Comparison of data and derived quantities for the middle atmosphere of the Southern Hemisphere

Before data from satellites can be used with confidence in dynamical studies of the middle atmosphere an assessment of their reliability is necessary. To this end, independently analysed data from different instruments may be compared. In this paper, this is done for the Southern Hemisphere as a prelude to the dynamical studies of the middle atmosphere being fostered by the MASH project of the Middle Atmosphere Program. Data from two infrared radiometers are used: a limb scanner (LIMS) and a nadir sounder (SSU). While there is usually qualitative agreement between basic fields (temperatures, winds), substantial quantitative differences are found, with more pronounced differences in fields of Eliassen-Palm flux divergence and Ertel's potential vorticity.The fidelity of the base-level analysis to which satellite data are tied is important for calculating quantities of relevance to dynamical theory. In the Southern Hemisphere, conventional data are sparse and, through the analysis procedure, this introduces errors into derived fields for the middle atmosphere. The impact of using base-level analyses from different sources is assessed. Large discrepancies are found in fields computed by differentiation.Several techniques are suggested whereby the reliability of fields derived from satellite data may be gauged.     

Evidence for spatially variable friction from tidal amplification and asymmetry in the Pertuis Breton (France)

The semi-diurnal tides are amplified and distorted as they propagate into the Pertuis Breton, a semi-enclosed shallow basin in the north-eastern part of Bay of Biscay, in France. This paper investigates the influence of bottom friction on amplification and phase lag of the tidal constituent M2 and its overtide M4 in the Pertuis Breton. A fine resolution two-dimensional (2D) numerical model is implemented to simulate tidal propagation. The model solves the depth-averaged shallow-water equations on a finite element grid using the TELEMAC 2D software. A two-zone parameterisation of friction coefficient is introduced to evaluate the impact of smooth mud flats on the tidal asymmetry and amplification in the Pertuis. Fitting the model to observed amplitudes and phases of M2 and M4 evaluates the decrease of Chezy friction coefficient from the mud flats to the rest of the Pertuis as 100:60. This conclusion is supported by the direct estimation based on morphology and composition of sea bed in the Pertuis Breton.     

A comparative study of winds and tidal variability in the mesosphere/lower-thermosphere region over Bulgaria and the UK

Meteor radars located in Bulgaria and the UK have been used to simultaneously measure winds in the mesosphere/lower-thermosphere region near 42.5°N, 26.6°E and 54.5°N, 3.9°W, respectively, over the period January 1991 to June 1992. The data have been used to investigate planetary waves and diurnal and semidiurnal tidal variability over the two sites. The tidal amplitudes at each site exhibit fluctuations as large as 300% on time scales from a few days to the intra-seasonal, with most of the variability being at intra-seasonal scales. Spectral and cross-wavelet analysis reveals closely related tidal variability over the two sites, indicating that the variability occurs on spatial scales large compared to the spacing between the two radars. In some, but not all, cases, periodic variability of tidal amplitudes is associated with simultaneously present planetary waves of similar period, suggesting the variability is a consequence of non-linear interaction. Calculation of the zonal wave number of a number of large amplitude planetary waves suggests that during summer 1991 the 2-day wave had a zonal wave number of 3, but that during January/February 1991 it had a zonal wave number of 4.     

Interpretation,reliability and accuracies of parameters deduced by the spaced antenna method in middle atmosphere applications

The spaced antenna method has proved to be an important and relatively inexpensive radar technique for making measurements of atmospheric wind velocities and other parameters. This discussion examines the reliability and accuracies of various parameters which can be measured with the technique.After a brief introduction, a series of comparisons of winds measured by the spaced antenna method and simultaneously by other techniques are presented. It is concluded that when using weak partial reflections in the height range 0–100km, the spaced antenna technique provides reliable estimates of the neutral air motion. Following this the assumptions made in applying the method are considered in more detail. The possibility of systematic errors and the likelihood of erroneous measurements are examined, and the accuracy of any particular measurement of wind speed is discussed. Previous objections to the technique are discussed, and in general shown to be invalid.Other parameters apart from wind speeds can be measured with the spaced antenna technique, such as pattern scale, the rate of natural fading, and angles of arrival. The meanings of these parameters are discussed in terms of physical quantities such as turbulent energy dissipation rates, small-scale gravity wave velocity fluctuations, and aspect sensitivities of scatterers, and it is indicated when and how these derived parameters can be applied to deduce meaningful physical quantities. The need for great caution in making these interpretations is discussed; for example it is not always possible to use the rate of natural fading to estimate the intensity of turbulence, although in some cases this is possible. Finally, interferometric applications of spaced antenna systems are discussed.     

Longitudinal (UT) effect in the onset of auroral disturbances over two solar cycles as deduced from the AE-index

Statistical study on the universal time variations in the mean hourly auroral electrojet index (AE-index) has been undertaken for a 21 y period over two solar cycles (1957–1968 and 1978–1986). The analysis, applied to isolated auroral substorm onsets (inferred from rapid variations in the AE-index) and to the bulk of the AE data, indicates that the maximum in auroral activity is largely confined to 09–18 UT, with a distinct minimum at 03–06 UT. The diurnal effect was clearly present throughout all seasons in the first cycle but was mainly limited to northern winter in the second cycle. Severe storms (AE > 1000 nT) tended to occur between 9–18 UT irrespective of the seasons whereas all larger magnetic disturbances (AE > 500 nT) tended to occur in this time interval mostly in winter. On the whole the diurnal trend was strong in winter, intermediate at equinox and weak in summer. The implication of this study is that Eastern Siberia, Japan and Australia are mostly at night, during the period of maximum auroral activity whereas Europe and Eastern America are then mostly at daytime. The minimum of auroral activity coincides with near-midnight conditions in Eastern America. It appears that the diurnal UT distribution in the AE-index reflects a diurnal change between interplanetary magnetic field orientation and the Earths magnetic dipole inclination.     

Fault configuration produced by initial arc rifting in the Parece Vela Basin as deduced from seismic reflection data

Abstract   The Parece Vela Basin (PVB), which is a currently inactive back-arc basin of the Philippine Sea Plate, was formed by separation between the Izu-Ogasawara Arc (IOA) and the Kyushu-Palau Ridge (KPR). Elucidating the marks of the past back-arc opening and rifting is important for investigation of its crustal structure. To image its fault configurations and crustal deformation, pre-stack depth migration to multichannel seismic reflection was applied and data obtained by the Japan Agency for Marine-Earth Science and Technology and Metal Mining Agency of Japan and Japan National Oil Corporation (Japan Oil, Gas and Metals National Corporation). Salient results for the pre-stack depth-migrated sections are: (i) deep reflectors exist around the eastern margin of KPR and at the western margin of IOA down to 8 km depth; and (ii) normal fault zones distributed at the eastern margin of the KPR (Fault zone A) and the western margin of the IOA (Fault zone B) have a total displacement of greater than 500 m associated with synrift sediments. Additional normal faults (Fault zone C) exist 20 km east of the Fault zone B. They are covered with sediment, which indicates deposition of recent volcanic products in the IOA. According to those results: (i) the fault displacement of more than 500 m with respect to initial rifting was approximately asymmetric at 25 Ma based on PSDM profiles; and (ii) the faults had reactivated after 23 Ma, based on the age of deformed sediments obtained from past ocean drillings. The age of the base sediments corresponds to those of spreading and rotation after rifting in the PVB. Fault zone C is covered with thick and not deformed volcanogenic sediments from the IOA, which suggests that the fault is inactive.     

Anomalous crustal strain prior to the 1923 Kanto,Japan, earthquake as deduced from analysis of old triangulation data

Horizontal earth's strains preceding the Kanto, Japan, earthquake of 1, September 1923, are deduced from the analysis of the old triangulation data. The anomalous strains that are several times larger than usual tectonic strain are found in the western part of Tokyo Bay, Sagamihara district, Japan for the observational period 1882/91–1898/1910, while any significant strain is not revealed in the other region of the Kanto district. The Kanto district was surveyed twice during the period 1883/85–1890/92 in the west and during the period 1890/92–1897/99 in the east respectively. The polarity of the detected anomalous strains, the directions and the signs of the principal strains, are quite the same as those of the postseismic crustal strains during the period 1924–74, and are reversed as compared to the coseismic one.The Philippine Sea plate thrusts under the South Kanto district with N25°W direction and pulls down the land during the interseismic period. The aseismic reverse faulting would begin several decades before the 1923 Kanto earthquake along the deep interface between the Asian plate and the convergent Philippine Sea plate. The down-going along the locked part of the interface would be accelerated, thus the compressional stress on the earth's surface might be concentrated over the deep fault plane together with the acceleration of the subsidence at the tip of the peninsula close to the Sagami trough.     

60年来大气中二氧化碳浓度数据的趋势方程研究

通过讨论已有的60年来大气中CO₂浓度数据的分布状态,采用趋势分析方法,给出了具体趋势方程形式.与冰芯分析或观测数据对比结果表明,趋势方程曲线与已有数据基本符合,随后初步给出了2010年至2016年间大气中CO₂浓度预测值.     

Historical measurements of the Earth's magnetic field compared with remanence directions from lava flows in Italy over the last four centuries

Direct measurements of the Earth's magnetic field in Italy since 1640 a.d. have been used to check the remanence directions derived from historically dated volcanic rocks of Etna and Vesuvius. Direct measurements consist of the records of L’Aquila and Pola geomagnetic observatories, the repeat stations of the Italian Magnetic Network and the data base of the Historical Italian Geomagnetic Data Catalogue. All have been relocated to the same reference site (Viterbo — lat. 42.45°N, long. 12.03°E) in order to draw a reference secular variation (SV) curve. The direction of the Earth's field at Viterbo has also been calculated from the historical records (2000-1600) of ref. [Jackson, A., Jonkers, A.R.T., Walker, M.R., 2000. Four centuries of geomagnetic secular variation from historical records. Phil. Trans. R. Soc. London, Ser. A 358, 957-990] database. The remanence directions from Etna show a general agreement with the trend of the SV curve, although their inclination is usually lower than that from the direct measurement. The directions from Vesuvius are more scattered. Large discrepancies occur at both volcanoes and in some cases have been ascribed in the literature to poor geographic information, making it difficult to identify the flows actually emplaced during the eruptions reported in the chronicles. Closer examination shows that the great majority of the best-defined remanence directions (semi-angle of confidence α₉₅ < 2.5°) deviate significantly from the geomagnetic direction measured at the time of the emplacement, the angle between the two directions being larger than the α₉₅ value. The value of 2.5-3.0° can thus be regarded as a conservative evaluation of the error when dealing with dating Etna and Vesuvius lava flows older than 17th century, even when the accuracy attained in remanence measurements is higher. In default of a SV curve for Italy derived from archaeological artefacts, a further error in dating is introduced when reference is made to SV curves of other countries, even if well-established, as these are from regions too far from Italy (>600 km) to confidently relocate magnetic directions.     

Comparison of Brewer spectrophotometer ultraviolet data from similar latitudes in the Northern and Southern Hemisphere

This paper presents UV data recorded over 12 months in 2000/2001 at a southern hemisphere (Toowoomba) and similar latitude northern hemisphere (BigBend and Everglades) sites using Brewer Spectrophotometers. The peak daily erythemally weighted UV (DUV) at the Toowoomba site was 8.8% and 25% higher compared to that at the BigBend and Everglades sites, respectively. The Everglades site exposures are lower than the BigBend site exposures in summer and spring. This may be due to the higher cloud cover for the Everglades site. The summer DUV values between Toowoomba and BigBend are similar. In spring, the average DUV value is the greatest at BigBend compared to the other two sites. Results indicate that variability between the measurement sites is due to varying cloud and ozone, and possibly due to aerosol and airmass differences of each region. Of these different site-specific local atmospheric conditions, cloud was the main contributor to the differences in UV between the sites.     

夏季青藏高原地区近地层水汽进入平流层的特征分析

青藏高原为亚洲季风区的典型代表区域,研究其水汽进入平流层的过程和机理对认识全球气候和大气环境变化具有一定的现实意义.本文基于中尺度气象模式(WRF)的模拟输出结果(2006年8月20日至8月26)驱动拉格朗日大气输送模式FLEXPART,通过追踪并解析气块的三维轨迹以及温度、湿度等相关物理量的相关变化特征,初步分析了夏季青藏高原地区近地层-对流层-平流层的水汽输送特征.研究结果表明,源于高原地区近地层的水汽在进入平流层的过程中受南亚高压影响下的大尺度环流和中小尺度对流的共同影响.首先,在对流抬升作用下,气块在短时间内(24 h)可抬升到9～12 km的高度,然后在南亚高压闭合环流影响下,相当部分气块在反气旋的东南侧穿越对流层顶进入平流层中,并继续向低纬热带平流层输送,进而参与全球对流层-平流层的水汽循环过程.在对流抬升高度上气块位置位于高原的西北侧,然而气块拉格朗日温度最小值主要分布于高原南侧,两个位置上气块的平均位温差值可达15～35 K,这种显著的温度差异将导致气块进入平流层时“脱水”.比较而言,夏季青藏高原地区近地层水汽进入平流层的多寡主要和大尺度汽流的垂直输送有关,而深对流的作用相对较弱.     

A 2.8 Ma record of environmental evolution and tectonic events inferred from the Cuoe core in the middle of Tibetan Plateau

Based on a multi-proxy investigation into the deep core of the Cuoe Lake in the middle of Tibetan Plateau, a 2.8 Ma paleoclimatic and paleoenvironmental evolution is reconstructed. The result of magnetic stratum indicates that the lake basin was formed at about 2.8 MaBP, while the multi-proxy analyses of lithology, grain size, magnetic susceptibility and geochemical elements reveal that there have been three major environmental evolution stages and at least two intensive uplifts of the Tibetan Plateau in the lake basin area, i.e. during 2.8-2.5 MaBP, the lake basin came into being as a result of the disaggregation of the planation surface and rapid rising of the Tibetan Plateau. During 2.5-0.8 MaBP, with gradual uplift of the Tibetan Plateau, the environment of this area was more effectively controlled by the climatic cycle of the alternative glacial-interglacial stages. After 0.8 MaBP, the middle part of the Plateau accelerated its uplift and entered cryoshere.     

Evaluation of linear regression methods as downscaling tools in temperature projections over the Pichola Lake Basin in India

In this paper, downscaling models are developed using various linear regression approaches, namely direct, forward, backward and stepwise regression, for obtaining projections of mean monthly maximum and minimum temperatures (Tmax and Tmin) to lake‐basin scale in an arid region in India. The effectiveness of these regression approaches is evaluated through application to downscale the predictands for the Pichola lake region in the state of Rajasthan in India, which is considered to be a climatically sensitive region. The predictor variables are extracted from (i) the National Centers for Environmental Prediction (NCEP) reanalysis dataset for the period 1948–2000 and (ii) the simulations from the third‐generation Canadian Coupled Global Climate Model (CGCM3) for emission scenarios A1B, A2, B1 and COMMIT for the period 2001–2100. The selection of important predictor variables becomes a crucial issue for developing downscaling models as reanalysis data are based on a wide range of meteorological measurements and observations. A simple multiplicative shift was used for correcting predictand values. Direct regression was found to yield better performance among all other regression techniques for the training data set, while the forward regression technique performed better in the validation data set, explored in the present study. For trend analysis, the Mann–Kendall non‐parametric test was performed. The results of downscaling models show that an increasing trend is observed for Tmax and Tmin for A1B, A2 and B1 scenarios, whereas no trend is discerned with the COMMIT scenario by using predictors. Copyright © 2010 John Wiley & Sons, Ltd.     

Underplating in the middle-lower Yangtze Valley and model of geodynamic evolution:Constraints from geophysical data

~~Underplating in the middle-lower Yangtze Valley and model of geodynamic evolution:Constraints from geophysical data1)East China petroleum Administration,1988(unpublished data). 1)Measurements of regional rock densities and magnetic parameters of Anhui Province(research report),Annul Geological and Mineral Bureau,1987(unpublished). 1)In this paper South China Block(SCB)includes the Yangtze block and Cathaysian block.In the Neoproterozoic the Yangtze block had been sutured to th…     

Modeling of Rayleigh waves dispersion in the Sannio region (Southern Italy) from seismic active refraction data

Short period surface waves, recorded during a seismic refractionsurvey in the Sannio region (Southern Italy), have been modeled to infera shallow velocity model for the area. Based on the decrease of resolutionwith depth, due to the bias on group velocity estimates arising frominterference of the Rayleigh waves with higher modes, we carried out aprocedure of fitting, with synthetic seismograms, of selected filtered traceswith a gaussian filter, having a width at half height equal to 1 Hz and acentral frequency lying in the range [1,4] Hz. We estimated the likelihoodbetween synthetic and observed seismograms by measuring their semblance.In this way we were able to infer a more refined local velocity modelcharacterized by a high Vp and Vs vertical gradient in the sedimentarycover. Two ad hoc resolution studies, based on group velocity andamplitude data respectively, indicate that the local velocity model is a goodvelocity model also for the entire studied area. The increase in the numberof available data when using amplitude information allows us to make amore selective choice in the model parameter space (Vp and Vs of eachlayer) and to solve for the Vp/Vs ratio. The inferred Vp velocity in thehalf-space is equal to 2.8 km/s. This value is in excellent agreement withthat inferred by other authors (3 km/s) by modeling P-wave travel timevs. distance. The best-fit model furnish low Vp/Vs for the sedimentarycover so indicating a high degree of the sediment's compaction in thestudied area. The inferred shallow high-velocity gradient indicates thatthe shallow sedimentary layer in the area could trap and focus the energytraveling into it.     

Study of the closure temperature of (U-Th)/He in detrital zircon obtained from natural evolution samples

Zircon(U-Th)/He thermochronometry was used as an effective indicator to study the thermal history of deep sedimentary basins with high temperatures.Zircon He ages and closure temperatures are very important parameters.In this paper,detrital zircon He closure temperature was studied by establishing the evolutionary pattern between zircon He ages and burial depth of borehole samples obtained from the Cenozoic strata in the Bohai Bay and Tarim basins,which have different thermal settings.The results show that the zircon He closure temperature of natural evolution samples is 200℃,which is higher than the temperature obtained from thermal simulation experiments(183℃).The temperature range of 140-200℃corresponds to the zircon helium partial retention zone.By properly understanding zircon He closure temperatures,zircon(U-Th)/He thermochronometry can provide a correct explanation of the He ages,and significant guidance in the study of the evolution of source rocks and the process of hydrocarbon accumulation in deep sedimentary basins.     

Geochemistry of coral from Papua New Guinea as a proxy for ENSO ocean–atmosphere interactions in the Pacific Warm Pool

A Porites sp. coral growing offshore from the Sepik and Ramu Rivers in equatorial northern Papua New Guinea has yielded an accurate 20-year history (1977–1996) of sea surface temperature (SST), river discharge, and wind-induced mixing of the upper water column. Depressions in average SSTs of about 0.5–1.0 °C (indicated by coral Sr/Ca) and markedly diminished freshwater runoff to the coastal ocean (indicated by coral δ¹⁸O, δ¹³C and UV fluorescence) are evident during the El Niño – Southern Oscillation (ENSO) events of 1982–1983, 1987 and 1991-1993. The perturbations recorded by the coral are in good agreement with changes in instrumental SST and river discharge/precipitation records, which are known to be diagnostic of the response of the Pacific Warm Pool ocean–atmosphere system to El Niño. Consideration of coastal ocean dynamics indicates that the establishment of northwest monsoon winds promotes mixing of near-surface waters to greater depths in the first quarter of most years, making the coral record sensitive to changes in the Asian–Australian monsoon cycle. Sudden cooling of SSTs by 1°C following westerly wind episodes, as indicated by the coral Sr/Ca, is consistent with greater mixing in the upper water column at these times. Furthermore, the coral UV fluorescence and oxygen isotope data indicate minimal contribution of river runoff to surface ocean waters at the beginning of most years, during the time of maximum discharge. This abrupt shift in flood-plume behaviour appears to reflect the duration and magnitude of northwest monsoon winds, which tend to disperse flood plume waters to a greater extent in the water column when wind-mixing is enhanced. Our results suggest that a multi-proxy geochemical approach to the production of long coral records should provide comprehensive reconstructions of tropical paleoclimate processes operating on interannual timescales.     

Erodibility of some crust forming soils/sediments from the Southern Aral Sea Basin as determined in a wind tunnel

Severe dust storms in the Southern Aral Sea Basin have become common with the desiccation of the sea. The high incidence of dust in the area has had severe ecological consequences. Within the framework of efforts to reduce this phenomenon, deflatability as well as deflatability‐related characteristics of some prominent soils/sediment surfaces in the Southern Aral Sea Basin were examined. The materials included a salt crust from a developed Solonchak, a Takyr crust and a Takyr‐like soil, and salt crusts from undeveloped Solonchaks formed on the exposed bottom of the Aral Sea. Characteristics determined were particle size distribution, dry aggregate size distribution and salt, carbonate and organic carbon contents. Deflatability was examined using a suction type wind tunnel with a SENSIT‐type sensor to detect airborne unconsolidated material, on materials treated to different moisture levels and with a chemical stabilizer, and on restored crusts created from the unconsolidated materials. Fine sand dominates in the materials, and in the Takyr crust and Takyr‐like soils is accompanied by significant amounts of silt and clay. All materials contain moderate amounts of carbonate and are low in organic matter. All soils/sediments contain salts, but in the salt crusts of the Solonchaks the salt fraction dominates. They all have more than 50 per cent PM₈₅₀ (particles with diameter <850 µm), indicating a relatively high deflatability potential. The materials from the Takyr crusts and Takyr‐like soil with a high proportion of fine aggregates had the lowest threshold friction velocities, while the salt crusts of the Solonchaks with a high proportion of coarse aggregates had the highest. This suggests that Takyrs and Takyr‐like soils are the most deflatable and Solonchak soils the least deflatable. These differences are attributed to the presence of salts that create stable, large aggregates in the Solonchak crusts. Wetting of the materials to three moisture levels considerably increased threshold friction velocity. The increase was most prominent in the salt‐rich materials, and was attributed to the rapid formation of surface films by drying in the course of the wind tunnel determinations. Applications of chemical stabilizers at two levels also considerably increased threshold friction velocity. On the restored crusts, threshold friction velocity dramatically increased, occasionally to non‐recordable values. This increase was monitored with both the salt crusts characteristic for the Solonchak soils and the fine‐grained crusts characteristic for the Takyr soils. The stability was attributed to the tightly packed salt particles in the salt crusts, and to the cohesive properties of the fine‐grained materials in the Takyr crusts. Once the crusts were ruptured, however, strong deflation commenced. These results suggest that by maintaining moisture in the soils/sediments (for example, by maintaining a high water table in the Amu‐Darya river flood plain) deflation can be reduced. By the same means, deflation can be reduced by creating new crusts or by preserving existing crusts. Copyright © 2005 John Wiley & Sons, Ltd.     

Historical trophic evolutions and their ecological responses from shallow lakes in the middle and lower reaches of the Yangtze River: Case studies on Longgan Lake and Taibai Lake

The evolutions of diatom floras and the total phosphorous (TP) concentrations in the historical period were reconstructed for two lakes, Longgan and Taibai in the middle Yangtze River,based on high resolutional fossil diatom study from two sediment cores and an established regional diatom-TP transfer function. The TP concentration in Longgan Lake changed slightly in the range of 36-62 μg/L and kept its middle trophic level in the past 200 years. The changes of diatom assemblages reflect a macrophyte-dominated history of the lake. During the nineteenth century, the lake TP concentration increased comparatively, corresponding to the increase in abundance of benthic diatoms. The progressive increase of epiphytic diatoms since the onset of the twentieth century indicates the development of aquatic plants, coinciding with the twice drops of water TP level. The TP concentration in Taibai Lake kept a stable status about 50 μg/L before 1953 AD, while diatoms dominated by facultative planktonic Aulacoseira granulata shifted quickly to epiphytic diatom species, indicating a rapid expansion of aquatic vegetation. During 1953-1970 AD, the coverage of aquatic plants decreased greatly inferred by the low abundance of epiphytic diatoms as well as declined planktonic types, and the reconstructed TP concentration shows an obvious rising trend firstly, suggesting the beginning of the lake eutrophication. The lake was in the eutrophic condition after 1970, coinciding with the successive increase of planktonic diatoms. The comparison of the two lakes suggests the internal adjustment and purification function of aquatic plants for nutrients in water. The discrepancy of TP trends in the two lakes after 1960 reflects two different patterns of lake environmental response to human disturbance. Sediments in Taibai Lake clearly recorded the process of lake ecological transformation from the macrophyte-dominated stage to the algae-dominated stage. The limits of TP concentration (68-118 μg/L) in the transitional state can be considered as the critical value between the two stable ecosystems. Further work will be necessary to provide more evidence from the sediments in more eutrophic lakes for the primary inference. The reconstructive TP level and the inference of aquatic plants from fossil diatoms in different lakes, as well as their comparison provide a scientific basis for ecological restoration of eutrophic lakes in research regions.     

近百年来我国西南地区酸沉降背景下湖泊环境变化——来自四面山龙潭湖沉积硅藻证据

大气酸沉降会导致地表水体酸化和富营养化,同时气候变暖也会影响湖泊水体环境,湖泊生态系统如何响应二者相互作用是全球变化研究的热点之一.我国西南地区是全球酸沉降负荷最高的地区之一,然而目前对酸沉降影响下地表水环境变化规律知之甚少.本文以重庆四面山龙潭湖为研究对象,通过沉积岩芯的²¹⁰Pb测年和硅藻分析,并结合流域历史资料数据,揭示龙潭湖过去近百年来水环境变化及其主要驱动因子.结果表明,1926-1968年,龙潭湖以Achnanthidium minutissimum和Encyonema silesiacum占优势;随后,Lindavia bodanica快速增加至峰值后逐渐减少,伴随Aulacoseira alpigena和Aulacoseira ambigua含量上升.硅藻组合中以适应中性和弱碱性水体的属种为主,嗜酸性属种含量很少,表明目前水体总体维持弱碱性,这得益于当地紫色砂页岩和森林土壤对于酸沉降的缓冲作用.尽管目前龙潭湖仍维持弱碱性,1980s以来硅藻推导水体酸碱度下降、中富营养种增加,表明酸沉降降低水体碱度并提高营养水平.气候变暖加剧水体热力分层和营养富集,导致适应扰动、贫营养环境的L.bodanica减少.2010年以来,中富营养属种略有减少反映湖泊富营养化有所减缓.