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1.
古海洋溶解氧研究方法综述 总被引：15，自引：0，他引：15
介绍了国内外在古海洋溶解氧研究领域内的主要方法和动态，并分别对沉积构造法、沉积硫法、同位素法、微量元素法、稀土元素法、有机地球化学法及古生态法等研究方法进行了全面的分析和评论，指出,古海洋溶解氧含量重建对于理解大洋循环、古气候、生物绝灭、地质事件以及有机质演化具有十分重要的科学意义，它有赖于溶解氧含量的替代性指标的建立、富氧问题研究的进一步深入以及综合分析气圈、水圈、沉积圈、生物圈等各子系统内部及它们之间氧的物质输送和转化。 相似文献
2.
Mixing in the surface layers in association with internal waves during winter in the northwestern Bay of Bengal 总被引：1，自引：1，他引：0
The upper ocean has complex and variable temperature stratification, and the surface layers in the northwest Bay of Bengal
in winter indicate the presence of transient thermal inversions that wane with the advancement of the season. During winter,
the sea surface loses heat and the surface waters of the coastal regions of the east coast of India are fairly stratified
with the residual freshwater atop from the preceding southwest monsoonal discharge. The vertical stability favors the formation
and sustenance of temperature inversions. To investigate the mechanism and the influence of ubiquitous internal waves that
thrive on stability, a three-dimensional Princeton Ocean Model is configured for the east coast of India and is applied to
study the process in the surface layers in association with the internal waves. The model domain constitutes a variable curvilinear
grid, and the input fields comprise bathymetry, initial temperature and salinity, wind stress, air-sea heat fluxes and tidal
forcing at the open boundaries. The numerical experiments demonstrate that vertical stability alone cannot cause, support
or augment the internal wave oscillations, if the stratification is attributed to salinity only. Internal waves may therefore
be perceived in stable layers, essentially from temperature-induced stratification. Despite stratification and enough vertical
density gradient in the upper ocean, the conditions may not suit for the occurrence of internal waves due to thermal diffusive
processes that overpower the salinity gradients. The vertical spreading of heat due to double diffusion is believed to be
transparent to tidal forcing as the generation of internal waves is subdued even under density stratification. The model simulations
indicate that the horizontal convergence/divergence motions, required for the manifestation of internal waves at the surface
are inhibited in the presence of temperature inversion. The available SAR imageries in winter endorse the model simulations
to this effect. 相似文献
3.
Control of estuarine stratification and mixing by wind-induced straining of the estuarine density field 总被引：2，自引：0，他引：2
Observations from the York River Estuary, Virginia, demonstrate that the along-channel wind plays a dominant role in governing the estuarine exchange flow and the corresponding increase or decrease in vertical density stratification. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away estuarine stratification, our results demonstrate that the wind can play a more important role in straining the along-channel estuarine density gradient. Down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase vertical stratification. Up-estuary winds tend to reduce, or even reverse the vertical shear, reducing vertical stratification. In two experiments each lasting approximately a month, the estuarine exchange flow was highly correlated with the along-channel component of the wind. The changes in stratification caused by the exchange flow appear to control the amount of vertical mixing as parameterized by the vertical eddy viscosity. The degree of stratification induced by wind straining also appears to play an important role in controlling the effectiveness of wind and tidal mixing. 相似文献
4.
Vertical extrusion and horizontal channel flow of orogenic lower crust: key exhumation mechanisms in large hot orogens? 总被引：1，自引：0，他引：1
K. SCHULMANN O. LEXA P. &#;TÍPSKÁ M. RACEK L. TAJ&#;MANOVÁ J. KONOPÁSEK J.-B. EDEL A. PESCHLER J. LEHMANN 《Journal of Metamorphic Geology》2008,26(2):273-297
A large database of structural, geochronological and petrological data combined with a Bouguer anomaly map is used to develop a two‐stage exhumation model of deep‐seated rocks in the eastern sector of the Variscan belt. An early sub‐vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks. These events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18?20 kbar to depths equivalent to 8?10 kbar, and for coeval burial of upper crustal rocks to depths equivalent to 8–9 kbar. Following the folding and vertical extrusion event, sub‐horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening. Fabrics that record the early vertical extrusion originated between 350 and 340 Ma, during building of an orogenic root in response to SE‐directed Saxothuringian continental subduction. Fabrics that record the later sub‐horizontal exhumation event relate to an eastern promontory of the Brunia continent indenting into the rheologically weaker rocks of the orogenic root. Indentation initiated thrusting or flow of the orogenic crust over the Brunia continent in a north‐directed sub‐horizontal channel. This sub‐horizontal flow operated between 330 and 325 Ma, and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re‐equilibration. The erosion depth as well as the degree of reworking decreases from south to north, pointing to an outflow of lower crustal material to the surface, which was subsequently eroded and deposited in a foreland basin. Indentation by the Brunia continental promontory was highly noncoaxial with respect to the SE‐oriented Saxothuringian continental subduction in the Early Visean, suggesting a major switch of plate configuration during the Middle to Late Visean. 相似文献
5.
Stéphane Bodin Nicolas Fiet Alexis Godet Virginie Matera Stéphane Westermann Arnaud Clément Nico M.M. Janssen Peter Stille Karl B. Föllmi 《Cretaceous Research》2009,30(5):1247-1262
Stable carbon, oxygen, and strontium isotope records were obtained from uppermost Hauterivian to lowermost Aptian belemnite rostra, which were collected in well-dated sections from the Vocontian Trough (southeastern France). This data set complements previously published belemnite-isotope records from the uppermost Berriasian-Hauterivian interval from the same basin. The belemnite carbon and oxygen isotope record is compared to the carbonate bulk-rock isotope record from the same sections, and from additional Italian sections. With regards to their long-term trends, both belemnite and whole-rock δ18O records are well correlated, except for the uppermost Hauterivian-lower Barremian interval, within which they deviate. This discrepancy is interpreted to be linked to the latest Hauterivian Faraoni oceanic anoxic event and its early Barremian aftermath. The Faraoni level is characterized by enhanced sea-water stratification, probably induced by the onset of a warmer and more humid climate along the northern Tethyan margin. The early Barremian was characterized by stronger vertical sea-water mixing reflected by a decrease in density contrast between sea-surface and deeper waters. The belemnite oxygen isotope record shows a more stable evolution with smaller fluctuations than its bulk-rock counterpart, which indicates that deeper water masses were not as much subjected to density fluctuations as sea-surface water. The comparison of belemnite and bulk-rock carbon isotope records allows observing the impact of regional influence exerted by platform carbonate ooze shedding on the carbon cycle. Discrepancies in the two records are observed during time of photozoan carbonate platform growth. The strontium isotopic record shows a gradual increase from the uppermost Berriasian to the uppermost lower Barremian followed by a rapid decrease until the uppermost Barremian and a renewed small increase within the lowermost Aptian. The major inflection point in the uppermost lower Barremian appears to predate the onset in the formation of the Ontong-Java volcanic plateau. 相似文献
6.
综合介绍了世界上几个典型蛇绿岩带中伸展变形构造的实例,如何形成辉长质或其它不同层次的糜棱岩,可造成辉绿岩墙群的倾斜旋转,可使得原始洋壳厚度变薄等,蛇绿岩中伸展构造记录了已经消失的洋盆的生成,拆离和消减过程中的关信息,研究蛇绿岩中伸展变形构造,有助于恢复古洋盆的规模的演化过程,也是蛇绿岩研究的新方向这一。 相似文献
7.
Sarah N. Giddings Derek A. Fong Stephen G. Monismith C. Chris Chickadel Kathleen A. Edwards William J. Plant Bing Wang Oliver B. Fringer Alexander R. Horner-Devine Andrew T. Jessup 《Estuaries and Coasts》2012,35(2):665-681
Estuarine fronts are well known to influence transport of waterborne constituents such as phytoplankton and sediment, yet
due to their ephemeral nature, capturing the physical driving mechanisms and their influence on stratification and mixing
is difficult. We investigate a repetitive estuarine frontal feature in the Snohomish River Estuary that results from complex
bathymetric shoal/channel interactions. In particular, we highlight a trapping mechanism by which mid-density water trapped
over intertidal mudflats converges with dense water in the main channel forming a sharp front. The frontal density interface
is maintained via convergent transverse circulation driven by the competition of lateral baroclinic and centrifugal forcing.
The frontal presence and propagation give rise to spatial and temporal variations in stratification and vertical mixing. Importantly,
this front leads to enhanced stratification and suppressed vertical mixing at the end of the large flood tide, in contrast
to what is found in many estuarine systems. The observed mechanism fits within the broader context of frontogenesis mechanisms
in which varying bathymetry drives lateral convergence and baroclinic forcing. We expect similar trapping-generated fronts
may occur in a wide variety of estuaries with shoal/channel morphology and/or braided channels and will similarly influence
stratification, mixing, and transport. 相似文献
8.
珠江磨刀门水道地处亚热带季风气候区,直面南海,容易受到热带风暴的袭击。选取台风"纳沙",采用SCHISM模型建立磨刀门水道三维水流盐度数值模型,通过数值试验对比,结合势能异常分析法,探究了台风期间波浪和局地风对磨刀门水道混合与层化的影响。结果显示:由于近岸及河道内水深较浅,波高整体较小,波浪对水体层化过程影响不大,对流速分布有一定的调整作用,使其分布更为均匀,并在一定程度上加强水体掺混,利于外海高浓度盐水向河口和海岸扩散。而台风期间强劲的局地风对垂向水体状态以及势能异常变化率各项均有显著作用,影响着盐淡水的层化混合过程。 相似文献
9.
The short-term variability observed in the near surface meteorological parameters and in the vertical thermal structure of
the upper layers of the northern Bay of Bengal during a weak monsoonal regime is examined with the aid of time series measurements.
The variability of the mixed layer depth is interpreted in terms of forced mixing caused by the surface wind stress and free
mixing by buoyancy flux, Ekman pumping controlled by the curl of the surface wind stress, convergence associated with a clockwise
gyral circulation and stratification caused by freshwater discharges from rivers. The daily-averaged current vectors in the
upper layers indicate the presence of clockwise gyral circulation in the polygon area. 相似文献
10.
Turbidity currents and pyroclastic density currents may originate as stratified flows or develop stratification during propagation. Analogue, density‐stratified laboratory currents are described, using layers of salt solutions with different concentrations and depths to create the initial vertical stratification. The evolving structure of the flow depends on the distribution of the driving buoyancy between the layers, B* (proportional to the layer volumes and densities), and their density ratio, ρ*. When the lower layer contains more salt than the upper layer, and so has a greater proportion of the driving buoyancy (B* < 0·5), this layer can run ahead leading to streamwise or longitudinal stratification (ρ*→0), or the layers can mix to produce a homogeneous current (ρ*→1). If the upper layer contains more salt and thus buoyancy (B* > 0·5), this layer travels to the nose of the current by mixing into the back of the head along the body/wake density interface to produce a homogeneous flow (ρ*→1) or overtaking, leading to streamwise stratification (ρ*→0). Timescales describing the mixing between the layers and the streamwise separation of the layers are used to understand these flow behaviours and are in accordance with the experimental observations. Distance–time measurements of the flow front show that strongly stratified flows initially travel faster than weakly stratified flows but, during their later stages, they travel more slowly. In natural flows that are stratified in concentration and grain size, internal features, such as stepwise grading, gradual upward fining and reverse grading, could be produced depending on B* and ρ*. Stratification may also be expected to affect interactions with topography and overall fan architecture. 相似文献
11.
扬子地区发育地层所记录的南沱冰期在时间上与Marinoan冰期相当,被认为是"雪球事件"的产物,受到广泛关注。借助于重庆秀山长河桥剖面的南沱组冰碛岩上覆陡山沱组盖帽白云岩和页岩样品的精细采集,选取冰碛岩之上2.5 m内的地层进行同位素比值和元素含量测试,并尝试性的使用酸不溶物的元素地球化学数据,对冰期后可能出现的环境变化进行了综合分析,结果表明:盖帽白云岩C同位素数据基本符合海水分层混合模式特征;U/Th值也反映出该地区在雪球后经历了由缺氧-贫氧环境向贫氧-氧化环境的迅速转变,可能反映了雪球后缺氧富有机质的深部大洋水随上升洋流上涌并被迅速氧化的过程;强烈的Eu正异常的出现,可能与埃迪卡拉纪海底火山、热液活动频繁出现或近源热液活动相关;盖帽碳酸盐岩样品中出现Ce轻微负异常,表明其形成于弱氧化环境;87Sr/86Sr值和Y/Ho值均呈现伴有大量陆源碎屑物输入的特点;盖帽碳酸盐岩的酸不溶物可以反映当时的大陆风化背景,其化学蚀变指数（CIA）稳定在72左右,说明当时的大陆环境具有温暖湿润的特征,化学风化作用强度中等。综上所述,在南陀冰期结束后,秀山长河桥剖面的沉积环境经历了缺氧-贫氧环境向贫氧-氧化环境的迅速转变,同时,随着上升洋流的出现原有的冰期海水分层被迅速破坏,并可能伴随着地表径流的不断增强,同时热液活动在这一时期也频繁发生。在这一时期,剖面附近的大陆环境也迅速由冰期过渡为温暖湿润的环境。 相似文献
12.
北极海冰与全球气候变化 总被引：7，自引：2，他引：7
最近有关北极海冰在全球气候系统中作用的研究发现，北冰洋边缘海域大洋深水的形成与海冰发育有关，海冰冻融过程对盐度层结具有重要影响，海冰变化可引起盐度突变层的灾变和热盐环流的突然停止，热盐环流的变化与北大西洋海冰１０年际变化相联系，北大西洋气候的不稳定性与热盐环流变化密切相关。北极海冰－海洋－大气间耦合作用，使北极海冰构成了北大西洋和全球气候反馈循环中的重要环节。 相似文献
13.
LUKAS JONKERS MAARTEN A. PRINS GEERT‐JAN A. BRUMMER MARTIN KONERT BRYAN C. LOUGHEED 《Sedimentology》2009,56(7):2192-2206
Grain‐size measurements of fine‐grained sediments based on laser diffraction may contain spurious information due to the over‐estimation of the size and proportion of platy particles. Consequently, some regard the use of laser diffraction particle sizing in palaeoceanography inappropriate. Here, it is shown experimentally that such concerns are not warranted. Laser diffraction particle sizing is known to be fast, precise and allows for detailed particle sizing over a broad size range; it is therefore potentially a very powerful technique if the complications associated with it can be overcome. As most sediments are mixtures of different components transported by different mechanisms, inferences of past environmental parameters require decomposition of the grain‐size record in question. Useful decomposition can only be performed if changes in the contribution of the components are registered predictably by the measuring device. This study reports on mixing experiments which show that the Fritsch A22 laser diffraction particle sizer does indeed register small changes in the contribution of the mixing components in a predictable way. Mixing proportions estimated from the measurements do, however, differ from the initial mixing proportions, but these can be converted with only small errors. Application of the conversion equations to a North Atlantic grain‐size record that showed consistent slowdown of deep‐ocean circulation in response to millennial scale ice‐rafting events during the last glacial does not quantitatively affect the original inferences. Laser diffraction particle sizing of fine‐grained sediments therefore yields reproducible and useful data for palaeoceanographic reconstructions. 相似文献
14.
Abstract The Mianlüe suture extends from Derni‐Nanping‐Pipasi‐Kangxian to the Lüeyang‐Mianxian area, then traverses the Bashan arcuate structure eastward to the Huashan region, and finally to the Qingshuihe area of the southern Dabie Mountains. From east to west, with a length of over 1500 km, the ophiolitic melangé associations are distributed discontinuously along the suture. The rock assemblages include ophiolite, island‐arc and oceanic island rock series, indicating that there existed a suture zone and a vanished paleo‐ocean basin. The Mianlüe paleo‐ocean basin experienced its main expansion and formation process during the Carboniferous‐Permian and closed totally in the Triassic. It belongs to the northern branch of the eastern paleotethys, separated from the northern margin of the Yangtze block under the paleotethys mantle dynamic system. 相似文献
15.
古海水pH值代用指标——海洋碳酸盐硼同位素研究进展 总被引：1，自引：0，他引：1
仪器测量的海水pH记录太短，无法评估海水pH自然变化的频率和幅度,并预测未来大气CO2急剧增加后海水酸度的响应。海相碳酸盐的硼同位素是目前恢复古海洋pH的有效途径，倍受古气候—环境学家的重视。评述了近年来海洋碳酸盐的硼同位素的最新研究成果和研究现状，重点探讨了海相碳酸盐的硼同位素的测定方法、硼同位素—pH模型和古海水pH恢复等前沿内容，旨在提供一个系统的海洋碳酸盐硼同位素—pH系统的基本概念及研究思路，以利于气候学、地质学界了解这一交叉领域的发展动态。 相似文献
16.
P. K. Gautam A. C. Narayana P. Kiran Kumar P. G. Bhavani M. G. Yadava A. J. T. Jull 《第四纪科学杂志》2021,36(1):138-151
The Indian monsoon carries large amounts of freshwater to the northern Indian Ocean and modulates the upper ocean structure in terms of upwelling and productivity. Freshwater-induced stratification in the upper ocean of the Bay of Bengal is linked to the changes in the Indian monsoon. In this study, we test the usefulness of δ18O and δ13C variability records for Globigerina bulloides and Orbulina universa to infer Indian monsoon variability from a sediment core retrieved from the southwestern Bay of Bengal encompassing the last 46 kyr record. Results show that the northeast monsoon was dominant during the Last Glacial Maximum. Remarkable signatures are observed in the δ18O and δ13C records during the Marine Isotope Stage (MIS) 3 to MIS-1. Our study suggests that Indian monsoon variability is controlled by a complex of factors such as solar insolation, North Atlantic climatic shifts, and coupled ocean–atmospheric variability during the last 46 kyr. 相似文献
17.
P. Lindsay 《Environmental Geology》1996,28(4):201-212
Spatial variations in the density and velocity fields have been observed in the Gareloch (Scotland) during surveys in 1987–1988
and 1993–1994. The variation of the density field has been analyzed on a variety of time scales from semidiurnal to seasonal
in order to quantify effects caused by the forcing factors of tidal mixing, freshwater input, and wind. Initial results indicate
that water density in the loch is controlled (to a major degree) by the freshwater input from runoff from the local catchment
area and from freshwater entering on the flood tide from the Clyde Estuary. It is estimated that during winter periods the
high freshwater flows from the rivers Leven and Clyde into the Clyde Estuary account for up to 75% of the freshwater creating
the density structure in the loch. Analysis of long-term dissolved oxygen data reveals that major bottom water renewals occurred
between July and January in the years 1987–1994. Major bottom water dissolved oxygen renewals have a general trend but during
the year sporadic renewals can take place due to abnormal dry spells increasing the density of the water entering from the
Clyde, or consistently strong winds from the north reducing stratification in the loch and producing better mixed conditions.
Velocities vary spatially, with the highest velocities of up to 0.6 m s–1 being associated with the velocity jet effect at the constriction at the sill of the loch. Observed near-surface mid-loch
velocities increased as the vertical density gradients in the upper layers increased. This indicates for the observed conditions
that increased stratification in the upper layers inhibits the entrainment rate and hence rate of gain of thickness of the
wind-driven surface layer, resulting in increased surface velocities for a given wind speed and direction. The main flow is
concentrated in the upper 10 m and velocities below 10 m are low. Observed mean spring tide surface velocities are on average
30% greater than mean neap tide surface velocities.
Received: 22 May 1995 · Accepted: 23 August 1995 相似文献
18.
第四纪冰期的千年尺度气候突变事件——Dansgaard-Oeschger Event (D-O事件),一直是古气候学领域关注的重点。近年来,数值模拟的研究发现,北大西洋副极地地区年际-年代际气候变率的振幅在D-O事件中的冰阶冷期远大于间冰阶暖期,这一现象为理解该区域海温代理指标的气候学意义提供了重要参考价值,但其动力机制尚不清晰。本文利用海气耦合气候模型(COSMOS),通过模拟氧同位素(MIS)3阶段的一个典型D-O事件过程,探讨了冰阶冷期北大西洋气候变率的放大机制。结果显示,北大西洋副极地海域的季节性海冰通过调控海气间热量交换,影响当地气候变率的幅度。冰阶期,热带暖水向北输送导致海洋次表层逐渐升温,削弱了表层-次表层海水的密度层结,有利于次表层暖水上涌,促进海冰融化及海表温度升高。这将激发出海平面气压的负异常,引起气旋式风切变,并通过Ekman抽吸作用加速表层-次表层海水的垂直混合,进一步促进次表层暖水的上涌。这一正反馈机制造成海洋次表层热量的迅速释放,海表温度快速升高。当次表层热量释放结束后,海表将无暖水补充,导致海表温度下降,海冰增多。该过程激发的海表气压正异常(即反气旋式风切变)将抑制垂直混合发生,促进次表层热量积累,为下一次放热过程提供条件。在间冰阶暖期,随着北大西洋季节性海冰消失,海气间热交换不再受海冰变化影响,海洋次表层与大气间的热交换始终处于准平衡态,气候变率的振幅显著下降。本研究结果显示,北大西洋季节性海冰的存在可以调控海洋次表层热量积累-释放的过程,产生“电容器”效应,这对理解冰期年际-年代际气候变率放大现象有重要启示意义。 相似文献
19.
20.
Both seismology and geochemistry show that the Earth's mantle is chemically heterogeneous on a wide range of scales. Moreover, its rheology depends strongly on temperature, pressure and chemistry. To interpret the geological data, we need a physical understanding of the forms that convection might take in such a mantle. We have therefore carried out laboratory experiments to characterize the interaction of thermal convection with stratification in viscosity and in density. Depending on the buoyancy ratio B (ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly), two regimes were found: at high B, convection remains stratified and fixed, long-lived thermochemical plumes are generated at the interface, while at low B, hot domes oscillate vertically through the whole tank, while thin tubular plumes can rise from their upper surfaces. Convection acts to destroy the stratification through mechanical entrainment and instabilities. Therefore, both regimes are transient and a given experiment can start in the stratified regime, evolve towards the doming regime, and end in well-mixed classical one-layer convection. Applied to mantle convection, thermochemical convection can therefore explain a number of observations on Earth, such as hot spots, superswells or the survival of several geochemical reservoirs in the mantle. Scaling laws derived from laboratory experiments allow predictions of a number of characteristics of those features, such as their geometry, size, thermal structure, and temporal and chemical evolution. In particular, it is shown that (1) density heterogeneities are an efficient way to anchor plumes, and therefore to create relatively fixed hot spots, (2) pulses of activity with characteristic time-scale of 50–500 Myr can be produced by thermochemical convection in the mantle, (3) because of mixing, no ‘primitive’ reservoir can have survived untouched up to now, and (4) the mantle is evolving through time and its regime has probably changed through geological times. This evolution may reconcile the survival of geochemically distinct reservoirs with the small amplitude of present-day density heterogeneities inferred from seismology and mineral physics. 相似文献