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1.
C and O isotope composition of Middle-Upper Miocene and Lower Pliocene carbonates from Kerch-Taman Region (Eastern Paratethys) have been studied in order to reconstruct palaeoenvironmental variability and post-sedimentation changes. The δ13C and δ18О values of the Upper Sarmatian to Lower Pliocene organogenic carbonates reflect the desalinization of paleobasins, global Late Miocene Cooling, and increase in seasonal temperature fluctuations. Isotopic composition of the Middle Sarmatian organogenic carbonates was strongly influenced by evaporation processes, high bioproductivity, and local submarine methane emissions. Warm climate and low bioproductivity together with unstable hydrological regime during the Late Chokrakian and the Karaganian times influenced the isotope composition of primary carbonates. Calcite shell of Spiratella sp. (δ13C =–0.4‰ and δ18О =–0.4‰) from Tarkhanian sediments was formed in warm marine environment. Dolomitization prevails over other secondary mineralization in the studied carbonate rocks. Two groups of secondary dolomites that are characterized by negative and positive δ13C values have been recognized. Lowe δ13C values (up to–31.4‰) in dolomites indicate the influence of both dissolved inorganic carbon (DIC) from oxidized organic matter (Сorg) and methane. Dolomites with positive δ13C values (7.0 and 7.8‰) associat with migration of CO2- and CH4-containing saline groundwater.  相似文献   

2.
We investigate the growth of the northern Tibetan Plateau and associated climate change by applying oxygen and carbon isotopic compositions in Cenozoic strata in the southwestern Qaidam basin. The X-ray diffraction and isotopic studies reveal that the carbonate minerals are mainly authigenic and they do not preserve any evidence for detrital carbonate and diagenesis. The isotope data show large fluctuations in the δ18O and δ13C values in the middle–late Eocene, indicating relatively warm and seasonal dry climate. The positive correlation of the δ18O and δ13C values in the Oligocene and the positive shift of the δ13C values from the Eocene to Oligocene suggest that the climate changed to arid in the Oligocene. However, the δ18O values show negative shift, which is closely related to the global cooling event. During the Miocene, the δ13C values vary between –2‰ and –4‰, whereas the δ18O values show continuous negative shift. The mean δ18O values decrease from –8.5‰ in the early Miocene to –10.0‰ in the late Miocene. The stable isotope-based paleoaltimetry results suggest that the elevation of the southwestern Qaidam basin was approximately 1500 m in the middle–late Eocene and Oligocene. Subsequently, during Miocene the crustal uplift process started and the elevation reached approximately 2000 m in the early Miocene and 2500 m in the late Miocene, which suggests large-scale growth of the northern Tibet Plateau during the Miocene.  相似文献   

3.
The Early Jurassic period was characterized by extreme environmental changes, as reflected by major global carbon isotope anomalies and abrupt changes in oxygen isotope and elemental records of marine organisms. Available data suggest an overall warm Early Jurassic climate interrupted by periods of severe cooling, with a climatic optimum during the early Toarcian. Available geochemical studies, however, have mainly focused on the northern margin of the Tethys Ocean, so that the palaeogeographic extent of these environmental perturbations, latitudinal palaeotemperature gradients and climate belt boundaries remain poorly constrained. Here we report the first stable isotope records of brachiopod shells (δ13C and δ18O values) from the Upper Sinemurian-Middle Toarcian interval in the southern margin of the Tethys Ocean (northwest Algeria). These data were used to better constrain the palaeoenvironmental evolution of the North Gondwana margin during the Early Jurassic, which likely played an important role on supra-regional climate. The diagenetic history of the analysed brachiopod shells was monitored using scanning electron microscopy, and elemental (manganese and strontium) compositions. The brachiopod δ13C and δ18O data show very similar trends as those reported for various Tethyan regions, and record negative carbon and oxygen isotope excursions near the Sinemurian–Pliensbachian and Pliensbachian–Toarcian transitions and during the Toarcian oceanic anoxic event (T-OAE). Despite these similarities, the carbon and oxygen isotope records are systematically offset towards more positive δ13C values (average +0.5‰) and more negative δ18O values (−1.0‰) compared to those obtained from sites of higher palaeolatitudes in the northern Tethyan margin. These offsets suggest a spatial heterogeneity in the stable isotope composition of dissolved inorganic carbon in the Early Jurassic Ocean and a marked latitudinal temperature gradient between the southern and northern margins of the Tethys.  相似文献   

4.
The δ13C and δ18O values of well-preserved carbonate rhizoliths (CRs) provide detailed insights into changes in the abundance of C3 and C4 plants in response to approximately decadal-scale changes in growing-season climate. We performed stable isotope analyses on 35-40 CRs sampled at 1-cm intervals from an 18-cm-thick paleosol formed in southern Illinois during Wisconsin interstadial 2. Minimum δ13C values show little variation with depth, whereas maximum values vary dramatically, and average values show noticeable variability; maximum δ18O values vary less than the minimum δ18O values. These findings indicate that a diverse and stable C3 flora with a limited number of C4 grass species prevailed during this interval, and suggest that the maximum growing-season temperatures were relatively stable, but minimum growing-season temperatures varied considerably. Two general patterns characterize the relationships between the δ13C and δ18O values obtained from the 1-cm samples. In some cases, low δ13C values correspond to low δ18O values and high δ13C values correspond to high δ18O values, suggesting that cooler growing-season temperatures favored C3 and warmer growing-season temperatures favored C4 plants. In other cases, low δ13C values correspond to high δ18O values, likely suggesting that wetter growing-season conditions were favorable to C3 plants. The high density of well-preserved CRs in this paleosol provides a unique opportunity to study detailed ecological responses to high-resolution variability in growing-season climate.  相似文献   

5.
Vegetation inherited from a Pliocene subtropical climate evolved through obliquity oscillations and global cooling leading to modern conditions. An integrated, highly time-resolved record of pollen and stable isotopes (δ18O and δ13C of Globigerina bulloides) was obtained to understand vegetation responses to Early Pleistocene climate changes. Continental and marine responses are compared in the Central Mediterranean region with a particular consideration of environmental changes during anoxic events.Pollen data illustrate vegetation dynamics as follows: [1] development of mesothermic elements (warm and humid conditions); [2] expansion of mid- and high-altitude elements (cooler but still humid conditions); and [3] strengthening of steppe and herb elements (cooler and dry conditions). These successions correlate with precession. δ18O variations recorded by Globigerina bulloides define two cycles (MIS 43-40) related to obliquity. At northern low- to mid-latitudes, the pollen signal records temperature and wetness changes related to precession even during global climate changes induced by obliquity. This may result in unexpected increasing wetness during glacial periods, which has to be considered specific to the Central and Eastern Mediterranean region. Lastly, an analysis of anoxic events reveals that enhanced runoff is indicated by increasing frequency of the riparian trees Liquidambar and Zelkova.  相似文献   

6.
High resolution δ13C and δ18O profiles recorded in precisely dated speleothems are widely used proxies for the climate of the past. Both δ13C and δ18O depend on several climate related effects including meteorological processes, processes occurring in the soil zone above the cave and isotope fractionation processes occurring in the solution layer on the stalagmite surface. Here we model the latter using a stalagmite isotope and growth model and determine the relationship between the stable isotope values in speleothem calcite and cave parameters, such as temperature, drip interval, water pCO2 and a mixing coefficient describing mixing processes between the solution layer and the impinging drop.The evolution of δ13C values is modelled as a Rayleigh distillation process and shows a pronounced dependence on the residence time of the solution on the stalagmite surface and the drip interval, respectively. The evolution of δ18O values, in contrast, is also influenced by buffering reactions between the bicarbonate in the solution and the drip water driving the δ18O value of the bicarbonate towards the value expected for equilibrium isotope fractionation between drip water and calcite. This attenuates the dependence of the δ18O values on drip interval. The temperature dependence of δ18O, however, is more pronounced than for δ13C and in a similar range as expected for fractionation under equilibrium conditions.We also investigate the isotopic enrichment of the δ13C and δ18O values along individual growth layers and, thus, the slopes expected for Hendy tests. The results show that a positive Hendy test is only possible if isotope fractionation occurred under disequilibrium conditions. However, a negative Hendy test does not exclude that isotope fractionation occurred under disequilibrium conditions. A more reliable indicator for disequilibrium fractionation is the enrichment of the δ13C values along an individual growth layer.  相似文献   

7.
The oxygen and carbon isotopic composition of surficial carbonates is a key technique for reconstructing past environments and climates. The understanding of modern isotopic systems is, however, a vital first step before applying these techniques to fossil examples. In this study the δ18O of aragonite shells from three different freshwater mollusc species and the δ18O of tufa stromatalites are analysed and compared to the modern temperature and isotopic regime of the river system in which they form (the river Gipping, Sproughton, southeast England). In all cases this range of carbonates appear to form in isotopic equilibrium with the δ18O of modern waters at water temperatures experienced during the summer months. It is, therefore, likely that in Quaternary interglacial deposits of the UK and western Europe the δ18O of fossil freshwater molluscs and tufa stromatolite carbonates will provide an indication of past summer temperature regimes and not mean annual temperatures, as has been previously suggested. The paper concludes by discussing the implications of this study for the isotopic analysis of interglacial deposits in Britain and western Europe.  相似文献   

8.
Tufa sediments are freshwater carbonates that precipitate in karst regions after degassing of carbon dioxide from groundwater in contact with the atmosphere. When laminated, these carbonates can provide high‐resolution records for the study of climate, hydrological and environmental conditions at the time of their precipitation. The formation of these carbonates directly depends on the hydrological regime, and in karst regions discontinuous discharges are often recorded. This study investigates the record of recent laminated tufa sediments precipitated downstream overflow springs in Trabaque Canyon (central Spain). The hydrological dynamics of the karst system were monitored for over three years and a stable isotope record was obtained from laminated tufa carbonates precipitated from an overflow spring. Additionally, a hydrological model of overflow springs was generated and a tufa δ18O record under constrained parameters was simulated. Temperature is the dominant control of the variation in tufa δ13C and δ18O values within each lamina, although when comparing different laminae, δ13CDIC and δ18O of river water are also major controls. The positive correlation between tufa δ13C values and water temperature is caused by the fractionation occurred by carbon dioxide degassing due to the thermal dependence of carbon dioxide solubility. Additionally, the system recorded a temperature‐independent degassing process caused by the large gradient between groundwater and atmospheric carbon dioxide that is limited to the proximity of the spring. This study cautions on the risk of assuming continuous deposition when studying laminated tufa sediments and highlights the potential of their stable isotope records to provide hydrological information of their aquifers during the past.  相似文献   

9.
The Kaiparowits Formation contains an exceptionally rich history of tectonic, climatic, and biologic conditions within the Western Interior of North America during the Campanian. Here we reconstruct aspects of the southern Cordilleran foreland basin's paleohydrology using δ18O and δ13C values determined from unionoid bivalve shells and pedogenic carbonate nodules derived from a suite of lithofacies associations. Unionoid shells derived from fluvial deposits display average water δ18O estimates of −13.7‰ ± 2.1 (1σ) (VSMOW) and shell δ13C values of −4.0‰ ± 1.5 (VPDB), whereas pedogenic carbonate nodules display average values of −6.0‰ ± 0.5 and −8.7‰ ± 0.8, respectively. Unionoid shells derived from pond deposits fall in between the two other environments with average values of −9.5‰ ± 1.8 and −5.7‰ ± 2.1, in δ18O and δ13C values respectively. Water δ18O estimates are interpreted to represent high altitude runoff within river systems, low elevation precipitation within the basin onto floodplain soils, and varying degrees of mixing between these two components within floodplain ponds. δ13C values track the isotopic composition of dissolved inorganic carbon within river, soil, and pond waters with high values likely reflecting greater contribution from chemically weathered marine carbonates exposed in the hinterland and lower values reflecting greater contributions from the in situ degradation of plant matter. Up-section there is a shift to lower δ18O values and higher δ13C values in fluvially-derived unionoid shells that post-dates an incursion of the Western Interior Seaway, but coincides with a shift in sediment provenance, an increase in basin sedimentation rates, and a change to a more anastomosed-style channel morphology within the basin foredeep depocentre. By combining the isotopic patterns with previously published sedimentologic, climate model, and paleofloral records we find: 1) additional evidence for humid, wet, and potentially monsoonal conditions within the region, 2) support for a tectonic uplift event, potentially related to Laramide deformation, and 3) greater aggradation and overbank flooding within the alluvial system in response to the uplift event.  相似文献   

10.
Carbon and oxygen stable isotopic composition of Cenozoic lacustrine carbonates from the intramontane Qaidam Basin yields cycles of variable length and shows several distinct events driven by tectonics and climate changes. From Eocene to Oligocene, the over-all trend in the δ13C composition of lacustrine carbonates shows a shift toward higher values, possibly related to higher proportions of dissolved inorganic carbon transported to the lake or lower input of soil derived CO2. At the same time, the δ18O composition of lacustrine carbonates is decreasing in accordance with the global cooling trend and northwards drifting of the whole region. During the Miocene, distinct isotopic events can be recognized, although their interpretation and linkage to a certain tectonic event remains difficult. These events may be related to uplift in the Himalayas, to the strongest phase of uplift in the Altyn Mountains, to pronounced subsidence of the Qaidam Basin or to the expansion of C4 plants on land. Generally cold, highly evaporative conditions can be deduced from enrichment of δ18O isotopic compositions during Pliocene and Quaternary times.  相似文献   

11.
A high resolution analysis of benthic foraminifera as well as of aeolian terrigenous proxies extracted from a 37 m-long marine core located off the Mauritanian margin spanning the last ~ 1.2 Ma, documents the possible link between major continental environmental changes with a shift in the isotopic signature of deep waters around 1.0–0.9 Ma, within the so-called Mid-Pleistocene Transition (MPT) time period. The increase in the oxygen isotopic composition of deep waters, as seen through the benthic foraminifera δ18O values, is consistent with the growth of larger ice sheets known to have occurred during this transition. Deep-water mass δ13C changes, also estimated from benthic foraminifera, show a strong depletion for the same time interval. This drastic change in δ13C values is concomitant with a worldwide 0.3‰ decrease observed in the major deep oceanic waters for the MPT time period. The phase relationship between aeolian terrigeneous signal increase and this δ13C decrease in our record, as well as in other paleorecords, supports the hypothesis of a global aridification amongst others processes to explain the deep-water masses isotopic signature changes during the MPT. In any case, the isotopic shifts imply major changes in the end-member δ18O and δ13C values of deep waters.  相似文献   

12.
Stable isotopes were measured in the carbonate and organic matter of palaeosols in the Somma–Vesuvius area, southern Italy in order to test whether they are suitable proxy records for climatic and ecological changes in this area during the past 18000 yr. The ages of the soils span from ca. 18 to ca. 3 kyr BP. Surprisingly, the Last Glacial to Holocene climate transition was not accompanied by significant change in δ18O of pedogenic carbonate. This could be explained by changes in evaporation rate and in isotope fractionation between water and precipitated carbonate with temperature, which counterbalanced the expected change in isotope composition of meteoric water. Because of the rise in temperature and humidity and the progressive increase in tree cover during the Holocene, the Holocene soil carbonates closely reflect the isotopic composition of meteoric water. A cooling of about 2°C after the Avellino eruption (3.8 ka) accounts for a sudden decrease of about 1‰ in δ18O of pedogenic carbonate recorded after this eruption. The δ13C values of organic matter and pedogenic carbonate covary, indicating an effective isotope equilibrium between the organic matter, as the source of CO2, and the pedogenic carbonate. Carbon isotopes suggest prevailing C3 vegetation and negligible mixing with volcanogenic or atmospheric CO2. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

13.
MIS 11 is often considered to be the best climatic analogue for the Holocene. Many studies have suggested, however, that it is a period of extreme climate warmth comparable in temperatures to the Middle and Late Pliocene. In Britain deposits of the Hoxnian interglacial are correlated to MIS 11 and multi-proxy techniques can be used to reconstruct the climate of this interglacial. Soil, groundwater and freshwater carbonates are common in Hoxnian deposits and the stable isotopic composition of these precipitates can be used to increase our understanding of MIS 11 environments in Britain. Carbonates from Marks Tey, Clacton, Swanscombe, Elveden and West Stow are studied, the stratigraphic context of which indicates that their formation is broadly synchronous (in the mid-Hoxnian, pollen zones Ho II to Ho III). The carbon isotopic composition of groundwater and pedogenic carbonates is typically depleted with respect to δ13C (ca −9 to −8‰ VPDB) reflecting uptake of plant respired CO2 during water migration/recharge. The carbon isotopic composition of lacustrine carbonate is more enriched with respect to δ13C (ca 0-1‰VPDB) reflecting the equilibration of lake waters with atmospheric CO2. The δ18O of groundwater and pedogenic carbonates is slightly more enriched than modern soil carbonates but not as enriched as soil carbonates formed under interglacials that were warmer than the Holocene (i.e. the Cromerian). The stable isotopic composition of Hoxnian carbonates does not, therefore, indicate that this interglacial was characterised by uniquely warm climates in the context of other Middle Pleistocene interglacials and the Holocene. This is contrary to many marine and littoral records from around the world but consistent with environmental records from Britain and Europe.  相似文献   

14.
Lithological, chemical, and stable isotope data are used to characterize lacustrine tufas dating back to pre-late Miocene and later unknown times, capping different surfaces of a Tertiary carbonate (Sinn el-Kedab) plateau in Dungul region in the currently hyperarid south-western Egypt. These deposits are composed mostly of calcium carbonate, some magnesium carbonate and clastic particles plus minor amounts of organic matter. They have a wide range of (Mg/Ca)molar ratios, from 0.03 to 0.3. The bulk-tufa carbonate has characteristic isotope compositions: (δ13Cmean = −2.49 ± 0.99‰; δ18Omean = −9.43 ± 1.40‰). The δ13C values are consistent with a small input from C4 vegetation or thinner soils in the recharge area of the tufa-depositing systems. The δ18O values are typical of fresh water carbonates. Covariation between δ13C and δ18O values probably is a reflection of climatic conditions such as aridity. The tufas studied are isotopically similar to the underlying diagenetic marine chalks, marls and limestones (δ13Cmean = −2.06 ± 0.84‰; δ18Omean = −10.06 ± 1.39‰). The similarity has been attributed to common meteoric water signatures. This raises large uncertainties in using tufas (Mg/Ca)molar, δ13C and δ18O records as proxies of paleoclimatic change and suggests that intrinsic compositional differences in material sources within the plateau may mask climatic changes in the records.  相似文献   

15.
Samples of chert nodules, diagenetic carbonates and evaporites (gypsum/anhydrite) collected from the gypsiferous limestones of the Kef Eddour Member (Ypressian‐Priabonian) near Metlaoui and Sehib (Tunisia) show selective silicification with great variety in the silicified by‐products. Based on δ13C values, which support an organic origin for the carbon, carbonates replaced evaporites microbially through bacterial sulphate reduction. Observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites (anhydrite/gypsum). In the absence of early diagenetic carbonate phases, silica with δ18O values from +25 to +28·6‰ [standard mean ocean water (SMOW)] replaced the outer parts of anhydrite nodules at pH < 9. In contrast, pore‐fluid pH values > 9 in the innermost parts of the anhydrite nodules prevented silica precipitation. The record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus. The microbial diagenetic replacement of evaporites (bacterial sulphate reduction) by carbonates (calcite, aragonite and dolomite) favoured silica replacement of carbonates rather than evaporites. Silica, with δ18O signature of +21 to +26‰ (SMOW), replaced carbonates on a volume‐for‐volume basis, yielding a more siliceous groundmass, and accounting for 90–95% of the nodules. The relatively higher δ18O values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed (meteoric/marine) to marine water. The lower δ18O values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage. The silica supply for chert formation could be derived from the reworked bio‐siliceous deposits (diatomites) to the west of the basin [vestiges of an opal‐CT precursor undetectable by X‐ray diffraction (XRD) were revealed by δ29Si magic‐angle‐spinning nuclear magnetic resonance investigations], diagenesis of the extraformational and overlying clay‐rich beds (the host limestones are clay‐poor as shown by XRD measurements), and minor volcanogenic and hydrothermal contributions during early diagenetic stages.  相似文献   

16.
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.  相似文献   

17.
《International Geology Review》2012,54(15):1909-1921
This paper reports the carbon and oxygen isotope compositions of lacustrine carbonate sediments from the Palaeogene Shahejie Formation, Qikou depression, Bohaiwan Basin, with the aim of determining the palaeoenvironmental conditions in the region. Results from Es2, the second member of the Shahejie Formation, showed values of δ13C and δ18O from –1.2‰ to +2.4‰ (average +0.6‰) and from –6.8‰ to –4.7‰ (average –5.7‰), respectively, suggesting a relatively hot climate attending deposition. The slightly closed nature of the lake, which contains brackish water, resulted in higher carbonate δ13C and δ18O values than in a meteoric environment. The values of δ13C and δ18O preserved within the carbonates of the overlying lower Shahejie I (Es1) varied between +1.3‰ and +4.9‰ (average +3.2‰) and from ?4.4‰ to ?1.8‰ (average ?3.1‰), respectively, indicating that the climate became colder at that time. Subsequently, a marine transgression caused the salinity of the lake water to increase. The values of δ13C and δ18O were controlled by salinity. The high δ13C values were also influenced by the rapid burial of the lake organisms and by algal photosynthesis. Values of δ13C and δ18O from carbonates in upper Es1 ranged from ?8.0‰ to +11.0‰ (average +10.1‰) and from ?5.0‰ to ?1.5‰ (average ?3.4‰), respectively, indicating a slight increase in the temperature over time. In the closed and reducing environment, extremes in δ13C values resulted from biochemical fermentation. The positive δ13C excursion recorded in the carbonates of the Shahejie Formation in the Qikou depression indicates that the palaeoclimate underwent a significant transformation during the Eocene and the Oligocene.  相似文献   

18.
《China Geology》2023,6(1):50-60
The Lomagundi-Jatuli Event (LJE) refers to the significant positive carbon isotope excursion in seawater constituents that occurred immediately after the increase in atmospheric oxygen content during the Paleoproterozoic (2.22–2.06 Ga). The δ13C values of 46 dolostone samples collected from the Paleoproterozoic Yongjingshao Formation varied in the range of 0.05 ‰–4.95 ‰ (V-PDB; maximum: 4.95‰) in this study, which may be related to the multicellular eukaryotes in the Liangshan Formation in the Yimen Group. They are much higher than the δ13C values of marine carbonates (?1.16‰ on average). The δ13C values of other formations in the Paleoproterozoic Yimen Group are negative. The notable positive carbon isotope anomalies of the Yongjingshao Formation indicate the response to the LJE at the southwestern margin of the Yangtze Block, which is reported for the first time. Furthermore, they are comparable to the δ13C values of carbonates in the Dashiling Formation of the Hutuo Group in the Wutaishan area in the North China Craton, the Wuzhiling Formation of the Songshan Group in the Xiong’ er area, Henan Province, and the Dashiqiao Formation of the Liaohe Group in the Guanmenshan area, Liaoning Province. Therefore, it can be further concluded that the LJE is a global event. This study reveals that LJE occurred in Central Yunnan at 2.15–2.10 Ga, lasting for about 50 Ma. The macro-columnar, bean-shaped, and microfilament fossils and reticular ultramicrofossils of multicellular eukaryotes in this period were discovered in the Liangshan Formation of the Yimen Group. They are the direct cause for the LJE and are also the oldest paleontological fossils ever found. The major events successively occurring in the early stage of the Earth include the Great Oxygenation Event (first occurrence), the global Superior-type banded iron formations (BIFs), the Huronian glaciation, the Great Oxygenation Event (second occurrence), the explosion of multicellular eukaryotes, the positive carbon isotope excursion, and the global anoxic and selenium-rich sedimentary event. The authors think that the North China Craton and the Yangtze Craton were possibly in different tectonic locations of the same continental block during the Proterozoic.©2023 China Geology Editorial Office.  相似文献   

19.
Petrography demonstrates the presence of three types of fibrous calcite cement in buildup deposits of the Kullsberg Limestone (middle Caradoc), central Sweden. Translucent fibrous calcite has intrinsic blue luminescence (CL) indicative of pure calcite. This cement has 2–5 mol% MgCO3, low Mn and Fe (≤ 100 p.p.m.), and is considered to be slightly altered to unaltered, primary low- to intermediate-Mg calcite. Grey turbid fibrous calcite has variable but generally low MgCO3 content (most analyses <2 mol%) and variable CL response, with Mn and Fe concentrations up to 1200 and 500 p.p.m., respectively. The heterogeneous characteristics of this variety of fibrous calcite are caused by diagenetic alteration of a translucent fibrous calcite precursor. Light-brown turbid fibrous calcite has low MgCO3 (near 1 mol%) and variable Mn (up to 800 p.p.m.) and Fe (up to 500 p.p.m.) concentrations, with an abundance of bright luminescent patches, which formed during alteration caused by reducing diagenetic fluids. The δ13C and δ18O values of all fibrous calcite form a tight field (δ13C=1·7 to 3·1‰ PDB, δ18O= ? 2·6 to ? 4·1‰ PDB) compared with fibrous calcite isotope values from other units. Fibrous calcite δ18O values are larger than adjacent meteoric or burial cements, which have δ18O δ ? 8‰ PDB. Consequently, most diagenetic alteration of Kullsberg fibrous calcite is interpreted to have occurred in the marine diagenetic realm. First-generation equant and bladed calcite cements, which pre-date fibrous calcite, are interpreted as unaltered, low-Mg calcite marine cements based on δ13C and δ18O data (δ13C = 2·3 to 2·7‰ PDB, δ18O= ? 2·8 to ? 3·5‰ PDB). Unlike fibrous cement, which reflects global sea water chemistry, first-generation equant and bladed calcite are indicators of localized modification of seawater chemistry in restricted settings. Kullsberg abiotic marine cements have larger δ18O values than most Caradoc marine precipitates from equatorial Laurentia. Positive Kullsberg δ18O values are attributed to lower seawater temperatures and/or slightly elevated salinity on the Baltic platform relative to seawater from which other marine precipitates formed.  相似文献   

20.
In a semiarid climatic zone, such as the Eastern Mediterranean region, annual rainfall variations and fractionation processes in the epikarst zone exert a profound influence on the isotopic compositions of waters seeping into a cave. Consequently, the isotopic compositions of speleothems depositing from cave waters may show complex variations that need to be understood if they are to be exploited for paleoclimate studies. This is confirmed by a four-year study of the active carbonate-water system in the Soreq cave (Israel). The δ18O (SMOW) values of cave waters range from −6.3 to −3.5%.. The highest δ18O values occur at the end of the dry season in waters dripping from stalactites, and reflect evaporation processes in the epikarst zone, whereas the lowest values occur in rapidly dripping (fast-drip) waters at the peak of the rainy seasons. However, even fast-drip waters are about 1.5%. heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual evaporated water in the epikarst zone. δ13C (PDB) values of dissolved inorganic carbon (DIC) vary from −15.6 to −5.4%., with fast-drip waters having lower δ13C values (mostly −15.6 to −12%.) and higher DIC concentrations relative to pool and stalactite-drip water. The low δ13C values of fast-drip waters and their supersaturation with respect to calcium carbonate indicates that the seepage waters have dissolved both soil-CO2 derived from overlying C3-type vegetation and marine dolomite host rock.The δ18O (PDB) values of various types of present-day low-magnesium calcite (LMC) speleothems range from −6.5 to −4.3%. and δ13C values from −13 to −5.5%. and are not correlated with speleothem type. An analysis of δ18O values of present-day calcite rafts and pool waters shows that they form in oxygen isotope equilibrium. Similarly, the measured ranges of δ13C and δ18O values for all types of present-day speleothems are consistent with equilibrium deposition at cave temperatures. The δ13C–δ18O range of contemporary LMC thus reflects the variations in temperatures and isotopic compositions of the presentday cave waters. The 10%. variation in the δ13C values in waters can be modeled by a simple Rayleigh calculation of the carbon isotope fractionation accompanying CO2-degassing and carbonate precipitation. These variations may obscure the differences in the carbon isotopic composition of speleothems that could arise when vegetation cover changes from C3 to C4-type plants. This consideration emphasizes that it is necessary to characterize the full range of δ13C values associated with contemporaneous speleothems in order to clarify the effects of degassing from those due to differing vegetation types.Isotopic studies of a number of different types of fossil LMC speleothems show many of them to exhibit isotopic trends that are similar to those of present-day LMC, but others show both higher and lower δ18O ranges. In particular, the higher δ18O range has been shown by independent age-measurements to be associated with a period of drier conditions. The results of the study thus indicate that it is necessary to work on a well calibrated cave system in semiarid climates and that the fossil speleothem record should be obtained from different types of contemporaneous deposit in order to fully characterize the δ18O–δ13C range representative of any given climatic period.  相似文献   

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