Carbonate Oxygen Isotope Research Articles

Origin of authigenic albite in a lacustrine mixed-deposition sequence (Lucaogou Formation, Junggar Basin) and its diagenesis implications

Authigenic albites occur widely in clastic reservoirs with important implications for diagenesis and reservoir formation. The middle Permian Lucaogou Formation in the Jimusaer Sag (Junggar Basin, NW China), where major exploration breakthroughs in shale oil have been achieved, reveals a new phenomenon that authigenic albites are abundant in unique mixed carbonate–volcanic–clastic sequences. This has not been reported in the literatures. To fill the knowledge gap, the origin of these authigenic albites and their relationship with dissolution pores (i.e. diagenesis implications) were investigated. Results show that two types (I and II) of authigenic albite were identified within the shale oil reservoirs. Euhedral Type I authigenic albites with 3–10 μm only occur in dolarenite intraclasts and are symbiotic with amorphous dolomite minerals with a pure chemical composition of >99% albite-end-member content. Larger Type II authigenic albites with 10–50 μm are widely distributed in reservoirs, primarily in dissolution pores, and coexist with authigenic dolomite minerals or dolomite overgrowths. Their chemical composition is less pure with anorthite-end-member contents that range from undetectable to 9.77%, with an average of 1.34%. A symbiotic relationship, pure chemical composition, size, and euhedral morphology indicate that Type I authigenic albites precipitated during syngenetic hydrothermal action. However, the morphology of dissolution pores, residual symbiotic “orthoclase”, impure chemical composition and carbon–oxygen isotope indicate that Type II were the products of the dissolution and reprecipitation of “perthite” crystal pyroclasts influenced by acid organic fluids in latter diagenesis. The differential dissolution of “orthoclase” and “albite” components in “perthite” crystal pyroclasts formed enormous intergranular secondary pores in the presence of dolomite minerals in the shale oil reservoirs.

Holocene Hydroclimatic Reorganizations in Northwest Canada Inferred From Lacustrine Carbonate Oxygen Isotopes

AbstractSub‐centennial oxygen (δ18O) isotopes of ostracod and authigenic calcite from Squanga Lake provides evidence of hydroclimatic extremes and a series of post‐glacial climate system reorganizations for the interior region of northwest Canada. Authigenic calcite δ18O values range from −16‰ to −21‰ and are presently similar to modern lake water and annual precipitation values. Ostracod δ18O record near identical trends with calcite, offset by +1.7 ± 0.6‰. At 11 ka BP (kaBP = thousands of years before 1950), higher δ18O values reflect decreased precipitation−evaporation (P−E) balance from residual ice sheet influences on moisture availability. A trend to lower δ18O values until ∼8 ka BP reflects a shift to wetter conditions, and reorganization of atmospheric circulation. The last millennium and modern era are relatively dry, though not as dry as the early Holocene extreme. North Pacific climate dynamics remained an important driver of P−E balance in northwest Canada throughout the Holocene.

Geochemical characteristics of carbonates and indicative significance of the sedimentary environment based on carbon-oxygen isotopes, trace elements and rare earth elements: case study of the Lower Paleozoic carbonates in the Gucheng area, Tarim Basin, China

Geochemical characteristics of carbonates and indicative significance of the sedimentary environment based on carbon-oxygen isotopes, trace elements and rare earth elements: case study of the Lower Paleozoic carbonates in the Gucheng area, Tarim Basin, China

Extracellular, Surface, and Intracellular Biomineralization of Bacillus subtilis Daniel-1 Bacteria

Even though biomineralization induced by bacteria has been studied for many years, some unsolved problems are still present in the biomineralization, especially the intracellular biomineralization, which should be further explored. In this study, the Bacillus subtilis Daniel-1 strain was identified by 16S rDNA homology, used to induce calcium carbonate biomineralization at various Mg/Ca molar ratios (0, 2, 4, 6, and 8). The results showed that pH still increased at the decline stage in the bacterial growth process, and could increase to about 8.69, resulting from the influence of carbonic anhydrase activity and ammonia. X-ray diffraction (XRD) results showed that calcite, Mg-rich calcite, and aragonite minerals were induced in the extracellular biomineralization process. The diversified morphologies and the presence of fatty acids, protein, and other organic substances in calcium carbonate analyzed by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) proved the biogenesis of these minerals. The stable carbon isotope values (–19.37‰ to −19.79‰) and oxygen isotope values (–6.37‰ to −8.91‰) presented in this study were relatively negative, also confirmed the biogenesis of the minerals. Magnesium carbonate contents in Mg-rich calcite increased from 8.79% to 25.71% with increasing Mg/Ca ratios. Aragonite could be grown not only in cell surface but also inside the cell by analyzing ultrathin slices of bacteria with HRTEM-EDS-SAED. Aragonite in cell surface was elongated whereas intracellular aragonite was granular. To the best of our knowledge, the intracellular biotic crystalline aragonite has rarely been reported. This study is helpful to further understand the mechanism of carbonate mineral biomineralization, and may also provide useful evidence for further studies on intracellular crystal structures.

Filling Provenance in Fracture Cavity Formation within Aksu Area, Tarim Basin, NW China: Indicators from Major and Trace Element, Carbon-Oxygen, and Strontium Isotope Compositions

Abstract With an aim to increase the understanding about sedimentary environment and isotopic and chemical characteristics of fillings in fracture cavities with multiple compositions, we conducted scanning electron microscope (SEM), fluid inclusion testing (FIT), common and trace element chemistry, full analysis testing, isotopic compositions (δ13C, δ18O, 87Sr/86Sr), and apatite fission track testing to study the formation environment of Aksu area, Tarim Basin. According to outfield and microscope observations, combined with SEM results, three textural and compositional type fractures and cavities were distinguished. Through fine analysis of geochemistry characteristics on fractures, cavities, multiple filling periods, and environments were interpreted. Constrained by rare earth element (REE) pattern diagram, relationships between carbon and oxygen isotopes, strontium isotope, the compositional patterns, and generation environment of the fracture and cavities were determined. The results show that (1) cavity, fracture filling, and wall rock primarily consist of calcite, with a proportion of 56.85%, 80.48%, and 81.00%, respectively. (2) Four fracture sets have been distinguished in the Ordovician limestone of the karst cave, Middle-Late Caledonian (Set 1), Early Hercynian (Set 2), Indo-Yanshanian (Set 3), and Himalayan orogeny (Set 4). Two stages of cave filling deposition are distinguished. Stage I was coeval with the Middle-Late Caledonian Set 1 fractures and is attributable to the circulation of freshwater fluid. Stage II was coeval with the Early Hercynian Set 2 fractures and is attributable to deep hydrothermal fluid circulation. (3) Cavity, fracture filling, and wall rock in Ordovician strata are slightly influenced by diagenesis alteration and territorial supply. Three significant filling stages were distinguished, freshwater fluid with strong oxidizing environment (Middle-Late Caledonian), hydrothermal fluid with authigenic abnormal enrichment (indicating obvious hypoxic sedimentary water, Early Hercynian), and high-temperature hydrothermal fluid from deep earth (primarily influenced by magmatism, Indo-Yanshanian, and Himalayan).

Holocene Lake Evolution and Glacial Fluctuations Indicated by Carbonate Minerals and Their Isotopic Compositions in the Sediments of a Glacial Melt Recharge Lake on the Northwestern Tibetan Plateau

Lakes and glaciers are widely distributed on the Tibetan Plateau and are linked via hydrological processes. They are experiencing rapid changes due to global warming, but their relationships during the Holocene are less well known due to limited coupled geological records. Here, we analyzed the δ13C-VPDB and δ18O-VPDB values and ion content of calcite and aragonite in a 407-cm-long sediment core from Guozha Co, a closed basin on the northwestern Tibetan Plateau supplied by glacial meltwater, in order to understand how the lake responded to glacier changes during the Holocene. Our results indicate that the glacial meltwater lowered the lake’s temperature and the δ18Olake water and δ18Oendogenic + authigenic carbonate values and diluted the ion concentrations in the lake water. Three stages of evolution, 8.7–4.0, 4.0–1.5, and 1.5 kyr BP to present, are distinguished based on the decrease in glacial meltwater recharge. Guozha Co has been a closed basin since at least 8.7 kyr BP, and it has changed from a fresh water lake during 8.7–1.5 kyr BP to a brackish lake from 1.5 kyr BP to present due to several climate events. The famous 4.2 kyr BP cold event was identified in the core at 4.0 kyr BP, while warm events occurred at 6.2, 3.9, 2.2, 0.9, and 0.4 kyr BP. Both glaciers and lakes in this area are controlled by climate, but they exhibit opposite changes, that is, glaciers retreat and lakes expand, and vice versa. Our results provide an accurate interpretation of the cold events based on carbonate minerals and carbon–oxygen isotopes in glacial meltwater–recharged lake sediments.

Global tree intrinsic water use efficiency is enhanced by increased atmospheric CO2 and modulated by climate and plant functional types.

We conducted a meta-analysis of carbon and oxygen isotopes from tree ring chronologies representing 34 species across 10 biomes to better understand the environmental drivers and physiological mechanisms leading to historical changes in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (Anet) to stomatal conductance (gs), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO2 (Ca), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While Ca was a dominant environmental driver of iWUE, the effects of increasing Ca were modulated either positively or negatively by climate, including vapor pressure deficit (VPD), temperature, and precipitation, and by leaf and wood functional types. A dual carbon-oxygen isotope approach revealed that increases in Anet dominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases in Anet, whereas reductions in gs occurred in the remaining ∼17%. This meta-analysis provides a strong process-based framework for predicting changes in tree carbon gain and water loss across biomes and across wood and leaf functional types, and the interactions between Ca and other environmental factors have important implications for the coupled carbon-hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions in gs as the major driver of increasing tree iWUE and will better inform Earth system models regarding the role of trees in the global carbon and water cycles.

Triple Oxygen Isotopes in Meteoric Waters, Carbonates, and Biological Apatites: Implications for Continental Paleoclimate Reconstruction

Research Article| January 01, 2021 Triple Oxygen Isotopes in Meteoric Waters, Carbonates, and Biological Apatites: Implications for Continental Paleoclimate Reconstruction Benjamin H. Passey; Benjamin H. Passey Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, MI, 48103, USA * E-mail: passey@umich.edu Search for other works by this author on: GSW Google Scholar Naomi E. Levin Naomi E. Levin Department of Earth and Environmental Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, MI, 48103, USA * E-mail: nelevin@umich.edu Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2021) 86 (1): 429–462. https://doi.org/10.2138/rmg.2021.86.13 Article history first online: 04 Jan 2021 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Benjamin H. Passey, Naomi E. Levin; Triple Oxygen Isotopes in Meteoric Waters, Carbonates, and Biological Apatites: Implications for Continental Paleoclimate Reconstruction. Reviews in Mineralogy and Geochemistry 2021;; 86 (1): 429–462. doi: https://doi.org/10.2138/rmg.2021.86.13 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search The usefulness of triple isotope studies of natural systems is contingent on the existence of resolvable differences in mass-dependent fractionation exponents θ [where θ = ln(17/16α)/ln(18/16α)] among processes that are prevalent in the system(s) of interest, or on the existence of resolvable non-mass-dependent fractionation in the system. Both such contingencies are satisfied in continental hydroclimate systems. The process of evaporation of water involves molecular diffusion of water vapor through air, which carries a θ value of 0.5185 (Barkan and Luz 2007) that differs strongly from the value of 0.529 for... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The δ13C–δ18O variations in marble in the Hida Belt, Japan

AbstractMarble has a great potential to understand a history of various geological events occurring during tectonic processes. In order to decode metamorphic–metasomatic records on C–O isotope compositions of marble at mid‐crustal conditions, we conducted a C–O–Sr isotope study on upper amphibolite‐facies marbles and a carbonate–silicate rock from the Hida Belt, which was once a part of the crustal basement of the East Asian continental margin. Carbon and oxygen isotope analyses of calcite from marbles (Kamioka area) and a carbonate–silicate rock (Wadagawa area) show a large variation of δ13C [VPDB] and δ18O [VSMOW] values (from −4.4 to +4.2 ‰ and +1.6 to +20.8 ‰, respectively). The low δ13C values of calcites from the carbonate–silicate rock (from −4.4 to −2.9 ‰) can be explained by decarbonation (CO2 releasing) reactions; carbon–oxygen isotope modeling suggests that a decrease of δ13C strongly depends on the amount of silicate reacting with carbonates. The occurrence of metamorphic clinopyroxene in marbles indicates that all samples have been affected by decarbonation reactions. All δ18O values of calcites are remarkably lower than the marine‐carbonate values. The large δ18O variation can be explained by the isotope exchange via interactions between marble, external fluids, and/or silicates. Remarkably low δ18O values of marbles that are lower than mantle value (~+5 ‰) suggest the interaction with meteoric water at a later stage. Sr isotope ratios (87Sr/86Sr = 0.707255–0.708220) might be close to their protolith values. One zircon associated with wollastonite in a marble thin‐section yields a U–Pb age of 222 ± 3 Ma, which represents the timing of the recrystallization of marble, triggered by H2O‐rich fluid infiltration at a relatively high‐temperature condition. Our isotope study implies that the upper amphibolite‐facies condition, like the Hida Belt, might be appropriate to cause decarbonation reactions which can modify original isotope compositions of marble if carbonates react with silicates.

Detrital zircon age and biostratigraphic and chemostratigraphic constraints on the Ediacaran–Cambrian transitional interval in the Irkutsk Cis–Sayans Uplift, southwestern Siberian Platform

AbstractA number of ecological and geochemical transformations occurred during late Ediacaran and early Cambrian time, the effects of which are difficult to overestimate. However, the strong linkage of biostratigraphic and chemostratigraphic methods with lithofacies makes the localization of the Precambrian–Cambrian boundary and its correlation with lithologically contrasting sections highly debatable. We analyse the taxonomy and stratigraphic distribution of small skeletal fossils and trace fossils, the carbonate carbon and oxygen isotope composition, and U–Pb detrital zircon age in the Ediacaran–Cambrian transitional interval of the Irkutsk Cis–Sayans Uplift (southwestern Siberian Platform). This interval (Moty Group) comprises a transgressive succession with red-coloured alluvial to deltaic siliciclastic deposits (Shaman Formation) and overlying shallow-marine carbonates (Irkut Formation). The lower Irkut Formation hosts sporadic and poorly preserved tubularCambrotubulusfossils, which are known from both the terminal Ediacaran Period (c. 550–541 Ma) and the Terreneuvian Epoch (541–521 Ma), and typical Fortunian trace fossils, including an index ichnotaxon of the Cambrian boundaryTreptichnus pedum. The biostratigraphic and carbonate carbon isotope data and U–Pb concordia ages of 531.1 ± 5.2 Ma (mean weighted, 530.6 ± 5.3 Ma) of the five youngest zircon grains from the lower Irkut Formation indicate that at least the shallow-marine carbonates of the upper Moty Group correspond to the Cambrian Stage 2 (c. 529–521 Ma). In the Irkutsk Cis–Sayans Uplift, the Cambrian Period tentatively began before or during the accumulation of the alluvial to deltaic siliciclastic Khuzhir and Shaman formations, and this crucial divide remained unmarked in the palaeontological and isotopic records.

Characteristics and formation of sinian (Ediacaran) carbonate karstic reservoirs in Dengying Formation in Sichuan Basin, China

The Gaoshiti-Moxi Gas Field has been discovered recently in the central part of Sichuan Basin, which might be the biggest gas field in China. In this gas field, a large amount of gas is from Neoproterozoic Sinian (Ediacaran) karstic carbonate rocks. The gas was originated from cracking of oil, and the oil source rocks are the Sinian and Cambrian strata. In this study, the characteristics of these carbonate reservoir rocks have been studied based on subsurface data and outcrop observation. The carbonate rocks are at the Sinian Dengying Formation, and the reservoir properties are controlled by sedimentary facies and diagenesis. The Dengying Formation is composed of algal mounds and grainstone build-ups, which were developed in a restricted platform setting from glacial period to interglacial period. Due to tectonic movement and eustatic cycle in Sichuan Basin during Neoproterozoic Era, five sedimentary cycles of transgression and regression have been developed in the Dengying Formation. According to the sedimentary facies and carbon-oxygen isotope variation, five sedimentary cycles have been recognized. Due to the penecontemporaneous dolomitization, together with hypergene karstification and late burial dissolution, the reservoir properties of the carbonates rocks have been improved greatly.

Evaluation of oxygen isotopes and trace elements in planktonic foraminifera from the Mediterranean Sea as recorders of seawater oxygen isotopes and salinity

Abstract. The Mediterranean Sea is characterized by a relatively strong west to east salinity gradient, which makes it an area suitable for testing the effect of salinity on foraminiferal shell geochemistry. We collected living specimens of the planktonic foraminifer Globigerinoides ruber albus to analyse the relation between element ∕ Ca ratios, stable oxygen isotopes of their shells, and surface seawater salinity, isotopic composition and temperature. The oxygen isotopes of sea surface water also correlate with salinity in the Mediterranean during winter, which is when sampling for this study took place. Seawater oxygen and hydrogen isotopes are positively correlated in both the eastern and western Mediterranean Sea, although the relationship differs from previously reported values, especially in the eastern region. The slope between salinity and seawater oxygen isotopes is lower than previously published results. Still, despite the rather modest slope, seawater and foraminiferal carbonate oxygen isotopes are correlated in our dataset, albeit with large residuals and high residual variability. This scatter could be due to either biological variability in vital effects or environmental variability. Numerical models backtracking particles show that ocean-current-driven mixing of particles of different origins might dampen sensitivity and could result in an offset caused by horizontal transport. Results show that Na ∕ Ca is positively correlated with salinity and independent of temperature. As expected, foraminiferal Mg ∕ Ca increases with temperature, which is in line with earlier calibrations, and in the high salinity environment. By using living foraminifera during winter, the previously established Mg ∕ Ca–temperature calibration is extended to temperatures below 18 ∘C, which is a fundamental prerequisite of using single foraminifera for reconstructing past seasonality.

Water depth variation and its impact on carbonate content and oxygen isotopes: A study from a satellite lake near Lake Qinghai over the past 7.6 kyr

The oxygen isotopic composition of sedimentary carbonate (δ18Ocarb) provides information on the regional water balance between precipitation and evaporation in a lacustrine system. A volume effect was proposed to explain the abnormally low δ18Ocarb values in Lake Qinghai on the northern Tibetan Plateau, under the conditions of high evaporation and low water levels, as observed during the early Holocene. Volume effect theory states that the small volume of the lake and its sensitivity to isotopic signals from freshwater discharge control the δ18Ocarb values. In this study, we present 7.6-kyr records of carbonate content and oxygen isotopes from Lake Gahai, a small and saline satellite lake near Lake Qinghai. The records demonstrate that the water level and lake size of Lake Gahai considerably changed over the past 7.6 kyr, and the volume effect was responsible for the δ18Ocarb variations. The records also suggest contrasting responses of carbonate precipitation to water depth across different timescales. For example, over the past 7.6 kyr, the carbonate content decreased with increasing lake water levels. This can be explained by gradual water freshening under low evaporation conditions and enhanced connection between Lake Qinghai and Lake Gahai. On the centennial scale, however, the carbonate content varied significantly and was positively correlated with Lake Gahai's water level. Such a variation is probably due to the periodically lower lake water levels and a stronger influence of terrestrial clastic material, which contributed to the dilution of the carbonate content under centennial cool and dry periods. Therefore, in order to advance our understanding of past hydroclimatic variations in the climatically sensitive region, information across different timescales should be separated.

Comparing contemporary biogeochemical archives from Mercenaria mercenaria and Crassostrea virginica: Insights on paleoenvironmental reconstructions

Reconstruction of past environmental conditions from modern and fossil organisms is predicated on predictable biological responses to environmental variation. Thus, understanding how different species record commonly experienced environmental conditions is critical. This is particularly important in settings where multiple biogeochemical archives are available for study (e.g., a species rich fossil assemblage). Previous work has documented species-specific thermal tolerances and growth rates, which suggests proxy-based reconstructions may reflect a combination of biological controls and environmental variation. To investigate this potential complication, we compared oxygen isotope (δ18O) profiles from shells of two contemporary marine bivalve mollusks (Mercenaria mercenaria and Crassostrea virginica) growing in the same locality (Cape Lookout region, North Carolina, USA). Carbonate oxygen isotope samples (δ18Oc) were collected from shell material deposited between August 2016 and April 2019. These data were calibrated with predicted δ18Oc profiles (aragonite and calcite) calculated from high resolution environmental records (hourly temperature records and weekly water samples). M. mercenaria specimens were periodically stained with calcein during the study interval, facilitating transformation of the δ18Oc profiles into the time domain. δ18Oc profiles from C. virginica were fit directly to the predicted δ18Oc profile. Our results suggest that M. mercenaria and C. virginica living in the Cape Lookout region are capable of depositing shell material throughout the year. Shell growth in both species slows significantly, or halts altogether, during short term large magnitude freshwater input events. M. mercenaria examined in this study show a strong preference for growth during the warmest hours of the day. This observation suggests optimal growth temperatures lie close to the upper limit of temperatures recorded at the study site. C. virginica does not show a preference for shell growth during the warm or cold hours of the day. Thus, a major conclusion of this study is that two different species living in the same place at the same time may record different aspects of commonly experienced environmental conditions. Nevertheless, our findings suggest the fossil record may contain more detailed archives of past environmental conditions than previously thought. Future studies should focus on high-resolution, short-duration calibrations where environmental data is recorded at the time, location, and resolution.

Lake-Level Oscillation Based on Sediment Strata and Geochemical Proxies Since 11,000 Year From Tengger Nuur, Inner Mongolia, China

A 794-cm section and core from Tengger Nuur (42°27´N, 110°42´E) in the Inner Mongolia Plateau had been collected. 8 effective AMS 14C data had been determined to set an age-depth model after minus about 1920 years carbon reservoir effective. Based on the multi-proxies grain size, carbonate-content, TOC-content, ratios of Mg/Ca and Sr/Ca, and carbonate carbon and oxygen isotope, palaeoenvironmental change had been reconstructed since the last deglacial. Tengger Nuur was very shallow during the last deglacial under cool and wet climate, especially much shallower at interval cold YD event. Although, temperature and humidity increased from the early Holocene ~10500 to 8750 cal a BP, low lake-level indicated that summer monsoon did not strengthen the modern monsoon boundary in Inner Mongolia. High monsoon precipitation made lake to expand during 8750~5000 cal a BP, but lake-level oscillated in a shallow state under higher evaporation. A low lake-level event interrupted with the cold-wet climate at interval 5450~5100 cal a BP. Summer monsoon receded gradually to keep a deep lake under high effective humidity during 5000~2000 cal a BP, and low lake-level events punctuated at intervals 4300~3980 cal a BP and 3700~2750 cal a BP. Lakes shrank gradually to be salinization or completely desiccation in the cold and dry westerly after 2000 cal a BP, but oscillated much lower lake-levels at four periods of 1900~1740 cal a BP, 1500~1050 cal a BP, 550~400 cal a BP (Little Ice Age), 100 cal a BP ~AD 1985. A weak eastern summer monsoon was in the early Holocene, and lake-level oscillation was controlled by the effective humidity in the arid and semi-arid area.

Environmental conditions at the Last Interglacial (Eemian) site Neumark‐Nord 2, Germany inferred from stable isotope analysis of freshwater mollusc opercula

Mollusc biogenic carbonates are valuable records of past environmental conditions. In particular, carbonate oxygen (δ18O) and carbon (δ13C) stable isotopes can be used to reconstruct different physical and chemical parameters, according to the different genera used (marine, freshwater or terrestrial). The Last Interglacial (early Eemian) palaeolake of Neumark‐Nord 2 (NN2), Germany provides an excellent example of a Neanderthal archaeological site with abundant freshwater carbonate remains. As in other European contexts, one of the most abundant species is Bithynia tentaculata. In order to provide a robust regional baseline for the interpretation of the archaeological data, this study includes a calibration phase on modern B. tentaculata opercula. The results indicate that these calcitic structures are likely to be subjected to a growth slowdown/cessation during summer, which influences their geochemistry, reflecting mainly the water properties of the rest of the year. This modern calibration, together with the existing palaeoenvironmental reconstructions developed for NN2 (e.g. pollen data), represents a valuable opportunity to establish B. tentaculata opercula as reliable environmental proxies applicable to several other freshwater contexts. The isotope data of the NN2 opercula, in agreement with the pollen record, indicate that the major archaeological horizon was formed during a rather wet period and potentially in a semi‐forested environment. However, human occupation occurred also during drier phases at the site and within a wide temperature range, indicating the absence of restricted environmental preferences by the local Neanderthal groups.

Geochemical Significance of Clay Minerals and Elements in Paleogene Sandstones in the Center of the Northern Margin of the Qaidam Basin, China

The average thickness of Paleogene sandstones reaches about 3000–4000 m at the northern margin of the Qaidam Basin. However, the provenance and sedimentary environment of these sandstones are uncertain; thus, more comprehensive research is needed. Integrated research is conducted on the provenance and weathering process based on petrographic characteristics, clay minerals, and geochemical compositions of sandstones in the center of the northern Qaidam Basin. The results of lithofacies analysis show that the Paleogene sandstones were mainly derived from an active continental magmatic arc, subduction accretion, or a fold-thrust belt. The average illite content in the Paleogene clay minerals is more than 50%, followed by chlorite and smectite, which reflect climatic and environmental characteristics that were arid to semi-arid, whereas the characteristics of carbon–oxygen isotopes reveal a mainly freshwater sedimentary environment. The corrected chemical index of alteration (CIAcorr) is between 56.3 and 75.7, with an average value of 66.5. These results indicate that the provenance of the Paleogene sandstones in the center of the northern Qaidam Basin mainly formed under cold and dry climatic conditions and experienced limited chemical weathering with a small amount that underwent intermediate chemical weathering under warm and humid conditions.

Effects of progressive dolomitization on reservoir evolution: A case from the Permian–Triassic gas reservoirs of the Persian Gulf, offshore Iran

Dolostones are among the most important reservoir units within the Permian–Triassic Dalan and Kangan formations throughout the central Persian Gulf. In this study, petrographic and geochemical (carbonate carbon and oxygen isotopes) analyses led to the identification of several dolomitization phases from sabkha and hypersaline to burial diagenetic realms. An integration of the estimated temperatures of dolomites and their textural properties results in the differentiation of three thermal domains of dolomitization including low (LTD; < 30 °C), moderate (MDT; 30–50 °C), and high (HDT; > 50 °C) temperature dolomites. They are respectively associated with primary/fabric retentive and early micro-fabric destructive, late micro-fabric destructive, and fabric destructive (recrystallization) and saddle dolomites. Combination of petrographic, geochemical, and petrophysical results (core porosity and permeability, and pore-throat size distribution) shows a good correlation between the higher estimated temperatures of dolomites and progressive destruction of the primary depositional textures, larger crystal sizes, and higher porosity and permeability values. In general, the late micro-fabric destructive, wholly fabric destructive, and recrystallized dolomites led to the improvement of flow capacity in the studied intervals via increasing pore-throat size, decreasing tortuosity, and enlarging the crystal size.

THE TEMPLE OF QUETZALCOATL, TEOTIHUACAN: NEW DATA ON THE ORIGINS OF THE SACRIFICIAL VICTIMS

AbstractContinuing isotopic investigation of the sacrificial burials and trophies beneath the Feathered Serpent Pyramid (Temple of Quetzalcoatl) in ancient Teotihuacan, Mexico, has produced new results. Isotopic proveniencing using bioapatite strontium and structural carbonate oxygen isotopes in tooth enamel was applied to 39 samples, 24 from the sacrificial victims and 15 from the trophy jaws. Both the strontium and oxygen isotope ratios suggest that most or all of the sacrificial victims came from the central highlands of Mexico, which includes the area of Teotihuacan. In this sense, we find somewhat less multiethnicity represented among the military at Teotihuacan than previously thought. Analysis of carbon isotope ratios in enamel structural carbonate indicated a childhood diet dominated by maize, relatively homogeneous among the victims at the pyramid, and typical for much of pre-Hispanic Mesoamerica.

Resetting Southern Tibet: The serious challenge of obtaining primary records of Paleoaltimetry

Abstract Over the last several decades a number of studies have attempted to reconstruct the rise of the Himalaya and Tibet since India-Asia collision began in the early Cenozoic, with rather contradictory results. Here, we evaluate the efforts at reconstructing the history of this major mountain-building event as archived in oxygen, carbon, and clumped isotope records from carbonates exposed on the southern Tibetan Plateau. We find that a number of potential isotopic records of paleoaltimetry from Tibet – using both conventional oxygen and clumped isotope systems–may have been reset during burial and heating. Without exception, the marine δ18O values of Cretaceous and Paleogene marine carbonates across the orogen have been reset from their primary values of 0 ± 4‰ (VPDB) to lower values between −5 and −20‰, most conspicuously in the Indo-Asian suture zone. For this and other reasons, we view isotopic records of paleoaltimetry from the suture zone and adjacent Gangdese arc with great caution, especially early Cenozoic basin sediments that have experienced similar burial and heating as the underlying marine limestones. Outside the suture zone/arc, marine carbonates retain the isotopic imprint of low-elevation, meteoric diagenesis, a result that supports paleoaltimetric reconstruction from these areas using younger non-marine carbonates. We are even more cautious about temperature reconstructions using clumped isotope analyses, which previous studies suggest are sensitive to resetting at modest burial temperatures. With a few possible exceptions, primary clumped isotope values have probably not withstood the elevated temperatures of the suture zone and magmatic arc, nor burial depths of >3 km in basins outside the suture. In light of susceptibility to alteration of oxygen isotopes in carbonates in orogenic belts, future paleoaltimetric reconstruction prior to the Miocene on the Tibetan Plateau should couple δ18O, Δ47, and Δ'17O analysis of carbonates with analysis of noncarbonate archives—such as silicates and organic matter— that are less susceptible to resetting. Carbon, unlike oxygen, isotopic values from paleosol carbonate are well preserved from all periods, due to the very low C/O ratio of most altering fluids. Samples from within the suture zone yield a record of paleo-vegetation change covering much of the Cenozoic. Carbon isotopic values from paleosols have no analog among modern Tibetan soils and most resemble in appearance and chemical composition the vegetated soils in the lowlands of northern India today. Carbon isotopes from paleosols depict a major reduction in vegetation cover in the suture zone since 20 Ma, probably due to a combination of uplift of the suture zone and global cooling. Northward on the Lhasa and Qiantang terranes, the landscape was less vegetated in the Oligocene compared to the contemporaneous suture zone, but more vegetated and less arid than it is today.