The exploration of indicator system establishment in ecotourism environmental impact assessment

Ecotourism environmental impact assessment is a traveling activity relevant influence prediction and appraisal. And the setting-up of the index system is very important to implement the appraisal of environmental impact of ecological travel. According to …     

Digital Geological Mapping of Sinus Iridum Area of the Moon Based on the Chang’E-I Data

Based on the CCD images,IIM data and DEM data of China＇s lunar exploration project （Chang＇E-Ⅰ） and related processed and analytic results,an integrated study of regional geology of Sinus Iridum and its adjacent area was conducted,and a series of relevant researches and analyses were carried out,including analysis of impact craters and their extrusive and accumulative materials,division of stratigraphic and tectonic units and classification of rock types,integrated analysis of chronology and lunar evolution history.In consideration of crater＇s shape features,quantity and preserving status of filling materials,the lunar impact craters can be divided into 7 types and 11 subtypes,and the accumulative materials of craters are divided into 6 types and 9 accumulative groups.According to the content and distribution of TiO2 and image characteristics,the basalts are divided into high-TiO2,medium-TiO2 and low-TiO2 basalts.Discussion was made on division of tectonic units and evolution features in the study area.The geological map of the Sinus Iridum Quadrangle （LQ-4） at a scale of 1∶2.5 M was preliminarily compiled with the ArcGIS system,and the spatial database of the map was established.Related technical specification,procedure and method for lunar geological mapping have been worked out,so as to lay a foundation for the forthcoming geological mapping of the global Moon in China by using the data of Chang＇E-Ⅱ and also for comprehensive study and geological mapping of other celestial bodies in the future.     

Distribution and concentrations of anthropogenic platinum group elements in the surface environment of Hong Kong

Ecotourism environmental impact assessment is a traveling activity relevant influence prediction and appraisal. And the setting-up of the index system is very important to implement the appraisal of environmental impact of ecological travel. According to the index system, natural ecological environment, humane social environment, the environment of tourist resources and the atmosphere of capacity of tourist environment, four major systems form the first index. Each system formed the second index of several key elements. In addition, it is a key step to appraise the sureness of the factor weight, which influences the rationality of the result directly. Daily method of weighting has the Delphi＇s and the AHP. According to ＂Grade form of characteristic value of Standard of rating for quality of tourist attractions＂ which the National Tourism Administration issues, the paper takes the Jingshan mountains ecological tourist zone as an example and adopts AHP to weight the factors mainly, assisted with the Delphi＇s by the help of computer. The Jingshan ecological tourist zone is good. The result of calculation, comprehensive value of 8.35 after establishment, shows that it doesn＇t change dramatically, attributing to the index of natural ecological environment like atmosphere, quality of surface water, acoustics environment and vegetation coverage. Improvement of traffic states and sanitation caused by the project in the region increases the comprehensive value, while the water and soil loss in partial area caused by construction decrease.     

Correlation of the largest craters,stratigraphic impact signatures,and extinction events over the past 250 Myr

The six largest known impact craters of the last 250 Myr(≥70 km in diameter),which are capable of causing significant environmental damage,coincide with four times of recognized extinction events at 36(with 2 craters),66,and 145 Myr ago,and possibly with two provisional extinction events at 168 and215 Myr ago.These impact cratering events are accompanied by layers in the geologic record interpreted as impact ejecta.Chance occurrences of impacts and extinctions can be rejected at confidence levels of99.96%(for 4 impact/extinctions)to 99.99%(for 6 impact/extinctions).These results argue that several extinction events over the last 250 Myr may be related to the effects of large-body impacts.     

Prognosis of the state （eutrophication and pollution） of freshwater ecosystems on the example of long-term research into Lake Baikal

Systematic （monthly） observations of changes in the components of trophic status in the pelagic zone of Baikal were carried out during years of spring blooms of almost pure cultures： Gymnodinium （1965）, Aulocoseira bai-calensis （1968）, Synedra acus （1969）, Nitzschia acicularis （1984 and 1987）, picocyanobacteria （2004-2005）. The research period coincided with various anthropogenic impacts on Baikal ecosystem： （1） the period of the natural state of the lake （until 1969）; （2） the period of weak anthropogenic impact （until 1985）; and （3） the period of acute anthropogenic impact. In the third period the following was found in the deep-water part of the lake： the episodic appearance of ammonium and nitrite nitrogen; change in the components of suspended matter （SM）; disturbed patterns of seasonal changes in SM content, chlorophyll-a, organic forms of carbon, nitrogen and phosphorus; the average annual concentrations of suspended organic carbon, nitrogen and phosphorus have increased considerably; the vertical distribution pattern of minerals and organic forms of nitrogen and phosphorus have been disturbed; finally, an observed inverse correlation between winter nitrogen content and the spring concentration of chlorophyll-a （1986-1989） would indicate the presence of toxins in the lake. This led to the study of POPs in Baikal and the Baikal region. Investigations undertaken during 2004-2005 in the pelagic coastal zones of the lake indicated the presence of toxic pollutants in southern Baikal （the correlation between winter nitrogen content and spring concentration of chlorophyll-a is inverse and equals 0.99 and 1.00 respectively）. Toxicants that enter the lake during the iced-over period mainly enter the trophogenic layer, as wastewaters are warmer than the lake waters, depress the development of endemic spring diatomic algae and lead to a rise in the trophic status.     

Surface Processes Driving Intracontinental Basin Subsidence in the Context of India–Eurasia Collision: Evidence from Flexural Subsidence Modeling of the Cenozoic Southern Tarim Basin along the West Kunlun Foreland, NW Tibetan Plateau

The India–Eurasia collision has produced a number of Cenozoic deep intracontinental basins, which bear important information for revealing the far-afield responses to the remote collision. Despite their significance, their subsiding mechanism remains the subject of debate, with end-member models attributing it to either orogenic or sedimentary load. In this study, we conduct flexural subsidence modeling with a two-dimensional finite elastic plate model on the Hotan–Mazatagh section along the southern Tarim Basin, which defines a key region in the foreland of the West Kunlun Orogen, along the NW margin of the Tibetan Plateau. The modeling results indicate that the orogenic load of West Kunlun triggers the southern Tarim Basin to subside by up to less than ~6 km, with its impact weakening towards the basin interiors until ~230 km north from the Karakax fault. The sedimentary load, consisting of Cenozoic strata, forces the basin to subside by ~2 to ~7 km. In combination with the retreat of the proto-Paratethys Sea and the paleogeographic reorganization of the Tarim Basin, we propose that surface processes, in particular a shift from an exorheic to an endorheic drainage system associated with the consequent thick sedimentary load, played a decisive role in forming deep intracontinental basins in the context of the India–Eurasia collision.     

Geochemical time series from lake sediments of the Central Asia as chronologic evidence of environmental change over the Late Holocene

Lake sediments provide archives of various parameters correlative with climatic oscillations such as seasonal temperature and humidity. However, geochemical records from many European and Asian lakes cannot be directly correlated with meteorological data within densely populated areas, because of anthropogenic pollution there. The human impact has greatly increased within the instrumental period of the last 150-200 years. However, the pollution problem is not essential for the mountain lakes of Siberia and Mongolia remote from industrial and agricultural centers. For such lakes a correlation between sedimentological-geochemical （inorganic） parameters and climatic data is correct enough. It is able to estimate any type of transfer functions for quantification of that dependence.     

Arsenic emission of high-arsenic coal combustion from southwestern Guizhou,China

With increasing use of coal, the growing impact on the environment from potentially hazardous trace elements becomes a great concern. Southwestern Guizhou, China, is a typical area of endemic arsenosis related to coal combustion. The use of locally mined,…     

The assessment of heavy metal stream in the air basin of Yerevan

Yerevan-the capital city of Armenia used to be a highly developed industrial center whose enterprises were integrated into the former USSR＇s industrial network. For this, the USSR disintegration entailed stagnation of most plants. Lately, some of them resumed operating, new enterprises were created, conditioning qualitative and structural changes in the city＇s industrial complex. Naturally,first of all, such changes manifest themselves in the extent of the impact of operating plants upon the environment and the air basin. This atmo-geochemical research was performed through studying natural depositing medium - snow cover, as the composition of its insoluble fraction is identical to that of solid constituent of air pollutants. The solid phase was analyzed through the method of quantitative spectral analysis for a wide set of heavy metals. Based on the data on heavy metal contents in separate samples, specialized geochemical maps of the city＇s territory were produced applying GIS ArcView 3.2 a. These initial data allow to calculate in-day dust load per square kilometer -38.3 kg/sq, km＊day, respectively, for the whole of the city＇s territory （225 sq. km） -8.6 tons. Data on element contents in dust allow to calculate the load of separate elements throughout the city landscape; so in-day load of Zn is 2.3 kg, Cu 1.1 kg. The determined contents of heavy metals are standardized by the element background and the following ranked geochemical series of heavy metal contents in ambient dust throughout the city was made up： Mo-Ag-Sn-Zn-Ni-Cu-Cr-Pb. The maps clearly reflect the morphology and extent of pollution zones and the structure of pollution. To characterize complex pollution, we have calculated a Summary Index of Pollution （SIP） that represents the sum of heavy metal contents in samples standardized by the background.     

Discovery of Cosmic Spherules from the Mesoproterozoic Strata and its Significance—in Case of the Ming Tombs Area, Beijing

This study covers cosmic spherules derived from the Mesoproterozoic Dahongyu Formation in the Ming Tombs area, Beijing. The cosmic spherules include iron oxide cosmic spherules, carbonaceous chondrites, and atomic iron “steely bead”-shaped cosmic spherules. The mineral assemblage of silicon carbide, forsterite, zircon, and glass spherules and fragments were picked from melt-silicified carbonate of the Mesoproterozoic Dahongyu Formation (ca. 1625 Ma). Cosmic spherule assemblages are solely discovered from sedimentary rocks in China. Platinum group elements (PGE) were determined for the first time in cosmic spherules and associated minerals. PGE comparative observation between meteorite and cosmic spherules is presented in this study. It is recognized that an extraterrestrial meteorite impact event might have occurred in the Dahongyu Stage. The main evidence is a large number of iron cosmic spherules in silicified oncolitic limestone, and associated cosmic silicon carbide, glass spherules, and fragments, as well as the presence of forsterite. The impact-volcanic crater is characteristic of a big black shale block dropped into the bended silicified limestone.     

Duration and synchroneity of the largest negative carbon isotope excursion on Earth: The Shuram/Wonoka anomaly

Carbonate δ¹³C values provide a useful monitor of changes in the global carbon cycle because they can record the burial ratio of organic to carbonate carbon. The most pronounced isotope excursions in the geologic record occur during the Neoproterozoic and have assumed a central role in the interpretation of biogeochemical events preceding the Ediacaran and Cambrian radiations. The most profound negative carbon isotope excursion is best recorded in the Ediacaran-aged Shuram Formation of Oman and has potential equivalents worldwide including the Wonoka Formation of South Australia and other sections in China, India, Siberia, Canada, Scandinavia and Brazil. All these excursions are less well understood than those in the Phanerozoic because of their unusual magnitude, long duration (> 1 Ma) and the difficulty in correlating Neoproterozoic basins to confirm independently that they do indeed record global change in the mixed ocean reservoir. Alternatively, these δ¹³C anomalies could reflect diachronous diagenetic processes. Currently none of these excursion are firmly time constrained and critical to their interpretation is a coherent reproducibility and synchroneity at the global ocean scale. Here we use available strontium isotope record as an independent chronometer to test the timing and synchroneity of the Shuram δ¹³C and its potential equivalents. The use of the ⁸⁶Sr/⁸⁷Sr ratio allows the reconstruction of a coherent, global δ¹³C record calibrated independently against time. The calibrated δ¹³C curve indicates that the Shuram negative anomaly spans several tens of millions of years and reaches values below −10‰. This carbon isotopic anomaly therefore represents a meaningful oceanographic event that fundamentally challenges our understanding of the carbon cycle as defined in the Phanerozoic.     

A pronounced negative δ13C excursion in an Ediacaran succession of western Yangtze Platform: A possible equivalent to the Shuram event and its implication for chemostratigraphic correlation in South China

A pronounced negative δ¹³C shift that can be potentially correlated with the Shuram excursion has been reported from middle Ediacaran strata in the Yangtze Gorges area of South China. Whether it represents a perturbation to the ocean carbon cycle or a record of post-depositional alteration is still open to debate. Resolving this controversy will help clarify if δ¹³C variations can be used for chemostratigraphic correlation of Ediacaran successions. To further understand the regional pattern of Ediacaran carbon isotopic excursions in the Yangtze platform, we carried out a detailed δ¹³C analysis of the Lianghong section in the western part of the Yangtze platform. The Ediacaran System at Lianghong is overlain by the Maidiping Formation yielding early Cambrian small shelly fossils and underlain by the Cryogenian Lieguliu Formation diamictite and tuffaceous siltstones. It comprises the Guanyinya and Hongchunping formations, which have been traditionally correlated with the Doushantuo and Dengying formations, respectively, in the Yangtze Gorges area. Two negative δ¹³C excursions occur in the Lianghong section. The lower one at the uppermost Guanyinya Formation, with a nadir at − 8.6‰, may be correlated with the pronounced negative δ¹³C shift (EN3) in the uppermost Doushantuo Formation in the Yangtze Gorges area and possibly with the well known Shuram event in Oman. The upper negative δ¹³C excursion occurs in the upper Hongchunping Formation and may be correlated with negative excursions (EN4) near the Ediacaran/Cambrian boundary. Other negative δ¹³C excursions (e.g., EN1 and EN2) are not expressed in the Lianghong section because the lower Guanyinya Formation is dominated by siliciclastic rocks. Combined with previously published Ediacaran δ¹³C profiles, our results indicate that the EN3 excursion (likely a Shuram equivalent) may occur widely in South China and can be a useful chemostratigraphic feature for regional and global stratigraphic correlation.     

Nitrogen isotope chemostratigraphy of the Ediacaran and Early Cambrian platform sequence at Three Gorges,South China

The appearance of multicellular animals and subsequent radiation during the Ediacaran/Cambrian transition may have significantly changed the oceanic ecosystem. Nitrogen cycling is essential for primary productivity and thus its connection to animal evolution is important for understanding the co-evolution of the Earth's environment and life. Here, we first report on coupled organic carbon and nitrogen isotope chemostratigraphy from the entire Ediacaran to Early Cambrian period by using drill core samples from the Yangtze Platform, South China. The results show that δ¹⁵N_TN values were high (~ + 6‰) until middle Ediacaran, gradually dropping down to − 1‰ at the earliest Cambrian, then rising back to + 4‰ in the end of the Early Cambrian. Organic carbon and nitrogen contents widely varied with a relatively constant C/N ratio in each stratigraphic unit, and do not apparently control the carbon and nitrogen isotopic trends. These observations suggest that the δ¹⁵N_TN and C/N trends mainly reflect secular changes in nitrogen cycling in the Yangtze Platform. Onset of the observed negative N isotope excursion coincided with a global carbon isotope excursion event (Shuram excursion). Before the Shuram event, the high δ¹⁵N probably reflects denitrification in a nitrate-limited oceanic condition. Also, degradation of dissolved and particulate organic matter could be an additional mechanism for the ¹⁵N-enrichment, and may have been significant when the ocean was rich in organic matter. At the time of the Shuram event, both δ¹³C_carb and δ¹⁵N_TN values were dropped probably due to massive re-mineralization of organic matter. This scenario is supported by an anomalously low C/N ratio, implying that enhanced respiration resulted in selective loss of carbon as CO₂ with recycled organic nitrogen. After the Shuram event, the δ¹⁵N value continued to decrease despite that δ¹³C_carb rose back to + 4‰. The continued δ¹⁵N drop appears to have coincided with a decreasing phosphorus content in carbonate. This suggests that ocean oxygenation may have generated a more nitrate-rich condition with respect to phosphorus as a limiting nutrient. Similar to the Shuram event, another negative δ¹³C_carb event in the Canglanpuan stage of the Early Cambrian is also characterized by carbon isotopic decoupling as well as the low C/N ratio. The results strongly support that the two stages of the decoupled negative δ¹³C_carb excursions reflect a disappearance of a large organic carbon pool in the ocean. The two events appear to relate with the appearance of new metazoan taxa with novel feeding strategies, suggesting a link between ocean oxygenation, nutrient cycling and the appearance and adaptation of metazoans. The nitrogen isotope geochemistry is very useful to understand the link between the environmental, ecological and biological evolutions.     

Glaciation and ~ 770 Ma Ediacara (?) Fossils from the Lesser Karatau Microcontinent,Kazakhstan

The Cambrian explosion, c. 530–515 Ma heralded the arrival of a diverse assembly of multicellular life including the first hard-shelled organisms. Fossils found in Cambrian strata represent the ancestors of most modern animal phyla. In contrast to the apparent explosiveness seen in the Cambrian fossil record, studies of molecular biology hint that the diversification observed in Cambrian strata was rooted in ancestry extending back into the Ediacaran (635–542 Ma). Fossil evidence for this mostly cryptic phase of evolution is derived from the soft-bodied fossils of the Ediacaran biota found throughout the world and bilaterian embryos found in the Doushantuo lagerstätte in South China. The first appearance of Ediacara fauna is thought to have followed the last of the ~ 750–635 Ma Neoproterozoic glacial episodes by 20–30 million years. In this paper, we present evidence for the oldest discovery of the ‘Ediacara’ discoidal fossils Nimbia occlusa and Aspidella terranovica (?) that predate the early Cryogenian glaciations by more than fifty million years. There is considerable disagreement over the significance of discoidal Ediacaran fossils, but our findings may support earlier suggestions that metazoan life has roots extending deeper into the Proterozoic Eon. We also confirm the presence of a Late Cryogenian (e.g. “Marinoan”) glaciation on the Lesser Karatau microcontinent including dropstones and striated clasts within the glacial strata.     

The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the Three Gorges area,South China

The Ediacaran period was one of the most important times for the evolution of life. However, the scarcity of well-preserved outcrops of Ediacaran rocks still leaves ambiguity in decoding ambient surface environmental changes and biological evolution.The Ediacaran strata in South China are almost continuously exposed, comprise mainly carbonate rocks with subordinate black shales and sandstones, and they contain many fossils, suitable for study of environmental and biological changes in the Ediacaran. We conducted drilling through the Doushantuo Fm at four sites in the Three Gorges area to obtain continuous, fresh samples without surface alteration and oxidation. We analyzed ⁸⁷Sr/⁸⁶Sr and ⁸⁸Sr/⁸⁶Sr ratios of the fresh carbonate rocks, selected on the basis of microscopic observations and the geochemical signatures of Sr contents, Mn/Sr and Rb/Sr ratios, and δ¹⁸O values, with a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS).The chemostratigraphy of the ⁸⁷Sr/⁸⁶Sr ratios of the drilled samples displays a smooth curve and two large positive shifts during Ediacaran time. The combination of the detailed chemostratigraphies of δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values and Mn and Fe contents enables us to decode the surface environmental changes and their causes in the Ediacaran. The first large positive excursion of ⁸⁷Sr/⁸⁶Sr occurred together with negative δ¹³C and positive δ¹⁸O excursions. The higher ⁸⁷Sr/⁸⁶Sr values indicate an enhancement of continental weathering, whereas the positive δ¹⁸O excursion suggests global cooling. Global regression due to global cooling enhanced the oxidative decay of exposed marine organic sediments and continental weathering. Accelerated influx of nutrients promoted primary productivity, resulting in oxidation of dissolved organic carbon (DOC), whereas active sulfate reduction due to a higher sulfate influx from the continents caused remineralization of the large DOC, both of which caused a negative δ¹³C anomaly. The 580 Ma Gaskiers glaciation accounts for the close correlation among the positive ⁸⁷Sr/⁸⁶Sr, negative δ¹³C and positive δ¹⁸O excursions.The second large positive shift of ⁸⁷Sr/⁸⁶Sr firstly accompanied a positive δ¹³C excursion, and continued through the Shuram δ¹³C negative excursion. The positive correlation of δ¹³C and ⁸⁷Sr/⁸⁶Sr values is consistent with an enhanced continental weathering rate due to continental collisions that built Trans-Gondwana mountain chains, and with a higher primary activity due to the enhancement of continental weathering and consequent higher nutrient contents in seawater. The accompanied increase in Mn and Fe contents implies a gradual decline of the seawater oxygen content due to more active aerobic respiration and oxidation of reductive materials flowing in the oceans. In the Shuram excursion, higher ⁸⁷Sr/⁸⁶Sr values and a transition from increase to decrease in Mn and Fe contents were accompanied by the large negative δ¹³C excursion. The higher ⁸⁷Sr/⁸⁶Sr values are the first compelling evidence for enhanced continental weathering, which was responsible for the large δ¹³C anomaly through the remineralization of the DOC by more active sulfate reduction due to a higher sulfate influx. Higher Mn and Fe contents in the early and middle stages of the excursion suggest a decline in the oxygen content of seawater due to oxidative decay of the DOC, whereas in the late stages the decrease in Mn and Fe contents is consistent with oceanic oxygenation.The emergence of Ediacara biota after the Gaskiers glaciation and the prosperity of the latest Ediacaran is concomitant with the formation of more radiogenic seawater with high ⁸⁷Sr/⁸⁶Sr values, suggesting that enhanced continental weathering, and the consequent higher influx of nutrients, played an important role in biological evolution.     

澄江动物群的核心价值:动物界成型和人类基础器官诞生

澄江动物群以富含多门类精美软躯体构造化石享誉学界,是寒武纪大爆发主幕的见证者;它亲眼目睹了地球动物树上几乎所有主要门类祖先的首次大聚会。对该动物群30多年的大规模调查研究大体上可以划分为3个阶段。始自1984年的第一个10年(1984—1994年),早期动物树上的基础动物亚界和原口动物亚界的大量门类被发现,然而对后口动物亚界全然无知。第二个10年研究(1995—2005年),基础动物亚界和原口动物亚界中的更多门类被发现;而最重要的进展,是西北大学团队主导的早期后口动物大发现使后口动物亚界的基本框架被构建,由此地球三分动物树首次成型。第三个10年及其以后时期(2005年之后),3个动物亚界中进一步添加了一些新的门类认知;综合20世纪40年代以来对埃迪卡拉生物群、寒武纪第一世和第二世的珍稀动物群研究成果,学界开始思考三分动物树分步成型与多幕式寒武纪大爆发事件的内在关联,提出了三幕式寒武纪大爆发新假说。澄江动物群研究具有多方面的科学价值,包括动物群结构研究,生态环境研究等,然其核心学术价值主要集中在如下两个方面:(1)作为寒武纪大爆发的主幕见证者,澄江动物群首次创建了地球上三分动物树的完整谱系框架。三幕式寒武纪大爆发假说揭示了地球上动物门类由低等到高等、分阶段爆发创造的本质内涵,即历时约0.4亿年的三幕式寒武纪大爆发与动物树阶段性快速成型的内在耦合性。第一幕发生于埃迪卡拉纪晚期,它诞生了众多基础动物亚界门类(包括大量绝灭门类),也可能出现了少量原口动物的先驱分子;第二幕发生于寒武纪的第一世,除了延续基础动物门类的繁盛外,更创生了原口动物亚界里的主要门类(包括蜕皮类和触手担轮类);第三幕发生于寒武纪第二世(以澄江动物群为代表),不仅延续了基础动物和原口动物门类的兴盛,更重要的是诞生了后口动物亚界里的所有门类。至此,整个三分动物树的框架成型,寒武纪大爆发创新事件宣告基本结束。文章还对几个重要动物类群的生物学属性进行了评述。第一鱼昆明鱼目是已知唯一的最古老脊椎动物,而云南虫、海口虫既不是脊椎动物(即高等脊索动物),也不属于低等脊索动物,而是一类特殊的低等后口动物;长江海鞘是可信的始祖尾索动物,它支持了经典的尾索动物起源假说;古虫动物门首创鳃裂构造,对后口动物亚界的起源探索提供了关键信息。(2)澄江动物群具有深刻的人文哲学意义,所发现的古虫动物门创造的“第一鳃裂”,昆明鱼目创造的“第一头脑”“第一脊椎”“第一心脏”,为达尔文在《人类的由来》中关于人类的主要基础器官起源悬案的破解提供了关键证据。澄江动物群承上启下,其各种形态解剖学信息对解读埃迪卡拉生物群以及寒武纪相关化石库中绝大多数生物类型都具有重要的参考价值。     

埃迪卡拉纪—寒武纪之交微生物岩特征对比及古海洋学意义: 以汉南—米仓山地区为例*

寒武纪初期不仅发生了宏体生物大爆发,而且也出现了地质历史时期少见的蓝细菌鞘体大规模钙化事件。埃迪卡拉纪—寒武纪之交海水化学性质的转变对真核生物的演化起到了重要作用,但是这种转变对微生物岩发育特征以及蓝细菌钙化事件的产生有无影响,目前尚不明确。鉴于此,对华南上扬子北缘汉南—米仓山地区上埃迪卡拉统—寒武系第二统多个典型微生物岩发育剖面进行了系统野外调查和室内岩石学分析,结果表明:上埃迪卡拉统灯影组叠层石广泛发育,以平铺状、缓波状特征为主,而凝块石既可以呈补丁状分布于叠层石纹层间,又可以呈细小的凝絮状、粘结状特征构成厚层岩层;寒武系第二统仙女洞组叠层石丰度显著降低,以高大、坚硬的丘状隆起为特点,包括单独的凝块石丘,以及微生物与古杯的联合建丘。虽然寒武系第二统微生物岩的层状结构和凝块结构与埃迪卡拉系相比并无太大差异,但是寒武系微生物岩内部保存有大量的钙化微生物化石,已识别出附枝菌(Epiphyton)、肾形菌(Renalcis)和葛万菌(Girvanella)等多种类型。在收集、整理前人有关微生物岩特征和发育资料的基础上,本次研究初步整理出华南寒武系第二统微生物岩的时空分布特点,发现寒武纪第二世第三期是钙化微生物大量发育的一个时期,在随后的第四期达到一个小的高峰。对于此次蓝细菌钙化作用幕的启动机制,除前人提出的海水高钙离子浓度和蓝细菌体内二氧化碳浓缩机制等认识外,寒武纪早期海水性质的转变(方解石质原生矿物受成岩改造程度较低)、适度的陆源碎屑输入(黏土组分保护作用)也有利于钙化微生物结构的保存,应引起重视。     

Birth and early evolution of metazoans

The reconstruction of the phylogenetic tree of animals (TOA) has long been one of the central interests in biological and paleobiological sciences. We review the latest results of paleontological and stratigraphical studies on the Ediacaran–Cambrian sequences mainly from South China for revising the TOA in accordance with modern genome biology. A particular focus is given to the pattern of animal diversification based on the fossil first appearances of high-rank clades chiefly in phylum-level. The results show an abrupt divergence of lineages during the Ediacaran–Cambrian transition; however, the appearances of metazoan phyla were obviously diachronous, with three major phases recognized herein. The first phase is marked by the appearances of basal metazoan phyla in the latest Ediacaran. Very few unequivocal bilaterian clades were present at this phase. The second phase occurred in the Terreneuvian (Cambrian Stages 1–2), represented by the occurrences of many lophotrochozoan lineages. This phase also involves the appearances of calcified basal metazoan lineages, and possibly, those of contentious ecdysozoans in the latest Terreneuvian, but no deuterostome has been known from this age. The third and also the largest phase occurred in the Cambrian Stage 3, which involve all the three supraphylogenetic clades of the Eubilateria. A number of lophotrochozoan lineages, the bulk of ecdysozoans, and all deuterostome phyla, appeared for the first time in this phase. Since there is no unambiguous evidence for bilaterians in the Ediacaran, the Cambrian explosion sensu stricto was an abrupt diversification of bilateral lineages in a short time of ca. 25 million years across the Ediacaran–Cambrian boundary. Next critical issues in research include high-resolution chrono- and chemostratigraphic analyses, correlations between biotic events and environmental perturbations, physiological approach to the biological connotation of biomineralization, and exploration for the lost mid-oceanic biota and environments, which are crucial in understanding the entire picture of the Cambrian explosion.     

New high‐resolution age data from the Ediacaran–Cambrian boundary indicate rapid,ecologically driven onset of the Cambrian explosion

The replacement of the late Precambrian Ediacaran biota by morphologically disparate animals at the beginning of the Phanerozoic was a key event in the history of life on Earth, the mechanisms and the time‐scales of which are not entirely understood. A composite section in Namibia providing biostratigraphic and chemostratigraphic data bracketed by radiometric dating constrains the Ediacaran–Cambrian boundary to 538.6–538.8 Ma, more than 2 Ma younger than previously assumed. The U–Pb‐CA‐ID TIMS zircon ages demonstrate an ultrashort time frame for the LAD of the Ediacaran biota to the FAD of a complex, burrowing Phanerozoic biota represented by trace fossils to a 410 ka time window of 538.99 ± 0.21 Ma to 538.58 ± 0.19 Ma. The extremely short duration of the faunal transition from Ediacaran to Cambrian biota within less than 410 ka supports models of ecological cascades that followed the evolutionary breakthrough of increased mobility at the beginning of the Phanerozoic.     

Rapid changes of magnetic Field polarity in the late Ediacaran: Linking the Cambrian evolutionary radiation and increased UV-B radiation

Novelty in the biological world is the culmination of genetic changes often triggered by the physical environment. The most radical phase of biological evolution took place during the Cambrian Evolutionary Radiation (CER). Prior to the CER, bacterial matgrounds and associated communities of Ediacaran organisms dominated the shallow seafloor. Near the end of the Ediacaran Period, ~ 550 million years ago, many soft-bodied biota went extinct. In the Early Cambrian, animals with the ability to burrow vertically altered the ecology of the seafloor and biomineralization became commonplace. Here we link the terminal Ediacaran extinction, the Cambrian substrate revolution and the diversification of biomineralizing organisms to changes associated with the reversal frequency of the Earth's magnetic field. Beginning around 550 Ma and continuing through much of the Cambrian, the Earth's magnetic field was rapidly reversing. Models, and limited paleointensity studies, indicate that rapid reversals are a feature of an overall weaker dipole. A weakened dipole reduces the dimensions of the magnetosphere that provides a barrier to incoming cosmic radiation. Here we show that the environmental effects of that collapse include increased dosing of UVB radiation into the shallow marine environment. Increased UVB radiation in the shallow marine environment provided selective pressure favoring organisms that could detect and avoid UVB damage by burrowing vertically, moving up or down in the water column, growing protective shells and other ‘flight from light’ mechanisms. These changes took place in advance of the CER, but effectively cleared the ecological space for the subsequent changes in the later Cambrian.