Carbon isotopic compositions of pore and matrix carbonates in carbonate nodules, and origin of carbonate formation

Carbon isotopes of pedogenic carbonate are often used to study paleoenvironments, but the existence of detrital carbonate changes the carbon isotopic composition. To develop an experimental method to determine existence of detrital carbonate in carbonate nodules, and to avoid it during isotope analysis, 23 pedogenic carbonate nodules in Miocene loess from the Loess Plateau of China were studied through micromorphology and carbon isotope analysis. The difference in carbon isotopic composition between matrix carbonate (B) and pore carbonate (A) (δ¹³C_(B-A)) ranges from 0.27‰ to 0.44‰ in nodules containing detrital carbonate and –0.16‰ to 0.13‰ in nodules where detrital carbonate is absent. The latter is within measurement error, but the former is beyond it. Here we propose an isotopic approach to determine if nodules contain detrital carbonate: if δ¹³C_(B-A) is within the measurement error, the nodules do not contain detrital carbonate, and vice versa. We suggest that it is better to analyze pore carbonate instead of matrix carbonate when using carbon isotope of carbonate nodules to reconstruct paleoenvironments.     

δ13C difference between plants and soil organic matter along the eastern slope of Mount Gongga

Carbon isotope ratios (δ¹³C) of plants, litter and soil organic matter (0–5 cm, 5–10 cm and 10–20 cm) on the eastern slope of Mount Gongga were measured. The results show that δ¹³C values of plants, litter and soil organic matter all decrease first and then increase with altitude, i.e δ¹³C values gradually decrease from 1200 to 2100 m a.s.l., and increase from 2100 to 4500 m a.s.l. The δ¹³C altitudinal variations are related to the distribution of C₃ and C₄ plants on the eastern slope of Mount Gongga, because C₄ plants are observed to grow only below 2100 m, while C3 plants occur at all altitudes. There are significantly positive correla-tions among δ¹³C of vegetation, δ¹³C of litter and δ¹³C of soil organic matter, and litter, 0–5 cm, 5–10 cm and 10–20 cm soil or-ganic matter are 0.56‰, 2.87‰, 3.04‰ and 3.49‰ greater in δ¹³C than vegetation, respectively. Considering the influences of rising concentration of atmospheric CO₂ and decreasing δ¹³C of atmospheric CO₂ since the industry revolution on δ¹³C of plants, 1.57‰ is proposed to be the smallest correction value for reconstruction of paleovegetation using δ¹³C of soil organic matter.     

Vegetation evolution on the central Chinese Loess Plateau since late Quaternary evidenced by elemental carbon isotopic composition

There are many controversial issues in loess studies such as natural vegetation types on the Chinese Loess Plateau during the historical periods and the spatial and temporal evolution of C3/C4 plants. Elemental carbon isotopic composition （δ^13Cec） in the loess section may offer new evidence for these problems. Elemental carbon （EC） is produced by incomplete combustion of vegetation, and its carbon isotopic composition has a very small difference from that of the formal vegetation, then δ^13Cec can be used as a record to recover the changes of vegetation. Elemental carbon was extracted by applying the oxidation method from the Ioess-paleosol sequence in the central Chinese Loess Plateau, and its car- bon isotope composition was analyzed by the isotope mass spectrometer. The results showed that the vegetation in this region was a mixed type of C3 and C4 plants, dominated with C3 plants in most of the time. Since late Quaternary, C3/C4 plants may not follow a simple glacial-interglacial cycle mode on the Chinese Loess Plateau, but showing fluctuations. C3 plants increased gradually in L4 period, and more C3 plants occurred during $3 period, and C4 plants increased again during L3-- L2 periods, after that, Cs plants dominated again during S1 --S0 periods. During periods of paleosol development, C3 plants were abundant in S3 and S1, and there were more Ca plants in S2 and SO. During periods of loess sedimen- tation, there were more C3 plants in L4 and L1, and there were more C4 plants in L3 and L2. On the orbital timescale, the vegetation variations revealed by δ^13Cec record are consistent with the results of pollen data and also similar to the results obtained by organic carbon isotopic composition since the last glacial period.     

Carbon and nitrogen stable isotope analyses of mammal bone fossils from the Zhongba site in the Three Gorges Reservoir region of the Yangtze River,China

Based on AMS ¹⁴C dating data, carbon and nitrogen isotope analyses were conducted on mammal bone collagen of deer, cattle and pigs from the Zhongba site in the Three Gorges Reservoir region of the Yangtze River. These analyses were conducted to reconstruct palaeodiets of mammals, palaeoecology, palaeoenviroment and previous human activities in the study area. Results show that the collagen loss of bone did not change the in vivo isotopic composition of carbon and nitrogen stable isotopes, and most of the bone fossils were well preserved. The bone collagen of samples from deer had a mean δ¹³C of -23.1‰ and a mean δ¹⁵N of 4.7‰, suggesting that deer subsisted in a closed habitat and fed on branches and leaves. The bone collagen of cattle had a mean δ¹³C of –19.6‰ and a mean δ¹⁵N of 5.2‰, which indicates that cattle subsisted in an open habitat and fed on grasses and stems. The δ¹³C values show that both deer and cattle fed on C₃ plants and lived in the same ecosystem, but the t-test results show that deer δ¹³C and δ¹⁵N values were both more negative than those of cattle, indicating that they inhabited different niches. The δ¹³C and δ¹⁵N values of cattle partially overlapped those of deer, suggesting some competition in diets between them. The t-tests show that the δ¹³C and δ¹⁵N values of pigs were more positive than those of cattle and deer, which signifies that pigs occupied a higher trophic level compared to cattle and deer. The wide range of pig δ¹³C values demonstrates that pig trading had been taking place from early Neolithic Age to late Bronze Age. There were no significant differences in deer δ¹³C and δ¹⁵N values among different archaeological periods, making it clear that climatic, ecological and environmental conditions were kept relatively stable from 2200 to 4200 a BP. This stability may have been responsible for the extensive and complete cultural layers at the Zhongba site. The minimum number of samples required to estimate the mean δ¹³C values of deer, pigs and cattle are 8, 73 and 16, respectively, and for mean δ¹⁵N values of deer, pigs and cattle, the minimum numbers are 4, 5 and 6, respectively.     

Carbon isotopic composition,turnover and origins of soil CO<Subscript>2</Subscript> in a monsoon evergreen broadleaf forest in the Dinghushan Biosphere Reservoir,South China

Carbon isotopic compositions of soil CO2 in rainy season (July) from two natural soil profiles (DHLS & DHS) in the monsoon evergreen broadleaf forest in the Dinghushan Biosphere Reservoir (DBR), South China, are presented. Turnover and origins of soil CO₂ are preliminarily discussed in this paper. Results show that the content of soil CO2 varies between 6120 and 18718 ppmv, and increases with increasing depth until 75 cm, and then it declines. In DHLS, soil CO₂ δ¹³C ranges from −24.71‰ to −24.03‰, showing a significant inverse correlation (R²＝0.91) with the soil CO₂ content in the same layer. According to a model related to soil CO₂ δ¹³C, the soil CO₂ is mainly derived from the root respiration (>80%) in DHLS. While in DHS, where soil CO₂ ? 13C ranges from −25.19‰ to −22.82‰, soil CO₂ is primarily originated from the decomposition of organic matter (51%–94%), excluding the surface layer (20 cm, 90%). Radiocarbon data suggest that the carbon in soil CO₂ is modern carbon in both DHLS and DHS. Differences in ¹⁴C ages between the “oldest” and “youngest” soil CO₂ in DHLS and DHS are 8 months and 14 months, respectively, indicating that soil CO₂ in DHLS has a faster turnover rate than that in DHS. The ¹⁴C values of soil CO₂, which range between 100.0‰ and 107.2‰ and between 102.5‰ and 112.1‰ in DHLS and DHS, respectively, are obviously higher than those of current atmospheric CO₂ and SOC in the same layer, suggesting that soil CO₂ is likely an important reservoir for Bomb-¹⁴C in the atmosphere.     

The elemental carbon record in Weinan loess section since the last 21 ka

We studied the records of elemental carbon (EC) of the last 21 ka in the Weinan loess section, Shanxi Province. The variations of EC abundance and δ¹³C_EC value along with depth (or age) were presented. There are four large peaks of EC abundance around the following years: 20.16 ka, 17.76 ka, 11.97 ka and 4.49 ka. Climatic situation was changed rapidly during these periods. The peaks around 11.97 ka and 20.16 ka are particularly sharp, occurring over intervals of tens to hundreds of years, which could represent short-duration intense events. δ¹³C_EC values in the upper 4 m of the Weinan loess section vary between −11.71‰ and −21.34‰, which suggests that the vegetation pattern of the last 21 ka on the Loess Plateau is C₄-dominated grasses.     

Variation of δ13C in karst soil in Yaji Karst Experiment Site, Guilin

This study deals with δ₁₃ C variation in karst soil system of Yaji Karst Experiment Site, Guilin, a typical region of humid subtropical karst formations. Samples of near ground air, plant tissue, soil and water (soil solution and karst spring) were respectively collected on site in different seasons during 1996–1999. Considerable variation of δ₁₃ C values are not only found with different carbon pools of soil organic carbon, soil air CO₂ and soil water HCO₃⁻, but also with the soil depths and with different seasons during a year. The °₁₃ C values of CO₂ both of near ground air and soil air are lower in July than those in April by 1‰–4‰ PDB. Our results indicate that the δ₁₃ C values of carbon in the water and air are essentially dependent on interface carbon interaction of air-plant—soil-rock—water governed by soil organic carbon and soil CO₂ in the system.     

Relationship between climatic conditions and the relative abundance of modern C3 and C4 plants in three regions around the North Pacific

Using −24‰ and −14‰ as the endpoints of stable carbon isotopic composition of total organic carbon (δ¹³C_TOC) of surface soil under pure C₃ and C₄ vegetation, and surface soil δ¹³C_TOC data from eastern China, Australia and the Great Plains of North Amer- ica, we estimate the relative abundance of C₃/C₄ plants (i.e., the ratio of C₃ or C₄ biomass to local primary production) in modern vegetation for each region. The relative abundance of modern C₃/C₄ vegetation from each region is compared to the corresponding climatic parameters (mean annual temperature and precipitation) to explore the relationship between relative C₄ abundance and climate. The results indicate that temperature controls the growth of C₄ plants. However, even where temperature is high enough for the growth of C₄ plants, they will only dominate the landscape when precipitation declines as temperatures increase. Our results are consistent with those of other investigations of the geographic distribution of modern C₄ plant species. Therefore, our results provide an important reference for interpretation of past C₃/C₄ relative abundance records in these three regions.     

Variations of organic carbon isotopic composition and its environmental significance during the last glacial on western Chinese Loess Plateau

The last glacial period is characterized by a cold and dry climate with low atmospheric CO2 concentration. The relatively arid climate and low CO2 concentration are favorable to the growth of C4 plants, but the low temperature limits the development of th…     

Stable isotopes of modern herbivore tooth enamel in the Tibetan Plateau: Implications for paleoelevation reconstructions

Fossil tooth enamel from herbivores is considered one of the best proxies for paleoclimate and paleoelevation reconstructions, due to its low susceptibility to diagenetic alteration. A synthesis of oxygen and carbon isotope analyses of modern tooth enamel from herbivores such as Tibetan yaks, asses and antelopes is assessed. The average δ 13C(PDB) value of herbivore tooth enamel in the Lhasa and south Qiangtang terrains is-11.3‰± 1.1‰, whereas in the north Qiangtang and Hoh Xil terrains value is-10.2‰± 1.4‰...     

CO<Subscript>2</Subscript> gas emplacement age in the Songliao Basin: Insight from volcanic quartz <Superscript>40</Superscript>Ar-<Superscript>39</Superscript>Ar stepwise crushing

Due to a lack of suitable minerals, the gas/oil emplacement ages have never been accurately obtained before. CH₄-CO₂-saline- bearing secondary inclusions are found in quartz from the volcanic rocks of the Yingcheng Formation, the container rocks of the deep CO₂ gas reservoir in the Songliao Basin. The inclusion fluid was trapped into microcracks in quartz during the gas emplacement and accumulation, providing an optimal target for the ⁴⁰Ar-³⁹Ar stepwise crushing technique to determine the CO₂ gas emplacement age. ⁴⁰Ar-³⁹Ar dating results of a quartz sample by stepwise crushing yield a highly linear-regression isochron with an age of 78.4±1.3 Ma, indicating that the accumulation of the deep CO₂ gas reservoir in the Songliao Basin occurred in the late Cretaceous. This is the first time to report an exact isotopic age for a CO₂ gas reservoir, which indicates that the ⁴⁰Ar-³⁹Ar dating can serve as a new technique to date the oil/gas emplacement ages.     

Carbon isotopic records in paleosols over the Pliocene in Northern China: Implication on vegetation development and Tibetan uplift

Carbon isotopic composition of pedogenic carbonate can be used to estimate the proportion of C₄ and C₃ plants. Here we present carbon isotopic data of carbonate in a red earth section at Xifeng, central Loess Plateau. Results show that C₄ vegetation increased in ∼4.4 Ma B.P., stabilized between 4.0 and 3.0 Ma B.P. The character and timing of C₄ expansion on the Loess Plateau are similar, but different with other localities, e.g. Pakistan and Africa, implying that regional climate changes were main factors driving the expansion of C₄ plants. This event is comparable in timing with increased aridity evidenced by Xifeng grain size and North Pacific eolian dust records. Therefore we argue that the Pliocene expansion of C₄ plants in northern China might have been caused by the increased aridity, which in turn might be related to rapid uplift of the Tibetan Plateau.     

The occurrence and significance of C25HBI in Cenozoic saline lacustrine source rocks from the Western Qaidam Basin,NW China

The saline lacustrine deposit of the Oligocene Lower Ganchaigou Formation is the main source rock for the Western Qaidam Basin,NW China.In this study,abundant highly branched isoprenoids with 25 carbon atoms (C25HBI) were detected in the upper section of the Lower Ganchaigou Formation.C25HBI is a biomarker for diatoms,and can provide information regarding biogeochemical processes during production and preservation of sedimentary organic matter.The carbon isotopic values of C25HBI in these source rocks were in the range of-18‰ to-20‰.The relative enrichment in 13C of C25HBI suggests that these isoprenoids were derived from diatom blooms.In this case,the diatoms used HCO3-as an additional carbon source to dissolved CO2 in water column due to their high biological productivity.Conversely,the diatom blooms indicated an abundant nutrient supply to the environment and high primary productivity.Thus,the occurrence of 13C-enriched C25HBI could be an ideal marker for good source rocks.The decreased concentration of CO2(aq) in the water column induced by diatom blooms can result in an enrichment in 13C of organic matter synthesized by primary producers.This may be a possible reason for the occurrence of 13C-enriched organic matter in the Cenozoic source rocks of the Western Qaidam Basin.Previous studies have shown that the high carbon number n-alkanes sourced from diatoms have no carbon preference.Hence,the sources of n-alkanes in regional source rocks are complex,and should be considered when discussing the sources of organic matter in future studies.     

Effect of elevated CO2 on CH4 emission

Global CH₄ emission may increase under CO₂ enrichment condition, which is projected for the future. CO₂ enrichment could affect CH₄ emission in two ways: (i) Photosynthesis of plants that also include plants in rice paddies and natural wetlands will be stimulated under CO₂ enrichment condition. CH₄ emission rate may be increased due to the accumulation of more plant biomass, root exudes and soil organic matters. (ii) Combined with other global warming forces, CO₂ enrichment may bring a change of atmospheric temperature and precipitation around the world. CH₄ emission will also be changed with the variation of the area and distribution of rice paddies and natural wetlands.     

Re-examining the reliability of tree-ring isotope ratio as a historical CO2 proxy

To examine the reliability of using tree ring δ¹³C and the model of isotopic fractionation in reconstructing atmospheric CO₂ levels, we studied the variations of some important parameters of several subtropical species under natural field conditions. It was found that, different from other researchers’ results, leaf conductance to CO₂ transfer, g, did not change in proportion to the change in rate of CO₂ assimilation, A, with the result that intercellular concen- tration of CO₂, C_i, could not keep constant. Thus, we conclude that the use of tree-ring isotope ratios in the reconstruction of atmospheric CO₂ variation based on the presupposition that C_i keeps constant during assimilation is not reliable under current circumstances.     

Geochemical characteristics and formation process of natural gas in Kela 2 gas field

 On the basis of a large amount of natural gascomponents and the carbon isotope as well as some otheranalysis data in Kela 2 gas field, the geochemical character-istics, source, origin, and formation process of natural gashave been discussed. The components of gas in the field tendto be "dry", and the drying coefficient is close to 1.0. Thecarbon isotope tends to be heavier, for instance, the averageof δ¹³C₁ is -27.36‰ and that ofδ¹³C₂ is -18.5‰. Compre-hensive analysis shows that humic natural gas in the Kuqapetroleum system comes mainly from Triassic and Jurassicsource rocks, and the contribution of Jurassic source rocks tothe pool maybe is more than that of Triassic rocks. The maincause that the gas tends to be dry and bears heavier isotopecomposition lies in the fact that Kela 2 natural gas is the ac-cumulation of late production of humic source rocks, and itis affected by the abnormal high pressure as well. Consider-ing the hydrocarbon generating and structural history, wecan regard the gas pool formation processes as twice fillingand twice adjusting (destroying), that is, the filling anddestroying process in the early Himalayan movement and thefilling and adjusting in the late Himalayan movement.     

Trend, seasonal and diurnal variations of atmospheric CO2 in Beijing

The concentration of atmospheric CO₂ in Beijing increased rapidly at a mean growth rate of 3.7% · a⁻¹ from 1993 to 1995. After displaying a peak of (409.7±25.9) μmol · mol⁻¹ in 1995, it decreased slowly. Both the almost stable anthropogenic CO₂ source and increasing biotic CO₂ sink contribute to the drop of CO₂ concentration from 1995 to 2000. The seasonal variation of CO₂ concentration exhibits a clear cycle with a maximum in winter, averaging (426.8±20.6) μmol · mol⁻¹, and a minimum in summer, averaging (369.1±6.1) μmol·mol⁻¹. The seasonal variation of CO₂ concentration is mainly controlled by phenology. The mean diurnal variation of atmospheric CO₂ concentration for a year in Beijing is highly clear: daily maximum CO₂ concentration usually occurs at night, but daily minimum CO₂ concentration does in the daytime, with a mean diurnal difference more than 34.7 μmol·mol⁻¹. It has been revealed that the interannual variations of atmospheric CO₂ concentration in winter and autumn regulated the interannual trend of atmospheric CO₂, whereas the interannual variation of CO₂ concentration in summer affected the general tendency of atmospheric CO₂ in a less degree.     

Effect of Ca on CO2 corrosion properties of X65 pipeline steel

The effect of Ca²⁺ on CO₂ corrosion to X65 pipeline steel was investigated in the simulated stratum water of an oil field containing different concentrations of Ca²⁺. It is found that Ca²⁺ can enhance the corrosion rate, especially in the Ca²⁺ concentration from 256 to 512 mg/L, which can be attributed to the growing grain size and loosing structure of corrosion scales with increasing Ca²⁺ concentration. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) investigations reveal that a complex carbonate (Fe, Ca)CO₃ forms at high Ca²⁺ concentration due to the gradual replacement of Fe²⁺ in FeCO₃ by Ca²⁺.     

Indicators of δ13C and δ18O of gas hydrate-associated sediments

The analyses of δ13C and δ18O of gas hydrate-associated sediments from two cores on Hydrate Ridge in Cascadia convergent margin offshore Oregon, eastern North Pacific show the values of d 13C from -29.81‰ to -48.28‰ (PDB) and d 18O from 2.56‰ to 4.28‰ (PDB), which could be plotted into a group called typical carbonate minerals influenced by the methane in cold venting. Moreover, the values of d 13C and d 18O show a consistent trend in both cores from top to bottom with increasing of d 13C and decreasing of d 18O. This trend could be explained as an effect caused by the anaerobic oxidation of methane (AOM) in depth and the oxygen fraction during the formation of gas hydrate in depth together. These characteristics of d 13C and d 18O indicate that the gas hydrate-associated sediments are significantly different from the normal marine carbonates, and they are deeply influenced by the formation and evolution of gas hydrate. So, the distinct characteristics of d 13C and d 18O of gas hydrate-associated sediments could be undoubtedly believed as one of parameters to determine the presence of gas hydrates in other unknown marine sediment cores.     

High-resolution carbon isotope record for the Paleocene-Eocene thermal maximum from the Nanyang Basin,Central China

The Paleocene-Eocene thermal maximum (PETM) was a transient episode of global warming, associated with massive atmospheric greenhouse gas input that occurred at the Paleocene/Eocene boundary. Biostratigraphic and isotope stratigraphic studies indicate that the PETM event is well documented in the marl deposits of the Yuhuangding section in the Nanyang Basin, Central China, with a carbon isotope negative excursion of ~6.1‰ within 19-m-thick marl deposits. This is the highest resolution record of the PETM so far found in the world. The PETM event was triggered within 2-cm-thick marl sediments, with a decrease of δ¹³C (stable carbon isotope ratio) from –3.2‰ to –5.2‰, suggesting a massive methane hydrate release for a transient period that was possibly caused by a catastrophic event. A comparison between marine and terrestrial records indicates a “Three-Phase Model” for the PETM event. Initially there is a rapid negative excursion in the δ¹³C record, followed by a slowly decreasing trend, and then a gradual positive recovery, corresponding respectively to a rapid dissociation of oceanic methane hydrate, followed by a slow release of methane and then the consumption of the released methane.