Baruch Spiro

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.

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 ...

Effects of the mineral matrix on the distribution of geochemical markers in thermally affected sedimentary sequences

The distribution of geochemical markers in bitumen fractions, both free and associated with minerals, was studied in bituminous chalk and black shale samples (Upper Cretaceous, Israel; Upper Cretaceous, West Greenland) affected by combustion metamorphism and the latter also by dyke intrusion. The bitumen fractions were obtained by stepwise extraction in conjunction with stepwise dissolution of the carbonate and subsequently silicate minerals. They show differences in their bulk composition and in the distribution of n-alkanes, alkylcyclohexanes, isoalkanes, isoprenoids, steranes and triterpanes. At low maturity, in the country rocks, the distribution of the biomarkers in the various bitumen fractions reflects mainly the source or organic matter and minerals (detrival vs autochthonous) associated during transport and sedimentation. With increasing maturation, processes take place like: (i) the formation of hydrogen-bonded associations of clay minerals with organic matter, (ii) the isomerisation of steranes which reaches different degrees; free bitumen>carbonate associated>silicate associated in the bituminous chalk, while bound > free in the black shales, (iii) the formation of isoalkanes and alkylcyclohexanes which occur only in the mineral-associated fractions. The particular effects of mineral groups on the geochemical markers during sedimentation, generation and primary migration necessitates the consideration of the composition of the mineral matrix for the interpretation of the geochemical markers. The generation of hydrocarbons during maturation probably involves the mobilization of bitumen from minerals and kerogen.

Kuwaiti dolocrete: petrology, geochemistry and groundwater origin

Abstract Near-surface sediments in southern Kuwait show extensive development of duricrusts. The host materials are siliciclastic sandstones of the post-Eocene Kuwait Group. Inland, the duricrusts are dominantly pedogenic calcrete (with some silcrete and gypcrete), whereas within 10–20 km of the coast, dolomite is the dominant duricrust mineral. Both these dolocretes and the inland calcretes display a similar maturation sequence in which carbonate-rich nodules develop and coalesce, carbonate progressively replacing and displacing detrital grains. The dolomite of the dolocretes forms mosaics of crystals typically 10–70 μm in size, varying from simple rhombs to spherulites. An intermediate morphology, named artichoke dolomite from its appearance in SEM, is particularly abundant. Authigenic palygorskite is associated with the dolomite. Dissolution of cores or zones within dolomite crystals has occurred. Calcite is present as sparry crystals (always post-dating dolomite) and is the expected precipitate from present-day soil and groundwaters. Chemical analyses of dolomite show highly negative values of δ 13 C (− to − 10.7‰ PDB) and δ 18 O varying from + 0.6 to + 3.3‰ PDB. The oxygen isotope values are interpreted as reflecting evaporation of a marine-based fluid. Manganese values of around 1000 ppm show that this fluid was reducing. Strontium data show variability reflecting mineral-fluid reactions. The general absence of metastable carbonates and presence of zoning in dolomite crystals suggests that meteoric dilution of seawater also occurred. Given that the duricrusts lack biogenic features, the light carbon isotope values are taken to indicate oxidation of seeping hydrocarbons. The dolocretes are interpreted as groundwater precipitates near the water table of a brackish water body formed at a time of higher relative sea level than today.

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.

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.

Geochemical and mineralogical variations in the upper Mercia Mudstone Group (Late Triassic), southwest Britain: correlation of outcrop sequences with borehole geophysical logs

The Mercia Mudstone Group comprises up to 1200m of predominantly red mudstones and siltstones laid down in rift-related basins during a period of regional subsidence. The mudstones are commonly dolomitic and contain horizons of sulphate and halide salts. Up to 150m of undifferentiated red mudstones of Norian and Rhaetian age (Late Triassic) were examined in coastal outcrops in Devon and Somerset, southwest Britain, and mineralogical and stable isotopic studies were carried out in order to identify any subtle changes in the succession. Gamma-ray measurements were taken at outcrop to provide correlation with published borehole logs in which the Mercia Mudstone Group has been subdivided on the basis of gamma-ray and sonic response. One of the borehole subdivisions was identified at outcrop within a sequence of otherwise undifferentiated red mudstones and was related to a transition in clay mineral assemblage and stable isotopic composition over 20 to 30 m of section. The red mudstones below this transition have a higher proportion of magnesian clay minerals and enriched carbonate oxygen isotope compositions, indicating deposition from Mg2+-rich marine-derived waters. Clay mineral assemblages in the succeeding red mudstones are dominated by illite, and oxygen isotopic compositions are relatively depleted, indicating a greater influence of K+-rich continental-derived waters.

S-isotope studies of shallow water, laminated gypsum and associated evaporites, Upper Permian, East Greenland

Abstract Three types of primary subaqueous gypsum occur in the Karstryggen Formation, Upper Permian of East Greenland: (1) planar laminated gypsum, having a narrow δ 34 S range of 10.2 to 10.4ℵ CDT, (2) algal laminated gypsum (9.2 to 12.0ℵ), and (3) bottom nucleated gypsum crystals (11.6 to 11.7ℵ). Type 3 was deposited from a residual brine in the basin while types 1 and 2 consist of seawater or mixed fresh water-seawater-derived sulphate with varying contribution of reoxidized sulphur originally deposited as sulphide or organic sulphur. Diagenetic nodular gypsum (7.3 to 10.8ℵ) was formed by redistribution of marine or mixed fresh water-seawater-derived brines which precipitated the laminated gypsum with contribution of sulphate originally precipitated as sulphide or organic sulphur in the organic-rich sediments.

Lavas con bitumen en el Cretácico Inferior de Copiapó, Región de Atacama, Chile: petroquímica e importancia metalogénica

espanolEl nivel de andesitas cretacicas (70 m de espesor promedio), emplazado en la parte alta (Formacion Pabellon) del Grupo Chanarcillo en la Region de Atacama, ha sido considerado por mas de 30 anos como un filon-manto. Las principales evidencias, que confirman que se trata de una sucesion de coladas de lava derramadas sobre un fondo marino somero, son: la perfecta concordancia con las calizas supra e infrayacientes a lo largo de decenas de km, la presencia de autobrechas y tobas entre flujos lavicos sucesivos, la ocurrencia de endofosiles en posicion de vida en lentes calcareos que engranan lateralmente con las lavas o se intercalan entre flujos sucesivos y el reconocimiento de, al menos, un centro de emision (Quebrada Cerrillos), donde las lavas alcanzan 300 m de potencia. Petrograficamente, las lavas son andesitas porfiricas (ocoitas), derivadas de magmas calcoalcalinos ricos en K, con una impronta geoquimica de intraplaca mas que de subduccion, las cuales fueron eruptadas a traves de una corteza continental delgada en el sector de trasarco del sistema arco-cuenca del Cretacico Inferior. La presencia de migrabitumen hidrotermal, en globulos, ocupando vesiculas primarias o en venillas, es un rasgo caracteristico de estas lavas, y su ocurrencia coincide con mineralizacion metalica, principalmente sulfuros de Cu. Los resultados de geoquimica organica demuestran que el bitumen se origino de materia organica marina, emplazandose en las lavas antes que las soluciones mineralizantes que dieron origen a los sulfuros de Cu. La asociacion bitumen-sulfuros de Cu, particularmente notable en la region de Copiapo, donde demarca un metalotecto regional, se ha observado, tambien, otros yacimientos a lo largo de la franja de lavas ocoiticas del Cretacico Inferior entre Copiapo e Illapel, sugiriendo asi que dicha asociacion tiene un control ligado a la evolucion global de la cuenca de trasarco. EnglishThe bitumen-rich andesitic horizon emplaced in the upper part (Pabellon Formation) of the Lower Cretaceous marine sequences in the Atacama Region (Chanarcillo Group) has been formerly considered a sill. The main evidence for interpreting this horizon as submarine lava flows includes: perfect concordance with both the overlying and underlying marine limestones along kilometers, the presence of autobreccias, tuffs and thin chert beds between the flows, the presence of endofossils in undisturbed positions within calcareous lenses that grade laterally to lavas or between successive beds, and the recognition of, at least, one emission center in Quebrada Cerrillos where the lavas reach 300 meters thick. Petrographically, the lavas are porphyritic andesites, derived from K-rich calc-alkaline magmas. Trace and minor element geochemistry point to an intraplate more than to a subduction related origin. These lavas are associated with a back-arc environment and were emplaced in a relatively thin continental crust during the Early Cretaceous. The presence of hydrothermal bitumen is a distinct feature of this horizon in many localities between Copiapo and Illapel. The bitumen occurs as the infilling of vesicles or veins, and is directly associated with metallic mineralisation, copper sulphides essentially. Organic geochemical results demonstrate that the bitumen originated from marine organic matter, migrating and being emplaced before the Cu-rich hydrothermal fluids. This association between bitumen and copper sulphides has become a regional metallotect in the Copiapo region. The same association has been found in numerous deposits along the Lower Cretaceous porphyritic lavas between Copiapo and Illapel, suggesting that this is a regional control associated to the evolution of the back-arc basin.