Stratigraphy and sedimentology of lower carboniferous (Dinantian) boreholes from West Co. Meath,Ireland

The 2-km deep Athboy Borehole (1439/2) together with the lower part of boreholes EP30 and N915 form a standard type section for strata of Dinantian (Courceyan to Asbian) age in west Co. Meath. Above a thin basal red-bed siliciclastic sequence, the marine Courceyan shelf succession is almost 600 m thick. It comprises the Liscartan, Meath, and Moathill Formations of the Navan Group and the Slane Castle Formation of the succeeding Boyne Group. The shallow-water limestones include micrites, oolites, and sandy bioclastic packstones and grainstones with subordinate skeletal wackestones and shales. Lateral facies changes from north to south in the Navan area suggest deepening across a shelf towards a depocentre further to the south around Trim. The deeper-water Waulsortian Limestones of late Courceyan to Chadian age (Feltrim Formation, ca. 213 m thick) form a series of five sheet-like mudbanks, interbedded with generally thin units of nodular crinoidal limestones and shales. The mudbanks are formed of bryozoan-rich peloidal wackestones and lime-mudstones with phase C and D components. Rare soft-sediment breccias occur at the bottom and top of banks. The succeeding Fingal Group commences with a thin interval (3–20 m) of black shales, laminated packstones, and micritic limestones of Chadian age, the Tober Colleen Formation. This is followed by the Lucan Formation (Chadian to Asbian) predominantly of laminated and graded calciturbidites, laminated sandstones, cherts, and black shales, which is over 1300 m thick. Ten sedimentary units have been informally defined, based on lithofacies and facies associations. The oldest unit, the Tara Member, is characterized by proximal debris-flow breccia deposits and nodular mudstones. A thick bioturbated micrite and shale unit (Ardmulchan Member) in the middle of the formation is overlain directly by a coarse oolitic and crinoidal grainstone unit (Beauparc Member). Near the top of the formation is a distinctive unit of coarse-grained laminated sandstones and shales (Athboy Member). The highest rocks in the Borehole are clean thickly-bedded limestones of the Asbian Naul Formation (>90 m thick). The youngest Dinantian strata in the area, the Brigantian Loughshinny Formation, marks a return to shale-dominant basin sedimentation. The significance of this work lies in the fact that the Athboy borehole is the longest continuously cored borehole in the Carboniferous of Ireland and provides a continuous sedimentary and biostratigraphic record for the northern part of the Dublin Basin. Foraminiferal biozones (Cf2–Cf6) have been recognized in this and in borehole N915, and Stage boundaries identified, which can be applied throughout the Basin. The sedimentary record for the Lucan Formation indicates four tectonic pulses during the Viséan, in the late Chadian/early Arundian, mid-Arundian, Holkerian, and late Holkerian/early Asbian.     

The formation of farm mounds on the Island of Sanday,Orkney

A recent archaeological site survey of the island of Sanday on Orkney has identified a distinct concentration of 15 farm mounds. An immediate need is to determine the nature, age, and processes of formation of these very distinctive landform features. This paper reports a geoarchaeological investigation of three mounds at Westbrough, Langskaill, and Skelbrae. For the island as a whole, the mounds vary in diameter from 50 to 205 m, with deposits up to a thickness of 4.3 m. The mounds vary in their morphology, reflecting complex processes of accumulation, often intense disturbance, and in some cases, erosion by the sea. Sections at the three sites are described with analytical data also being given for particle size, percent loss-on-ignition, total phosphate, δ¹³C, and radiocarbon dating. The Westbrough and Skelbrae mounds were formed post-7th century A.D.; more detailed dating was possible for the Langskaill site, which was initiated in the early to mid 13th century. A steady and relatively rapid sedimentation rate (ca. 1.9 cm year⁻¹) took place over the 200 years following the initiation of the mound. The lower part of the Westbrough mound is distinguished by the presence of peat, interpreted as discarded bedding material, but the remainder of the investigated sections are composed dominantly of mineral sediments containing organic matter of terrestrial origin. It is suggested that these materials accumulated from the ash of domestic hearths as well as from the residues of bedding material from the byres (cattle sheds). Turves (vegetation plus some underlying soil) would have been cut, dried, and then used in the byres and such turves would have retained a significant mineral content. The implication is that the inherent fertility of the island made it unnecessary for the farmers to spread the dung over the fields. This may explain why there is the distinct concentration of these mounds on the island of Sanday, an island noted in earlier times for its fertile soils developed on calcareous sands.     

Effect of tropospheric temperature change on the zonal mean circulation and SH winter extratropical cyclones

This study aims to understand the mechanisms which cause an overall reduction of SH extratropical cyclone activity with a slight increase in the high latitudes in a warmer climate simulated in general circulation models (GCMs) with increasing CO₂. For this purpose, we conducted idealized model experiments by forcing warm temperature anomalies to the areas where climate change models exhibit local maximum warming—the tropics in the upper troposphere and the polar regions in the lower troposphere—simultaneously and separately. The Melbourne University atmospheric GCM (R21) coupled with prescribed SST was utilized for the experiments. Our results demonstrate that the reduction of SH extratropical cyclone frequency and depth in the midlatitudes but the slight increase in the high latitudes suggested in climate change models result essentially from the tropical upper tropospheric warming. With this tropical warming, the enhanced static stability which decreases baroclinicity in the low and midlatitudes turns out to be a major contributor to the decrease of cyclone activity equatorward of 45°S whereas the increased meridional temperature gradient in the high latitudes seems an important mechanism for the increase of cyclone activity over 50°–60°S.     

The impact of inhomogenous background errors on a global wave data assimilation system

One of the main limitations in current wave data assimilation systems is the lack of an accurate representation of the structure of the background errors. In this work, models for the observational error variance, background error variance and background error correlations are developed based on the results of previous studies. These are tested in a global wave data assimilation system and the resulting wave forecasts are verified against independent observations from buoys. Forecasts of significant wave height show substantial improvement over the Australian Bureau of Meteorology's current operational wave forecasting system. However, forecasts of peak period are not similarly improved. The regional impacts of the new assimilation scheme are found to vary on a seasonal basis. Overall, it is shown that the inclusion of a latitudinally dependent background error, and improved specification of the background and observational error variances can reduce the root-mean-square error of 24-hour forecast Significant Wave Height by almost 10%.     

Large scale features and energetics of the hybrid subtropical low ‘Duck’ over the Tasman Sea

New aspects of the genesis and partial tropical transition of a rare hybrid subtropical cyclone on the eastern Australian coast are presented. The ‘Duck’ (March 2001) attracted more recent attention due to its underlying genesis mechanisms being remarkably similar to the first South Atlantic hurricane (March 2004). Here we put this cyclone in climate perspective, showing that it belongs to a class within the 1 % lowest frequency percentile in the Southern Hemisphere as a function of its thermal evolution. A large scale analysis reveals a combined influence from an existing tropical cyclone and a persistent mid-latitude block. A Lagrangian tracer showed that the upper level air parcels arriving at the cyclone’s center had been modified by the blocking. Lorenz energetics is used to identify connections with both tropical and extratropical processes, and reveal how these create the large scale environment conducive to the development of the vortex. The results reveal that the blocking exerted the most important influence, with a strong peak in barotropic generation of kinetic energy over a large area traversed by the air parcels just before genesis. A secondary peak also coincided with the first time the cyclone developed an upper level warm core, but with insufficient amplitude to allow for a full tropical transition. The applications of this technique are numerous and promising, particularly on the use of global climate models to infer changes in environmental parameters associated with severe storms.     

Vitrinite anisotropy under differential stress and high confining pressure and temperature: preliminary observations

The orientation of the optical indicating surface of vitrinite in reflected light has been determined following deformation at 350 and 500°C, confining pressures of 500 and 800 MPa and a strain rate of 10⁻⁵ s⁻¹. High temperature and large strain have facilitated reorientation of the indicating surface, increase in anisotropy (bireflectance) and an increase in maximum vitrinite reflectance. In a specimen deformed at 500°C and 23% axial strain the maximum vitrinite reflectance has been reoriented more than 70° from close to parallel to σ₁ in the undeformed state to perpendicular to σ₁ following deformation. Orientation of the optical indicating surface of some of the deformed specimens suggests the orientation of the maximum reflectance is a composite product of the original orientation of the indicating surface and an orientation produced during deformation.     

热带大气10～20天振荡的大气环流模式数值模拟

利用大气环流模式对热带大气10～20天振荡进行了数值模拟研究，其结果同观测资料分析基本一致。模式资料的功率谱及动能的分析表明，10～20天振荡确实比30～60天振荡更突出地存在于热带大气中。同热带大气30～60天振荡的结构有些不同，对于在热带10～20天振荡，除纬向1波外，纬向2波也很明显；流动的斜压结构特征欠佳；系统以较快速度沿赤道西移。暖SST事件对热带大气10～20天振荡的影响也进行了模拟，对结果的比较分析表明，暖STT将使10～20天大气振荡减弱，使其结构更趋正压性。     

Zirconolite and xenotime U–Pb age constraints on the emplacement of the Golden Mile Dolerite sill and gold mineralization at the Mt Charlotte mine,Eastern Goldfields Province,Yilgarn Craton,Western Australia

In situ SHRIMP U–Pb dating of magmatic zirconolite (CaZrTi₂O₇) in the Golden Mile Dolerite from the Mt Charlotte gold deposit (Yilgarn Craton, Australia) has yielded the first robust emplacement age (2,680 ± 9 Ma) for the principle host-rock of gold mineralization in the Kalgoorlie district. In contrast, co-magmatic zircon gave ages from ~2.68 Ga to ~2.17 Ga, reflecting isotopic resetting of high-U and -Th crystals. In situ SHRIMP analysis of hydrothermal xenotime (YPO₄), which co-exists with gold in alteration pyrite, provided a Pb/Pb isochron age of 2,655 ± 13 Ma. This date indicates that the youngest deposit in the Kalgoorlie district (Mt Charlotte) formed at ~2.65 Ga, and provides a new minimum age for the structurally older Golden Mile deposit. Our results indicate that gold mineralization at Mt Charlotte is ~50 million years older than indicated by recent ⁴⁰Ar/³⁹Ar dating and places new constraints on the timing of late-stage regional faulting (D₄) in the province.