Aeolianites are integral components of many modern and ancient carbonate depositional systems. Southern Australia contains some of the most impressive and extensive late Cenozoic aeolianites in the modern world. Pleistocene aeolianites on Yorke Peninsula are sculpted into imposing seacliffs up to 60 m high and comprise two distinct imposing complexes of the Late Pleistocene Bridgewater Formation. The lower aeolianite complex, which forms the bulk of the cliffs, is a series of stacked palaeodunes and intervening palaeosols. The diagenetic low Mg‐calcite sediment particles are mostly bivalves, echinoids, bryozoans and small benthic foraminifera. This association is similar to sediments forming offshore today on the adjacent shelf in a warm‐temperate ocean. By contrast, the upper aeolianite complex is a series of mineralogically metastable biofragmental carbonates in a succession of stacked lenticular palaeodunes with impressive interbedded calcretes and palaeosols. Bivalves, geniculate coralline algae and benthic foraminifera, together with sparse peloids and ooids, dominate sediment grains. Fragments of large benthic foraminifera including Marginopora vertebralis, a photosymbiont‐bearing protist, are particularly conspicuous. Palaeocean temperatures are interpreted as having been sub‐tropical, somewhat warmer than offshore carbonate factories in the region today. The older aeolianite complex is tentatively correlated with Marine Isotope Stage 11, whereas the upper complex is equivalent to Marine Isotope Stage 5e. Marine Isotope Stage 5e deposits exposed elsewhere in southern Australia (Glanville Formation) are distinctive with a subtropical biota, including Marginopora vertebralis. Thus, in this example, palaeodune sediment faithfully records the nature of the adjacent inner neritic carbonate factory. By inference, aeolianites are potential repositories of information about the nature of long‐vanished marine systems that have been removed due to erosion, tectonic obliteration or are inaccessible in the subsurface. Such information includes not only the nature of marine environments themselves but also palaeoceanography. 相似文献
Bermuda is a reef atoll along the northern edge of Caribbean coral province. Although investigated by seismic and some shallow drilling, the Pleistocene marine depositional geohistory is poorly constrained. Islands along the southern rim are built by tropical calcareous aeolianites that range in age from Holocene to early Pleistocene (ca 880 kyr). These dunes are composed of particles that were derived from adjacent Pleistocene marine environments at the time of formation. Thus, the aeolianites should contain a record of marine deposition through the Early to Late Pleistocene. Carbonate grains from all aeolian deposits can, via Ward cluster analysis, be separated into two distinct groups: (i) a Halimeda‐rich group; and (ii) a crustose coralline‐rich group. Distribution of these two groups is interpreted to broadly reflect low‐energy (lagoonal) and high‐energy marginal reef (coralline algae and cup‐reef) environments, respectively. Unlike the beach sources, coral particles are perplexingly absent in the aeolianites. This conundrum is interpreted to partly reflect the domal nature of Bermudan corals, which are not incorporated into aeolian deposits due to their relatively large size. Aeolianites from Marine Isotope Stages 7, 9 and 11 record sediments produced in adjacent shallow marine settings that were similar to those present today. The spatially consistent sediment trends are not, however, present in aeolianites from Marine Isotope Stage 5E, where the aeolian bioclastic components are uniformly rich in Halimeda along both southern and northern shores. Such a distribution, where coralline‐rich sediments would be expected, suggests an extrinsic oceanographic control, interpreted herein to be elevated seawater temperature brought in by the Gulf Stream. This interpretation is consistent with palaeozoological studies of Bermuda, as well as North America, the Mediterranean, Japan and Western Australia. 相似文献
Extreme meteorological conditions favor the development of severe storms and tornadoes that may have largely impacts on the population despite its relatively short life. Tornadic severe storms have been documented around the World. In Mexico (MEX), the study of the occurrence of tornadoes and severe storms is relatively new. In this research, we have selected an event of severe tornadic storm in Ciudad Acuña, Mexico. The storm was driven by a frontal system moving southward from USA converging with a warmer moist air flux from the Gulf of Mexico. The tornado strikes on the Northeast of Mexico, in Coahuila State, on May 25, 2015. Imagery of infrared channel from GOES 13 satellite and the presence of a hook echo in radar data of May 25, 2015, indicate a supercell structure. The maximum values of radial velocity were about ?20 and 15 m s?1. In this study, the WRF model was used in order to simulate the mesoscale meteorological conditions of the tornado. Model simulations capture atmospheric features observed in Doppler radar. The simulated storm-relative helicity values were between 400 and 500 m2 s?2. The simulated convective available potential energy values were of 3000 J kg?1. These values were higher than values for convective storms, located over the region of Ciudad Acuña in Mexico and Del Rio in USA. The supercell was a result of high humidity and temperature gradients, conditioned by frontal activity and moisture flux intensifications from the Gulf of Mexico.
During the twentieth century sea surface temperatures in the Atlantic Ocean exhibited prominent multidecadal variations. The source of such variations has yet to be rigorously established—but the question of their impact on climate can be investigated. Here we report on a set of multimodel experiments to examine the impact of patterns of warming in the North Atlantic, and cooling in the South Atlantic, derived from observations, that is characteristic of the positive phase of the Atlantic Multidecadal Oscillation (AMO). The experiments were carried out with six atmospheric General Circulation Models (including two versions of one model), and a major goal was to assess the extent to which key climate impacts are consistent between the different models. The major climate impacts are found over North and South America, with the strongest impacts over land found over the United States and northern parts of South America. These responses appear to be driven by a combination of an off-equatorial Gill response to diabatic heating over the Caribbean due to increased rainfall within the region and a Northward shift in the Inter Tropical Convergence Zone (ITCZ) due to the anomalous cross-equatorial SST gradient. The majority of the models show warmer US land temperatures and reduced Mean Sea Level Pressure during summer (JJA) in response to a warmer North Atlantic and a cooler South Atlantic, in line with observations. However the majority of models show no significant impact on US rainfall during summer. Over northern South America, all models show reduced rainfall in southern hemisphere winter (JJA), whilst in Summer (DJF) there is a generally an increase in rainfall. However, there is a large spread amongst the models in the magnitude of the rainfall anomalies over land. Away from the Americas, there are no consistent significant modelled responses. In particular there are no significant changes in the North Atlantic Oscillation (NAO) over the North Atlantic and Europe in Winter (DJF). Additionally, the observed Sahel drying signal in African rainfall is not seen in the modelled responses. Suggesting that, in contrast to some studies, the Atlantic Multidecadal Oscillation was not the primary driver of recent reductions in Sahel rainfall. 相似文献
Atlantic Multidecadal Variability (AMV) is investigated in a millennial control simulation with the Kiel Climate Model (KCM), a coupled atmosphere–ocean–sea ice model. An oscillatory mode with approximately 60 years period and characteristics similar to observations is identified with the aid of three-dimensional temperature and salinity joint empirical orthogonal function analysis. The mode explains 30 % of variability on centennial and shorter timescales in the upper 2,000 m of the North Atlantic. It is associated with changes in the Atlantic Meridional Overturning Circulation (AMOC) of ±1–2 Sv and Atlantic Sea Surface Temperature (SST) of ±0.2 °C. AMV in KCM results from an out-of-phase interaction between horizontal and vertical ocean circulation, coupled through Irminger Sea convection. Wintertime convection in this region is mainly controlled by salinity anomalies transported by the Subpolar Gyre (SPG). Increased (decreased) dense water formation in this region leads to a stronger (weaker) AMOC after 15 years, and this in turn leads to a weaker (stronger) SPG after another 15 years. The key role of salinity variations in the subpolar North Atlantic for AMV is confirmed in a 1,000 year long simulation with salinity restored to model climatology: No low frequency variations in convection are simulated, and the 60 year mode of variability is absent. 相似文献
Lake Fidler is an ectogenic meromictic lake with a monimolimnion maintained by periodic incursions of brackish water from the lower Gordon River estuary. A dam across the middle reaches of the Gordon River has restricted these incursions of brackish water and meromictic stability has rapidly declined. A palaeolimnological study was carried in order to assess the historical development of meromixis and the impact of the dam on the microbiological communities in the lake. Fossil pigments in a 17 m sediment core were analysed using reverse phase high performance liquid chromatography (rp-HPLC) and mass spectrometry (MS). In addition, taphonomic studies of pigment production, deposition and degradation in the water column and surface sediments were used to identify planktonic and benthic pigment degradation processes and constrain the stratigraphic interpretation. Results comparing the pigment composition of pelagic sediment traps and littoral surface sediments indicated that the core from the centre of the lake would permit a historical reconstruction of planktonic bacterial and algal communities. Marked increases in prokaryotic pigments ca 3500 yr B.P. suggested the possible colonisation of a chemocline by phototrophic bacteria. Further changes in chlorophyll: carotenoid ratios and changes in relative abundances of both chlorophyll a and bacteriochlorophyll c derivatives also indicated that a change in the depositional environment had occurred; possibly due to altered stratification or anoxia. From this we infer the onset of either intermittent or permanent meromixis. Further increases in prokaryotic pigment abundance suggested that the present state of permanent meromixis was firmly established by 2070 ±50 14C yr B.P., and diatom analysis confirmed the development of a stable mixolimnion. High resolution studies of the top 10 cm of sediments measured pigments in mean concentrations of 15.1 ng g-1 with a mean S.D. of only 2.78 indicating little change in pigment abundance since the construction of the dam. Thus, Lake Fidler still retains most of the features of meromixis. However, evidence from nearby Lake Morrison and Sulphide Pool has shown that any further declines in meromictic stability will cause a rapid reversion to holomixis. Palaeolimnological evidence from the early stages of meromictic development of Lake Fidler suggests that such reversion to holomixis may not permanently eliminate all the microbiological communities, and that, given time, they may return and prosper with re-establishment of a suitable chemocline. These studies will guide recommendations for a management strategy to prevent the further decay of meromixis in the Gordon River lakes. 相似文献
The rotational form of the vertically averaged equations of motion is applied to derive a formula, linear friction included, which establishes a direct connection between sense of rotation of tidal currents and features of tidal amphidromic systems. Two factors in the formula, called and , influence the sense of rotation of tidal currents; the factor involves the frequency of the tidal signal , the Coriolis parameter f, and the linear friction coefficient r. The sign of the cross-product of the logarithm of sea-surface elevation (), and phase () gradients determines whether the factor favors clockwise or anticlockwise sense of rotation. is a unit vector and is the angle between ln and . The limits ||0, ||0 and 0 lead to a clockwise sense of rotation in the Northern Hemisphere. 0 favors anticlockwise rotation in the Northern Hemisphere. Friction and low frequencies favor an anticlockwise sense of rotation. The theory works well in semi-enclosed regions like the North Sea. Although only linear friction and sea-surface elevation gradients were considered, there are ocean regions where the agreement between theory and observations is also good.Responsible Editor: Hans Burchard 相似文献