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1.
The nature and dynamics of climate change in central Asia since the late Pleistocene are controversial. Moreover,most of the published studies focus mainly on the evolution of moisture conditions, and there have been few attempts to address changes in seasonality. In this study, records of δ13Corg, TOC, TN, C/N and grain size were obtained from lacustrine sediments at Yili Basin, Xinjiang, NW China. Our aim was to reconstruct the trend in seasonality of precipitation from the last glaciation to the Holocene. The organic matter content of the sediments is derived predominantly from terrestrial plants. The δ13Corgvalues vary from-19.4‰ to-24.8‰, indicating that the vegetation was dominated by C_3 plants. Winter-spring precipitation is identified as the factor determining the relative proportions of C_3 and C_4 plants in the region. A negative trend in δ13Corgcorresponding to an increase in the relative abundance of C_3 plants indicate a trend of increasing winter-spring precipitation from the last glaciation to the Holocene. The increased incidence of wintertime storms in the interior of Asia is suggested to result in the increase of winterspring precipitation in the Holocene.  相似文献   

2.
It is a consensus that marine carbonate archives the isotopic composition of seawater dissolved inorganic carbon(DIC,δ~(13)C_(sw)),the largest active C reservoir in the hydrosphere.Carbonate carbon isotope(δ~(13)C_(carb)) excursions have been used to reflect perturbations of the global carbon cycle and related environmental change.However,the deep time δ~(13)C_(carb) records indicate faster and more pronounced perturbations of the carbon cycle compared to the present day.Here,we report δ~(13)C_(carb) and elemental compositions of Late Paleozoic carbonate sections from South China,showing negative correlations between δ~(13-)C_(carb) and Fe~(2+) content of carbonate(Fe_(carb)).We suggest that,because Late Paleozoic carbonate was mainly produced by benthic carbonate-secreting organisms,δ~(13)C_(carb)recorded the isotopic composition near the seafloor,where benthic flux derived from anaerobic organic matter degradation delivers both Fe~(2+) and ~(13)C-depleted DIC from porewater.The binary mixing between seawater and benthic flux would result in the deviation of δ~(13)C_(carb) fromδ~(13)C_(sw).The negative correlation implies that δ~(13)C_(carb) is influenced by benthic flux and is affected by the seafloor redox and sedimentation rate.The deep time spatially heterogeneous and temporally oscillatory δ~(13)C_(carb) records in the basin-scale could be alternatively attributed to the variations of local environmental factors rather than aδ~(13)C_(sw) depth-gradient.Thus,the seafloor carbonate precipitation is continuously affected by diagenetic reactions in sediments,suggesting that δ~(13)C_(carb) recording the seawater DIC composition is conditional.Our study urges that the interpretation of δ~(13)C_(carb) should also consider the sedimentary process and depositional environment of marine carbonate.  相似文献   

3.
Stable isotope paleoaltimetry has provided unprecedented insights into the topographic histories of many of the world's highest mountain ranges. However, on the Tibetan Plateau(TP), stable isotopes from paleosols generally yield much higher paleoaltitudes than those based on fossils. It is therefore essential when attempting to interpret accurately this region's paleoaltitudes that the empirical calibrations of local stable isotopes and the relations between them are established. Additionally,it is vital that careful estimations be made when estimate how different isotopes sourced from different areas may have been influenced by different controls. We present here 29 hydrogen isotopic values for leaf wax-derived n-alkanes(i.e., δD_(wax) values,and abundance-weighted average δD values of C_(29) and C_(31)) in surface soils, as well as the δD values of soil water(δD_(sw)) samples(totaling 22) from Mount Longmen(LM), on the eastern TP(altitude ~0.8–4.0 km above sea level(asl), a region climatically affected by the East Asian Monsoon(EAM). We compared our results with published data from Mount Gongga(GG). In addition,47 river water samples, 55 spring water samples, and the daily and monthly summer precipitation records(from May to October,2015) from two precipitation observation stations were collected along the GG transect for δD analysis. LM soil δD_(wax) values showed regional differences and responded strongly to altitude, varying from.160‰ to.219‰, with an altitudinal lapse rate(ALR) of.18‰ km~(-1)(R~2=0.83; p0.0001; n=29). These δD_(wax) values appeared more enriched than those from the GG transect by ~40‰. We found that both the climate and moisture sources led to the differences observed in soil δD_(wax) values between the LM and GG transects. We found that, as a general rule, ε_(wax/rw), ε_(wax/p) and εwax/sw values(i.e., the isotopic fractionation of δD_(wax) corresponding to δD_(rw), δD_p and δD_(sw)) increased with increasing altitude along both the LM and GG transects(up to 34‰ and 50‰, respectively). Basing its research on a comparative study of δD_(wax), δD_p, δD_(rw)(δD_(springw)) and δD_(sw), this paper discusses the effects of moisture recycling, glacier-fed meltwater, relative humidity(RH), evapotranspiration(ET), vegetation cover, latitude,topography and/or other factors on ε_(wax/p) values. Clearly, if ε_(wax-p) values at higher altitudes are calculated using smaller ε_(wax-p) values from lower altitudes, the calculated paleowaterδD_p values are going to be more depleted than the actual δD values, and any paleoaltitude would therefore be overestimated.  相似文献   

4.
The hydrogen isotopic composition(δD) of leaf wax long-chain n-alkanes(C27, C29, and C31) from lacustrine sediments has been widely applied to reconstruct terrestrial paleoclimatic and paleohydrological changes. However, few studies have addressed whether the aquatic-derived n-alkanes can affect the δD values of lake sedimentary long-chain n-alkanes, which are usually regarded as a recorder of the terrestrial hydrological signals. Here we systematically investigated δD values of long-chain n-alkanes from modern aquatic plants, both near-shore and off-shore surface sediments, surrounding terrestrial plant litters, as well as river water and lake water in Lake Qinghai and its satellite lakes on the northeastern Qinghai-Tibet Plateau. Our data showed that(i) δD values of long-chain n-alkanes from aquatic plants varied from-184‰ to-132‰ for n-C27, from-183‰ to-138‰ for n-C29, and from-189‰ to-130‰ for n-C31, respectively, with no significant differences among the three n-alkanes homologues;(ii) δD values of long-chain n-alkanes from aquatic plants were generally more positive than those from surrounding terrestrial plants, possibly because that they recorded the D-enrichment of lake water in this semi-arid region;(iii) δD values of long-chain n-alkanes from surface sediments showed significant differences among the three n-alkanes homologues, due to the larger aquatic input of n-C27 to the sedimentary lipid pool than that of n-C31, and(iv) n-C27 δD values of near-shore aquatic plants and near-shore sediments are more negative than those from off-shore as a result of lower δD values of near-shore lake water. Our findings indicate that in this region(i) the offset between sedimentary n-C27 and n-C31 δD values(ΔδDC27-C31) could potentially be used to evaluate if sedimentary long-chain n-alkanes are derived from a single source;(ii) while δD values of n-C27 may be influenced by lake water hydrological changes, sedimentary n-C31 is derived predominantly from terrestrial plants and thus its δD can serve as a relatively reliable indicator for terrestrial paleoclimatic and paleohydrological reconstructions.  相似文献   

5.
Carbon preference index (CPI) of long-chain n-alkanes preserved in surface soil increases gradually from southeastern China to the north margin of Loess Plateau.Along this latitudinal transect,the CPI value correlates to relative humidity,precipitation,and temperature with a negative linear relationship,respectively,whereas the correlation of CPI to temperature is relatively weak.In the Wuyi,Shennongjia,and Tianshan Mountains,CPI values do not change systemically with altitude increasing (or temperature decreasing).However,mean value of CPI for the individual mountain increases in turn from the humid mountain to the arid.These results jointly suggest that aridity (or humidity) is a dominate climate factor in altering soil CPI value.High CPI values of geological records therefore indicate the arid paleoclimate.Though long-chain n-alkanes in soil are derived mainly from leaf wax of terrestrial vascular plants,the regular latitudinal variations of soil CPI might not be caused by the change of vegetation.We speculate that increased long-chain n-alkanes from microbes and/or enhanced biodegradation in the humid climate lead to the decrease of soil CPI.  相似文献   

6.
Along the Proterozoic stratigraphic sections of Jixian, Tianjin, the δ13C values of carbonates are mostly negative in the Changchengian (Changcheng System), varying approximately within a range of (0±1 )‰in the Jixianian (Jixian System), and have commonly positive values of (2±2)‰ in the Qingbaikouan (Qingbaikou System). From Chuanlinggou Formation to Dahongyu Formation, i.e. from ca. 1700 Ma to 1600 Ma, the δ13C values of carbonates increase from about -3‰ to 0‰, but oscillate with large excursion in Dahongyu Formation. Two large excursions occur in the Jixianian, one in Yangzhuang Formation and the other in the third sub-member of Wumishan Formation overlaying. These integrated and continuous records of carbon isotope reflect secular variations of δ13C value in ancient seawater from ca. 1700 Ma to ca. 800 Ma. The increase and subsequent oscillation of seawater δ13C value occurring from the ages of ca. 1700 to ca. 1600 Ma and <1300 Ma may signify two correlative global tectonic events.  相似文献   

7.
Chen  Chaojun  Yuan  Daoxian  Cheng  Hai  Yu  Tsailuen  Shen  Chuanchou  Edwards  R. Lawrence  Wu  Yao  Xiao  Siya  Zhang  Jian  Wang  Tao  Huang  Ran  Liu  Ziqi  Li  Tingyong  Li  Junyun 《中国科学:地球科学(英文版)》2021,64(10):1761-1773
It is conducive to the sustainable development of human beings in karst regions to research the mechanism of karst rocky desertification(KRD) expansion. Whether the large-scale KRD in southwestern China is caused by climate change or human activities is still controversial. In this study, the evolution of the KRD in southwestern China over the past 2000 years was reconstructed through the high-precision δ~(13) C record of stalagmites from Shijiangjun(SJJ) Cave, Guizhou Province, China. Theδ~(13) C of the stalagmites from SJJ Cave exhibited heavy values from the Medieval Warm Period(MWP) to the Little Ice Age(LIA). Furthermore, the δ~(13) C records of other stalagmites and tufa from southwestern China also showed the same significant heavy trend. Because the stalagmite δ~(13) C could record the change of ecological environment, it indicated that the consistent change of the stalagmites δ~(13) C may record the process of KRD expansion in the karst regions of southwestern China. During the MWP, the stronger Asian summer monsoon and the northward movement of the rain belt led to a dry period in southwestern China and a wet period in northern China. In contrast, it was wet in southwestern China and dry in northern China during the LIA.In addition, after the Jing-Kang event(JK event, AD1127) occurred at the end of the Northern Song dynasty, the political and economic center of China migrated to southern China for the first time, which changed the population distribution pattern of larger population in the north and smaller population in the south. Therefore, the expansion of KRD in southwestern China was exacerbated in the MWP due to the change of climate in southwestern China, the migration of a large number of people, wars, the large-scale reclamation of arable land, and the cultivation of large areas of crops.  相似文献   

8.
The significance of the various biogeochemical pathways that drive carbon cycling and the relative fractions of dissolved inorganic carbon(DIC) produced by these reactions within the sulfate-methane transition zone(SMTZ) are still being debated. Unraveling these processes is important to our understanding of the benthic DIC sources and their contributions to the global carbon cycle. Here, we measure pore water geochemistry(chlorine, sulfate, methane, Ca~(2+), Mg~(2+), DIC and δ~(13)C-DIC) as well as solid geochemistry(sedimentary organic carbon(SOC) and δ~(13)C of SOC) in nearshore sediments from Qi'ao Island in the Pearl River Estuary of the Southern China Sea. Our analysis indicates that SOC originates from the mixing of carbon from terrestrial and marine sources, and that terrestrial materials dominate the net loss of SOC during the degradation of organic matter, especially at sites located near the river outlets. Sulfate reduction via SOC degradation is not appreciable in the upper sediment layer due to conservative mixing-dilution by freshwater. However, below this layer, the anaerobic oxidation of methane(AOM) and methanogenesis occur. Within the SMTZ, the δ~(13)C mass balance shows that the proportions of DIC derived from organoclastic SO_4~(2-) reduction(OSR) and AOM are 50.3% to 66.7% and 0.1% to 17.9%, respectively, whereas methanogenesis contributes 17.0% to 43.9%. This study reveals that the upward diffusion of DIC from ongoing methanogenesis significantly influences carbon cycling within the SMTZ in these estuarine sediments. As a result, we suggest that the plots of the ratio of change in sulfate to change in DIC in pore water should be used with caution when discriminating between sulfate reduction pathways in methane-rich sediments.  相似文献   

9.
In this study,39 leaf samples of three angiosperms(Betula albo-sinensis,tree species;and Caragana jubata and Berberis diaphana,shrub species) were collected in the middle-east parts at 2300–3640 m asl of the Qilian Mountains to study the variations of leaf characteristics of angiosperms with altitude change in inland high-altitude regions of China.Five leaf indexes,viz.epidermal cell density(ED),stomatal density(SD),stomatal index(SI),leaf vein density(VD) and carbon isotopic ratio(δ13C),were analyzed in laboratory.The results show that there are significant or even very significant linear correlations between the five indexes and altitude,of which SD,SI and VD exhibit a negative correlation with altitude,while ED and δ13C exhibit a positive correlation with altitude.Such a correlation assemblage is quite different from the situation in the low-altitude humid environment.Generally,only an assemblage of positive correlations can be observed between the indexes(viz.SD,SI and δ13C,etc.) and the altitude in the low-altitude humid environment,which were caused mainly by the plants' responses to the change of atmospheric CO2 concentration(Cα).However,an assemblage of the negative and positive correlations found here may be attributed mainly to the plants' responses to the change of physiological drought caused by change of low temperature,and here it is preliminarily called the inland high-altitude pattern of plant leaf variations.  相似文献   

10.
Total 26 modern soil samples were collected from various regions under different climate conditions from tropical to arid temperate in China and systematically analyzed for their organic matters by GC/MS in order to evaluate climatic impacts on soil organic components. Abundant lipids molecules were recognized, including n-alkanes, n-alkenones, and long-chain branched alkanes. We find the pre- dominance of main peaks of long-chain n-alkanes (nC29, nC31, nC33) and short-chain ones (nC16, nC17, nC18) records information of soil generation in warm-humid and cold-dry regions. The proportion of n-alkanes (nC16 nC17 nC18) to (nC29 nC31 nC33) varies in good agreement with moisture-heat conditions and thus probably can serve as a new index for climate change. The ratios of C21-/nC22 , nC17/nC31 and (nC15 nC17 nC19) / ( nC27 nC29 nC31) of n-alkanes, indicating respectively input ratios of lower bacterial alga, aquatic organisms, and higher plants and terraneous organisms, co-vary well in different climate regions from forest to grassland and desert, suggesting that they have also reflected the difference of climates between monsoon region and inland one. The C21-/C22 ratio of n-alkan-2-one records largely the discrepancy of temperature from north to south of China bordered by the Qinling Mountains, but less humidity. The C21-/C22 ratio of n-alkan-3-ones changes well with climatic factors, such as tem- perature and humidity. The biogenic source of series A-D long-chain branched alkanes may be derived from some kinds of special epiphyte that most likely live in weak oxic-anoxic and moisture-heat envi- ronments, suggesting their distribution record as well some information on climatic change. All these researches demonstrate that the distributions of lipids molecules in modern soils in China record well signals of climates from quite different climatic regions, and can serve as important climatic proxies to reveal climatic change over China.  相似文献   

11.
Tan  Liangcheng  Liu  Wen  Wang  Tianli  Cheng  Peng  Zang  Jingjie  Wang  Xiqian  Ma  Le  Li  Dong  Lan  Jianghu  Edwards  R. Lawrence  Cheng  Hai  Xu  Hai  Ai  Li  Gao  Yongli  Cai  Yanjun 《中国科学:地球科学(英文版)》2020,63(10):1622-1632
Evaluating anthropogenic impacts on regional vegetation changes during historical time is not only important for a better understanding of the Anthropocene but also valuable in improving the vegetation-climate models. In this study, we analyzed stable isotopes(δ~(18)O, δ~(13)C) and trace elements(Mg/Ca, Sr/Ca) of a stalagmite from Huangchao Cave in central Shandong, northern China.~(230)Th and AMS~(14)C dating results indicate the stalagmite deposited during 174BC and AD1810, with a hiatus between AD638 and 1102. Broad similarities of the δ~(18)O and trace elements in the stalagmite suggest they are reliable precipitation indexes. The δ~(13)C of the stalagmite, a proxy of vegetation change, was generally consistent with local precipitation and temperature variations on a centennial-scale before the 15th century. It typically varied from –9.6‰ to –6.3‰, indicating climate controlled C3 type vegetation during this period. However, a persistent and marked increasing trend in the δ~(13)C record was observed since the 15th century, resulting in δ~(13)C values from –7.7‰ to –1.6‰ in the next four centuries. This unprecedented δ~(13)C change caused by vegetation deterioration cannot be explained by climate change but is fairly consistent with the dramatically increasing population and farmland in Shandong. We suggest that the increasing deforestation and reclamation in central Shandong began to affect vegetation in the mountain region of central Shandong since the 15th century and severely destroyed or even cleared the forest during the 16th–18th century.  相似文献   

12.
The hydrogen isotopic composition(δD) of n-alkanes in lacustrine sediments is widely used in palaeoenvironmental studies, but the heterogeneous origins and relative contributions of these lipids provide challenges for the interpretation of the increasing dataset as an environment and climatic proxy. We systematically investigated n-alkane δD values from 51 submerged plants(39 Potamogeton, 1 Myriophyllum, and 11 Ruppia), 13 algae(5 Chara, 3 Cladophora, and 5 Spirogyra) and 20 terrestrial plants(10 grasses and 10 shrubs) in and around 15 lakes on the Tibetan Plateau. Our results demonstrate that δD values of C_(29) nalkane are correlated significantly with the lake water δD values both for algae(R~2=0.85, p0.01, n=9) and submerged plants(R~2=0.90, p0.01, n=25), indicating that δD values of these algae and submerged plants reflect the δD variation of lake water. We find that apparent hydrogen isotope fractionation factors between individual n-alkanes and water(εa/w) are not constant among different algae and submerged plants, as well as in a single genus under different liminological conditions, indicating that the biosynthesis or environmental conditions(e.g. salinity) may affect their δD values. The δD values of submerged plant Ruppia in the Xiligou Lake(a closed lake) are significant enriched in D than those of terrestrial grasses around the lake(one-way ANOVA,p0.01), but the algae Chara in the Keluke Lake(an open lake) display similar δD values with grasses around the lake(one-way ANOVA, p=0.8260.05), suggesting that the n-alkane δD values of the algae and submerged plants record the signal of D enrichment in lake water relative to precipitation only in closed lakes in arid and semi-arid area. For each algae and submerged plant sample, we find uniformed δD values of different chain length n-alkanes, implying that, in combination with other proxies such as Paq and Average Chain Length, the offset between the δD values of different chain length n-alkanes can help determine the source of sedimentary n-alkanes as well as inferring the hydrological characteristics of an ancient lake basin(open vs closed lake).  相似文献   

13.
The evolution of the East Asian summer monsoon(EASM) during the Holocene has long been of significant interest.Knowledge of past EASM variability not only increases our understanding of monsoon dynamics on a long timescale,but it also provides an environmental and climatic background for research into Chinese cultural development.However,the timing of the EASM maximum remains controversial.The popular concept of an "early Holocene maximum" is mainly based on speleothemδ~(18)O(δ~(18)O_c) records from caves in southern China;however,the interpretation of δ~(18)O_C as a reliable proxy for EASM intensity is being increasingly challenged.The present paper is a critical review of the climatic significance of the δ~(18)O_C record from China.Firstly,we suggest that precipitation in northern China is an appropriate index of EASM intensity,the variation of which clearly indicates a mid-Holocene monsoon maximum.Secondly,an interregional comparison demonstrates that the precipitation record in northern China is quite different from that in southern China on a range of timescales,and is inconsistent with the spatial similarity exhibited by speleothem oxygen isotope records.Furthermore,both modeling and observational data show that the δ~(18)O_C records from southern China indeed reflect changes in precipitation δ~(18)O(δ~(18)O_P) rather than precipitation amount,and therefore that their use as an EASM proxy is inappropriate.Finally,we address several significant monsoon-related issues-including the driving mechanism of the EASM on an orbital timescale,the climatic significance of speleothem oxygen isotopes,and the relationship between atmospheric circulation and precipitation in monsoonal regions.  相似文献   

14.
Source rock extracts and crude oils from the Songliao Basin were analyzed by high-temperature gas chromatography (HTGC), gas chromatography-mass spectrometry (HTGC-MS) and gas chromatography-isotope ratio-mass spectrometry (GC-IRMS), for high molecular-weight alkanes. The distributions of n-alkanes in the Nenjiang Formation extracts are in the C14―C63 range; a bimodal distribution occurs in the C-21 and C21―40 regions. The C30―C37 n-alkanes are accompanied by C29―C35 hopanes, whereas the high molecular-weight C45―C47 n-alkanes co-occur with abundant isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The high δ 13C values of the n-alkanes and the microscopic maceral compositions indicate a highly diversified organic source input for the Nenjiang Formation source rocks, ranging from aquatic plants, blue alge-bacteria, to land plant material. In contrast, n-alkanes in the rock extracts of the Qingshankou Formation are characterized by a single modal distribution, with relatively low abundances of C29―C35 hopanes, but high molecular-weight isoalkanes, alkylcyclohexanes and alkylcyclopentanes. The relatively low δ 13C values of C22―C44 n-alkanes and organic material compositions indicate that the source rocks in the Qingshankou Formation contain dominantly type I algal organic matter. The relative abundance of C 40 compounds in source rocks changes little at low maturity stage, but decreases drastically at higher maturity levels, with a concurrent reduction in the odd/even carbon predominance. In crude oils, in contrast, the relative abundance of C 40 compounds appears to relate closely with the oil source and oil viscosity.  相似文献   

15.
Scleractinian corals exhibit a dual trophic pattern of autotrophic photosynthesis and heterotrophic predation.However,whether corals can adjust their trophic status under contrasting environmental conditions remains unclear.In our study,70 scleractinian corals(Favia palauensis) were collected from Sanya and the Xisha and Nansha Islands in the South China Sea.We measured the zooxanthellae density(ZD) and δ~(13)C of zooxanthellae(δ~(13)C_z) and host tissue(δ~(13)C_h) and analyzed the difference between δ~(13)C_h and δ~(13)C_z(i.e.,△~(h-z)~(13)C).The relatively high ZD and δ~(13)C_z values in the samples from Sanya indicate that these corals might have higher photosynthetic rates and autotrophic abilities than those from Xisha and Nansha.In contrast,the relatively low δ~(13)C_h and △~(h-z) ~(13)C values in the samples from Xisha and Nansha suggest that these corals might have a higher heterotrophic ability than those from Sanya.In addition,we tested the coral tissue biomass and skeletal δ~(13)C(δ~(13)C_s) in the samples from Sanya and examined their correlations with △~(h-z)~(13)C.The results showed a negative correlation,indicating that the more the organic material produced by the coral,the stronger its heterotrophic ability.Our results show that corals can adjust their trophic status under different environmental and physiological conditions,which is essential for increasing their adaptability to different environmental conditions.  相似文献   

16.
Neogene eolian successions are one of the most important terrestrial palaeoenvironmental archives in East Asia.However, they have received far less attention than Quaternary loess deposits, especially in the case of lipid biomarker analysis.In order to obtain a better insight into the early-middle Miocene palaeoenvironment, we conducted a study of n-alkane biomarkers from sediments of the QA-I section(Qinan) in the western Chinese Loess Plateau, and compared the results with those of previous n-alkane analyses of eolian and aquatic sediments of varying age. Our principal results are as follows:(1) All QA-I samples contain n-alkanes ranging from C_(14) to C_(35), among which the relative content of short-chain n-alkanes(C_(14)–C_(20)) from microorganisms is significantly greater than that of long-chain n-alkanes(C_(26)–C_(35)) from the waxes of terrestrial higher plants;the main peak is at C_(16)–C_(18). All samples have a relatively lower abundance of medium-chain n-alkanes(C_(21)–C_(25)) than that of long-and short-chain n-alkanes, similar to strongly weathered palaeosols in Quaternary loess and Late Miocene-Pliocene Hipparion Red-Earth; however, this distribution is significantly different from that in weakly-weathered loess of Quaternary loess and Late Miocene-Pliocene Hipparion Red-Earth, as well as from aquatic sediments.(2) Despite some odd-over-even carbon predominance of long-chain n-alkanes in the QA-I samples, the carbon preference index(CPI) values are significantly lower than those of most of the weakly-weathered sediments. Our results show that strong weathering and microbial processes have significantly altered the n-alkanes in the Miocene eolian deposits in Qinan, and led to a significant oxidation and degradation of long-chain n-alkanes and the predominance of short-chain n-alkanes from bacteria. Therefore, the contribution of microorganism to total organic carbon(TOC) and its resulting in carbon isotopic composition should be carefully assessed in future studies.  相似文献   

17.
Aquatic plants are major input sources of autochthonous organic matter in lake sediments, but investigations on fatty acid(FA) and n-alkane distributions in aquatic plants are currently limited, greatly hindering the applications of their isotope geochemistry in lacustrine environments. Here, the reported n-alkyl lipid distributions of aquatic plants in globally studied lakes,together with newly obtained aquatic plant n-alkyl lipid data in Chinese lakes(Yunnan and Inner Mongolia), are used to understand their distribution characteristics. The results show that aquatic plants have predominantly mid-chain lipids(C_(23)–C_(25) n-alkanes and C_(22)–C_(24) FAs), differing from that of terrestrial plants(dominant by long-chain lipids), but the long-chain n-alkanes(e.g., C_(27) and C_(29)) and long-chain FAs(e.g., C_(26) and C_(28)) also show high abundances in most samples. Submerged plants have high concentrations of long-chain n-alkanes(avg. 47 μg g~(-1)) and long-chain FAs(avg. 170 μg g~(-1)), close to those in terrestrial plants, indicating that submerged plants may make large contributions of long-chain n-alkyl lipids to lake sediments, while the contributions of long-chain n-alkyl lipids derived from algae to lake sediments may be small because of their low concentrations(avg. 2 μg g~(-1) for n-alkanes and 9 μg g~(-1) for FAs). We find that lipid molecular proxies(including ACL_(14-32) and ATR_(14-18)) can be reliably used to distinguish the FAs sourced from algae and other plants, and Paq′ values can be utilized to distinguish the nalkane sources between submerged plants and terrestrial plants. Aquatic plants do not have significant δD differences among different chain-length n-alkanes and FAs for each sample, suggesting that the offset between δD values of different chain-length n-alkyl lipids in lake sediments can help determine sedimentary lipid input sources and infer paleohydrological changes.  相似文献   

18.
Carbon stable isotope techniques were extensively employed to trace the dynamics of soil organic carbon(SOC)across a land-use change involving a shift to vegetation with different photosynthetic pathways.Based on the isotopic mass balance equation,relative contributions of new versus old SOC,and SOC turnover rate in corn fields were evaluated world-wide.However,most previous research had not analyzed corn debris left in the field,instead using an average corn plant δ~(13)C value or a measured value to calculate the proportion of corn-derived SOC,either of which could bias results.This paper carried out a detailed analysis of isotopic fractionation in corn plants and deduced the maximum possible bias of SOC dynamics study.The results show approximately 3‰ isotopic fractionation from top to bottom of the corn leaf.The ~(13)C enrichment sequence in corn plant was tassel﹥stalk or cob﹥root﹥leaves.Individual parts accounting for the total dry mass of corn returned distinct values.Consequently,the average δ~(13)C value of corn does not represent the actual isotopic composition of corn debris.Furthermore,we deduced that the greater the fractionation in corn plant,the greater the possible bias.To alleviate bias of SOC dynamics study,we suggest two measures:analyze isotopic compositions and proportions of each part of the corn and determine which parts of the corn plant are left in the field and incorporated into SOC.  相似文献   

19.
The Wuliu-Zengjiayan section in Jianhe County, Guizhou Province, China has been suggested as a potential Global Stratotype Section and Point(GSSP) for the defined base of Cambrian Series 3. A molecular organic geochemical study on the Cambrian Series 2Series 3 boundary interval was carried out to reveal the biotic and environmental change during this transition. The hydrocarbons extracted from the Kaili Formation were proved to be indigenous. The various geochemical proxies such as biomarker parameter, δ13Corg, atomic H/C value of kerogen, and TOC content, co-vary along with the sedimentary column, and show a quick and significant change just across the Cambrian Series 2Series 3 boundary. The less abundance of isoprenoid hydrocarbons, the relative enrichment of midchain monomethyl branched alkanes(mmb-alkanes), the relatively negative value of δ13Corg, and the higher TOC contents may suggest that the upper Cambrian Series 2 was deposited in a relatively reducing environment with a higher organic input from cyanobacteria-predominated benthic microbial mats. On the other hand, the relative enrichment of isoprenoid hydrocarbons, the less abundance of mmb-alkanes, the relatively positive value of δ13Corg, the lower TOC contents, and the lower atomic H/C values of kerogen are combined to indicate an enhanced phytoplankton production and an increased oxygen content and nutrients in the ocean during the early Cambrian Series 3, which could have benefited the explosion of the Kaili Biota. The Wuliu-Zengjiayan section may provide a typical case to understand the co-variation of marine microbe, animal, and environment.  相似文献   

20.
In 2013, the China Geological Survey and Guangzhou Marine Geological Survey conducted the second Chinese gas hydrate expedition in the northern South China Sea(SCS) and successfully obtained visible gas hydrate samples. Five of the thirteen drilling sites were cored for further research. In this work, Site GMGS2-08 is selected for the stable isotopic analysis of foraminifera present in the boreholes in order to reveal the carbon isotopic characteristics of the foraminifera and their response to methane release in the gas hydrate geological system. Our results show that the methane content at Site GMGS2-08 is extremely high, with headspace methane concentrations up to 39300 μmol L~(-1). The hydrocarbon δ~(13)C values, ranging from-69.4‰ to-72.3‰ PDB, distinctly indicate biogenic generation. Based on the δD analytical results(~(-1)83‰ to~(-1)85‰ SMOW), headspace methane is further discriminated to be microbial gas, derived from CO_2 reduction. By isotopic measurement, five light δ~(13)C events are found in the boreholes from Site GMGS2-08, with foraminiferal δ~(13)C values being apparently lower than the normal variation range found in the glacial-interglacial cycles of the SCS. The δ~(13)C values of benthic Uvigerina peregrina are extremely depleted(as low as~(-1)5.85‰ PDB), while those of planktonic Globigerinoides ruber reach-5.68‰ PDB. Scanning electron micrograph(SEM) studies show that foraminiferal tests have experienced post-depositional alteration, infilled with authigenic carbonate, and the diagenetic mineralization is unlikely to be related to the burial depths. The correlation calculation suggests that the anaerobic oxidation of organic matter has only weak influences on the δ~(13)C composition of benthic foraminifera. This means that the anomalous δ~(13)C depletions are predominantly attributed to the overprinting of secondary carbonates derived from the anaerobic oxidation of methane(AOM). Furthermore, the negative δ~(13)C anomalies, coupled with the positive δ18O anomalies observed at Site GMGS2-08, are most likely the critical pieces of evidence for gas hydrate dissociation in the geological history of the study area.  相似文献   

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