全文获取类型
收费全文 | 184篇 |
免费 | 14篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 21篇 |
地球物理 | 41篇 |
地质学 | 77篇 |
海洋学 | 10篇 |
天文学 | 28篇 |
自然地理 | 20篇 |
出版年
2022年 | 1篇 |
2021年 | 1篇 |
2020年 | 1篇 |
2019年 | 6篇 |
2018年 | 2篇 |
2017年 | 4篇 |
2016年 | 8篇 |
2015年 | 2篇 |
2014年 | 12篇 |
2013年 | 10篇 |
2012年 | 8篇 |
2011年 | 14篇 |
2010年 | 3篇 |
2009年 | 10篇 |
2008年 | 9篇 |
2007年 | 11篇 |
2006年 | 9篇 |
2005年 | 3篇 |
2004年 | 11篇 |
2003年 | 4篇 |
2002年 | 7篇 |
2001年 | 5篇 |
2000年 | 3篇 |
1999年 | 9篇 |
1998年 | 7篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1993年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1984年 | 5篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1973年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有202条查询结果,搜索用时 15 毫秒
1.
Gavin L. England Birger Rasmussen Neal J. McNaughton Ian R. Fletcher David I. Groves & Bryan Krapez 《地学学报》2001,13(5):360-367
SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment-hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime 207 Pb/206 Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post-diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event. 相似文献
2.
Northern Hemispheric“polar lows”,i.e.maritime,small,but fairly intense cyclonic systems have been studied formore than two decades.Recently researchers have turned their attention towards the occurrence of polar lows and othersmall-scale cyclonic disturbances on the Southern Hemisphere.In this paper a short account of our present knowledgeof Northern Hemispheric polar lows will be given,followed by some preliminary results of an investigation of the natureand occurrence of polar lows on the Southern Hemisphere. 相似文献
3.
Kaare L. Rasmussen Hans J. F. Olsen Raymond Gwozdz Evgeniy M. Kolesnikov 《Meteoritics & planetary science》2001,36(7):1001-1006
Abstract— Jull et al. propose an alternative interpretation of our depth vs. 14C data measured on a peat core from the central Tunguska impact site (Rasmussen et al., 1999). We find that the proposed alternative is untenable. 相似文献
4.
Birger Rasmussen Ian R. Fletcher Janet R. Muhling 《Contributions to Mineralogy and Petrology》2011,162(6):1259-1277
Xenotime is a widespread accessory mineral in lower greenschist to upper amphibolite facies metasedimentary rocks from the
Palaeoproterozoic Mount Barren Group, southwestern Australia. Xenotime is closely associated with detrital zircon, commonly
forming syntaxial outgrowths, in samples of sandstone, micaceous quartzite, slate, phyllite, garnet-bearing semi-pelites,
and in kyanite-, garnet-, and staurolite-bearing mica schists. In situ geochronology of xenotime from lower greenschist sandstones
has previously yielded multiple U–Pb ages with peaks at ~2.0, ~1.7, and ~1.65 Ga, interpreted to represent the age of detritus,
early diagenesis, and a later thermal event, respectively. New U–Pb dating of xenotime in slate yields a major population
at ~1.7 Ga with a minor population at ~1.2 Ga, reflecting diagenetic and metamorphic growth, respectively, whereas xenotime
in phyllite forms a minor age population at ~1.7 Ga and a main peak at ~1.2 Ga. Mid-greenschist facies semi-pelitic schists
(quartz-muscovite-garnet) contain xenotime that formed before 1.8 Ga and at 1.2 Ga, representing detrital and peak metamorphic
ages, respectively. Xenotime in samples of amphibolite facies schist (650°C and ~8 kbars) yields U–Pb ages of ~1.2 Ga, coinciding
with the time of peak metamorphism. A single analysis of a xenotime core from an amphibolite facies schist gave an age of
~1.8 Ga, consistent with the presence of detrital xenotime. Our results suggest that detrital xenotime may be preserved under
greenschist facies conditions, but is largely replaced during upper amphibolite facies conditions. Detrital xenotime is replaced
through dissolution–reprecipitation reactions forming compositionally distinct rims during greenschist and amphibolite facies
metamorphism at 1.2 Ga. Diagenetic xenotime is present in lower greenschist facies samples, but was not observed in metasedimentary
rocks that had experienced temperatures above mid-greenschist facies metamorphism (450°C). The apparent disappearance of detrital
and diagenetic xenotime and appearance of metamorphic xenotime during prograde metamorphism indicates that some of the yttrium,
heavy rare earth elements, and phosphorus needed for metamorphic xenotime growth are probably derived from the replacement
of detrital and diagenetic xenotime. 相似文献
6.
主要介绍了近 2 0年来稻田甲烷排放的模式研究和排放量的估算以及减少稻田甲烷排放的措施。数值模式是估算稻田甲烷排放量的一条有效途径 ,模式的研究现在正处于发展阶段。介绍了几个主要的模型 ,既有物理过程模型也有经验模型。年排放量的估算范围为 6 79~ 4 1 4Tg ,随着技术的发展和大量实验的进行估算值的精度正得到不断的提高。减排措施是减少稻田甲烷排放的必要手段 ,但是目前的减排技术均处于研究阶段 ,应用还不成熟 相似文献
7.
Imogen O.H. Fielding Simon P. Johnson Sebastien Meffre Jianwei Zi Stephen Sheppard Ross R. Large Birger Rasmussen 《地学前缘(英文版)》2019,10(1):89-105
Proterozoic orogens commonly host a range of hydrothermal ores that form in diverse tectonic settings at different times. However, the link between mineralization and the regional-scale tectonothermal evolution of orogens is usually not well understood, especially in areas subject to multiple hydrothermal events.Regional-scale drivers for mineral systems vary between the different classes of hydrothermal ore, but all involve an energy source and a fluid pathway to focus mineralizing fluids into the upper crust. The Mount Olympus gold deposit in the Proterozoic Capricorn Orogen of Western Australia, was regarded as an orogenic gold deposit that formed at ca. 1738 Ma during the assembly of Proterozoic Australia. However,the trace element chemistry of the pyrite crystals closely resembles those of the Carlin deposits of Nevada,with rims that display solid solution gold accompanied by elevated As, Cu, Sb, Hg, and Tl, surrounding gold-poor cores. New SHRIMP UeP b dating of xenotime intergrown with auriferous pyrite and ore-stage alteration minerals provided a weighted mean~(207) Pb*/~(206) Pb* date of 1769 ± 5 Ma, interpreted as the age of gold mineralization. This was followed by two discrete episodes of hydrothermal alteration at 1727 ± 7 Ma and 1673 ± 8 Ma. The three ages are linked to multiple reactivation of the crustal-scale Nanjilgardy Fault during repeated episodes of intracratonic reworking. The regional-scale drivers for Carlin-like gold mineralization at Mount Olympus are related to a change in tectonic regime during the final stages of the intracratonic 1820 -1770 Ma Capricorn Orogeny. Our results suggest that substantial sized Carlin-like gold deposits can form in an intracratonic setting during regional-scale crustal reworking. 相似文献
8.
Palaeo-bottom current strength of the West Spitsbergen Current (WSC) and the influence of the Svalbard-Barents Sea Ice Sheet (SBIS) on the depositional environment along the northern Svalbard margins are poorly known. Two gravity cores from the southern Yermak Plateau and the upper slope north of Nordaustlandet, covering marine isotope stage (MIS) 1 to MIS 5, are investigated. Five lithofacies, based on grain size distribution, silt/clay ratio, content and mean of sortable silt (SS), are distinguished to characterise the contourite-dominated sedimentary environments. In addition, depositional environments are described using total organic carbon (TOC), total sulphur (TS) and calcium carbonate (CaCO3) contents of sediments. Facies A, containing coarse SS, suggests strong bottom current activity and good bottom water ventilation conditions as inferred from low TOC content. This facies was deposited during the glacial periods MIS 4, MIS 2 and during the late Holocene. Facies B is dominated by fine SS indicating weak bottom current and poor ventilation (cf. high TOC content of 1.2–1.6%), and correlates with the MIS 4/3 and MIS 2/1 transition periods. With an equal amount of clay and sand, fine SS and high content of TOC, facies C indicates reduced bottom current strength for intervals with sediment supply from proximal sources such as icebergs, sea ice or meltwater discharge. This facies was deposited during the last glacial maximum. Facies D represents mass-flow deposits on the northern Svalbard margin attributed to the SBIS advance at or near the shelf edge. Facies E sediments indicating moderate bottom current strength were deposited during MIS 5 and MIS 3, and during parts of MIS 2. This first late Quaternary proxy record of the WSC flow and sedimentation history from the northern Svalbard margin suggests that the oceanographic conditions and ice sheet processes have exerted first-order control on sediment properties. 相似文献
9.
Kamila Sztybor Tine L. Rasmussen 《Boreas: An International Journal of Quaternary Research》2017,46(2):212-228
The effect of seeping of methane on marine sediment records has been studied in four gravity cores from Vestnesa Ridge, Svalbard margin. The area shows acoustic signs in the form of flares indicating active methane gas seepage. For a better understanding of the timing and variability of the flux of methane in the past and the effects on potential proxies, a detailed study of the diagenetic processes that may affect the composition and structure of both sediments and foraminiferal shells is needed. Here we discuss deep‐sea records from methane‐influenced environments in three cores from an active and very heterogeneous seep‐area (pockmark) and one core from outside the pockmark for background. The results include the distribution and stable isotopes of authigenic carbonates and of benthic and planktonic foraminifera, magnetic susceptibility, AMS‐14C dates, sedimentary data and biostratigraphy. Extremely low δ13C values recorded in both benthic and planktonic foraminifera during the Bølling‐Allerød interstadials indicate possible increased methane flux beginning at late Heinrich event H1. The recorded low values are mainly a result of diagenetic overprint by methane‐derived authigenic carbonates. The δ18O signals of authigenic carbonates are close to those of foraminiferal calcite and thus the δ18O records remain a valid stratigraphical tool in methane seep sites, except in the case of severely encrusted samples. In addition, the records from the active pockmark show nearly constant values of low magnetic susceptibility in contrast to higher and more variable magnetic susceptibility values from the control station and other published records from normal sediments west of Svalbard. This phenomenon is probably caused by dissolution of magnetic minerals in the reducing environmental conditions of methane seep sediments, associated with anaerobic oxidation of methane and formation of paramagnetic minerals (pyrite). This process enables magnetic susceptibility to be used as a common diagnostic tool for identifying methane‐related palaeo‐reductive environments. 相似文献
10.
I. R. Fletcher B. Rasmussen N. J. McNaughton 《Australian Journal of Earth Sciences》2013,60(5):845-859
SHRIMP (Sensitive High‐Resolution Ion MicroProbe) analytical procedures have been developed to enable dating of the small, early diagenetic xenotime overgrowths that commonly occur on zircons in siliciclastic sedimentary rocks. The method will be particularly useful in Precambrian terranes, where diagenetic xenotime dating could play a role equivalent to biostratigraphic dating in the Phanerozoic. Reliable 207Pb/206Pb data are more readily obtained than 206Pb/238U, which also favours application to the Precambrian. However, it is demonstrated that 206Pb/238U dating of larger overgrowths (>10 μm) is also viable and applicable to Phanerozoic samples. SHRIMP Pb/Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproterozoic sandstone in the Kimberley Basin has constrained diagenesis to a precision of ± 7 Ma. In contrast, greenschist‐facies metasediments of the Archaean Witwatersrand Basin, South Africa, contain both authigenic and alteration xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin. 相似文献