藏南舌羊齿植物群的发现和其在地质学及古地理学上的意义

喜马拉雅地区一直被传统地质学认为是一个典型的古地槽区,属于劳亚和冈瓦纳两古大陆之间的古地中海的一部分,具有很厚的古生代到老第三纪的海相沉积。本世纪上叶,魏格纳就提出大陆漂移的假说,设想陆块之间曾有过大规模的水平移动,可是大陆固定和海洋永存的传统观念在人们脑海中根深蒂固。     

从古地磁学看日本列岛的形成

根据古地磁学的原理,可由某地区在某时代的平均磁场强度方向求出该地区在该时代内的古地磁极位置。而由该地区不同时代的古地磁极位置,又可以进一步求出对应的APWP来(APWP为视极移曲线)。此时,如果我们再对不同地区的APWP进行对比,则可判断不同地区之间相对运动的历史状况。显而易见,古地磁学对大陆漂移、海底扩张和板块构造等学说起着极其重要的支柱作用。     

三叠纪古生物地理与大陆漂移

自板块说问世以来,大陆漂移理论获得了新的生命力,二十年来迅速发展,已经在许多学科领域内获得大量的证据。本文试图从三叠纪古生物地理和地层的角度,对其进行讨论。     

华南早新元古代莲沱组地层磁倾角偏低研究及其古地理意义

华南莲沱组最新的年龄结果表明,其时代结束于715Ma,因此,准确确定莲沱组的古纬度对"雪球地球"的研究具有重要意义。通过对莲沱组红层进行等温剩磁各向异性研究,获得其磁倾角校正因子为0.8719,校正后的磁倾角为70.4°,对比热退磁实验测得的莲沱组磁倾角为67.8°,则其磁倾角偏低量为2.6°。通过校正前后的磁倾角分别计算古纬度,获得磁倾角偏低所引起的古纬度变化为3.9°±6°。通过对比华南与澳大利亚-东南极板块的720Ma古地理位置,发现这一时期冰碛岩从中纬度到赤道广泛分布,验证了当时的"雪球地球"环境。     

中国晚元古代古构造与古地理

本文所指晚元古代时限为距今1000-600百万年。中国晚元古代可分为两个阶段,自1000-800百万年一段暂不建纪,因为这一时期的地层在全国的对比还存在很多问题,生物面貌远不明确,所以我们仍称青白口群,不用青白口系。自800-600百万年这一段即现称震旦纪。根据地层沉积类型和分区,我们编制了三幅古构造古地理图(图1-3)。     

古地磁学、工作方法及其应用(三)

古地磁的应用古地磁学在地球物理学和地质学方面的应用,其依据是地磁场在从前寒武纪到现在的全部演化过程中,磁极位置不断移动,极性经常倒转.也就是这一依据对大陆漂移和海底扩张等假说提供了重要证据.对地球膨胀说(即地球半径在缓慢地增大)提供了数据,可计算出各个地质史期的地球半径.用古地磁学研究宇宙星球(例如月球、陨石)上物质的磁性,从而了解地磁场的起源和演化等方面的问题.根据测得的地磁倾角,可算出样品产地的古纬度.     

四川渡口地区晚第四纪沉积物的孢粉分析及其地质意义

作者在四川渡口地区选择了桐子林、金江火车站、盐边新村三个露头剖面,做了系统的孢粉取样及分析。本文就分析结果,讨论了本区第四纪地层的划分、对比及古环境等方面问题。     

四川盆地雅安至天全白垩系一下第三系古地磁研究

对采自四川雅安新开店至天全始阳剖面白垩系21个采点197块样品、下第三系15个采点129块样品进行了古地磁研究。由特征剩磁方向统计,分别获得了采样区白垩纪和早第三纪的古纬度为32.5°N和31.7°N;古地磁极点位置分别为φ=172°E,λ=81.8°N和φ=75.7°E,λ=88.1°N;建立了白垩系和下第三系的磁极性地层剖面,并提出了有关地层划分和对比的意见。根据各个时期磁偏角的变化特征和古地磁极的游移轨迹可以看出,从白垩纪开始至早第三纪中晚期,采样区地块先顺时针方向旋转,然后又逆时针方向旋转。     

利用天然热发光划分和对比碳酸盐岩层

热发光方法在地质学中的应用,只是近年来才迅速、广泛地发展起来的。目前已用于地质年龄测定、考古学、地层对比、岩相古地理、矿床学、地质温度计、构造活动、变质作用、陨石以及月岩研究等方面,并且都取得了一定的效果。     

我国某些地区的海绿石特征及其对相环境分析的意义

海绿石矿物一般是在正常盐度的浅海环境中生成,可作为良好的指相矿物。在无化石的地层中海绿石可用来测定同位素年龄、进行地层对比和解释地层间断。此外,海绿石也可作为储油层沉积环境的综合指标之一。 本文通过各种方法对我国某些地区砂岩类型的自生海绿石的矿物学特征作了综合系统的研究。试图通过从已知到未知、从现代到古代作对比,对松辽盆地的某些储油层段相环境的确定,提出必要的实际资料,进而来讨论它们的沉积环境。     

Proterozoic (pre-Ediacaran) glaciation and the high obliquity,low-latitude ice,strong seasonality (HOLIST) hypothesis: Principles and tests

Sedimentological observations and palaeomagnetic data for Cryogenian glacial deposits present the climatic paradox of grounded glaciers and in situ cold climate near sea-level, glaciomarine deposition, and accompanying large (up to 40 °C) seasonal changes of temperature, all in low to near-equatorial (< 10°) palaeolatitudes (equated with geographic latitudes). Neither the “snowball Earth” nor the “slushball Earth” hypothesis can account for such strong seasonality near the palaeoequator, which together with findings from sedimentology, chemostratigraphy, biogeochemistry, micropalaeontology, geochronology and climate modelling argue against those scenarios. An alternative explanation of glaciation and strong seasonality in low palaeolatitudes is offered by a high (> 54°) obliquity of the ecliptic, which would render the equator cooler than the poles, on average, and amplify global seasonality. A high obliquity per se would not have been a primary trigger for glaciation, but would have strongly influenced the latitudinal distribution of glaciers. The principle of low-latitude glaciation on a terrestrial planet with high obliquity is validated by theoretical studies and observations of Mars. A high obliquity for the early Earth is a likely outcome of a single giant impact at 4.5 Ga, the widely favoured mechanism for lunar origin. This implies that a high obliquity could have prevailed during most of the Precambrian, controlling the low palaeolatitude of glaciations in the early and late Palaeoproterozoic and Cryogenian. It is postulated that the obliquity changed to < 54° between the termination of the last Cryogenian low-palaeolatitude glaciation at ≤ 635 Ma and the initiation of Late Ordovician–Early Silurian circum-polar glaciation at 445 Ma.The High Obliquity, Low-latitude Ice, STrong seasonality (HOLIST) hypothesis for pre-Ediacaran glaciation emerges favourably from numerous glacial and non-glacial tests. The hypothesis is in accord with such established or implied features of Cryogenian glaciogenic successions as extensive and long-lived open seas, an active hydrological cycle, aridity and palaeowesterly (reversed zonal) winds in low palaeolatitudes, and the apparent diachronism or non-correlation of some low-palaeolatitude glaciations. A pre-Ediacaran high obliquity also offers a viable solution of the faint young Sun paradox of a warm Archaean Earth. Furthermore, reduction of obliquity during the Ediacaran–early Palaeozoic would have yielded a more habitable globe with much reduced seasonal stresses and may have been an important factor influencing the unique evolutionary events of the Ediacaran and Cambrian. The palaeolatitudinal distribution of evaporites cannot discriminate unambiguously between high- and low-obliquity states for the pre-Ediacaran Earth. Intervals of true polar wander such as postulated by others for the Ediacaran and Early Cambrian imply major mass-redistributions within the Earth at those times, which may provide a potential mechanism for reducing the obliquity during the Ediacaran–early Palaeozoic.     

Phanerozic bauxite epochs

The Early Cambrian, Middle and Late Devonian, Middle and Late Carboniferous, Permian, Late Triassic-Early Jurassic, Late Cretaceous, Paleocene-Eocene, and Miocene epochs of bauxite formation have been the most productive. They lasted for no less than 10 Ma. The scope of bauxite deposition of various epochs is shown in the diagram, and the present-day localization of Cenozoic, Mesozoic, and Paleozoic bauxites is depicted in separate maps. The Cenozoic bauxite deposits are located in tropical and subtropical zones of the Southern and Northern hemispheres. The Mesozoic deposits occur in the Northern Hemisphere as far north as 50°N, and the Paleozoic deposits, as far north as 70°N. Palinspastic reconstructions show that during all the aforementioned epochs, bauxites were deposited at paleotropical latitudes. The current localization of the Paleozoic and Mesozoic bauxites at high latitudes up to the Polar Circle is caused by continental drift to the north in the Phanerozoic.     

Last Glacial Maximum and Hypsithermal in the Southern Hemisphere

A synthesis of the two contrasting climates of the upper Quaternary in the Southern Hemisphere, based on continental geological indicators, suggests that the Antarctic Anticyclone is the most important climatic system in the hemisphere. According to glaciological and oceanographic studies, the Antarctic Anticyclone covered an area in the LGM that was double of that during the Hypsithermal. This phenomenon produced a consequent shifting of the climatic belts by approximately 10° of latitude to the north in the LGM and a migration to the south during the Hypsithermal. Oceanic and continental anticyclones were active systems in shaping climates in South America, Australasia, and Southern Africa. These structures were enhanced throughout the LGM (provoking generalized droughts), and weakened at the Hypsithermal (permitting more rains in tropical latitudes). The results presented here were obtained by integrating our own results with regional and continental syntheses of other authors. The paleoclimatic proposals are amply validated by more than 1500 dates in the four continents. Geological and geomorphological features, such as dunefields, paleosols, fluvial terraces and lake levels were the main tools employed in the work.     

A geochemical view into continental palaeotemperatures of the end-Permian using oxygen and hydrogen isotope composition of secondary silica in chert rubble breccia: Kaibab Formation,Grand Canyon (USA)

The upper carbonate member of the Kaibab Formation in northern Arizona (USA) was subaerially exposed during the end Permian and contains fractured and zoned chert rubble lag deposits typical of karst topography. The karst chert rubble has secondary (authigenic) silica precipitates suitable for estimating continental weathering temperatures during the end Permian karst event. New oxygen and hydrogen isotope ratios of secondary silica precipitates in the residual rubble breccia: (1) yield continental palaeotemperature estimates between 17 and 22 °C; and, (2) indicate that meteoric water played a role in the crystallization history of the secondary silica. The continental palaeotemperatures presented herein are broadly consistent with a global mean temperature estimate of 18.2 °C for the latest Permian derived from published climate system models. Few data sets are presently available that allow even approximate quantitative estimates of regional continental palaeotemperatures. These data provide a basis for better understanding the end Permian palaeoclimate at a seasonally-tropical latitude along the western shoreline of Pangaea.     

Environmental change modelling for Central and High Asia: Pleistocene, present and future scenarios

A regional modelling concept was developed for late Quaternary climate reconstructions and future climate impact assessments. Based on estimates of different climate parameters covering the entire Central and High Asia in a grid-cell spacing of 1 km², climatic determinants of the recent spatial distribution of climate-sensitive environments (glacial and periglacial environments, forest) were explored. Simple climatic threshold functions were established, defining critical climate values for modelling the spatial extension of environments considered. Using palaeogeomorphological indicators as a basis, late Quaternary climatic conditions were modelled in a Last Glacial Maximum (LGM) scenario consistent with palaeogeomorphological and palaeoclimatological data. The results enabled a validation of LGM palaeoclimate simulations performed by the ECHAM GCM. To assess the magnitude of possible future climatic impacts on the spatial distribution of glaciers, permafrost and potential forest stands, two GCM-based IPCC-SRES climate change scenarios (IPCC special report on emission scenarios) for the time period 2070-2099 were considered. Assuming future climates to be perturbed for long enough to affect the environments, a distinct loss of glaciated areas and permafrost must be expected.     

大巴山与龙门山前晚三叠世构造运动的储集层沉积学响应*

通过对大巴山前樊哙剖面与龙门山前小塘子村剖面上三叠统沉积相垂向演化分析、砂泥岩X射线荧光光谱分析、储集层微观薄片研究,认为川东北与川西北地区上三叠统的沉积环境、沉积水体的深度和盐度及砂岩碎屑组分与印支晚幕构造运动有紧密的关系。印支晚幕运动早期,大巴山与龙门山前的小塘子组沉积水体盐度较高,属温暖潮湿气候下的潮坪沉积;须家河组二段沉积时期,随着大巴山与龙门山的低幅隆升,古气候由温暖潮湿逐渐变为较干冷,沉积水体变浅且盐度高,属辫状河三角洲沉积。须三段沉积时期,大巴山与龙门山前古气候又演变为温暖潮湿,沉积水体加深,湖泊沉积较发育,水体盐度逐渐降低。印支晚幕安县运动发生后,大巴山前与龙门山前的古气候变得干冷,沉积水体变浅,盐度降低,沉积物属典型的陆相淡水沉积。同时,小塘子组-须家河组砂岩碎屑成分也随着构造运动的演化具有显著的不同。     

Paleogene monsoons across India and South China: Drivers of biotic change

Monsoonal climates at low latitudes (< 32°N) are an inevitable consequence of seasonal migrations of the Inter-tropical Convergence Zone (ITCZ), but the character of these monsoons depends on continental configuration, orographic expression and the strength of Hadley circulation. To explore the evolution of monsoon systems across southern Asia we compare climate signatures archived in ten Paleogene floras from northern India, Tibet and southern China, occupying low palaeolatitudes at a time of extreme global warmth and elevated CO₂. Fossil leaf form reveals that under such 'hothouse' conditions megathermal early Eocene to earliest Miocene forests were exposed to strong monsoonal climates typical of those experienced today arising from annual migrations of the ITCZ, possibly enhanced by a lower equator-to-pole temperature gradient. Throughout the Paleogene an elevated Tibetan highland produced no discernable modification of this ITCZ monsoon, although rainfall seasonality similar to that of the modern South Asia Monsoon (SAM) is observed in northern India as early as the beginning of the Eocene, despite its near-equatorial palaeoposition. In South China rainfall seasonality increased progressively achieving modern monsoon-like wet season/dry season precipitation ratios by the early Oligocene. Despite evidencing weak rainfall seasonality overall, fossil leaves from South China have exhibited monsoon-adapted morphologies, comparable to those seen in today's Indonesia-Australia Monsoon, for at least 45 million years. Together, the Indian and South China fossil leaf assemblages show that the evolution of megathermal ecosystems across southern Asia has been influenced profoundly by monsoonal climates for at least the last 56 million years. The Paleogene ITCZ-driven monsoon system strongly impacted India as it transited the Equator likely eliminating Gondwanan taxa not able to adapt to seasonal precipitation extremes. Furthermore, powerful seasonally-reversing winds, and associated surface ocean currents, are likely to have facilitated two-way biotic transfer between India and Eurasia long before closure of the Tethys Ocean.     

Glacier fluctuations in the southern Peruvian Andes during the late‐glacial period,constrained with cosmogenic 3He

The occurrence of pronounced climate reversals during the last glacial termination has long been recognised in palaeoclimate records from both hemispheres and from high to low latitudes. Accurate constraint of both the timing and magnitude of events, such as the Younger Dryas and Antarctic Cold Reversal, is vital in order to test different hypotheses for the causes and propagation of abrupt climate change. However, in contrast to higher‐latitude regions, well‐dated records from the Tropics are rare and the structure of late‐glacial tropical climate remains uncertain. As a step toward addressing this problem, we present an in situ cosmogenic ³He surface exposure chronology from Nevado Coropuna, southern Peru, documenting a significant fluctuation of the ice margin during the late‐glacial period. Ten tightly clustered ages from a pair of moraines located halfway between the modern glacier and the Last Glacial Maximum terminus range from 11.9 to 13.9 ka and give an arithmetic mean age of 12.8 ± 0.7 ka (1σ). These data constitute direct evidence for a readvance, or prolonged stillstand, of glaciers in the arid Andes of southwestern Peru. Copyright © 2011 John Wiley & Sons, Ltd.     

晚古生代全球古气候特征及其研究方法

相对第四纪而言,人们对史前全球古气候的研究还比较薄弱,在方法学方面也需要进一步探讨。本文归纳了古气候演变原因,初步总结了全球晚古生代古气候特征,探讨了古气候的研究方法。一方面,可以据此进一步细化对史前古气候的研究,另一方面旨在古为今用,加强区域和全球不同尺度气候环境的预测,趋利避害,更好地服务于人类。     

大陆型冰川与海洋型冰川物质平衡对比研究——以天山和阿尔卑斯山典型冰川为例

物质平衡是冰川对气候变化最直接的响应,是冰川变化的重要参数.大陆型冰川与海洋型冰川发育在不同的水热环境下,它们对气候变化的响应程度、过程和机理存在很大差异,因此在全球变暖背景下对这两类不同性质冰川物质平衡变化特征及其机理做一全面的对比研究意义重大.以东、西天山的乌鲁木齐河源1号冰川和图尤克苏冰川以及阿尔卑斯山东、中、西部的欣特雷斯冰川、Caresèr冰川和Sarennes冰川为参照冰川,在对比分析这五条参照冰川近60 a来物质平衡变化幅度差异和阶段性差异基础上,对大陆型冰川与海洋型冰川物质平衡变化特征及其机理进行了对比研究.结果表明:在物质平衡阶段性变化上,阿尔卑斯山参照冰川物质平衡变化具有相似的阶段性,而天山和阿尔卑斯山参照冰川之间以及天山内部两条参照冰川之间物质平衡阶段性变化存在很大的差异,可见,无论不同性质冰川还是同一性质的不同冰川,其物质平衡的阶段性变化都可能存在差异,这与不同冰川所处环境水热变化的时间差异有关,而与冰川性质无关;在物质平衡变化幅度上,海洋型冰川变化幅度明显大于大陆型冰川,原因是不同性质冰川发育的水热环境及其对气候变化敏感性差异;在前人对冰川加速消融机理的研究基础上,本文也讨论了大陆型冰川与海洋型冰川物质平衡变化的机理及其异同.