Are topographic data sets fractal?

The scale invariant properties of fractal sets make them attractive models for topographic profiles because those profiles are the end product of a complex system of physical processes operating over many spatial scales. If topographic data sets are fractal, their power spectra will be well represented by lines in log-log space with slopess such that –3s<–1. The power spectra from a Digital Elevation Model (30 meter sample spacing) of the Sierra Nevada Batholith and from Seabeam center beam depths (425 meter sample spacing) along a flowline in the South Atlantic are curved. Straight sections in the spectra can be identified but the slopes of those sections are strongly dependent upon the particulars of the data analysis. Fractal geometry must be used with caution in the discussion of topographic data sets.     

Respiration rates and hypoxia on the Louisiana shelf

The spatial and temporal variation in water-column respiration, estimated from enzymatic respiratory electron-transport-system activity, was measured monthly on a cross-shelf transect on the Louisiana shelf from May through October 1991. In July 1991, water-column respiration was also determined on an alongshore transect, and in situ benthic respiration and photosynthesis rates were determined at jour stations on the cross-shelf transect. Bottom waters were persistently hypoxic (O₂<2 mg 1^?1) at most stations in July and August and sporadically hypoxic at other times. Water-column respiration rates were in the same range as earlier, less extensive studies and not unusually high for coastal and estuarine waters. They were highest in summer, decreased with distance offshore and depth, and increased with temperature. Their variation with pigment and oxygen concentrations were complex functions of season and depth. Oxygen depletion below the oxycline could occur within days to months, depending on the season and location. In July, benthic respiration rates were also not unusually high in comparison with other shallow sediments, although the ratio of benthic: total (water column+benthic) respiration was high. Combined water-column and benthic respiration could deplete the bottom water oxygen in approximately 1 mo. Because the system rarely goes anoxic (defined as observing sulfide), some mechanism(s) must exist to reaerate bottom waters. Most physical mechanisms are unlikely to provide significant reaeration at this time of year. Measured benthic and conservatively estimated bottom-water photosynthesis could resupply 23% of the oxygen lost daily by respiration. Although this is too limited a dataset from which to draw conclusions about the relative importance of bottom-water and benthic respiration and photosynthesis in determining bottom-water oxygen concentrations, it does suggest that all these processes must be considered.     

Application of sedimentary and chronological analyses to refine the depositional context of a Late Pleistocene vertebrate deposit,Naracoorte, South Australia

Cave deposits of infill sediments and associated vertebrate fossils provide a valuable source of information on terrestrial palaeoenvironments, climatic conditions and palaeocommunities. In the deposits of the Naracoorte Caves World Heritage Area, such records span the last 500 ka and are renowned for their rich, diverse vertebrate assemblages. Previous research into the Grant Hall deposit of Victoria Fossil Cave suggested that it may preserve the only peak last interglacial (ca. 125 ka) faunal community within the World Heritage Area. The current work tested this existing model for the age of faunal remains from Grant Hall using multiple techniques. Physical and geochemical properties of the visually homogeneous sediments were analysed at regular intervals through the sequence to establish meaningful stratigraphic divisions and sediment provenance. Optically stimulated luminescence dating of individual quartz grains indicates that sediments accumulated in Grant Hall from 93 ± 8 to 70 ± 5 ka. Minimum ages provided by U/Th dating of fossil teeth (72.3 ± 2.2 to 38.2 ± 0.8 ka) are consistent with the luminescence chronology, and show that the deposit represents a more recent faunal accumulation than previously modelled for the site. U/Th ages on calcite straws within the deposit are significantly older than the sediments and fossil teeth (>500 to 186.4 ± 1 ka). As such they provide no further constraint on the chronology of the deposit but do indicate that speleothem deposition was active over much of the Middle Pleistocene. Sedimentary analyses resulted in the identification of five depositional units, contrasting with previous divisions which were based only on visual observation of the sedimentary sequence. Sediments within each unit are broadly classified as sandy silts with soil structures and may be indirectly derived from the lunettes of nearby Bool Lagoon, although their ultimate provenance is unknown. As a result of this work, palaeoenvironmental reconstruction based on fossil remains in the deposit may be more accurately related to prevailing climatic and environmental conditions at the time of accumulation. It also contributes to an understanding of the temporal occurrence of regional vertebrate faunas through the Late Pleistocene, reinforcing the value of developing stratigraphically constrained chronologies for cave deposits based on multiple techniques.     

The measurement of lunar altitudes by photography—I. Estimating the true lengths of shadows

This paper provides a discussion of all known sources of error in the apparent length of a lunar shadow recorded photographically and measured with a microdensitometer. The discussion is general but is illustrated by reference to sunrise shadows of various lengths cast by the lunar Straight Wall. Measurements of shadow lengths made between half-density points of a microdensitometric scan along a shadow may contain systematic errors due to the non-linearity of the characteristic of the photographic emulsion. The most important sources of systematic-error are seeing (especially in the case of short shadows), penumbrae, and uncertain selenographic coordinates. Other errors, due to seeing, effects at a shadow-tip, and procedures of measurement, are also important but probably apply in a more nearly random manner.     

Wavelet Analysis and Filtering to Identify Dominant Orientations of Permeability Anisotropy

An accurate representation of permeability anisotropy is needed to model the rate and direction of groundwater flow correctly. We develop a wavelet analysis technique that can be used to characterize principal directions of anisotropy in both stationary and non-stationary permeability fields. Wavelet analysis involves the integral transform of a field using a wavelet as a kernel. The wavelet is shifted, scaled, and rotated to analyze different locations, sizes, and orientations of the field. The wavelet variance is used to identify scales and orientations that dominate the field. If the field is non-stationary, such that different zones of the field are characterized by different dominant scales or orientations, the wavelet variance can identify all dominant scales and orientations if they are distinct. If the dominant scales and orientations of different zones are similar, the wavelet variance identifies only the dominant scale and orientation of the primary zone. In this paper, we present a combined wavelet analysis and filtering approach to identify all dominant scales and orientations in a non-stationary permeability field. We apply the method to permeability data obtained in the laboratory from the Massillon sandstone.     

New U/Th ages for Pleistocene megafauna deposits of southeastern Queensland,Australia

Arguments over the extinction of Pleistocene megafauna have become particularly polarised in recent years. Causes for the extinctions are widely debated with climate change, human hunting and/or habitat modification, or a combination of those factors, being the dominant hypotheses. However, a lack of a spatially constrained chronology for many megafauna renders most hypotheses difficult to test. Here, we present several new U/Th dates for a series of previously undated, megafauna-bearing localities from southeastern Queensland, Australia. The sites were previously used to argue for or against various megafauna extinction hypotheses, and are the type localities for two now-extinct Pleistocene marsupials (including the giant koala, Phascolarctos stirtoni). The new dating allows the deposits to be placed in a spatially- and temporally constrained context relevant to the understanding of Australian megafaunal extinctions. The results indicate that The Joint (Texas Caves) megafaunal assemblage is middle Pleistocene or older (>292 ky); the Cement Mills (Gore) megafaunal assemblage is late Pleistocene or older (>53 ky); and the Russenden Cave Bone Chamber (Texas Caves) megafaunal assemblage is late Pleistocene (～55 ky). Importantly, the new results broadly show that the sites date prior to the hypothesised megafaunal extinction ‘window’ (i.e., ～30–50 ky), and therefore, cannot be used to argue exclusively for or against human/climate change extinction models, without first exploring their palaeoecological significance on wider temporal and spatial scales.     

Deconvolution with wavelets and vaguelettes

The use of wavelets for the solution of convolution equations is studied as a possible alternative to the well-established Fast Fourier Transform (FFT) technique. Two possible solution strategies are investigated: (1) The use of wavelets for the representation of both the given data and the unknown solution. This leads to an algorithm with good de-noising and data-compression properties. In terms of computational efficiency this algorithm is inferior to FFT. (2) The use of wavelets for the representation of the unknown solution and of so-called vaguelettes for the representations of the given data. This leads to an algorithm which is even faster than FFT. Received: 14 October 1998 / Accepted: 30 November 1999     

An explosive–intrusive subglacial rhyolite eruption at Dalakvísl,Torfajökull,Iceland

This paper describes unusual rhyolitic deposits at Dalakvísl, Torfajökull, Iceland that were emplaced during a Quaternary subglacial eruption. Despite its small volume (<0.2 km³), the eruption mechanisms were highly variable and involved both explosive and intrusive phases. The explosive phase involved vesiculation-driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian. Textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures. In contrast, the intrusive phase of the eruption generated vesicle-poor quench hyaloclastites associated with a variety of peperitic lava bodies. The presence of juvenile-rich fluvio-lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable. In order to explain the variable eruption mechanisms, a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent. When the cavity became completely filled by volcanic deposits, the vent became blocked and rising magma was forced to intrude through poorly consolidated debris. This led to arrested fragmentation and welding of foam domains to form vesicle-poor obsidian lava; the transition to an intrusive eruption has taken place. Although this vent-blocking mechanism is particularly relevant to subglacial eruptions, it may also apply to subaerial rhyolitic eruptions, where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone. Meanwhile, the variable style of a small-volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano-ice interactions.     

Proglacial sediment trapping in recently formed Silt Lake,upper Lillooet Valley,Coast Mountains,British Columbia

The sedimentology of proglacial Silt Lake was assessed by lake sediment coring and monitoring of lacustrine processes during a late‐summer period of high glacier melt to characterize sediment delivery from the heavily glacierized catchment and investigate the sediment trapping dynamics of this upland lake. A complete varve chronology was established for a distal basin of the lake which was exposed by Lillooet Glacier retreat between 1947 and 1962. The varve record showed decreasing sedimentation rates in the basin while the glacier retreated, and as the lake became free of ice contact in the early 1970s. Although recession has continued over recent decades, and glacier proximity to the lake has, therefore, continued decreasing, lacustrine sedimentation rates are now accelerating due to changing basin morphometry caused by delta progradation. Over shorter time scales, lake sedimentation patterns respond to changing runoff conditions, including late‐summer glacier melt intensity, intra‐annual flooding events, diumal runoff fluctuations, and within‐lake turbidity currents. Turbidity currents included quasi‐regular flows during high diurnal discharges and an episodic flushing of temporarily stored sediment from the sandur or delta at a time of low stage. Suspended sediment yield to Silt Lake is estimated to exceed 10³ Mg km^?2 a^?1, a magnitude that surpasses previous local and regional yield estimates for the glacierized headwaters of the Lillooet River valley. Since Silt Lake currently traps a significant prooportion of that upland sediment supply, and the trapping efficiency of the basin has been variable at decadal time scales, the formation and continued development of Lilt Lake has likely had a significant influence on downstream sediment delivery. Lacustrine sediment‐based proxies of long‐term hydroclimatic variability being developed in glacially distal settings should include provisions for dynamic sediment trapping effects in upstream water bodies that often form in the active proglacial environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Effects of hypoxia on benthic macrofauna and bioturbation in the Estuary and Gulf of St. Lawrence,Canada

The bottom water in the >300 m deep Lower St. Lawrence Estuary (LSLE) is persistently hypoxic in contrast to the normoxic bottom waters in the Gulf of St. Lawrence (GSL). We photographed the seabed at 11 stations in the Estuary and Gulf of St. Lawrence (EGSL) during the summers 2006 and 2007 and analysed the images to identify bioturbation traces (lebensspuren) and benthic macrofauna. The objective was to identify the environmental variables that influence the density and diversity of benthic macrofauna and bioturbation traces, and the differences that exist among regions with high, medium and low oxygen levels in the bottom water. The bottom water oxygen concentration is the variable that best explains the densities of total-traces as well as surface-traces. However, the density of these traces was higher in hypoxic regions than in well-oxygenated regions. The higher density of traces in the hypoxic region of the LSLE is mainly due to the activities of the surface deposit feeder Ophiura sp., which occurs in large numbers in this region. Possible explanations explored are stress behaviour of the organisms in response to hypoxia and different benthic macrofauna community structures between the hypoxic regions of the LSLE and the normoxic regions of the GSL. In the former, surface deposit feeders and low-oxygen tolerant species dominate over suspension feeders and low-oxygen intolerant species.