Inertial oscillations and frontogenesis driven by a quadratic horizontal density variation

We consider inviscid rotating flow driven by a horizontally quadratic density variation in a horizontally unbounded slab. This configuration permits a similarity solution, removing the dependence on the horizontal coordinate from the vorticity and temperature equations, which are then solved by numerical integration along characteristics. At large values of Rossby number, the flow proceeds to a singularity in a similar manner to the non-rotating flow with the same initial conditions. At small values of Rossby number there are inertial oscillations of growing amplitude, which have been analysed using the method of multiple scales. The oscillations become desynchronised between the upper and lower parts of the domain, and static instability appears for a small fraction of each oscillation period. Eventually the oscillations give way to the rapid formation of a singularity, in contrast to geostrophic adjustment theory which predicts that a singularity will form only if the Rossby number is sufficiently large.     

North Sea near-surface wind climate and its relation to the large-scale circulation patterns

The North Sea 10-m wind speed (WS10) climate is compared and related to circulation patterns based on the sea level pressure (SLP) extracted from three reanalysis and one high-resolution model dataset. The mean magnitude and the trends of WS10 depend considerably on the selected reanalysis. The variability of WS10 among the three reanalysis datasets is highly correlated in the recent period (1980–2000) but less so in the past period (1960–1980). The WS10 over the North Sea is well represented by the relatively low reanalysis resolution when compared to the high-resolution WS10 model data partially owing to the high spatial correlation of WS10. Exceptions are observed only at the coastal areas. The dominant mode of WS10 explains coherent variability of WS10 over the North Sea and is related to a SLP pattern similar to the North Atlantic oscillation (NAO). The increase of the magnitude of the dominant WS10 pattern is related to the increase of the magnitude of the NAO-like SLP pattern from 1960s to mid-1990s. The second dominant WS10 pattern—a dipole in WS10 to the north and south of Great Britain—is related to the differences in SLP between Scandinavia and Iceland. The relation between the second WS10 and SLP patterns is more prominent in the recent period. The extreme WS10 in the German Bight is related to the low SLP over Scandinavia. The extreme WS10 is strongly increasing from the early 1980s to the beginning of 1990s, which is not observed in the corresponding SLP time series over Scandinavia.     

Mining lakes as groundwater‐dominated hydrological systems: assessment of the water balance of Mining Lake Plessa 117 (Lusatia,Germany) using stable isotopes

In the present study, the stable isotopes δ¹⁸O and δ²H were used for assessment of the water balance in a heterogeneously structured catchment area in the Lusatian Lignite Mining District, in particular, for estimation of the annual groundwater inflow and outflow (I_GW and O_GW) of Mining Lake Plessa 117. The application of stable isotopes was possible since the water exchange in the catchment area had reached steady‐state conditions after the abandonment of mining activities in 1968 and the filling of the voids and aquifers by re‐rising groundwater in the years thereafter. Diverging slopes of the Evaporation Line and the Global Meteoric Water Line manifested as evaporation from the lake catchment area. The calculated isotope water balance was compared with the commonly used surface water balance, which is unable to differentiate between I_GW and O_GW, and with a local groundwater model. The groundwater model calculated an I_GW of about 811 000 m³ yr^?1 and an O_GW close to zero, whereas the isotope water balance showed fluxes of about 914 000 and 140 000 m³ yr^?1, respectively. Considering the contribution of the groundwater inflow to the total annual input into the lake (ΔI_T) and the mean residence time (τ), where the groundwater model and the isotope water balance calculated 42 and 47% for ΔI_T and 4·3 and 3·9 years for τ, respectively, it was shown that both water balance calculation methods led to comparable results despite the differences in I_GW and O_GW. Copyright © 2008 John Wiley & Sons, Ltd.     

Characteristics of oceanic impact‐induced large water waves—Re‐evaluation of the tsunami hazard

Abstract— The potential hazard of a meteorite impact in the ocean is controversial with respect to the destructive power of generated large ocean waves (tsunamis). We used numerical modeling of hypervelocity impact to investigate the generation mechanism and the characteristics of the resulting waves up to a distance of 100–150 projectile radii. The wave signal is primarily controlled by the ratio between projectile diameter and water depth, and can be roughly classified into deep‐water and shallow‐water impacts. In the latter, the collapse of the crater rim results in a wave signal similar to solitary waves, which propagate and decay in agreement with shallow‐water wave theory. The much more likely scenario for an asteroid impact on Earth is a relatively small body (much smaller than the water depth) striking the deep sea. In this case, the collapse of the transient crater results in a significantly different and much more complex wave signal that is characterized by strong nonlinear behavior. We found that such waves decay much more rapidly than previously assumed and cannot be treated as long waves. For this reason, the shallow‐water theory is not applicable for the computation of wave propagation, and more complex models (full solution of the Boussinesq equations) are required.     

Magma mixing in the Aleutian arc: evidence from cognate inclusions and composite xenoliths

The complexity of igneous processes in the Aleutian calc-alkaline magma series can be inferred from study of xenolithic fragments. Composite xenoliths and cognate inclusions provide direct evidence for magma—magma and wall-rock—magma mixing processes. Using distributions of Cr in clinopyroxene, compositional endmembers involved in mixing are identified within the xenoliths. The basaltic mixing endmember is more mafic than calc-alkaline lavas in the arc. Magma mixing and wall-rock assimilation within calc-alkaline basaltic to andesitic magmas is identified in phenocrystic assemblages as well as in xenoliths, and appears to be a widespread phenomenon in Aleutian calc-alkaline magmas.     

Multivariate statistical estimates of Holocene vegetation and climate change,forest-tundra transition zone,NWT, Canada

Newly derived fossil pollen data were obtained from four sites along a transect from the boreal forest limit into tundra in the eastern Northwest Territories. Multivariate statistical analyses were employed to interpret the pollen assemblages. Transfer functions were constructed between the pollen data and climatic data, and paleoclimatic estimates were derived. The objective nature of the reconstructions provides an independent verification of the general outlines of the chronology of tree-line movements during the mid- and late-Holocene as established in previous paleosol and pollen studies. Boreal forest extended to approximately 62°N, associated with mean July temperatures 1 to 3°C above modern means, from at least 5500 to 3700 yr B.P. Although a major episode of southward displacement of tree line at about 3700 yr B.P. is recorded, later events are not clearly represented. Considerations of the statistics, the time scales, and the nature of the pollen rain suggest only conservative interpretations of the results are possible. It is suggested that the pollen sites may have been sensitive recorders of regional vegetation change only when they were near the ecotone, corresponding to a climatic threshold.     

Griffith Taylor Lecture,Geographical Society of New South Wales, 2004: Australia and the ‘State of Nature/Native’

This account sets itself against the binarism of physical versus cultural determinism that drives population–environment debate over people's place on the Australian continent. In reading back into colonial times, it evokes ‘our’ unresolved sense of relation to nature on a continent that has pressed awkward buttons since the time the botanist on Cook's Endeavour, Joseph Banks, first mused over the apparent contradiction on the east coast of human presence and uncultivated land. Australia's ‘state of nature’, including its inhabitants, elicited tensions in Anglo-Celtic peoples’ ambiguous relation with nature that have never gone away or been resolved. Indeed, I argue, against the background of what was considered in eighteenth-century Enlightenment thought to be a distinctly human separation from, and capacity to rise above, nature, that the Australian state of nature/native precipitated a crisis in prevailing ideas of ‘the human’. As consternation grew into the nineteenth century about the very capacity of Aboriginal people for improvement, and particularly for cultivation, I argue they challenged the basis upon which the unity of humankind had been assumed in Enlightenment thought. They could not be comprehended, according to the prevailing conception of racial difference, as a mere variety of the human. The radical idea of innate human difference may thus be understood as arising out of this incomprehension, as an attempt to account for the ontologically inexplicable difference of the Australian Aborigine. This lecture's excursion into the origins of ‘race’ is by way of highlighting the uncertainty of the divide between the human and non-human worlds. Imagining an entangled world of living things may help to craft fresh lines of debate about modes of environmental belonging and becoming on this continent that overcome the stale binaries and blind spots of Australia's population–environment debate.     

Botryococcus—A planktonic green alga, the source of petroleum through the ages: Transmission electron microscopical studies of oil shales and petroleum source rocks

The hydrocarbon secreting alga Botryococcus has been identified in organic remains of sediments ranging from Precambrian to Recent, and is believed to have been a major source material for petroleum generation throughout the geological time. In some petroleum source rocks of Lower Palaeozoic and Precambrian age, identification of the alga is only possible by electron microscopy. Transmission electron microscopy (TEM) has been used in the present study to identify microstructures of the algal remains in a range of oil shales and petroleum source rocks. It has been established that Botryococcus is the predominant alga in the Kukersite oil shale of Estonia. Similarly, the alga has been shown to be a major contributor to petroleum source rocks in Cambrian and Precambrian sedimentary basins in Australia. TEM has been applied to observations of Botryococcus in torbanites and to products from simulated maturation experiments on torbanite. A comparison with algal remains from Cambrian and Precambrian sediments ranging from undermature to overmature, enabled the distinction of organic matter in various stages of oil generation. Maturation/thermal effects on alginite have been established by reflectance and fluorescence, and compared with experimental results.