The genesis of late Quaternary caliche nodules in Mission Bay, Texas: stable isotopic compositions and palaeoenvironmental interpretation

Caliche is a fairly widespread pedogenic carbonate that commonly has been used to reconstruct palaeoclimatological conditions. Stable isotopic analyses of three types of caliche nodules from Mission Bay, Texas, provide insights into the values and limitations in palaeoenvironmental interpretations. Soft incipient nodules (type I) and partially lithified chalky nodules (type II), composed of low Mg‐calcite, are in situ pedogenic products in the late Quaternary soils; they represent young to intermediate caliche nodules with no obvious diagenesis and, with constraints, can be used to investigate palaeoenvironmental conditions. The well‐lithified hard nodules (type III) dispersed on the beach and shallow bay exhibit complex shapes, fabrics, mineralogy and geochemical compositions. They are mature nodules that have undergone substantial diagenesis and, therefore, are believed to have lost their initial environmental signatures. The incipient nodules in the presently active grey soil and the older subjacent brown soil display significantly different δ¹³C values, ?8·4 and ?4·4‰, respectively, which indicates a change in palaeovegetation from C₃/C₄ mixed to C₃‐dominated flora. The δ¹³C values probably reflect a marked climate shift from warm and dry to cool and wet conditions in the middle Holocene. However, in the same grey soil, there is a sub‐set of incipient caliche nodules with δ¹³C values around 0·1‰, which is probably due to the input of localized carbon sources in the soil (e.g. shell fragments). The occurrence of essentially identical nodules appearing from the same modern soil horizon with significantly different δ¹³C values questions the universal reliability of this type of data for palaeoenvironmental interpretation. This study demonstrates that, whereas the stable isotopic compositions of caliche nodules can be used for palaeoenvironmental reconstruction, diagenesis and the influence of localized carbonate sources in the soils could lead to erroneous interpretations.     

Change of Organic δ13C in Ornithogenic Sediments of the Xisha Archipelago, South China Sea and its Implication for Ecological Development

Organic carbon isotopes in sediments have been frequently used to identify the source of organic matter.Here we present a study of organic δ~(13)C on two sediment profiles influenced by guano from Guangjin and Jinqing islands in the Xisha Archipelago,South China Sea.Organic matter from ornithogenic coral sand sediments has two main sources,guano pellets and plant residues,and their organic δ~(13)C(δ~(13)C_(OM)) are significantly different.Organic carbon δ~(13)C_(guano) is much higher thanδ~(13)C_(plants),and δ~(13)C_(OM)of bulk samples is intermediate.Based on a two-end-member mixing model,the proportions of guano-and plant-derived organic matter in the bulk samples were reconstructed quantitatively.The results showed that seabirds began to inhabit the islands around approximately1200-1400 AD,and that guano pellets have been an important source of soil organic matter since then.With the accumulation of guano-derived nutrients,plants began to develop prosperously on the islands in the last 200 years,which is reflected by the significant increase of plant-derived organic matter in the upper sediment layer.However,guano-derived organic matter decreased greatly in recent decades,indicating a rapid decrease in seabird population.Our results show that organic δ~(13)C can be effectively used to quantitatively determine different source contributions of OM to bulk ornithogenic coral sand sediments.     

Genesis of the Changba–Lijiagou Giant Pb-Zn Deposit, West Qinling, Central China: Constraints from S-Pb-C-O isotopes

The extensive Changba-Lijiagou Pb-Zn deposit is located in the north of the Xihe–Chengxian ore cluster in West Qinling. The ore bodies are mainly hosted in the marble, dolomitic marble and biotite-calcite-quartz schist of the Middle Devonian Anjiacha Formation, and are structurally controlled by the fault and anticline. The ore-forming process can be divided into three main stages, based on field geological features and mineral assemblages. The mineral assemblages of hydrothermal stage I are pale-yellow coarse grain, low Fe sphalerite, pyrite with pits, barite and biotite. The mineral assemblages of hydrothermal stage II are black-brown cryptocrystalline, high Fe shalerite, pyrite without pits, marcasite or arsenopyrite replace the pyrite with pits, K-feldspar. The features of hydrothermal stage III are calcite-quartz-sulfide vein cutting the laminated, banded ore body. Forty-two sulfur isotope analyses, twenty-five lead isotope analyses and nineteen carbon and oxygen isotope analyses were determined on sphalerite, pyrite, galena and calcite. The δ34 S values of stage I(20.3 to 29.0‰) are consistent with the δ34 S of sulfate(barite) in the stratum. Combined with geological feature, inclusion characteristics and EPMA data, we propose that TSR has played a key role in the formation of the sulfides in stage I. The δ34 S values of stage II sphalerite and pyrite(15.1 to 23.0‰) are between sulfides in the host rock, magmatic sulfur and the sulfate(barite) in the stratum. This result suggests that multiple S reservoirs were the sources for S2-in stage II. The δ34 S values of stage III(13.1 to 22‰) combined with the structure of the geological and mineral features suggest a magmatic hydrothermal origin of the mineralization. The lead isotope compositions of the sulfides have 206 Pb/204 Pb ranging from 17.9480 to 17.9782, 207 Pb/204 Pb ranging from 15.611 to 15.622, and 208 Pb/204 Pb ranging from 38.1368 to 38.1691 in the three ore-forming stages. The narrow and symmetric distributions of the lead isotope values reflect homogenization of granite and mantle sources before the Pb-Zn mineralization. The δ13 CPDB and δ18 OSMOW values of stage I range from-0.1 to 2.4‰ and from 18.8 to 21.7‰. The values and inclusion data indicate that the source of fluids in stage I was the dissolution of marine carbonate. The δ13 CPDB and δ18 OSMOW values of stage II range from-4 to 1‰ and from 12.3 to 20.3‰, suggesting multiple C-O reservoirs in the Changba deposit and the addition of mantle-source fluid to the system. The values in stage III are-3.1‰ and 19.7‰, respectively. We infer that the process of mineralization involved evaporitic salt and sedimentary organic-bearing units interacting through thermochemical sulfate reduction through the isotopic, mineralogy and inclusion evidences. Subsequently, the geology feature, mineral assemblages, EPMA data and isotopic values support the conclusion that the ore-forming hydrothermal fluids were mixed with magmatic hydrothermal fluids and forming the massive dark sphalerite, then yielding the calcite-quartz-sulfide vein ore type at the last stage. The genesis of this ore deposit was epigenetic rather than the previously-proposed sedimentary-exhalative(SEDEX) type.     

Dolomitization by hypersaline reflux into dense groundwaters as revealed by vertical trends in strontium and oxygen isotopes: Upper Muschelkalk,Switzerland

The Trigonodus Dolomit is the dolomitized portion of the homoclinal ramp sediments of the Middle Triassic Upper Muschelkalk in the south‐east Central European Basin. Various dolomitizing mechanisms, followed by recrystallization, have been previously invoked to explain the low δ¹⁸O, high ⁸⁷Sr/⁸⁶Sr, extensive spatial distribution and early nature of the replacive matrix dolomites. This study re‐evaluates the origin, timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the Trigonodus Dolomit at 11 boreholes in northern Switzerland. In each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth: crystal size increase, change from anhedral to euhedral textures, ultraviolet‐fluorescence decrease, δ¹⁸O_VPDB decrease from ?1·0‰ at the top to ?6·7‰ at the base and an ⁸⁷Sr/⁸⁶Sr increase from 0·7080 at the top to 0·7117 at the base. Thus, dolomites at the top of the unit record isotopic values similar to Middle Triassic seawater (δ¹⁸O_VSMOW = 0‰; ⁸⁷Sr/⁸⁶Sr = 0·70775) while dolomites at the base record values similar to meteoric groundwaters from the nearby Vindelician High (δ¹⁸O_VSMOW = ?4·0‰; ⁸⁷Sr/⁸⁶Sr = >0·712). According to water–rock interaction modelling, a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends. Instead, the combined isotopic, geochemical and petrographic data can be explained by dolomitization via seepage‐reflux of hypersaline brines into dense, horizontally‐advecting groundwaters that already had negative δ¹⁸O and high ⁸⁷Sr/⁸⁶Sr values. Evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization. This study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre‐existing groundwaters can be preserved during seepage‐reflux dolomitization in low‐angle carbonate ramps.     

Geochemical Characteristics of the Upper Triassic Xujiahe Formation in Sichuan Basin, China and its Significance for Hydrocarbon Accumulation

The alternative development of coal-bearing hydrocarbon source rocks and low-porosity and low-permeability tight sandstone reservoirs of the Triassic Xujiahe Formation in the Sichuan Basin is favorable for near-source hydrocarbon accumulation. The natural gas composition of the Xujiahe Formation in the Sichuan Basin is dominated by hydrocarbon gases, of which the methane content is80.16%-98.67%. Typically, the C_2~+ content is larger than 5% in main wet gas. The dry gas is mainly distributed in the western and northern regions of the basin. The non-hydrocarbon gases mainly contain nitrogen, carbon dioxide, hydrogen, and helium, with a total content of 2%. The carbon isotope ranges of methane and its homologues in natural gas are: δ~(13)C_1 of-43.8‰ to-29.6‰, δ~(13)C_2 of-35.4‰ to-21.5‰, δ~(13)C_3 of-27.6‰ to-19.8‰,and δ~(13)C_4 of-27.7‰ to-18.8‰. δ~(13)C_3δ~(13)C_4 occurs in some natural gas with a low evolution degree; such gas is mainly coal-related gas from humic-type source rocks of the Xujiahe Formation. As for the natural gas, δ~2 H_(CH4) values ranged from-195‰ to-161‰,δ~2 H_(C2H6) values ranged from-154‰ to-120‰, and δ~2 H_(C3H8) values ranged from-151‰ to-108‰. The dry coefficient,δ~(13)C and δ~2 H_(CH4) are all positively correlated with the maturity of source rocks. The higher the maturity of source rocks is, the larger the natural gas dry coefficient is and the larger the δ~(13)C and δ~2 H_(CH_4) values are, indicative of the characteristic of near-source accumulation. The δ~2 H_(C2H6) value of natural gas is influenced by paleosalinity to a relatively large extent; the higher the paleosalinity is, the larger the δ~2 H_(C2H6) value is. The Pr/Ph value of the condensate oil ranged from 1.60 to 3.43, illustrating light oxidization-light reduction and partial-oxidization characteristics of the depositional environment of coal-bearing source rocks of the Xujiahe Formation. The natural gas light hydrocarbon(C_5-C_7) from the Xujiahe Formation presented two characteristics: the first was the relatively high aromatic hydrocarbon content(19%-32.1%), which reveals the characteristic of natural gas with humic substances of high-maturity; the second was the low content of aromatic hydrocarbon(0.4%-9.3%),reflecting water-washing during the accumulation of the natural gas. The reported research outcomes indicate a potential mechanism for natural gas accumulation in the Xujiahe Formation, which will further guide natural gas exploration in this region.     

Characterization of C‐isotope variability in the Delhi Supergroup,northwestern India and the Mesoproterozoic ocean

Marine carbonate rocks of the Delhi Supergroup of northwestern India show little deviation in whole‐rock δ ¹³Ccarb and δ ¹⁸Ocarb values, which generally are around 0 and –10‰ respectively. These narrow ranges and almost constant δ ¹³Ccarb values persist despite close sampling through long sections. The data suggest that the global rate of organic carbon burial was probably constant during deposition of the Delhi Supergroup. The nearly invariant C isotopic profile of the Delhi Supergroup is similar to C isotopic profiles of Mesoproterozoic carbonates older than 1.3 Ga, as reported from different parts of world. Carbonate units on the western margin of the Delhi Supergroup however, have on average moderately positive δ ¹³C values (from 2 to +4.96‰). These high δ ¹³C carbonates may represent the Mesoproterozoic–Neoproterozoic transition (from ～1.25 to ～0.85 Ga), a period characterized by high positive δ ¹³C values globally.     

Techniques for assessing spatial heterogeneity of carbonate δ13C values: Implications for craton‐wide isotope gradients

The sedimentary record of carbonate carbon isotopes (δ¹³C_carb) provides one of the best methods for correlating marine strata and understanding the long‐term evolution of the global carbon cycle. This work focuses on the Late Ordovician Guttenberg isotopic carbon excursion, a ca 2·5‰ positive δ¹³C_carb excursion that is found in strata globally. Substantial variability in the apparent magnitude and stratigraphic morphology of the Guttenberg excursion at different localities has hampered high‐resolution correlations and led to divergent reconstructions of ocean chemistry and the biogeochemical carbon cycle. This work investigates the magnitude, spatial scale and sources of isotopic variability of the Guttenberg excursion in two sections from Missouri, USA. Centimetre‐scale isotope transects revealed variations in δ¹³C_carb and δ¹⁸O_carb greater than 2‰ across individual beds. Linear δ¹³C_carb to δ¹⁸O_carb mixing lines, together with petrographic and elemental abundance data, demonstrate that much of the isotopic scatter in single beds is due to mixing of isotopically distinct components. These patterns facilitated objective sample screening to determine the ‘least‐altered’ data. A δ¹⁸O_carb filter based on empirical δ¹⁸O_carb values of well‐preserved carbonate mudstones allowed further sample discrimination. The resulting ‘least‐altered’ δ¹³C_carb profile improves the understanding of regional as well as continental‐scale stratigraphic relations in this interval. Correlations with other Laurentian sections strongly suggest that: (i) small‐scale variability in Guttenberg excursion δ¹³C_carb values may result in part from local diagenetic overprinting; (ii) peak‐Guttenberg excursion δ¹³C_carb values of the Midcontinent are not distinct from their Taconic equivalents; and (iii) no primary continental‐scale spatial gradient in δ¹³C_carb (for example, arising from chemically distinct ‘aquafacies’) is required during Guttenberg excursion‐time. This study demonstrates the importance of detailed petrographic and geochemical screening of samples to be used for δ¹³C_carb chemostratigraphy and for enhancing understanding of epeiric ocean chemistry.     

Global significance of oxygen and carbon isotope compositions of pedogenic carbonates since the Cretaceous

Few global syntheses of oxygen and carbon isotope composition of pedogenic carbonates have been attempted,unlike marine carbonates.Pedogenic carbonates represent in-situ indicators of the climate conditions prevailing on land.The δ~(18)O and δ~(13)C values of pedogenic carbonates are controlled by local and global factors,many of them not affecting the marine carbonates largely used to probe global climate changes.We compile pedogenic oxygen and carbon isotopic data(N= 12,167) from Cretaceous to Quaternary-aged paleosols to identify potential trends through time and tie them to possible controlling factors.While discrete events such as the PaleoceneEocene Thermal Maximum are clearly evidenced,our analysis reveals an increasing complexity in the distribution of the δ~(18)O vs δ~(13)C values through the Cenozoic.As could be expected,the rise of C_4 plants induces a shift towards higher δ~(13)C values during the Neogene and Quaternary.We also show that the increase in global hypsometry during the Neogene plays a major role in controlling the δ~(18)O and δ~(13)C values of pedogenic carbonates by increasing aridity downwind of orographic barriers.Finally,during the Quaternary,an increase of 3‰ inδ~(18)O values is recorded both by the pedogenic carbonates and the marine foraminifera suggesting that both indicators may be used to track global climate signal.     

Dolomitization,gypsum calcitization and silicification in carbonate–evaporite shallow lacustrine deposits

This paper describes and interprets the mineral and facies assemblages that occur in carbonate–evaporite shallow lacustrine deposits, considering the importance of the processes pathway (i.e. dolomitization, gypsum calcitization and silicification). The Palaeogene deposits of the Deza Formation (Almazán Basin, central‐northern Spain) are selected as a case study to determine the variety of physicochemical processes taking place in carbonate–evaporite shallow lakes and their resulting diagenetic features. Dolostones are the predominant lithology and are composed mainly of dolomite with variable amounts of secondary calcite (5 to 50%), which mainly mimic lenticular gypsum (pseudomorphs). Five morphological types of dolomite crystal were identified as follows: dolomite tubes, dolomite cylinders, rhombohedral dolomite, spheroidal and quasi‐rhombohedral dolomite, and cocoon‐shaped dolomite. The dolomite cylinders and tubes are interpreted as the dolomitized cells of a widespread microbial community. The sequence of diagenetic processes started with growth of microlenticular interstitial gypsum in a calcareous mud deposited on the playa margin mudflats, and that sometimes included microbial sediments. Immediately following growth of gypsum, dolomite replaced the original calcite (or possibly aragonite) muds, the microbial community and the gypsum. Partial or total replacement of gypsum by dolomite was related mainly to the biomineralization of endolithic microbial communities on gypsum crystals. Later calcitization took place under vadose, subaerial exposure conditions. The development of calcrete in distal alluvial settings favoured the release of silica and subsequent silicification on the playa margin mudflats. Stable isotope compositions of calcite range from ?9·02 to ?5·83‰ δ¹³C_PDB and ?7·10 to 1·22‰ δ¹⁸O_PDB; for the dolomite, these values vary from ?8·93 to ?3·96‰ δ¹³C_PDB and ?5·53 to 2·4‰ δ¹⁸O_PDB. Quartz from the cherts has δ¹⁸O_SMOW values ranging from 27·1 to 31·1‰. Wide variation and relatively high δ¹⁸O_SMOW values for dolomite indicate evaporitic and closed hydrological conditions; increased influx of meteoric waters reigned during the formation of secondary calcite spar.     

Crassulacean acid metabolism influences D/H ratio of leaf wax in succulent plants

This study sought to characterize hydrogen isotopic fractionation during biosynthesis of leaf wax n-alkanes in succulent plants capable of crassulacean acid metabolism (CAM). The metabolic and physiological features of CAM represent crucial strategies for survival in hot and dry climates and have been hypothesized to impact hydrogen isotope fractionation. We measured the stable carbon and hydrogen isotopic compositions (δ¹³C and δD, respectively) of individual n-alkanes in 20 species of succulent plants from a global collection of the Huntington Botanical Gardens, San Marino, California. Greenhouse conditions and irrigation with water of constant δD value enabled determination of interspecies differences in net D/H fractionation between source water and leaf wax products. Carbon isotope ratios provide constraints on the extent of CAM vs. C₃ photosynthesis and indicate a wide range of CAM use, with δ¹³C values ranging from −33.01‰ to −18.54‰ (C₂₇–C₃₃ n-alkanes) and −26.66‰ to −17.64‰ (bulk tissue). Despite the controlled growth environment, we observed ca. 90‰ interspecies range in δD values from −193‰ to −107‰. A positive correlation between δ¹³C_bulk and δD_C31 values with R² = 0.60 (δ¹³C_C31 and δD_C31 values with R² = 0.41) implicates a metabolic isotope effect as the dominant cause of interspecies variation in the hydrogen isotopic composition of leaf wax n-alkanes in CAM-intermediate plants.     

Sr and Nd isotopes as tracers in pedogenic studies: Evidence for Saharan dust contribution to the soils of Muravera (Sardinia,Italy)

Sr and Nd isotopes were applied to 5 soil profiles from the Muravera area, in south-eastern Sardinia.All the soils, which have developed during the Quaternary on the Lower Paleozoic metamorphic basement except for one on Eocene carbonates, are located far from major sources of pollution. Therefore, they are suitable for testing pedogenic processes and geochemical evolution to benefit for environmental studies.The Sr isotopic ratios range largely (δ⁸⁷Sr = 1.7–65.9‰), even in each soil profile. In particular, the observed increase of δ⁸⁷Sr with depth in the most of the metamorphic rock-based soils can be accounted for by the downward decrease of Sr contributions from organic matter and Saharan dust, both displaying lower isotopic ratios than the soil bedrocks. The carbonate rock-based soil exhibits δ⁸⁷Sr higher (1.7–18.1‰) than the bedrock, indicating a significant contribution of radiogenic Sr from the siliciclastic fraction of the soil, and probably from dust input. The Nd isotopic ratios are slightly variable through the profiles (ɛ_Nd from −7.8 to −14.5), confirming little mobility of Nd and Sm during the pedogenesis. Among the minerals present in the soils, phosphates, albite, and calcite are those important in providing low radiogenic Sr and Nd to organic matter of the soils.Lastly, this isotopic study has in particular allowed for evaluating the potential proportion of contribution of Saharan dust to south-eastern Sardinia, thus corroborating the findings of other studies related to soils from the central-western Mediterranean.     

Stable isotope evidence for the origin of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel

The oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene-Pleistocene) dolomitization of these rocks. Diagenetic carbonates included early siderite (δ¹⁸O_SMOW=+24.4 to +26.5‰δ¹³C_PDB=?1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ¹⁸O_SMOW=+18.4 to +25.8‰; δ¹³C_PDB=?2.1 to +0.2‰), and calcite (δ¹⁸O_SMOW=+21.3 to +32.6‰; δ¹³C_PDB=?4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values. A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ¹⁸O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post-dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ¹⁸O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.     

Application of stable carbon isotopes for reconstructing salt‐marsh floral zones and relative sea level,New Jersey,USA

We investigated use of δ¹³C in bulk organic sediment to define the botanical origin of samples preserved in coastal sediment as a means to reconstruct relative sea level in New Jersey, USA. Modern transects at three sites demonstrated that low and high salt‐marsh floral zones dominated by C₄ species (Spartina alterniflora and Spartina patens) were associated with sediment δ¹³C values between ?18.9‰ and ?15.8‰ and occurred from mean tide level (MTL) to mean higher high water (MHHW). Brackish transitional settings vegetated by Phragmites australis with Iva fructescens and Typha sp. (C₃ species) and freshwater upland samples (C₃ species) were characterized by bulk sediment δ¹³C values of ?27.0‰ to ?22.0‰ and existed above MHHW. Parallel transects at one site suggested that intra‐site variability was not discernible. The utility of δ¹³C values for reconstructing relative sea level in New Jersey is limited by an inability to differentiate between brackish sediments related to sea level and freshwater upland samples. To facilitate this distinction in a 4.4 m core, we used a multi‐proxy approach (δ¹³C values with presence or absence of agglutinated foraminifera) to recognize indicative meanings for four sample types. Sediment with δ¹³C values greater than ?18.9‰ was derived from a vegetated salt‐marsh and formed between MTL and MHHW. Sediment with δ¹³C values less than ?22.0‰ and containing agglutinated foraminifera formed in a brackish transitional zone between MHHW and highest astronomical tide (HAT). This is the narrowest elevational range of the four sample types and most precise sea‐level indicator. Sediment with δ¹³C values less than ?22.0‰ and lacking foraminifera can only constrain the upper bound of former sea level. Samples with intermediate values (?22.0‰ to ?18.9‰) formed between MTL and HAT. Using these indicative meanings and radiocarbon dates, we suggest that a transition from brackish to salt‐marsh δ¹³C values recorded in the core took approximately 350 years (from 1800 to 1450 cal. a BP). Copyright © 2011 John Wiley & Sons, Ltd.     

Geochemical Constraints on the Origin and Evolution of Spring Waters in the Changdu‐Lanping‐Simao Basin,Southwestern China

Chemical and isotopic data were measured for 51 leached brine springs in the Changdu-Lanping-Simao Basin (CD-LP-SM), China. The predominance of Cl and Na, saturation indices of carbonate minerals, and Na/Cl and Ca/SO4 ratios of ~1 suggest that halite, sulphate, and carbonate are the solute sources. Integration of geochemical, δ18O, and δD values suggests that springs are mainly derived from meteoric water, ice-snow melt, and water-rock interactions. B concentrations range from 0.18 to 11.9 mg/L, with δ11B values of ?4.37‰ to +32.39‰, indicating a terrestrial source. The δ11B-B relationships suggest B sources of crustal origin (marine carbonates with minor crust-derived volcanics); we did not identify a marine or deep mantle origin. The δ11B values of saline springs (+4.61‰ to +32.39‰) exceed those of hot (?4.37‰ to +4.53‰) and cold (?3.47‰ to +14.84‰) springs; this has contributed to strong water-rock interactions and strong saturation of dissolved carbonates. Conversely, the global geothermal δ11B-Cl/B relationship suggests mixing of marine and non-marine sources. The δ11B-Cl/B relationships of the CD-LP-SM are similar to those of the Tibet geothermal belt and the Nangqen Basin, indicating the same B origin. These differ from thermal waters controlled by magmatic fluids and seawater, suggesting that B in CD-LP-SM springs has a crustal origin.     

Origin and Accumulation of Natural Gases in the Upper Paleozoic Strata of the Ordos Basin in Central China

The natural gases in the Upper Paleozoic strata of the Ordos basin are characterized by relatively heavy C isotope of gaseous alkanes with δ 13C1 and δ13C2 values ranging mainly from ?35‰ to ?30‰ and ?27‰ to ?22‰, respectively, high δ13C excursions (round 10) between ethane and methane and predominant methane in hydrocarbon gases with most C1/(C1-C5) ratios in excess of 0.95, suggesting an origin of coal-derived gas. The gases exhibit different carbon isotopic profiles for C1-C4 alkanes with those of the natural gases found in the Lower Paleozoic of this basin, and believed to be originated from Carboniferous-Permian coal measures. The occurrence of regionally pervasive gas accumulation is distinct in the gently southward-dipping Shanbei slope of the central basin. It is noted that molecular and isotopic composition changes of the gases in various gas reservoirs are associated with the thermal maturities of gas source rocks. The abundances and δ13C values of methane generally decline northwards and from the basin center to its margins, and the effects of hydrocarbon migration on compositional modification seem insignificant. However, C isotopes of autogenetic calcites in the vertical and lateral section of reservoirs show a regular variation, and are as a whole depleted upwards and towards basin margins. Combination with gas maturity gradient, the analysis could be considered to be a useful tool for gas migration.     

Carbon and sulfur isotopic fluctuations associated with the end-Guadalupian mass extinction in South China

Concentrations of total organic matter (TOC), carbon isotopic compositions of carbonate and organic matter (δ¹³C_carb, δ¹³C_org), and sulfur isotopic compositions of carbonate associated sulfate (δ³⁴S_sulfate) across the Guadalupian–Lopingian (G–L) boundary were analyzed from identical samples of Tieqiao section, Laibin, Guangxi province, South China. The δ¹³C_carb values show a positive excursion from − 0.45‰ to the peak of 3.80‰ in the Laibin limestone member of the Maokou Formation, followed by a drastic drop to − 2.60‰ in the lowest Heshan formation, then returned to about 1.58‰. Similar to the trends of the δ¹³C_carb values, Δ¹³C_carb–org values also show a positive excursion followed by a sharp negative shift. The onset of a major negative carbon isotope excursion postdates the end Guadalupian extinction that indicates subsequent severe disturbance of the ocean–atmosphere carbon cycle. The first biostratigraphic δ³⁴S_sulfate values during the G–L transition exhibit a remarkable fluctuation: a dramatic negative shift followed by a rapid positive shift, ranging from 36.88‰ to − 37.41‰. These sulfate isotopic records suggest that the ocean during the G–L transition was strongly stratified, forming an unstable chemocline separating oxic shallow water from anoxic/euxinic deep water. Chemocline excursions, together with subsequent rapid transgression and oceanic anoxia, were likely responsible for the massive diversity decline of the G–L biotic crisis.     

Primary and diagenetic isotopic signals in fossils and hemipelagic carbonates: the Lower Jurassic of northern Spain

Stable isotope and trace element analyses of 230 Jurassic (Pliensbachian–Toarcian) samples from northern Spain have been performed to test the use of geochemical variations in fossils (belemnites and brachiopods) and whole‐rock hemipelagic carbonates as palaeoceanographic indicators. Although the succession analysed (Reinosa area, westernmost Basque–Cantabrian Basin) has been subject to severe thermal alteration during burial diagenesis, the samples appear to be well preserved. The degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic, chemical and cathodoluminescence analyses. It is demonstrated that brachiopods and whole‐rock carbonates, although widely used for palaeoceanic studies, do not retain their primary marine geochemical composition after burial diagenesis. In contrast, there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock. Well‐preserved belemnite shells (non‐luminescent to slightly luminescent) typically show stable isotope values of +4·3‰ to –0·7‰δ¹³C, +0·7‰ to –3·2‰δ¹⁸O, and trace element contents of <32 μg g^–1 Mn, <250 μg g^–1 Fe, >950 μg g^–1 Sr and Sr/Mn ratios >80. This study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low‐Mg calcite fossil shells and hemipelagic bulk carbonates, behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated. Multiple non‐luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders, yet their δ¹³C and δ¹⁸O values exhibited moderate scatter. Such variability is likely to be related to the palaeoecological behaviour of belemnites and/or high‐frequency secular variations in sea‐water chemistry superimposed on the long‐term isotopic trend. A pronounced positive carbon‐isotope excursion (up to +4·3‰) is documented in the early Toarcian serpentinus biozone, which correlates with the Toarcian δ¹³C maximum reported in other European and Tethyan regions.     

Aragonite stromatolitic buildups from Santorini (Aegean Sea,Greece): Geochemical and palaeontological constraints of the caldera palaeoenvironment prior to the Minoan eruption (ca 3600 yr bp)

The pyroclastic deposits of the Minoan eruption (ca 3600 yr bp ) in Santorini contain abundant xenoliths. Most of these deposits are calcareous blocks of laminated‐botryoidal, stromatolite‐like buildups that formed in the shallow waters of the flooded pre‐Minoan caldera; they consist of (i) light laminae, of fibrous aragonite arranged perpendicular to layering, and (ii) dark laminae, with calcified filaments of probable biological origin. These microstructures are absent in the light laminae, suggesting a predominant inorganic precipitation of aragonite on substrates probably colonized by microbes. Internal cavities contain loose skeletal grains (molluscs, ostracods, foraminifera and diatoms) that comprise taxa typical of shallow marine and/or lagoon environments. Most of these forms are typical of warm water environments, although no typical taxa from hydrothermal vents have been observed. Past gasohydrothermal venting is recorded by the occurrence of barite, pyrolusite and pyrite traces. The most striking features of the stable isotopic data set are: (i) an overall wide range in δ¹³C_PDB (0·16 to 12·97‰) with a narrower variation for δ¹⁸O_PDB (?0·23 to 4·33‰); and (ii) a relatively uniform isotopic composition for the fibrous aragonite (δ¹³C = 12·40 ± 0·43‰ and δ¹⁸O = 2·42 ± 0·77‰, n = 21). The δ¹³C and δ¹⁸O values from molluscs and ostracods display a covariant trend, which reflects a mixing between sea water and a fluid influenced by volcano‐hydrothermal activity. Accordingly, ⁸⁷Sr/⁸⁶Sr from the studied carbonates (0·708758 to 0·709011 in fibrous aragonite and 0·708920 to 0·708991 in molluscs) suggests that the aragonite buildups developed in sea water under the influence of a hydrothermal/volcanic source. Significant differences in trace elements have been detected between the fibrous aragonite and modern marine aragonite cements. The caldera water from which the fibrous aragonite crusts formed received an input from a volcano‐hydrothermal system, probably producing diffuse venting of volcanogenic CO₂ gas and of a fluid enriched in Ca, Mn and Ba, and depleted in Mg and probably in Sr.     

Stable Carbon Isotope Variations in Cave Percolation Waters and their Implications in Four Caves of Guizhou, China

Monitoring and sampling of main plants,soil CO2,soil water,bedrock,spring water,drip water and its corresponding speleothem were performed at four cave systems of Guizhou,Southwest China,from April 2003 to May 2004,in order to understand stable carbon isotope ratios variations of dissolved inorganic Carbon(DIC) in cave percolation waters(δ13CDIC) and their implications for paleoclimate.Stable carbon isotopic compositions and ions(Ca,Mg,Sr,SO4,Cl etc.) were measured for all samples.The results indicate that there are significant differences among the δ13CDIC values from inter-cave,even inter-drip of intra-cave in the four caves.The δ13CDIC values from the Liangfeng Cave(LFC) is lightest among the four caves,where vegetation type overlying the cave is primary forest dominated by tall trees with lighter average δ13C value(–29.9‰).And there are remarkable differences in δ13CDIC values of different drip waters in the Qixing Cave(QXC) and Jiangjun Cave(JJC),up to 6.9‰ and 7.8‰,respectively.Further analyses show that the δ13CDIC values in cave drip waters are not only controlled by vegetation biomass overlying the cave,but also hydro-geochemical processes.Therefore,accurate interpreting of δ13C recorded in speleothems cannot be guaranteed if these effects of the above mentioned factors are not taken into consideration.     

Columnar calcites as testimony of diagenetic overprinting at the boundary between Upper Tournaisian dolomites and limestones (Belgium): multiple origins for apparently similar features

In topographic flat areas, sedimentary settings may vary from one outcrop to another. In these settings, calcite precipitates may yield macroscopically similar columnar features, although they are products of different sedimentary or diagenetic processes. Three columnar calcite crystal fabrics, i.e. rosettes, palisade crusts and macro-columnar crystal fans, have been differentiated near and at the contact between Upper Tournaisian dolomites and limestones along the southern margin of the Brabant-Wales Palaeohigh. Their petrographic characteristics, and geochemical and fluid inclusion data provide information on the (dia)genetic processes involved. Rosettes composed of non-luminescent columnar calcite crystal fans (1–5 cm in diameter) developed on top of one another, forming discrete horizons in repetitive sedimentary cycles. The cycles consist of three horizons: (I) a basal horizon with fragments from the underlying horizon, (II) a micrite/microspar horizon with incipient glaebules, (III) an upper horizon consisting of calcite rosettes, with desiccation features. The petrographical features and δ¹⁸O signatures of −10·0 to −5·5‰ and δ¹³C values of −5·5 to −3·2‰ support either evaporative growth, an evaporative pedogenic origin, or overprinting of marine precipitates. Palisade crusts, composed of a few to 10 mm long non-luminescent calcite crystals, coat palaeokarst cavities. Successive palisade growth-stages occur which are separated by thin laminae of micrite or detrital quartz, displaying a geopetal arrangement. Palisade crusts are interpreted as intra-Mississippian speleothems. This interpretation is supported by their petrographic characteristics and isotopic signature (δ¹⁸O = −8·7 to −6·5‰ and δ¹³C = −4·8 to −2·5‰). Macro-columnar crystals, 1–50 cm long, developed mainly perpendicular to cavity walls and dolomite clasts. Crystal growth stages in the macro-columnar crystals are missing. δ¹⁸O values vary between −16·4 and −6·8‰ and δ¹³C values between −5·2 and −0·9‰. These features possibly support a late diagenetic high temperature precipitation in relation to hydrothermal karstification.