Importance of wave-induced bed liquefaction in the fine sediment budget of Cleveland Bay,Great Barrier Reef   总被引：3，自引：0，他引：3  

J. Lambrechts  C. Humphrey  L. McKinna  O. Gourge  K.E. Fabricius  A.J. Mehta  S. Lewis  E. Wolanski 《Estuarine, Coastal and Shelf Science》2010

Data from a three-year long field study of fine sediment dynamics in Cleveland Bay show that wave-induced liquefaction of the fine sediment bed on the seafloor in shallow water was the main process causing bed erosion under small waves during tradewinds, and that shear-induced erosion prevailed during cyclonic conditions. These data were used to verify a model of fine sediment dynamics that calculates sediment resuspension by both excess shear stress and wave-induced liquefaction of the bed. For present land-use conditions, the amount of riverine sediments settling on the bay may exceed by 50–75% the amount of sediment exported from the bay. Sediment is thus accumulating in the bay on an annual basis, which in turn may degrade the fringing coral reefs. For those years when a tropical cyclone impacted the bay there may be a net sediment outflow from the bay. During the dry, tradewind season, fine sediment was progressively winnowed out of the shallow, reefal waters.  相似文献   

Long-Term Variability of Volume and Heat Transport in the Nordic Seas: A Model Study     

Sarah Lundrigan 《大气与海洋》2013,51(2):156-168

This article presents results from a model study of interannual and decadal variability in the Nordic Seas. Fifty years of simulations were conducted in an initial condition ensemble mode forced with the National Centers for Environmental Prediction (NCEP) reanalysis. We studied two major events in the interannual and interdecadal variability of the Nordic Seas during the past fifty years: the Great Salinity Anomaly in the 1960s and early 1970s and the warming of the Arctic and subarctic oceans in the late 1990s.

Previous studies demonstrated that the Great Salinity Anomaly observed in the subarctic ocean in 1960 was originally generated by intensified sea-ice and freshwater inflow from the Arctic Ocean. Our model results demonstrate that the increase in the transport of fresh and cold waters through Fram Strait in the 1960s was concurrent with a reduction in the meridional water exchange over the Greenland–Scotland Ridge. The resulting imbalance in salinity and heat fluxes through the strait and over the ridge also contributed to the freshening of the water masses of the Nordic Seas and intensified the Great Salinity Anomaly in the Nordic Seas.

The warming of the Atlantic Waters in the Nordic Seas and Arctic Ocean during the past two decades had an important impact on the variability of these two ocean basins. Some previous observational and model studies demonstrated that the warming of the subpolar Atlantic Ocean in the late 1990s and the meridional transport of the Atlantic Water mass (AW) into the Nordic Seas and Arctic Ocean contributed to this process. At the same time, observations show that the warming of the AW in the Nordic Seas started in the 1980s (i.e., earlier than the warming of the subpolar North Atlantic Ocean). Our model results suggest that this process was triggered by an imbalance in the lateral heat fluxes through Fram Strait and over the Greenland–Scotland Ridge. In the late 1980s the AW transport over the Greenland–Scotland Ridge was stronger than normal while the exchange through Fram Strait was close to normal. The related imbalance in the lateral heat fluxes through the strait and over the ridge warmed the Nordic Seas and caused an increase in the temperature of the AW inflow to the Arctic Ocean in the late 1980s (i.e., about a decade earlier than the warming of the source of the AW in the subpolar North Atlantic Ocean). Thus the model results suggest that the imbalance in lateral heat and salinity fluxes through the strait and over the ridge connecting the Nordic Seas to the North Atlantic and Arctic oceans could amplify the interannual variability in the subarctic ocean.

[Traduit par la rédaction] Cet article présente les résultats d'une étude par modèle de la variabilité interannuelle et décennale dans les mers nordiques. Nous avons effectué des simulations sur une période de cinquante ans en mode d'ensemble de conditions initiales forcé avec les réanalyses des NCEP (National Centers for Environmental Prediction). Nous avons étudié deux événements majeurs survenus dans la variabilité interannuelle et décennale des mers nordiques au cours des cinquante dernières années : la grande anomalie de salinité des années 1960 et du début des années 1970 et le réchauffement des océans Arctique et subarctique vers la fin des années 1990.

Des études précédentes ont démontrées que la grande anomalie de salinité observée dans l'océan subarctique en 1960 a été causée par une intensification de l'apport de glace de mer et d'eau douce depuis l'océan Arctique. Les résultats que nous avons obtenus du modèle montrent que l'accroissement du transport d'eau douce et froide à travers le détroit de Fram dans les années 1960 s'est produit en même temps qu'une réduction dans l’échange méridien d'eau au-dessus de la crête Groenland–Écosse. Le déséquilibre résultant dans les flux de salinité et de chaleur à travers le détroit et au-dessus de la crête a aussi contribué à l'adoucissement des masses d'eau des mers nordiques et a intensifié la grande anomalie de salinité dans les mers nordiques.

Le réchauffement des eaux atlantiques dans les mers nordiques et dans l'océan Arctique au cours des deux dernières décennies a eu un impact important sur la variabilité de ces deux bassins océaniques. Des études observationnelles et par modèle précédentes ont établi que le réchauffement de l'océan Atlantique subpolaire dans les années 1990 et le transport méridien de la masse d'eau atlantique dans les mers nordiques et dans l'océan Arctique ont contribué à ce processus. En même temps, les observations montrent que le réchauffement des eaux atlantiques dans les mers nordiques a commencé dans les années 1980 (c.–à–d. plus tôt que le réchauffement de l'océan Nord-Atlantique subpolaire). Les résultats du modèle suggèrent que ce processus a été déclenché par un déséquilibre dans les flux de chaleur latéraux à travers le détroit de Fram et au-dessus de la crête Groenland–Écosse. À la fin des années 1980, le transport des eaux atlantiques au-dessus de la crête Groenland–Écosse était plus fort que la normale alors que l’échange à travers le détroit de Fram était près de la normale. Le déséquilibre résultant dans les flux de chaleur latéraux à travers le détroit et au-dessus de la crête a réchauffé les mers nordiques et causé une augmentation de la température des eaux atlantiques parvenant à l'océan Arctique à la fin des années 1980 (c.-à-d. environ une décennie avant le réchauffement de la source d'eaux atlantiques dans l'océan Nord-Atlantique subpolaire). Donc, les résultats du modèle suggèrent que le déséquilibre dans les flux de chaleur et de salinité latéraux à travers le détroit et au-dessus de la crête reliant les mers nordiques à l'Atlantique Nord et à l'Arctique pourrait amplifier la variabilité interannuelle dans l'océan subarctique.  相似文献   

A record of Lateglacial/Holocene environmental change from a high‐elevation site in the Intermountain West,USA     

Andrea Brunelle  Thomas A. Minckley  Elliott Lips  Paul Burnett 《第四纪科学杂志》2013,28(1):103-112

Records of past vegetation and fire history can be complicated by changes in the depositional environment of a sampling location. However, these changes can alternatively be used as a measure of climate variability. Our study site, ca. 18.0 cal. ka BP record from Little Brooklyn Lake, Wyoming, located near the crest of the Snowy Range, records three moisture states. Initially, the lake was likely a glacier‐fed pond indicated by the presence of Pediastrum algae colonies. Around 13.0 cal. ka BP this pond transitioned to a meadow environment, suggested by the loss of Pediastrum algae colonies and slow sedimentation rates. Meadow conditions were maintained until ca. 5.0 cal. ka BP when Pediastrum algae colony abundance increased,indicating the formation of a shallow lake. From 18.0 to ca. 5.0 cal. ka BP, the pollen record is suggestive of alpine vegetation conditions with relatively high spruce and herbaceous taxa. Low charcoal influx also characterizes the period between 18.0 and 5.0 cal. ka BP. After 5.0 cal. ka BP, the coincidence of the formation of shallow lake and pollen data, indicating a shift to a spruce and fir forest, suggests an increase in effective moisture. Fire remained rare in this basin over the entire record, however, once the lake established sedimentation rates and charcoal influx increased. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Footwall dip of a core complex detachment fault: thermobarometric constraints from the northern Snake Range (Basin and Range,USA)     

F. J. COOPER  J. P. PLATT  R. ANCZKIEWICZ  M. J. WHITEHOUSE 《Journal of Metamorphic Geology》2010,28(9):997-1020

Low‐angle detachment faults are common features in areas of large‐scale continental extension and are typically associated with metamorphic core complexes, where they separate upper plate brittle extension from lower plate ductile stretching and metamorphism. In many core complexes, the footwall rocks have been exhumed from middle to lower crustal depths, leading to considerable debate about the relationship between hangingwall and footwall rocks, and the role that detachment faults play in footwall exhumation. Here, garnet–biotite thermometry and garnet–muscovite–biotite–plagioclase barometry results are presented, together with garnet and zircon geochronology data, from seven locations within metapelitic rocks in the footwall of the northern Snake Range décollement (NSRD). These locations lie both parallel and normal to the direction of footwall transport to constrain the pre‐exhumation geometry of the footwall. To determine P–T gradients precisely within the footwall, the ΔPT method of Worley & Powell (2000) has been employed, which minimizes the contribution of systematic uncertainties to thermobarometric calculations. The results show that footwall rocks reached pressures of 6–8 kbar and temperatures of 500–650 °C, equivalent to burial depths of 23–30 km. Burial depth remains constant in the WNW–ESE direction of footwall transport, but increases from south to north. The lack of a burial gradient in the direction of footwall transport implies that the footwall rocks, which today define a sub‐horizontal datum in the direction of fault transport, also defined a sub‐horizontal datum at depth in Late Cretaceous time. This suggests that the footwall was not tilted about the normal to the fault transport direction during exhumation, and hence that the NSRD did not form as a low‐angle normal fault cutting down through the lower crust. Instead, the following evolution for the northern Snake Range footwall is proposed. (i) Mesozoic contraction caused substantial crustal thickening by duplication and folding of the miogeoclinal sequence, accompanied by upper greenschist to amphibolite facies metamorphism. (ii) About half of the total exhumation was accomplished by roughly coaxial stretching and thinning in Late Cretaceous to Early Tertiary time, accompanied by retrogression and mylonitic deformation. (iii) The footwall rocks were then ‘captured’ from the middle crust along a moderately dipping NSRD that soled into the middle crust with a rolling‐hinge geometry at both upper and lower terminations.  相似文献   

Patterns of mixed siliciclastic‐carbonate sedimentation adjacent to a large dry‐tropics river on the central Great Barrier Reef shelf,Australia     

《Australian Journal of Earth Sciences》2013,60(5):665-683

The Great Barrier Reef represents the largest modern example of a mixed siliciclastic‐carbonate system. The Burdekin River is the largest source of terrigenous sediment to the lagoon and is therefore an ideal location to investigate regional patterns of mixed sedimentation. Sediments become coarser grained and more poorly sorted away from the protection of eastern headlands, with mud accumulation focused in localised ‘hot spots‘ in the eastern portion of embayments protected from southeast trade winds. The middle shelf has a variable facies distribution but is dominated by coarse carbonate sand. North of Bowling Green Bay, modern coarse carbonate sand and relict quartzose sand occur. Shore‐normal compositional changes show Ca‐enrichment and Al‐dilution seawards towards the reef, and shore‐parallel trends show Al‐dilution westwards (across bays) along a Ca‐depleted mixing line. Intermediate siliciclastic‐carbonate sediment compositions occur on the middle shelf due to the abundance of relict terrigenous sand, a pattern that is less developed on the narrow northern Great Barrier Reef shelf. Rates of sediment deposition from seismic evidence and radiochemical tracers suggest that despite the magnitude of riverine input, 80–90% of the Burdekin‐derived sediment is effectively captured in Bowling Green Bay. Over millennial time‐scales, stratigraphic controls suggest that sediment is being preferentially accreted back to the coast.  相似文献   

中国南极长城湾及附近海区1993/1994年夏季表层水样无机氮、磷酸盐的浓度及其分布状况     

陈皓文  朱明远  洪旭光  陆华  袁峻峰 《海洋湖沼通报》1996,(4)

1993／1994年南极夏季对中国南极长城湾及附近海区表层海水四种营养盐浓度及其分布状况作了调查。结果表明，NH4－N、NO3－N、NO2－N及PO4－P的大致浓度分别是2．13、7．07、0．74和1．12μg·dm-3。一般来说，湾内高于湾外氮盐于12月份常出现高值。四种形态的无机盐类之最高、最低浓度出现的时间和站位不尽一致。∑N／P递减次序大致是12月，2月，1月。水温变化与NH4－N、PO4－P浓度和∑N／P的变化常较一致，而与NO2－N则常不一，一些站位上营养盐浓度偏低可能与浮游植物摄取有关。结果显示了调研海区的浅水内湾特征。浮游植物所需营养充分。该湾仍维持正常状态。  相似文献   

神舟四号高度计波形数据预处理和信息提取   总被引：1，自引：0，他引：1  

纪永刚  张杰  张有广  孟俊敏 《海洋与湖沼》2007,38(6):487-494

神舟四号(SZ-4)高度计在国内首次提供了星载雷达高度计回波波形数据.本文中作者分析了SZ-4高度计回波波形的特点,完成波形数据的预处理,并在此基础上完成初步的信息提取.在数据预处理方面,通过SZ-4高度计水陆边界处波形的特点,提出了波形最大幅度控制的方法,筛选回波波形.在波形归一化处理过程中,发现SZ-4高度计波形中存在双峰现象,并指出第二个峰为异常波形区.在波形信息提取方面,利用波形重新跟踪得到的半功率点计算出SZ-4高度计高度跟踪补偿误差,并根据高度计天线指向角和回波波形下降沿斜率之间的关系,从波形后沿提取天线指向角信息.分析结果表明,SZ-4高度计天线指向比较平稳,而跟踪补偿由于变化较大,在计算海面高度时,应作为一项误差源被考虑到.  相似文献   

Remote estimation of surficial seafloor properties through the application Angular Range Analysis to multibeam sonar data   总被引：2，自引：2，他引：2  

Luciano Fonseca  Larry Mayer 《Marine Geophysical Researches》2007,28(2):119-126

The variation of the backscatter strength with the angle of incidence is an intrinsic property of the seafloor, which can be used in methods for acoustic seafloor characterization. Although multibeam sonars acquire backscatter over a wide range of incidence angles, the angular information is normally neglected during standard backscatter processing and mosaicking. An approach called Angular Range Analysis has been developed to preserve the backscatter angular information, and use it for remote estimation of seafloor properties. Angular Range Analysis starts with the beam-by-beam time-series of acoustic backscatter provided by the multibeam sonar and then corrects the backscatter for seafloor slope, beam pattern, time varying and angle varying gains, and area of insonification. Subsequently a series of parameters are calculated from the stacking of consecutive time series over a spatial scale that approximates half of the swath width. Based on these calculated parameters and the inversion of an acoustic backscatter model, we estimate the acoustic impedance and the roughness of the insonified area on the seafloor. In the process of this inversion, the behavior of the model parameters is constrained by established inter-property relationships. The approach has been tested using a 300 kHz Simrad EM3000 multibeam sonar in Little Bay, NH. Impedance estimates are compared to in situ measurements of sound speed. The comparison shows a very good correlation, indicating the potential of this approach for robust seafloor characterization.  相似文献   

第四次南极考察长城站越冬队极地海洋气象考察报告   总被引：1，自引：3，他引：1  

张文义 《海洋预报》1993,10(2):24-32

本文介绍了中国第四次南极考察队,在长城站进行气象考察期间,从获得的每日四次现场气象观测资料经统计分析,得出了1988年长城站地区气象要素分布特点。 利用长城站现场气象观测资料及接收到的地面天气图和卫星云图,开展了对长城站地区天气预报工作。并总结了影响长城站地区天气系统和出现的天气及影响长城站地区的气旋路径。对今后有关人员作好站区天气预报和科学考察工作顺利进行十分重要。  相似文献   

Tidally incised valleys on tropical carbonate shelves: An example from the northern Great Barrier Reef, Australia     

Peter T. Harris  Andrew Heap  Vicki Passlow  Michael Hughes  James Daniell  Mark Hemer  Ole Anderson 《Marine Geology》2005,220(1-4):181-204

The formation of incised valleys on continental shelves is generally attributed to fluvial erosion under low sea level conditions. However, there are exceptions. A multibeam sonar survey at the northern end of Australia's Great Barrier Reef, adjacent to the southern edge of the Gulf of Papua, mapped a shelf valley system up to 220 m deep that extends for more than 90 km across the continental shelf. This is the deepest shelf valley yet found in the Great Barrier Reef and is well below the maximum depth of fluvial incision that could have occurred under a − 120 m, eustatic sea level low-stand, as what occurred on this margin during the last ice age. These valleys appear to have formed by a combination of reef growth and tidal current scour, probably in relation to a sea level at around 30–50 m below its present position.

Tidally incised depressions in the valley floor exhibit closed bathymetric contours at both ends. Valley floor sediments are mainly calcareous muddy, gravelly sand on the middle shelf, giving way to well-sorted, gravely sand containing a large relict fraction on the outer shelf. The valley extends between broad platform reefs and framework coral growth, which accumulated through the late Quaternary, coincides with tidal current scour to produce steep-sided (locally vertical) valley walls. The deepest segments of the valley were probably the sites of lakes during the last ice age, when Torres Strait formed an emergent land-bridge between Australia and Papua New Guinea. Numerical modeling predicts that the strongest tidal currents occur over the deepest, outer-shelf segment of the valley when sea level is about 40–50 m below its present position. These results are consistent with a Pleistocene age and relict origin of the valley.

Based on these observations, we propose a new conceptual model for the formation of tidally incised shelf valleys. Tidal erosion on meso- to macro-tidal, rimmed carbonate shelves is enhanced during sea level rise and fall when a tidal, hydraulic pressure gradient is established between the shelf-lagoon and the adjacent ocean basin. Tidal flows attain a maximum, and channel incision is greatest, when a large hydraulic pressure gradient coincides with small channel cross sections. Our tidal-incision model may explain the observation of other workers, that sediment is exported from the Great Barrier Reef shelf to the adjacent ocean basins during intermediate (rather than last glacial maximum) low-stand, sea level positions. The model may apply to other rimmed shelves, both modern and ancient.  相似文献