Decarboxylation Of Organic Acids Research Articles

Influence of coal rock on tight sandstone reservoirs in coal seam roofs: A case study of the lower jurassic in the Taibei sag, Turpan-Hami Basin, China

After shale gas, coalbed methane and gas hydrate, coal measure gas has become an important new unconventional resource, for which exploration to depths of >3000 m are taking place in China. However, owing to unusual geological conditions and as yet inadequate exploration technology the current level of exploration is low. The acidic water medium in coal-bearing strata affects the diagenesis of adjacent tight sandstone reservoirs, thereby influencing reservoir quality. However, the current understanding of the impact mechanism of coal seams on neighboring tight reservoirs is insufficient, constraining the further exploration of coal measure gas. In order to reveal the influence of coal rock on a tight sandstone reservoir in its roof, we study the coal-bearing strata of the Xishanyao Formation in the Taibei Sag of the Turpan-Hami Basin using cast thin sections, physical property tests, XRD analysis, carbon and oxygen isotopes, high-pressure mercury intrusion and low-temperature N2 adsorption. The carbonate cement in the sandstone reservoir of the coal seam roof was formed by the decarboxylation of organic acid. The organic acid, containing CO2 generated by coal rock hydrocarbon generation, has modified the tight sandstone reservoir of the coal seam roof via dissolution. This has improved the physical properties and pore throat structure of the tight sandstone reservoir close to the coal-sandstone interface. The amount of improvement extends to about 2 m into the sandstone. During the upward transport of fluid, the pre-existing carbonate cement dissolves and then precipitates, which causes the CO2 generated by the organic acid decarboxylation to exchange carbon with the carbon isotopes in the pre-existing carbonate cement. Due to the slow upward diffusion rate and limited diffusion distance of the CO2-bearing fluids, the carbonate cement exhibits gradually heavier δ13CPDB values with increasing distance from the coal-sandstone interface.

Authigenic calcite as a record of geologic fluids in siliciclastic rocks: Evidences from the Upper Permian Wuerhe Formation, Junggar basin, NW China

The identification of geologic fluids and related fluid–rock interactions during diagenesis is the subject of much research in sedimentary petrology. Authigenic calcite potentially provides a record of geologic fluids and it occurs heterogeneously in the Upper Permian Wuerhe Formation (P3w) in the Shawan Sag, Junggar Basin, which has a complex history of geologic fluid activity. This provides an ideal opportunity to study the effectiveness of authigenic calcite in tracing fluids. We conducted optical, cathodoluminescence (CL), and scanning electron microscopic observations, as well as the major and trace element and stable carbon and oxygen isotopes of authigenic calcite. The results show that three generations of calcite were precipitated in the P3w Formation, and the diagenetic fluid was affected to varying degrees by paleo-meteoric water and hydrocarbon-bearing fluids. During early diagenesis, diagenetic fluid with low Mn contents precipitated the amorphous early-stage calcite (dark red in CL images, MnO <1.5%, δ13C = −8.6‰ to 2.1‰, VPDB). Its carbon source was mainly meteoric CO2. During mesodiagenesis, the limited hydrocarbon emplacement during the Middle Jurassic enriched the pore fluids in Mn and 13C-depleted organic derived CO2, subsequently precipitating the late-stage sparry calcite I (orange in CL images with MnO of 2.5%–4% and δ13C of −14.5‰ to −8.1‰). The carbon in this calcite came from the dissolution of early-stage calcite and CO2 generated by decarboxylation of organic acids. During the Early Cretaceous, large-scale hydrocarbon charging occurred and the pore fluids were further enriched in Mn and organic derived CO2, eventually precipitating the late-stage sparry calcite II (bright yellow in CL images with MnO of >4% and δ13C of −25.7‰ to −14.9‰). Its carbon source was mainly CO2 produced by the decarboxylation of organic acids. The precipitation of abundant late-stage sC-depleted calcite suggests that the hydrocarbons were oxidized to organic acids in the reservoir. The two periods of hydrocarbon charging caused the dissolution of laumontite and the early-stage calcite, forming secondary minerals and dissolution pores, which increased the porosity and permeability of the rock. Therefore, authigenic calcite is a useful tracer of fluid properties, fluid–rock interactions, and alteration processes in petroliferous basins.

Botryoidal quartz as an abiotic signature in Palaeoarchean cherts of the Pilbara Supergroup, Western Australia

Chemically oscillating reactions (COR) are abiotic processes that generate self-repeating circularly concentric morphologies during decarboxylation of organic acids. The geometry and millimetre to centimetre size dimensions of laminated quartz botryoids recorded in the Palaeoarchean Apex and Strelley Pool chert horizons in Western Australia simulate the self-similar fractal patterns arising in vitro from COR using classical and modified reactants of the Belousov-Zhabotinsky (B-Z) reaction. The botryoidal patterns akin to those developed by the COR include circular concentric laminae, open-book features, microbialite- and stromatolite-like laminations, wavy and domal structures, and rosettes. The mineral composition and organic matter (OM) distribution of these objects indicates an origin from the early diagenetic decarboxylation of carboxylic acids. Pore-water alkalinity decrease is likely due to the decomposition of OM, the generation of carbonic acid, and the cessation of chemical wave diffusion. The chemical waves developed from primary circular oxidation spots, under solubility equilibrium with respect to silica could trigger diagenetic precipitation of quartz. The presence of Fe2+-bearing hematite in various botryoidal geometries in the Apex chert is an analogue of ferroin-derived Fe in B-Z solutions, whereas the presence of Fe and Zn sulphides in the Strelley Pool quartz botryoids is akin to sulphur redox intermediates in B-Z experiments. Correlated microscopy and Raman spectral analysis corroborate that the metamorphosed OM associated with these Palaeoarchean botryoids is indigenous and syngenetic with the host chert. OM in botryoidal quartz displays circular concentric laminations as well as gradients of its density. Accordingly, an abiotic model of COR for the diagenetic growth of the studied Apex and Strelley Pool quartz botryoids is proposed. Comparisons with the chemical compositions, geometric morphology and range of size dimensions of self-similar patterns have been the criteria for development of this model expression. The explored ancient botryoids can thus represent abiotic sedimentological signatures of carbon cycling.

Origin and Evolution of Geofluids in the Eocene Red-Bed Sandstones of the Dongying Depression, China

Fluid inclusion and petrographic study focused on authigenic quartz, annealed microfractures in quartz grains (AMF) and carbonate cement, was performed in red-bed reservoir sandstones from the first member of the Kongdian Formation and the lower fourth member of the Shahejie Formation (Shengli oilfield, East China). Both hydrocarbon and aqueous inclusions are present. Microthermometry and Laser Raman spectroscopes of liquid-rich two-phase inclusions showed that the pressure-correction value of aqueous inclusions is < 15°C. Differences in homogenization temperature and salinity occur between inclusions in quartz and carbonate cements. The liquid-rich two-phase aqueous inclusions were classified into four groups based on the salinity-homogenization temperature plots. The liquid-rich two-phase hydrocarbon inclusions were classified into two groups. The timing of the first-stage diagenetic fluid was before 31.3Ma. Fluids were from syn-depositional water and diagenetic fluids of clay minerals in interbedded mudstones. The timing of the second-stage diagenetic fluid was between 31.3 and 26.4Ma. Fluids were rich in organic acids and were accompanied by filling of small amounts of low maturity oil. The timing of the third-stage diagenetic fluid was between 26.4 and 21.4Ma. Fluids were mainly influenced by dehydration of gypsum-salt rocks and decarboxylation of organic acids. The timing of the fourth-stage diagenetic fluid was after 21.4 Ma. Fluids were influenced by organic acids and carbon acids and were accompanied by filling of mature oil after 10Ma. During the burial history, the red-bed reservoirs experienced the evolution process of early alkaline diagenetic fluids, early acid diagenetic fluids, late alkaline diagenetic fluids and late acid diagenetic fluids, which caused the alternate evolution of multiple alkaline and acid diagenetic environments.

Formation Time and Material Source of Carbonate Cements in Permian Sandstone in the Ordos Basin, Western of China

Abstract Carbonate cements are an important type of cement in Permian tight gas reservoirs in the Ordos Basin. Moreover, the distribution of carbonate cements in sandstone provides an important basis for studying the fluid mechanism and genesis of cements and a scientific basis for investigating the physical properties of reservoirs and the underground distribution of oil and gas. Based on the observations of thin sections and temperature measurements of fluid inclusions, this study concludes that the carbonate cements in the study area are dominated by calcite and ferrous calcite and divided into three phases, where the latter two phases are dominant. The material source of carbonate cements is obtained via the thermal simulations of gold tubes in source rocks, temperature of fluid inclusions in carbonate cements, composition of fluid inclusions in quartz overgrowth, and analysis of carbon and oxygen isotopes. Results show that phase I calcite may be directly precipitated from the supersaturation of pore water by Ca2+ and CO32−. The CO32− in the phase II calcite and ferrous calcite stems from the CO2 generated in the hydrocarbon generation process of organic matter, while the CO32− in phase III calcite stems from the CO2 generated in the decarboxylation of organic acids. Ca2+ is derived from the dissolution of early carbonate cements in sandstone and conversion of smectite into illite in mudstone. The Permian sandstone in the Yishan Slope at the center of the basin is close to the source rock, and the carbon isotopic value of the carbonate cements, which is dominated by organic carbon, ranges between −7.5‰ and −19.0‰. However, the Permian sandstone in the Tianhuan Depression at the edge of the basin is far away from the source rock, with its carbon isotope value ranging from −5.9‰ to −8.7‰, which is probably due to the insufficient supply of organic carbon and a small amount of early inorganic CO2 remains in the reservoir.

Influence of multiphase carbonate cementations on the Eocene delta sandstones of the Bohai Bay Basin, China

This study deciphers the origin and distribution of multiphase carbonate cementations and assesses their implications for reservoir quality of the Eocene delta hydrocarbon sandstones in the Bohai Bay Basin. Petrographic observation and stable isotopic analysis identified two phases of carbonate cementations: (1) eodiagenetic phase including non-ferroan calcite cement precipitated at 25–42 °C (δ13CVPDB: 1.0‰ to +5.7‰ and δ18OVPDB: 10.7‰ to −7.5‰); and (2) mesodiagenetic phase which includes ferroan calcite cement (δ13CVPDB: 11.8‰ to −5.7‰ and δ18OVPDB: 17.0‰ to −12.6‰) and ankerite cement (δ13CVPDB: 13.7‰ to −5.9‰ and δ18OVPDB: 17.1‰ to −12.4‰) precipitated at 90–134 °C. The eodiagenetic non-ferroan calcite cement that is mainly sourced from microbial methanogenesis of organic matter in adjoining mudrocks, extensively cemented at or near sandstone-mudstone contact. Mesodiagenetic ferroan carbonate cements are derived from predominantly thermal decarboxylation of organic acids and subordinate dissolution of eodiagenetic calcite cements in sandstones. These ferroan carbonates are preferentially concentrated at distances of 0.5–1.0 m from sandstone-mudstone contacts. Kinetic modelling reproduces evolution pathways of different diagenetic minerals in the Es4s sandstones (the upper part of the fourth member of the Eocene Shahejie Formation). More importantly, results of modelling reveal that the dissolution of non-ferroan calcite cements would not enhance total reservoir quality due to the precipitation of more stable ferroan calcite and ankerite cements. Therefore, the occurrence of multi-phase carbonate cementations at different sections of sandstones leads to significant destruction of porosity and reservoir heterogeneity during progressive diagenesis.

Multiple organic–inorganic interactions and influences on heterogeneous carbonate‐cementation patterns: Example from Silurian deeply buried sandstones, central Tarim Basin, north‐western China

AbstractThis study investigates the importance of multiple organic–inorganic interactions on heterogeneous carbonate cementation patterns in Silurian deeply buried sandstones, central Tarim Basin, north‐western China, to evaluate their effects on reservoir quality. Petrographic observations and mineral geochemistry identify two stages of carbonate cementation: (i) eogenetic, poikilotopic blocky calcite precipitated at 20 to 50°C (δ13CVPDB from −0.3‰ to +6.8‰; δ18OVPDB between −6.6‰ and −2.9‰) and dolomite precipitated at 40 to 62°C (δ13CVPDB from +1.9‰ to +5.3‰; δ18OVPDB between −6.2‰ and −2.1‰); and (ii) mesogenetic, isolated pore‐filling ankerite formed at 70 to 120°C (δ13CVPDB from −14.4‰ to −6.6‰; δ18OVPDB between −11.6‰ and −7.8‰). The eogenetic carbonates are predominantly distributed as carbonate‐cemented beds or concretions along sandstone–mudstone contacts and were derived mainly from microbial methanogenesis of organic matter and dissolution of carbonate minerals in adjacent calcareous mudstones. The mesogenetic ankerite cements occur predominantly as carbonate‐cemented beds, concretions and patches, and are more concentrated in the central sections of sandstone bodies. It is inferred that ankerite cements were sourced from dissolution of eogenetic calcite and dolomite via in situ generation of organic CO2 related to thermal decarboxylation of organic acids within sandstones. Kinetic modelling results coupled with petrographic observations illustrate that mesogenetic dissolution of eogenetic calcite and dolomite cements did not enhance total reservoir porosity due to re‐precipitation of ankerite cements on a very local scale within sandstone bodies. The development of extensively carbonate‐cemented geometries (beds, concretions and patches) created by different generations of carbonate cements derived from external sources, led to reservoir heterogeneity and significant destruction of sandstone reservoir quality especially during diagenesis. The results of this study demonstrate the importance of multi‐stage, intra‐formational mass transfer induced by concentration gradients during organic–inorganic interactions in mediating a variety of spatial patterns of carbonate cementation under different diagenetic regimes. Better insights of these organic–inorganic interactions in a coupled sandstone–mudstone system could improve predictive models of heterogeneous diagenetic alterations in deeply buried sandstones of similar origin.

Formation mechanisms of calcite cements in tight sandstones of the Jurassic Lianggaoshan Formation, northeastern Central Sichuan Basin

The characteristics and formation mechanism of calcite cements in the tight sandstone of the Jurassic Lianggaoshan Formation in the northeastern Central Sichuan Basin were analysed using petrographic and isotopic techniques. In the tight sandstone of the Lianggaoshan Formation, cements are mostly calcite and occur as poikilitic, pore-filling, fracture-filling and replacement of clastic particles. Contents of Al, Si, Fe and Mn in the poikilitic calcites are significantly less than that in the dissolution pore-filling and metasomatic calcites. Three stages (early, middle and late) of authigenic calcites correspond to temperature ranges of <60, 60–100 and ≥100 °C, respectively, with most calcite cements formed under lower temperature (<100 °C) conditions. The δ18O values of the early–middle authigenic calcites are in equilibrium with connate water, and the δ18O values of late calcites are depleted in 18O indicating equilibrium at higher temperatures. The early authigenic calcites precipitated in a relatively open system associated with CO2 from bacterial fermentation at an immature to low-mature stage, and a Ca2+- and alkaline-rich environment owing to hydration–hydrolysis and dissolution of silicate minerals during phase A of eodiagenesis. The middle–late authigenic calcites precipitated in a relatively closed system with CO2 from decarboxylation of organic acids and Ca2+ from dissolution of silicate minerals and transformation of clay minerals during phase B of eodiagenesis to mesodiagenesis. Calcite cements mainly occur in the medium and fine sandstones of sand flats and beach bars. Authigenic calcite dissolution is extremely weak, and calcite cementation is pore-space destructive.

Characteristics and formation mechanisms of tight oil: A case study of the Huahai Depression, Jiuquan Basin, Northwest China

The exploration of tight oil reservoirs has achieved a big success, especially in the Huahai Depression, Jiuquan Basin, China, but the characteristics and formation mechanisms of the tight oil reservoir are unclear. In this paper, a series of analyses including thin section, scanning electron microscope, gas chromatography–mass spectrometry, isotope, and fluid inclusions were taken to study the tight reservoirs of the Xiagou Formation (K1g) and Zhonggou Formation (K1z) in the Huahai Depression. The results show that the main factors influencing the porosity and permeability of the tight reservoir are the compaction and two periods of carbonate cementation. The second type of carbonate cementation was related to the decarboxylation of organic acids in parts of the study area. The tight oil accumulation periods for different formations are confirmed, during 115–102 Ma in K1g1, 114–97 Ma in K1g2, and 103–85 Ma in K1z1, respectively. The tight oil migrations are executed for only several meters vertically but hundreds of meters even kilometers laterally. Two kinds of formation mechanisms exist in the study area. The main formation pattern is the “hydrocarbon accumulated after reservoir had tightened,” which has occurred in most of the study area. The other pattern, the “hydrocarbon accumulated during reservoir was tightening,” is found in several places of the study area.

Quantitative characterization of organic acid generation, decarboxylation, and dissolution in a shale reservoir and the corresponding applications—A case study of the Bohai Bay Basin

In order to study the generation of organic acids and their dissolution effects on reservoirs, the type, yield and stability of common organic acids in the stratum were analyzed by laboratory simulations of the organic acid generation from kerogen and the decarboxylation of these organic acids. A kinetic model for the generation of organic acids and thermodynamic models of the decarboxylation of organic acids and the dissolution reaction were established. Acetic acid was used as an example to characterize the integration of the organic acid generation, decarboxylation, and dissolution under geological conditions. In addition, the process of dissolution pore development in shale reservoirs of various ages was reproduced. The results show that among the organic acids produced in the kerogen thermal simulation, the most stable is acetic acid with a decarboxylation temperature higher than 250 °C, followed by formic acid, and oxalic acid is the least stable. A large amount of organic acids can be produced in the middle thermal maturation stage (Ro = 0.89%–1.25%) of organic matter (OM). The modeling method combining kinetics and thermodynamics can achieve the quantitative characterization of the process of dissolution pore development. According to the calculation results, if the organic acids dissolve the reservoir as a whole, the porosity increment produced by the dissolution of the reservoir’s minerals is not significant. In other words, the dissolution of the reservoir by organic acids does not have an overall significance. However, if the organic acids only move along the predominant pathway (such as fractures) after their generation and dissolved the surrounding minerals of this channel, obvious dissolution can be produced. Comparison of the mineral composition and crack development of shale reservoirs indicates that organic acids could preferentially dissolve the reservoir cracks, so as to improve the porosity and permeability of the reservoir.

Phylogenomic Analysis Identifies a Sodium-Translocating Decarboxylating Oxidoreductase in Thermotogae.

Bacterial sodium-dependent decarboxylases were the first enzymes exemplifying sodium-dependent bioenergetics. These enzyme complexes couple decarboxylation of organic acids with the export of sodium ions via a special membrane subunit. In 711 representative prokaryotic genomes, we have analyzed genomic neighborhoods of the genes that code the membrane subunit of sodium decarboxylases. In representatives of Thermotogae, the operons with the gene of this subunit lack the genes of subunits that perform non-oxidative decarboxylation. Instead, these operons contain the genes of alpha- and delta-subunits of decarboxylating oxidoreductases of alpha-ketoacids. The genes of beta- and gamma-subunits of the decarboxylating oxidoreductases were found within the genomes of respective Thermotogae species as separate, two-gene operons. We suggest that the described two operons code together for sodium-translocating decarboxylating oxidoreductases capable of coupling oxidative decarboxylation of alpha-ketoacids with the export of sodium ions, which is a novel type of bioenergetic coupling.

Pore fluid evolution, distribution and water-rock interactions of carbonate cements in red-bed sandstone reservoirs in the Dongying Depression, China

The compositions, distribution and its interaction with rocks of the evolving pore fluids controls the distribution of carbonate cements and reservoir storage spaces. The reservoir quality of the red-bed sandstone reservoirs in the Dongying Depression was investigated by an integrated and systematic analysis including carbonate cement petrology, mineralogy, carbon and oxygen isotope ratios and fluid inclusions. The investigation was also facilitated by probing the mineral origins, precipitation mechanisms, pore fluid evolution and distribution, and water-rock interaction of carbonate cements and their influences on reservoir quality. Diagenetic-evolving fluids in the interbedded mudstones are the main source for the precipitation of calcite cements that completely fill the intergranular volume (CFIV calcite) with heavier oxygen and carbon isotopes. The ferro-carbonate cements in the reservoir sandstone are enriched in lighter carbon and oxygen isotopes. In addition to the cations released by the conversion of clay minerals in reservoirs, products of organic acid decarboxylation and the associated feldspar dissolution process provide important sources for such carbonate cementation. The carbon isotopes of CO2 and the oxygen isotopic composition of fluids equilibrated with the CFIV calcite, ferro-calcite, dolomite and ankerite cements indicate that the pore in the red-bed reservoirs experienced high salinity fluids, which evolved from the early-formed interbedded mudstones, through organic acid input and to organic acid decarboxylation. Pore fluids from nearby mudstones migrated from the edge to the centre of sandbodies, causing strong calcite cementation along the sandbody boundaries and forming tight cementation zones. Pore fluids associated with organic CO2 and acids and organic acid decarboxylation are mainly distributed in the internal portion of sandbodies, causing feldspar dissolution and precipitation of ferro-carbonate cements. The distribution of pore fluids caused the zonal distribution of carbonate cements in sandbodies during different periods. This may be advantageous to preserve the porosity of reservoirs as exemplified by the distribution of high-quality reservoirs in the red-bed sandbodies.

Origin of dolomite−ankerite cement in the Bravaisberget Formation (Middle Triassic) in Spitsbergen, Svalbard

The organic carbon (OC)−rich, black shale succession of the Middle Triassic Bravaisberget Formation in Spitsbergen contains scattered dolomite−ankerite cement in coarser−grained beds and intervals. This cement shows growth−related compositional trend from non−ferroan dolomite (0-5 mol % FeCO3) through ferroan dolomite (5-10 mol % FeCO3) to ankerite (10-20 mol % FeCO3, up to 1.7 mol % MnCO3) that is manifested by zoned nature of composite carbonate crystals. The 13 C (−7.3‰ to −1.8‰ VPDB) and 18 O (−9.4‰ to −6.0‰ VPDB) values are typical for burial cements originated from mixed inor− ganic and organic carbonate sources. The dolomite−ankerite cement formed over a range of diagenetic and burial environments, from early post−sulphidic to early catagenic. It reflects evolution of intraformational, compaction−derived marine fluids that was affected by disso− lution of biogenic carbonate, clay mineral and iron oxide transformations, and thermal de− composition of organic carbon (decarboxylation of organic acids, kerogen breakdown). These processes operated during Late Triassic and post−Triassic burial history over a tem− perature range from approx. 40C to more than 100C, and contributed to the final stage of cementation of the primary pore space of siltstone and sandstone beds and intervals in the OC−rich succession. Key wor ds: Arctic, Svalbard, Middle Triassic, cementation, petrography, geochemistry, carbon and oxygen isotopes.

Overtone-Induced Decarboxylation: A Potential Sink for Atmospheric Diacids

Atmospheric photochemistry induced by solar excitation of vibrational overtone transitions has recently been demonstrated to be of importance in cleaving weak bonds (in HO(2)NO(2)) and inducing intramolecular rearrangement followed by reaction (in H(2)SO(4)). Here, we propose another potentially important process: the decarboxylation of organic acids. To demonstrate this possibility, we have calculated the decarboxylation pathways for malonic acid and its monohydrate. The barrier to the gas-phase decarboxylation was calculated to be in the range 26-28 kcal/mol at the B3LYP/6-311++G(3df,3pd) level of theory, in good agreement with previous results. The transition state is a six-membered ring structure which is accessed via concerted O-H and C-C stretches; excitation of v(OH) > or = 3 of either one of the OH stretching modes is sufficient to supply the energy needed for the decarboxylation. A low-energy isomer of the malonic acid-water complex forms an eight-membered, multiply hydrogen bonded structure, bound by 3-6 kcal/mol, somewhat less stable than the lowest energy, six-membered ring isomer. Decarboxylation of such complexes uses water as a catalyst; the water accepts an acidic proton from one malonic acid group and transfers a proton to the carbonyl of the other acid group. The barrier for this process is 20-22 kcal/mol, suggesting that complexes excited to v(OH) > or = 2 possess sufficient energy to react. Using estimated absorption cross sections for the OH overtone transitions, we suggest that the overtone-induced decarboxylation of malonic acid and its water complex is competitive with wet deposition of the acid and with gas-phase reaction with OH for removal of the acid.

CO(2)-concentrating: consequences in crassulacean acid metabolism.

The consequences of CO(2)-concentrating in leaf air-spaces of CAM plants during daytime organic acid decarboxylation in Phase III of CAM (crassulacean acid metabolism) are explored. There are mechanistic consequences of internal CO(2) partial pressures, p(i)(CO(2)). These are (i) effects on stomata, i.e. high p(i)(CO(2)) eliciting stomatal closure in Phase III, (ii) regulation of malic acid remobilization from the vacuole, malate decarboxylation and refixation of CO(2) via Rubisco (ribulose bisphosphate carboxylase/oxygenase), and (iii) internal signalling functions during the transitions between Phases II and III and III and IV, respectively, in the natural day/night cycle and in synchronizing the circadian clocks of individual leaf cells or leaf patches in the free-running endogenous rhythmicity of CAM. There are ecophysiological consequences. Obvious beneficial ecophysiological consequences are (i) CO(2)-acquisition, (ii) increased water-use- efficiency, (iii) suppressed photorespiration, and (iv) reduced oxidative stress by over-energization of the photosynthetic apparatus. However, the general potency of these beneficial effects may be questioned. There are also adverse ecophysiological consequences. These are (i) energetics, (ii) pH effects and (iii) Phase III oxidative stress. A major consequence of CO(2)-concentrating in Phase III is O(2)-concentrating, increased p(i)(CO(2)) is accompanied by increased p(i)(O(2)). Do reversible shifts of C(3)/CAM-intermediate plants between the C(3)-CAM-C(3) modes of photosynthesis indicate that C(3)-photosynthesis provides better protection from irradiance stress? There are many open questions and CAM remains a curiosity.

Origin of siderite veins in the Western Carpathians: I. P– T– X– δ13C– δ18O relations in ore-forming brines of the Rudňany deposits

Siderite–barite veins of the Rudňany ore field have precipitated from NaCl–CaCl 2–H 2O±CO 2 solutions with CaCl 2 weight fractions between 0.05 and 0.38, total salinities between 17 and 35 wt.%, and up to 1.8 mol% CO 2. Homogeneous and heterogeneous trapping modes have been distinguished according to volumetric phase ratios of the fluid inclusions, which have been trapped at 175–205 °C in veiny siderite, at 200–230 °C in siderite and quartz from drusy cavities, and at 90–180 °C in the siderite–postdating quartz–sulfidic assemblage. A sequential re-equilibration technique has been developed to locate precisely vapour-saturated halite liquidus of individual brine inclusions, which normally homogenise by halite dissolution. Slopes of the vapour-unsaturated halite liquidi constrained by isochores of coexisting two-phase aqueous inclusions have been approximately three to five times steeper in natural polycomponent brines than those in the experimental NaCl–H 2O system. Fluid pressures between 1 and 2 kbar have been determined using this method in drusy quartz, associating with siderite crystals. Oxygen isotope variations (16.8–19.9‰ V-SMOW) in siderite result from a temperature increase from ∼175 to ∼230 °C, whereas nearly constant δ 13C values (−3.9‰ to −4.6‰ V-PDB) indicate absence of CO 2 devolatilisation. Carbon isotope composition of the parental fluid changed from −9.5‰ to −8.8‰ V-PDB, while the oxygen isotope ratios remained essentially fixed at 7.7±0.3‰ V-SMOV during formation of siderite. The calculated δ 18O fluid value is attributed to formation water enriched in 18O during isotopic exchange with crustal rocks at low fluid/rock ratios. The δ 13C fluid values probably correspond to a mixture of CO 2 from dissolved matrix carbonates and that liberated by thermal decarboxylation of organic acids (acetates), which normally occur in formation waters. Leachate chemistry data have corroborated high concentrations of CaCl 2 and MgCl 2 in the ore-forming brines. High Br concentrations are similar to halite-fractionated basinal brines. Ca 2+ and K + concentrations in excess of those in normal basinal brines reflect extensive cation exchange with surrounding low- to medium-grade metamorphites, coincidental with calcite and K-feldspar dissolution, dolomitisation, and albitisation of plagioclases. High pressures of the siderite-forming fluids (1–2 kbar) must be linked with a deep circulation (∼4–8 km) of the basinal brines derived primarily from marine water, but significantly modified by interaction with low- to medium-grade Paleozoic rocks at low fluid-to-rock ratios. An anti-clockwise crystallisation PT path inferred from fluid inclusion and stable isotope data is inconsistent with the siderite precipitation during extensional tectonic regime coincidental with Permian–Triassic rifting suggested in previous models. More likely, the progressively increasing fluid pressures and temperatures could be attributed to compression and crustal thickening triggered by Jurassic subduction in the Meliatic–Hallstatt oceanic suture. The superimposed quartz–ankerite–sulfidic ores have likely crystallised from brines expelled during compression associated with a continental collision during Early to Middle Cretaceous times.

Diurnal changes in chlorophyll a fluorescence, CO(2)-exchange and organic acid decarboxylation in the tropical CAM tree Clusia hilariana.

Crassulacean acid metabolism (CAM) plants are dependent on the organic acids that accumulate overnight in the vacuoles as a source of CO(2) during the daylight deacidification period, when stomata are closed and high irradiances generally prevail. We performed an integrative analysis of diurnal changes in gas exchange, chlorophyll fluorescence parameters and organic acid decarboxylation to understand the adjustments in photochemical and non-photochemical processes during the different CAM phases in Clusia hilariana Schlecht., a dominant tree species in the sandy coastal plains of southeastern Brazil. A linear relationship was obtained between the quantum yields of photochemical and non-photochemical quenching, irrespective of the CAM phase and prevailing irradiance. Degradation of malic and citric acids during the midday stomatal closure period could lead to potential CO(2) fixation rates of 23 &mgr;mol m(-2) s(-1), whereas CO(2) losses, measured as CO(2) evolution, corresponded to about 3% of this value. Thus, decarboxylation of malate and citrate provided high internal CO(2) concentrations during phase III of CAM, even though the stomata were closed, allowing optimal utilization of light energy, as indicated by the non-saturating electron transport rates (ETR) in the light response curves, with highest rates of ETR occurring at midday in the diurnal curves. At the transition from phase III to IV of CAM, depletion of internal CO(2) sources and low stomatal conductances, which restricted the supply of exogenous CO(2), reduced the demand for photochemical energy to drive carbon assimilation. This was compensated by increases in thermal energy dissipation as indicated by higher rates of non-photochemical quenching, while high irradiances still prevailed. Shifts in the CAM phases and changes in protective thermal dissipation potential allowed C. hilariana to match changes in PPFD patterns for leaves of different orientations. Evidence that most of the decline in photochemical efficiency was probably related to the fast-relaxing component of non-photochemical quenching is provided by the high values of the quantum yield of photosystem II after 20 min of relaxation in darkness, and an almost complete recovery after sunset. These adjustments in photosynthetic machinery minimized the danger of photo-inhibition in C. hilariana, which is commonly found in fully exposed habitats.

Kaolin cements in limestones; potential indicators of organic-rich pore waters during diagenesis

ABSTRACT Kaolin (dickite and/or kaolinite) cement occurs locally in the Chalk Group (Upper Cretaceous-Lower Tertiary) in the North Sea and the Derbyhaven Beds (Lower Carboniferous) on the Isle of Man, United Kingdom. Stable-isotope and petrographic data indicate that the kaolin cements in both the Chalk Group and Derbyhaven Beds precipitated during late diagenesis in either marine or modified marine pore waters. Kaolin cements in both formations are associated with hydrocarbons, which suggests that organic complexing of aluminum may be responsible for precipitation of kaolin cement. Published experimental studies have demonstrated that the mobility of aluminum is greatly increased by complexing with organic acids. In such systems aluminum would be released and made available for clay-mineral precipitation when the organo-aluminum complexes are destabilized. Increased whole-rock 13C values in Chalk Group limestones from the Eldfisk Field, where kaolin cement was widespread, provides corroborating evidence for the late diagenetic decarboxylation of organic acids, which would have released complexed aluminum. The presence of kaolin cement may thus be an indicator of the former presence of organic-rich pore fluids in limestones and provide an additional tool for determination of the pore-water history of limestones and sedimentary basins.

Ecophysiology of the C3-CAM intermediate Clusia minor L. in Trinidad: seasonal and short-term photosynthetic characteristics of sun and shade leaves

The seasonal changes in crassulacean acid metabolism (CAM) activity in response to daily integrated photon flux density (PFD) and precipitation were compared in sun and shade leaves of the C3-CAM intermediate tree Clusia minor L. Measurements of CAM activity (ΔH+) showed that maximum leaf acidity consistently occurred 4 h after dawn, suggesting that new sampling procedures need to be adopted in order to quantify CAM in Clusia species. Whilst exposed leaves responded to intermittent dry conditions, shaded leaves showed a clear induction of CAM activity as conditions became drier. The magnitude of CAM activity correlated well with daily integrated PFD, such that the extent of decarboxylation of organic acids was consistently associated with increased acidification during the subsequent dark period. Over two sampling days, both sun and shade leaves exhibited the four phases of CAM, although PEPc remained active throughout phase II with the result that 50% of the maximum leaf acidity in shade leaves was accumulated during this time. During phase III, internal CO2 supply was augmented by substantial citrate decarboxylation, in addition to malic acid. Chlorophyll fluorescence characteristics were dominated by high rates of PSII electron transport, together with an extremely high potential for thermal dissipation, such that excess light was maintained within safe limits at times of maximum PFD. Photochemical stability was maintained by matching supply and demand for internal CO2: in the morning, C3 and C4 carboxylation processes were regulated by extended PEPc activity, so that decarboxylation was delayed until temperature and light stress were highest at midday.

Ecophysiology of the C3-CAM intermediate Clusia minor L. in Trinidad: seasonal and short-term photosynthetic characteristics of sun and shade leaves

The seasonal changes in crassulacean acid metabolism (CAM) activity in response to daily integrated photon flux density (PFD) and precipitation were compared in sun and shade leaves of the C 3 -CAM intermediate tree Clusia minor L. Measurements of CAM activity (ΔH + ) showed that maximum leaf acidity consistently occurred 4 h after dawn, suggesting that new sampling procedures need to be adopted in order to quantify CAM in Clusia species. Whilst exposed leaves responded to intermittent dry conditions, shaded leaves showed a clear induction of CAM activity as conditions became drier. The magnitude of CAM activity correlated well with daily integrated PFD, such that the extent of decarboxylation of organic acids was consistently associated with increased acidification during the subsequent dark period. Over two sampling days, both sun and shade leaves exhibited the four phases of CAM, although PEPc remained active throughout phase II with the result that 50% of the maximum leaf acidity in shade leaves was accumulated during this time. During phase III, internal CO 2 supply was augmented by substantial citrate decarboxylation, in addition to malic acid. Chlorophyll fluorescence characteristics were dominated by high rates of PSII electron transport, together with an extremely high potential for thermal dissipation, such that excess light was maintained within safe limits at times of maximum PFD. Photochemical stability was maintained by matching supply and demand for internal CO 2 : in the morning, C 3 and C 4 carboxylation processes were regulated by extended PEPc activity, so that decarboxylation was delayed until temperature and light stress were highest at midday.