatic plant photosynthesis. The median total dissolved inorganic carbon and Ca concentrations were 2.49 and 1.35 millimoles 1 −1 respectively. Calcium originated exclusively from limestone, and carbon dioxide mainly from the soil and dissolved limestone. South facing catchments provided springwaters with significantly higher levels of TDIC and Ca when compared with north facing catchments. The study suggests that acid rain made a measurable contribution to limestone dissolution. Carboniferous limestone denudation rates were estimated as 54 to 63 m 3 km −2 a −1 (54 to 63 mm 1000 years −1 ). About 50% of the Mg came from limestone and the remainder, together with most K, Na, SO 4 and Cl from precipitation. Concentrations of dissolved nutrients were low, medians for NO 3 , NH 4 , total PO 4 and SiO 3 were 24 μmol, 1.4 μmol, 0.64 μmol and 15.5 μmol 1 −1 respectively. The concentration of a further 23 trace elements was determined.

Calcification and photosynthesis in Corallina officinalis L. using the 14CO2 method

Rates of calcification and photosynthesis were highest in the apical intergenicular segments of Corallina. Rates were lower in the penultimate segment and fell to steady values in the 5th segment. Differences in the dry weight of the five ultimate segments showed a positive correlation with differences in photosynthetic rate. Calcification rates were variable but showed a positive correlation with photosynthetic rates. In apical segments the mean ratio of photosynthetic 14CO2 uptake to 14CO2 incorporation into calcite (Cr) was 1·03. Calcification in the dark was 30–50% of the light saturated rate and probably resulted from the combination of a delayed light dependent reaction and a physical absorption process.

Phototrophic microorganisms of the Pamukkale travertine, Turkey: Their distribution and influence on travertine deposition

The travertines at Pamukkale contain a diverse assemblage of phototrophs: 17 species of cyanobacteria, 16 diatoms, and 5 Chlorophyceae. Two communities were recognized on the active travertines: (1) surficial mats dominated by filamentous cyanobacteria, particularly Lyngbya (Phormidium) laminosum forming soft weakly mineralized layers to 10 mm thick, and (2) a predominantly endolithic assemblage, also dominated by cyanobacteria developing 2–5 mm below the travertine surface. The distribution of these communities is determined largely by water flow and the de‐greee of desiccation. Two further communities are briefly described from nondeposit‐ing areas. Most of the active travertine consists of alternating layers of micrite and sparite 0.25–0.75 mm in thickness, which probably result from short‐term fluctuations in water flow rather than diel events (photosynthesis, temperature). The presence of needle‐fiber calcite in surface samples suggests that evaporation of water may play some part in travertine forma...

Equilibrium and disequilibrium stable isotope effects in modern charophyte calcites: implications for palaeoenvironmental studies

Abstract Stable isotopes in fossil charophyte calcite may record palaeoenvironmental information but at present, very little is known about isotopic records in modern Chara stem encrustations or gyrogonites. This problem is addressed with isotopic data from present-day Chara from a wide spread of freshwater environments in Britain and Ireland. δ 13 C values of stem encrustations from low water flow environments (lakes, ponds, ditches) are more positive than their coexisting gyrogonite values, by between 0.6 and 3.0‰, reflecting microenvironmental metabolic effects caused by photosynthesis. In stronger water flows, where dissolved inorganic carbon is well mixed, stem encrustation δ 13 C values are close to equilibrium. δ 18 O values in Chara stem encrustations are not equilibrium values, being typically offset to isotopically lighter compositions by about 1.5‰ relative to equilibrium by kinetic effects during rapid calcification. Gyrogonite δ 18 O values are less negative than coexisting stem encrustation values by between 0.7 and 1.7‰. There is no clear evidence that gyrogonite δ 18 O or δ 13 C values are influenced by extensive disequilibrium but the data are difficult to interpret further without new experimental studies. Isotopic records in fossil lacustrine Chara marls (mainly stem encrustation calcites) should be interpreted with caution. Environmental records, essentially of relative changes in late summer water temperature or hydrology may be preserved, but these are unlikely to represent true equilibrium values. Gyrogonite calcification probably happens on a timescale of weeks, such that isotopic data represent ‘snapshots’ of the environmental conditions at the time of calcification. The timing of calcification in Chara may itself be variable depending on environment and temperature; it is a major unknown that currently hinders interpretation of isotopic data.