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1.
A detailed record (≈2-kyr intervals) of the difference in stable carbon isotopes (Δδ 13C) between Globigerina bulloides and Globigerinoides ruber (pink) is used to reconstruct changes in upwelling intensity off the Congo River for the last 190,000 yr. Comparisons of the oxygen and carbon isotope data from this core with records from the Niger Fan and from pelagic cores in the eastern equatorial South Atlantic indicate that the Congo Fan isotope records do not contain a strong freshwater signal as is described off other major rivers. The temporal pattern of the Δδ 13C signal correlates with the marine organic carbon record from the Congo Fan. Thus the planktonic Δδ 13C record, reflecting past changes in upwelling intensity and nutrient content, corroborates the signal provided by sedimentary organic carbon, which is presumed to indicate changes in the amount of biological productivity and export flux to the seafloor. The planktonic Δδ13C signal is characterized by a dominant 23-kyr periodicity which provides evidence for a strong response of upwelling fluctuations off the Congo to precessional forcing. Minima in the Δδ13C record are aligned with periods of minimum boreal summer insolation over Central Africa reflecting an increase of upwelling and biological productivity off the Congo at periods of enhanced zonal intensity of southeast trades and corresponding weak southeast monsoon over the eastern South Atlantic. A strong response to changes in river discharge probably did not occur, indicating that fertilization by river-derived nutrients has played only a minor role with respect to Late Quaternary changes in the total amount of primary productivity off the Congo. 相似文献
2.
A procedure, based on measurement of the stable carbon isotope 13C, has been developed for determining the extent to which petroleum carbon is incorporated into soil organic matter (SOM) by humification of biomass produced during biodegradation of the petroleum in soil. We have shown that a crude oil having a δ 13C of-27.4%, when biodegraded in a soil containing SOM with a δ 13C of-15.7%, resulted in a change of the δ 13C of the bound SOM reflecting that of petroleum carbon. Comparison of five soil biodegradation tests using different amounts and types of fertilizer to stimulate biodegradation of the oil in this soil showed that the extent of the δ 13C change in the bound SOM varied with the extent of oil biodegradation observed. To obtain 13C data on the SOM, the residual petroleum was first removed by rigorous extraction with dichloromethane using a Soxhlet apparatus. The extracted soil was then combusted to release bound carbon as CO 2, which was analyzed for 13C. Where the SOM has a δ 13C similar to that of petroleum, 14C measurements of SOM would give similar results. This type of data, referred to as the petroleum “footprint” in the SOM, could be useful in identifying or confirming intrinsic biodegradation of petroleum in contaminated soil. 相似文献
3.
The relationship between organic carbon accumulation rates and 13C/ 12C ratios of total organic carbon (TOC) was investigated in an highland peat bog core (Ru-3) from Equatorial Africa. This core yielded a sequence spanning the last 14 kyr and was analysed with a 100–300 yr resolution for TOC-δ 13C values. The Holocene section shows contrasted TOC accumulation regimes and TOC δ 13C varying between −28.5 and −19.5‰ with a few very short ‘isotopic excursions' (dated at ca. 9.3, 7.5, 4.2 ka B.P.). The organic carbon accumulation rates range from 2 to 20 mg C cm −2 yr −1. They increase when TOC becomes more depleted in 13C, notably between 12 and 9.8 ka B.P., 8.5 and 7.8 ka B.P. and after 1.6 ka B.P. Periods of restricted carbon storage correspond to heavier TOC accumulation at 9.3, and between 7.5 and 1.6 ka B.P. At the study site, the δ-variations can be related to variable C4-plant inputs, and possibly, to changes in the fractionation between CO 2 and the organic carbon in C3 vascular plants. The Ru-3 record indicates restricted carbon storage during the periods of increased contribution from C4 plants and/or of decreased fractionation between CO 2 and organic carbon in C3 plants. Changes in TOC-δ 13C values in core Ru-3 seem to match fluctuations of East Equatorial African lakes. High lake stands correspond to low δ 13C intervals and vice versa. This points to indirect climatic forcing of δ 13C changes in intertropical peats. 相似文献
4.
Two sediment cores retrieved from the continental slope in the Benguela Upwelling System, GeoB 1706 (19°33.7′S 11°10.5′E) and GeoB 1711 (23°18.9′S, 12°22.6′E), reveal striking variations in planktonic foraminiferal abundances during the last 160,000 years. These fluctuations are investigated to assess changes in the intensity and position of the upwelling centres off Namibia. Four species make up over 95% of the variation within the core, and enable the record to be divided into episodes characterized by particular planktonic foraminiferal assemblages. The fossil assemblages have meaningful ecological significance when compared to those of the modern day and the relationship to their environment. The cold-water planktonic foraminifer, Neogloboquadrina pachyderma sinistral [ N. pachyderma (s)], dominates the modern-day, coastal upwelling centres, and Neogloboquadrina pachyderma dextral and Globigerina bulloides characterize the fringes of the upwelling cells. Globorotalia inflata is representative of the offshore boundary between newly upwelled waters and the transitional, reduced nutrient levels of the subtropical waters. In the fossil record, episodes of high N. pachyderma (s) abundances are interpreted as evidence of increased upwelling intensity, and the associated increase in nutrients. The N. pachyderma (s) record suggests temporal shifts in the intensity of upwelling, and corresponding trophic domains, that do not follow the typical glacial-interglacial pattern. Periods of high N. pachyderma (s) abundance describe rapid, discrete events dominating isotope stages 3 and 2. The timing of these events correlates to the temporal shifts of the Angola-Benguela Front (Jansen et al., 1997) situated to the north of the Walvis Ridge. Absence of high abundances of N. pachyderma (s) from the continental slope of the southern Cape Basin indicates that Southern Ocean surface water advection has not exerted a major influence on the Benguela Current System. The coincidence of increased upwelling intensity with the movement of the Angola-Benguela Front can be interpreted mainly by changes in strength and zonality of the trade wind system. 相似文献
5.
Two late Quaternary sediment cores from the northern Cape Basin in the eastern South Atlantic Ocean were analyzed for their benthic foraminiferal content and benthic stable carbon isotope composition. The locations of the cores were selected such that both of them presently are bathed by North Atlantic Deep Water (NADW) and past changes in deep water circulation should be recorded simultaneously at both locations. However, the areas are different in terms of primary production. One core was recovered from the nutrient-depleted Walvis Ridge area, whereas the other one is from the continental slope just below the coastal upwelling mixing area where present day organic matter fluxes are shown to be moderately high. Recent data served as the basis for the interpretation of the late Quaternary faunal fluctuations and the paleoceanographic reconstruction. During the last 450,000 years, NADW flux into the eastern South Atlantic Ocean has been restricted to interglacial periods, with the strongest dominance of a NADW-driven deep water circulation during interglacial stages 1, 9 and 11. At the continental margin, high productivity faunas and very low epibenthic δ13C values indicate enhanced fluxes of organic matter during glacial periods. This can be attributed to a glacial increase and lateral extension of coastal upwelling. The long term glacial-interglacial paleoproductivity cycles are superimposed by high-frequency variations with a period of about 23,000 yr. Enhanced productivity in surface waters above the Walvis Ridge, far from the coast, is indicated during glacial stages 8, 10 and 12. During these periods, cold, nutrient-rich filaments from the mixing area were probably driven as far as to the southeastern flank of the Walvis Ridge. 相似文献
6.
Stable isotope analyses of fossil teeth and other authigenic minerals have been used to reconstruct the paleoenvironment and paleoelevation of the Tibetan Plateau. The accuracy of such reconstructions is limited by the lack of a comprehensive modern comparative database from the region. We analyzed the carbon and oxygen isotopic compositions ( δ13C and δ18O values) of tooth enamel from modern herbivores, the δ13C values of grasses and the δ18O values of water samples collected from various elevations within the Tibetan Plateau to examine their relationships with modern environment/elevation. The δ13C values of enamel samples from horses, yaks and goats display a narrow range of variation, with a mean of − 10.7 ± 1.4‰ ( n = 301), indicating that these modern herbivores were feeding predominantly on C3 plants, consistent with the current dominance of C3 vegetation in the region. Some of the samples have δ13C values between − 7.3 and − 10‰. Although these higher δ13C values could suggest consumption of some C4 plants by the animals, the lack of significant seasonal δ13C variations within individual teeth indicates that these higher enamel δ13C values are due to consumption of C3 plants experiencing water stress and/or some CAM plants rather than C4 plants. Our data show that the conservative “cut-off” δ13C value for a pure C3 diet within the Tibetan Plateau should be − 8‰ for modern herbivores and − 7‰ (or even − 6.5‰) for fossils if the region was as arid in the past as today. In contrast to the small intra-tooth δ13C variations within individual teeth, serial enamel samples display large intra-tooth δ18O variations, reflecting seasonal variations in the δ18O of meteoric water. The mean δ18O values of tooth enamel from yaks and horses show a strong correlation with water δ18O values, confirming that the δ18O of tooth enamel from obligate drinker generally tracks the δ18O of meteoric water. Unfortunately, elevation alone cannot explain most of the variance in the δ18O of precipitation and tooth enamel, suggesting that quantitative reconstruction of the paleoelevation of the Tibetan Plateau using re-constructed δ18O values of paleo-meteoric water from fossil enamel or other oxygen-bearing minerals is not warranted. For a given environment, horses have the lowest enamel– δ18O values while goats display the highest enamel– δ18O values among the species studied. The large inter-species δ18O variations are likely due to differences in physiology and diet/drinking behavior of the animals. This underscores the importance of species-specific studies when interpreting δ18O data of fossil mammalian teeth in a stratigraphic sequence as a record of paleoclimate changes. 相似文献
7.
Mussels, Mytilus edulis, were exposed for periods of 6 weeks at various locations in Dutch coastal waters during 1989 and 1990. “Survival in air” showed to be a sensitive response parameter for indicating pollution induced environmental stress in transplanted mussels sampled from eight field sites. Increased tissue contaminant levels, especially PCBs and PAHs, correlated with a reduced survival time during aerial exposure. Three weeks exposure of mussels in the laboratory to 1 μg · 1 −1 PCBs affected the aerial survival time negatively. Laboratory experiments did not indicate that lowered salinity influences the “Survival in air” response after sufficient acclimation (15 days), facilitating the use of this response parameter in both marine and estuarine waters. 相似文献
8.
A representative paleoenvironmental reconstruction of continental middle latitude from ca. 2 my to the upper part of Middle Pleistocene (279 ± 77 ky) was obtained from the carbon and oxygen stable isotopes analyzed in ostracode shells ( Cyprideis torosa) recovered in the Guadix-Baza Basin (SE Spain), an intramontaneous closed depression filled by alluvial and lacustrine sediments. This study was performed along a 356-m-thick composite section, dated previously by paleomagnetism and the amino acid racemization method. δ 13C and δ 18O profiles reflected changes in temperature, the evaporation/infill ratio in the water bodies and the amount of rain. δ 13C is also affected by changes in plant biomass: periods with high δ 13C and δ 18O values are associated with warm and dry regimes, and with less vegetation, which, in some cases, coincide with the development of displacive gypsum crystals, whereas low δ 13C and δ 18O values correlate with cold and humid episodes, which cause more vegetation biomass and, therefore, increasing the input of isotopically light carbon. Intermediate δ 18O values are linked to temperate dry or humid episodes when they coincide with high or low δ 13C values, respectively. 86 paleoclimatic events were distinguished in the Pleistocene record from the δ 13C and δ 18O profiles. From both the statistical analysis of the geochemical data and the geological observations, four Cold and Humid Long Periods (low δ 18O) and four Warm and Dry Long Periods (high δ 18O) were defined. This differs with respect to the paleoclimatological behavior established for the Northern Hemisphere where during cold periods (glacial), no water was available while permafrost conditions persisted, whereas in warm episodes (interglacial), higher precipitation rates occurred. Good correspondences between the Guadix-Baza Basin paleoclimatic record and a marine oxygen-isotope sequence, two continental cores and other long Mediterranean paleoenvironmental records (pollen sequences from Israel) were found, which suggested that climate changes in the Guadix-Baza Basin were in tune with global climatic changes. 相似文献
9.
Five positive carbon isotope excursions are reported from Platteville–Decorah strata in the Upper Mississippi Valley. All occur in subtidal carbonate strata, and are recognized in the Mifflin, Grand Detour, Quimbys Mill, Spechts Ferry, and Guttenberg intervals. The positive carbon isotope excursions are developed in a Platteville–Decorah succession in which background δ13C values increase upward from about −2‰ at the base to about 0‰ Vienna Pee Dee belemnite (VPDB) at the top. A regional north–south δ13C gradient, with lighter values to the north and heavier values to the south is also noted. Peak excursion δ13C values of up to +2.75 are reported from the Quimbys Mill excursion, and up to +2.6 from the Guttenberg excursion, although there are considerable local changes in the magnitudes of these events. The Quimbys Mill, Spechts Ferry, and Guttenberg carbon isotope excursions occur in units that are bounded by submarine disconformities, and completely starve out in deeper, more offshore areas. Closely spaced chemostratigraphic profiles of these sculpted, pyrite-impregnated hardground surfaces show that they are associated with very abrupt centimeter-scale negative δ13C shifts of up to several per mil, possibly resulting from the local diagenetic effects of incursions of euxinic bottom waters during marine flooding events. 相似文献
10.
通过对高寒草甸不同坡向条件下25科86种C3植物叶片稳定性碳同位素组成(δ 13C)的测定,研究了高寒草甸C3植物δ 13C和水分利用效率对坡向变化的响应以及环境因子对其产生的影响,分析了坡向上控制植物δ 13C变化的主要环境因子.结果表明: 从北坡到南坡的生境梯度上,土壤含水量不断减少,而土壤温度及光照强度不断增加,植物群落结构也发生了相应变化.5个坡向中,植物叶片δ 13C值为-31.19‰~-21.80‰,均值为(27.18±0.13)‰;生长季植物叶片δ 13C均值在南坡最高,其次为西南坡、西坡、西北坡,最低值出现在北坡.坡向间δ 13C值的差异主要是由不同坡向土壤含水量和土壤温度,以及光照强度的不同导致的,其中土壤含水量是主要的限制因子.北坡-南坡梯度上植物叶片δ 13C值随土壤含水量下降、土壤温度及光照强度升高而变大,表明不同坡向植物的水分利用状况对干旱胁迫的响应不同,植物逐渐提高了水分利用效率以适应干旱胁迫的生境. 相似文献
11.
Carbon isotope ratios ( 13C/ 12C) were measured for the leaves of the seagrass Thalassia testudinum Banks ex König and carbonates of shells collected at the seagrass beds from seven sites along the coast of southern Florida, U.S.A. The δ 13C values of seagrass leaves ranged from −7.3 to −16.3‰ among different study sites, with a significantly lower mean value for seagrass leaves from those sites near mangrove forests (−12.8 ± 1.1‰) than those far from mangrove forests (−8.3 ± 0.9‰; P < 0.05). Furthermore, seagrass leaves from a shallow water area had significantly lower δ 13C values than those found in a deep water area ( P < 0.01). There was no significant variation in δ 13C values between young and mature leaves ( P = 0.59) or between the tip and base of a leaf blade ( P = 0.46). Carbonates of shells also showed a significantly lower mean δ 13C value in the mangrove areas (−2.3 ± 0.6‰) than in the non-mangrove areas (0.6 ± 0.3‰; P <0.025). In addition, the δ 13C values of seagrass leaves were significantly correlated with those of shell carbonates (δ 13C seagrass leaf = −9.1 + 1.3δ 13C shell carbonate ( R2 = 0.83, P < 0.01)). These results indicated that the input of carbon dioxide from the mineralization of mangrove detritus caused the variation in carbon isotope ratios of seagrass leaves among different sites in this study. 相似文献
12.
利用设置在浙江省淳安县姥山林场和湖北省太子山石龙林场2个试验点的33年生马尾松种源试验林,选取不同纬度的10个代表性种源,研究其在种源间的差异、地理变异模式及对水热因子的响应.结果表明: 马尾松平均年轮δ 13C在种源间的差异极显著,高纬度种源的平均年轮δ 13C高于低纬度种源.马尾松年轮δ 13C呈纬向的地理变异模式,形成了对种源原地环境的适应性.年轮δ 13C与种源地年均温(MAT)、1月均温(T 1)、年降水量(MAP)、5—9月降水量(P 5-9)及≥10 ℃年积温(CT)均呈显著或极显著负相关,与干燥度指数(AI)呈显著正相关.淳安和太子山点马尾松年轮δ 13C对种源地干燥度指数的响应函数可分别解释年轮δ 13C变化的37.5%和42.5%,种源地AI是年轮δ 13C适应性的重要环境限制因子.马尾松稳定碳同位素δ 13C的年际变化与生长环境关系密切,湖北太子山试验点处于相对干旱的中西部地区,干燥度指数高,平均年轮δ 13C比淳安试验点高1.8%.太子山点和淳安点的马尾松年轮δ 13C分别对7月和8月的气温响应敏感,夏季降水量是年轮稳定碳同位素分馏的主要限制因子,而不同种源对未来气候变化的响应敏感性不同. 相似文献
13.
Aims The optimal patterns of plant community for water use and nutrient utilization, the responses of soil carbon and nitrogen turnover processes to forest succession, and the mechanisms of soil organic carbon accumulation, are three critical issues in forest ecosystem study. It is difficult to accurately detect these ecological processes with conventional methodologies in the short term, yet the application of 13C and 15N natural abundance technique may yield important information about these processes. Methods This study was conducted in Dinghushan Biosphere Reserve. We investigated the natural isotopic abundance of both 13C and 15N of plant-soil continuum along a successional gradient from Pinus massoniana forest (PF) to coniferous and broad-leaved mixed forest (MF), and monsoon evergreen broad-leaved forest (BF). We also analyzed the correlations of foliar stable carbon isotope ratio ( δ13C) and stable nitrogen isotope ratio ( δ15N) with foliar elemental contents and the variations of soil δ13C and δ15N along soil profiles at different successional stages. Important findings A significant positive correlation between foliar δ13C and foliar C:N was observed. In both litter and soil, the δ13C values tended to decrease along the forest succession, with the order as PF > MF > BF. Foliar δ15N was positively correlated with foliar N content. The δ15N values of litter and upper soil (0-10 cm) increased with successional status. Both soil δ13C and δ15N values increased with increasing soil depth at all three forests. Our results imply that 1) trade-off between water use efficiency and nitrogen use efficiency did not necessarily exist in subtropical forests of China; 2) the application of isotopic technique could assist understanding of the mechanisms of soil carbon accumulation in subtropical forests, especially in old-grow forests; 3) the 15N natural abundance of plant-soil continuum could be a potential indicator of soil nitrogen availability and ecosystem nitrogen saturation status. 相似文献
14.
Assessing petroleum biodegradation rates is an important part of predicting natural attenuation in subsurface sediments. Monitoring carbon dioxide (CO 2) and methane (CH 4) produced in situ, and their radiocarbon 14C), stable carbon ( 13C) and deuterium (D). signature provide a novel method to assess anaerobic microbial processes. Our objectives were to: (1) estimate the rate of anaerobic petroleum hydrocarbon (PH) mineralization by monitoring the production of soil gas CH 4 and CO 2 in the vadose zone of low-permeability sediment, (2) evaluate the dominant microbial processes using δ 13C and δD, and (3) determine the proportion of CH 4 and CO 2 attributable to anaerobic mineralization of PH using 14C analysis. Argon was sparged into the subsurface to dilute existing CO 2 and CH 4 concentrations. Vadose zone CO 2, CH 4, oxygen, total combustible hydrocarbons, and argon concentrations were measured for 75 days. CO 2 and CH 4 samples were collected on day 86 and analyzed for 14C, δ 13C, and δD. Based on CH 4 soil gas production, the anaerobic biodegradation rate was estimated between 0.017 to 0.055 mg/kg soil-d. CH 4 14C (2.6 pMC), δ 13C (-45.64‰), and δD (-316‰) values indicated that fermentation of PH was the sale source of CH 4 in the vadose zone. CO 2 14C (62 pMC) indicated that approximately 47% of the total CO 2 was from PH mineralization and 53% from plant root respiration. Although low-permeability sediment increases the difficulty of completely replacing in situ soil gas and assuring anaerobic conditions, this novel respiration method distinguished between anaerobic processes responsible for PH degradation. 相似文献
15.
Cellulose samples extracted from cornhusk have been successfully acetylated in an ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl). Without using any catalyst, cornhusk cellulose acetates (CCAs) with the degree of substitution (DS) in a range from 2.16 to 2.63 were prepared in one-step. Under the homogeneous state, the DS value of CCAs was easily controlled by the acetylation time. The obtained CCAs were characterized by means of FT-IR, 13C NMR, DSC, TGA, and a mechanical test. The NMR results showed that the distribution of the acetyl moiety among the three OH groups of the anhydroglucose unit shows a preference at the C 6 position. The CCAs exhibited good solubility in some organic solvents, such as acetone and DMSO. The cast CCA films from their acetone solutions had good mechanical properties. At the end of each acetylation of cornhusk cellulose, the ionic liquid AmimCl could be effectively recovered. Therefore, this study presents a promising approach and “green process” to make use of crop by-products. 相似文献
16.
Carbon isotope changes during most of Late Ordovician time (from the mid-Caradoc Kinnekulle K-bentonite until the beginning of the Silurian) were investigated. As the corresponding sequence of rocks is stratigraphically nearly complete in Estonia, an attempt was made to use it to elaborate the general pattern of carbon isotope changes in the Late Ordovician. Complications were caused by several local or regional hiatuses in the middle and late Caradoc and Hirnantian. A total of 385 whole rock samples were studied from eight drill cores in northern and central Estonia. The following positive carbon isotope events were observed: (1) the mid-Caradoc excursion (peak δ13C value 2.2‰) in the uppermost part of the Keila Stage, also known in Sweden; (2) the first late Caradoc excursion (1.9‰) in the lower part of the Rakvere Stage; (3) the second late Caradoc excursion (2.4‰) in the upper part of the Nabala Stage; (4) the early Ashgill excursion (2.5‰) in the lowermost part of the Pirgu Stage; (5) the widely known large Hirnantian excursion (in Estonia the peak value reaches 6.7‰) in the Porkuni Stage. The study interval comprises a long (10 Ma) period characterized by low-magnitude carbon isotope changes and a following brief (2 Ma) interval with large changes. No obvious lithological preference for hosting the positive shifts was recorded. In principle, the δ13C values exceeding the background values may occur in all types of rocks present in a sedimentary basin. Several δ13C positive excursions (values 1.5‰ to 3‰) in the Mohawkian of North America are evidence that the minor Caradoc and early Ashgill δ13C positive shifts in Baltoscandia may have counterparts in Laurentia. If correctly correlated, these shifts may have global significance. The Hirnantian excursion is usually linked to a major glacial event, even if some carbon cycling mechanisms are not completely understood. The environmental causes suggested for the earlier minor shifts range from global climatic and glacial events to very local changes in basin regime and sea level. Our study supports the primary role of climatic or climatically triggered oceanic processes. 相似文献
17.
分析了长白山北坡垂直样带3种典型原始森林地表凋落物及不同粒径土壤组分中有机质的δ 13C值,并将在岳桦林样地(EB,海拔1996 m)采集的20 cm土柱分别置换到云冷杉林(SF,海拔1350 m)和阔叶红松林(PB,海拔740 m),云冷杉林样地采集的土柱置换到阔叶红松林中,进行为期1年的野外模拟增温试验.结果表明:3种林型土壤的δ 13C值均显著高于凋落物的δ 13C值,凋落物和土壤有机质中的δ 13C值由地表凋落物向土壤下层逐渐增加,而土壤粒径中有机质的δ 13C值随粒径减小而增大.3种林型中,凋落物δ 13C值变化趋势为云冷杉林(-28.3‰)>阔叶红松林(-29.0‰)>岳桦林(-29.6‰),而土壤有机质的δ 13C值变化趋势为岳桦林(-25.5‰)>阔叶红松林(-25.8‰)>云冷杉林(-26.2‰).在土壤温度增加0.7 ℃~2.9 ℃条件下,土壤及其各粒级的δ 13C值均呈下降趋势,而且<2 μm粘粒和2~63 μm粉粒δ 13C值的降幅(0.48‰和0.47‰)高于>63 μm砂粒δ 13C值的降幅(0.33‰).未来气候变暖可能对储藏在细小颗粒中年龄较长的有机碳带来较大的影响. 相似文献
18.
In order to elucidate early Aptian marine paleotemperature evolution across the period of enhanced organic carbon (C org)-burial [Oceanic Anoxic Event (OAE) 1a], stable isotope analyses were performed on pelagic limestones at Deep Sea Drilling Project Site 463, central Pacific Ocean. The δ18O data exhibit a distinct anomaly by ~ − 2‰ spanning the OAE 1a interval (i.e., a ~ 6 m-thick, phytoplanktonic C org-rich unit constrained by magneto-, bio- and δ13C stratigraphy). Elucidation of paleotemperature significance of the δ18O shift is made by taking account of recent Sr/Ca evidence at the same section, which revealed that geochemical signals in carbonate-poor lithologies are relatively unaltered against burial diagenesis. By discriminating δ18O values from carbonate-poor samples (CaCO 3 contents = 5–30 wt.%), it appears that an abrupt rise in sea-surface temperatures (SSTs) by 8 °C (= − 1.7‰ shift in δ18O) occurred immediately before OAE 1a, whereas a cooling mode likely prevailed during the peak C org-burial. In terms of its stratigraphic relationship as to the C org-rich interval and to a pronounced negative δ13C excursion, as well as its timescale, the observed SST rise resembles those associated with the Paleocene–Eocene thermal maximum and, more strikingly, Jurassic Toarcian OAE. This observation is consistent with the hypothesis that these paleoenvironmental events were driven by a common causal mechanism, which was likely initiated by the greenhouse effect via massive release of CH 4 or CO 2 from the isotopically-light carbon reservoir and terminated by a negative productivity feedback. 相似文献
19.
North Africa has undergone drastic climatic changes over the past several hundred thousand years. The timing of humid intervals called pluvials was investigated by uranium-series disequilibrium dating of travertines from the Kurkur Oasis, Western Desert, Egypt. The youngest and best dated travertines (70–160 ka) are found in Wadi Kurkur and include spring and lacustrine units exposed as 2 to 3 m high terraces. Travertines having an age of approximately 191–220 ka are exposed by differential erosion as linear mounds produced by spring systems over fracture zones in ancient wadis. The oldest travertines, having ages >260 ka, are extensive, cap limestone units above the oasis, and were deposited in paludal and lacustrine environments. Oxygen isotope ratios were measured for the wadi travertines (δ 18O values ranging from 16.7 to 19.1‰ SMOW) and for spring mound travertines (18.5–20.5‰). Equilibrium oxygen isotope fractionation calculations indicate that the Kurkur travertines were deposited from waters having δ 18O values similar to ancient Western Desert groundwaters ( −11‰, Sonntag et al., 1978a,b; Thorweithe, 1990; Sultan et al., 1996). The ages of the travertines correspond to times of monsoonal maxima, eustatic sea level high stands and interglacial maxima. Rainfall producing these groundwaters (and travertines) was significantly fractionated during atmospheric transport, in contrast to modern meteoric waters (−2.09‰, Joseph et al., 1992), implying a distant source for the pluvial waters. Increased precipitation, recharge of Western Desert groundwaters, and resultant travertine deposition are interpreted to be consequences of Milankovitch insolation cycle forcing, through enhanced Atlantic and Indian Ocean monsoons during interglacial time periods. 相似文献
20.
Presynaptic modulation by opioids of electrically-evoked neurotransmitter release from superfused rat amygdala slices prelabelled with [ 3H]noradrenaline (NA) and [ 14C]choline was examined. Both [ 3H]NA and [ 14C]acetylcholine release were strongly inhibited by morphine, the mixed δ/μ-receptor agonist [
-Ala 2,
-Leu 5]enkephalin (DADLE) and the highly selective μ-agonist [
-Ala 2, MePhe 4, Gly-ol 5]enkephalin (DAMGO), whereas the highly selective δ-agonist [
-Pen 2,
-Pen 5]enkephalin and the κ-agonist bremazocine were without effect. The inhibitory effects were potently antagonized by naloxone but not by the selective δ-receptor antagonist fentanylisothiocyanate. When the selective uptake inhibitor desipramine was used to prevent uptake of [ 3H]NA into noradrenergic nerve terminals, but sparing the uptake into dopaminergic nerve terminals, the electrically evoked release of tritium was strongly inhibited by bremazocine but not by DADLE or DAMGO. The data indicate, that in the amygdala transmitter release from dopaminergic nerve fibres is inhibited only via activation of κ-receptors, whereas transmitter release from noradrenergic and cholinergic nerve fibers is subjected to inhibition by opioids via activation of μ-receptors only. Regional differences and similarities of modulation of neurotransmitter release by opioids in the rat brain are briefly discussed. 相似文献
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