Geophysical constraints on understanding the origin of the Illinois basin and its underlying crust

Interpretation of reprocessed seismic reflection profiles reveals three highly coherent, layered, unconformity-bounded sequences that overlie (or are incorporated within) the Proterozoic “granite–rhyolite province” beneath the Paleozoic Illinois basin and extend down into middle crustal depths. The sequences, which are situated in east–central Illinois and west–central Indiana, are bounded by strong, laterally continuous reflectors that are mappable over distances in excess of 200 km and are expressed as broad “basinal” packages that become areally more restricted with depth. Normal-fault reflector offsets progressively disrupt the sequences with depth along their outer margins. We interpret these sequences as being remnants of a Proterozoic rhyolitic caldera complex and/or rift episode related to the original thermal event that produced the granite–rhyolite province. The overall thickness and distribution of the sequences mimic closely those of the overlying Mt. Simon (Late Cambrian) clastic sediments and indicate that an episode of localized subsidence was underway before deposition of the post-Cambrian Illinois basin stratigraphic succession, which is centered farther south over the “New Madrid rift system” (i.e., Reelfoot rift and Rough Creek graben). The present configuration of the Illinois basin was therefore shaped by the cumulative effects of subsidence in two separate regions, the Proterozoic caldera complex and/or rift in east–central Illinois and west–central Indiana and the New Madrid rift system to the south. Filtered isostatic gravity and magnetic intensity data preclude a large mafic igneous component to the crust so that any Proterozoic volcanic or rift episode must not have tapped deeply or significantly into the lower crust or upper mantle during the heating event responsible for the granite–rhyolite.     

Coastal morphodynamics and Chenier-Plain evolution in southwestern Louisiana, USA: A geomorphic model

Using 28 topographic profiles, air-photo interpretation, and historical shoreline-change data, coastal processes were evaluated along the Chenier Plain to explain the occurrence, distribution, and geomorphic hierarchy of primary landforms, and existing hypotheses regarding Chenier-Plain evolution were reconsidered. The Chenier Plain of SW Louisiana, classified as a low-profile, microtidal, storm-dominated coast, is located west and downdrift of the Mississippi River deltaic plain. This Late-Holocene, marginal-deltaic environment is 200 km long and up to 30 km wide, and is composed primarily of mud deposits capped by marsh interspersed with thin sand- and shell-rich ridges (“cheniers”) that have elevations of up to 4 m.In this study, the term “ridge” is used as a morphologic term for a narrow, linear or curvilinear topographic high that consists of sand and shelly material accumulated by waves and other physical coastal processes. Thus, most ridges in the Chenier Plain represent relict open-Gulf shorelines. On the basis of past movement trends of individual shorelines, ridges may be further classified as transgressive, regressive, or laterally accreted. Geomorphic zones that contain two or more regressive, transgressive, or laterally accreted ridges are termed complexes. Consequently, we further refine the Chenier-Plain definition by Otvos and Price [Otvos, E.G. and Price, W.A., 1979. Problems of chenier genesis and terminology—an overview. Marine Geology, 31: 251–263] and define Chenier Plain as containing at least two or more chenier complexes. Based on these definitions, a geomorphic hierarchy of landforms was refined relative to dominant process for the Louisiana Chenier Plain. The Chenier Plain is defined as a first-order feature (5000 km²) composed of three second-order features (30 to 300 km²): chenier complex, beach-ridge complex, and spit complex. Individual ridges of each complex type were further separated into third-order features: chenier, beach ridge, and spit.To understand the long-term evolution of a coastal depositional system, primary process–response mechanisms and patterns found along the modern Chenier-Plain coast were first identified, especially tidal-inlet processes associated with the Sabine, Calcasieu, and Mermentau Rivers. Tidal prism (Ω) and quantity of littoral transport (M_total) are the most important factors controlling inlet stability. Greater discharge and/or tidal prism increase the ability of river and estuarine systems to interrupt longshore sediment transport, maintain and naturally stabilize tidal entrances, and promote updrift deposition. Thus, prior to human modification and stabilization efforts, the Mermentau River entrance would be classified as wave-dominated, Sabine Pass as tide-dominated, and Calcasieu Pass as tide-dominated to occasionally mixed.Hoyt [Hoyt, J.H., 1969. Chenier versus barrier, genetic and stratigraphic distinction. Am. Assoc. Petrol. Geol. Bull., 53: 299–306] presented the first detailed depositional model for chenier genesis and mudflat progradation, which he attributed to changes in Mississippi River flow direction (i.e., delta switching) caused by upstream channel avulsion. However, Hoyt's model oversimplifies Chenier-Plain evolution because it omits ridges created by other means. Thus, the geologic evolution of the Chenier Plain is more complicated than channel avulsions of the Mississippi River, and it involved not only chenier ridges (i.e., transgressive), but also ridges that are genetically tied to regression (beach ridges) and lateral accretion (recurved spits).A six-stage geomorphic process-response model was developed to describe Chenier-Plain evolution primarily as a function of: (i) the balance between sediment supply and energy dissipation associated with Mississippi River channel avulsions, (ii) local sediment reworking and lateral transport, (iii) tidal-entrance dynamics, and (iv) possibly higher-than-present stands of Holocene sea level. Consequently, the geneses of three different ridge types (transgressive, regressive, and laterally accreted) typically occur contemporaneously along the same shoreline at different locations.     

An easy-to-use Model for the Optical Thickness and Ambient Illumination within Cometary Dust Comae

We define a procedure which allows estimation of the optical thickness of a cometarydust coma and the ambient illumination of the nucleus for any given comet, if estimatesof the nucleus radius and the dust activity (Afρ) are available. The calculation isperformed for a singly scattering coma with a cos(ϑ) distribution of dust overits day side. We find that the ambient illumination is of the same order as the incidentsunlight if the optical thickness is of order one. The optical thickness increases, all elseequal, linearly with the nucleus radius. Therefore the effect of the presence of the comamay be neglected for small (≈ 1 km diameter) comets, but is important forcomets such as 1P/Halley and Hale–Bopp.     

Measurement requirements for a Near-Earth Asteroid impact mitigation demonstration mission

A concept for an Impact Mitigation Preparation Mission, called Don Quijote, is to send two spacecrafts to a Near-Earth Asteroid (NEA): an Orbiter and an Impactor. The Impactor collides with the asteroid while the Orbiter measures the resulting change in the asteroid's orbit, by means of a Radio Science Experiment (RSE) carried out before and after the impact. Three parallel Phase A studies on Don Quijote were carried out for the European Space Agency: the research presented here reflects the outcomes of the study by QinetiQ. We discuss the mission objectives with regard to the prioritisation of payload instruments, with emphasis on the interpretation of the impact. The Radio Science Experiment is described and it is examined how solar radiation pressure may increase the uncertainty in measuring the orbit of the target asteroid. It is determined that to measure the change in orbit accurately a thermal IR spectrometer is mandatory, to measure the Yarkovsky effect. The advantages of having a laser altimeter are discussed. The advantages of a dedicated wide-angle impact camera are discussed and the field-of-view is initially sized through a simple model of the impact.     

Projected changes in reported campylobacteriosis and cryptosporidiosis rates as a function of climate change: a New Zealand study

Two pathogens whose reported incidence rates may alter under climate change and variability were selected for study: the bacterium Campylobacter and the protozoan oocyst Cryptosporidium. Both are of particular importance in New Zealand, given its extensive and intensive agricultural farming systems, and therefore to other agriculturally-based economies. Local and international studies have indicated that rates of illnesses associated with these pathogens (campylobacteriosis and cryptosporidiosis) may increase as temperature rises and as rainfall becomes more intense. An existing calibrated linear SIR (Susceptible-Ill-Recovered) model was used to make predictions of the proportional change in the reported rates of these two zoonoses. This method uses analytical solutions of the SIR model and a simple exponential approach to describe the temporal changes in pathogen contact rates—and hence of reported disease rates. These changes reflect climate change impacts only and do not consider adaptation or mitigation measures. Projections cannot be made of the actual-but-unknown-illness rates because of under-reporting throughout the country. The SIR model outputs provide projected changes in reported disease incidence as a function of temperature and rainfall for the years 2015, 2040 and 2090. These are calculated for three climate change scenarios: low (B1), medium (A1B) and high (A2) emissions of greenhouse gases and for four seasons. Projections show the potential for substantial changes in reported rates by the year 2090 across New Zealand, with children most at-risk. Maximum increases in reported illness rates tend to occur in summer when pathogen contact rates are greatest. Average annual rates of increase of reported campylobacteriosis are predicted to rise by as much as 20 % and by 36 % for cryptosporidiosis (children, A2 scenario, 2090). To our knowledge, this is the first time that SIR modelling has been coupled with climate change projections.     

Vertical distributions of chlorophyll in deep, warm monomictic lakes

The factors affecting vertical distributions of chlorophyll fluorescence were examined in four temperate, warm monomictic lakes. Each of the lakes (maximum depth >80 m) was sampled over 2 years at intervals from monthly to seasonal. Profiles were taken of chlorophyll fluorescence (as a proxy for algal biomass), temperature and irradiance, as well as integrated samples from the surface mixed layer for chlorophyll a (chl a) and nutrient concentrations in each lake. Depth profiles of chlorophyll fluorescence were also made along transects of the longest axis of each lake. Chlorophyll fluorescence maxima occurred at depths closely correlated with euphotic depth (r ² = 0.67, P < 0.01), which varied with nutrient status of the lakes. While seasonal thermal density stratification is a prerequisite for the existence of a deep chlorophyll maximum (DCM), our study provides evidence that the depth of light penetration largely dictates the DCM depth during stratification. Reduction in water clarity through eutrophication can cause a shift in phytoplankton distributions from a DCM in spring or summer to a surface chlorophyll maximum within the surface mixed layer when the depth of the euphotic zone (z _eu) is consistently shallower than the depth of the surface mixed layer (z _SML). Trophic status has a key role in determining vertical distributions of chlorophyll in the four lakes, but does not appear to disrupt the annual cycle of maximum chlorophyll in winter.     

Optical and infrared observations of the Centaur 1997 CU26

Minor planet 1997 CU₂₆ is a Centaur, and is probably undergoing dynamical evolution inwards from the Kuiper Belt. We present optical and infrared ( VRIJHK ) photometry which gives mean colours of V − R =0.46±0.02, V − I =1.02±0.02, V − J =1.74±0.02, V − H =2.15±0.02 and V − K =2.25±0.02. The resulting relative reflectance spectrum lies between those of Chiron and Pholus (although closer to that of Chiron). A 1.6–2.6 μm spectrum confirms the broad absorption feature at 2.05 μm associated with water ice reported by Brown et al. 1997 CU₂₆ displays no significant light curve variation and (unlike Chiron) has no observable coma. We place an upper limit to the dust production rate of 1.5 kg s⁻¹. J -band data taken at phase angles of 1.°7 to 4.°0 give a phase parameter of G _J =0.36±0.1, and are consistent with a phase parameter of G =0.15 in the V band (a value often assigned to low-albedo objects when no other information is available) if we assume a phase reddening of 0.017 mag deg⁻¹ in the J band. We find V (1, α =4.°1) =7.022±0.02, from which we deduce, by assuming G =0.15±0.1, an absolute visual magnitude of H _V =6.64±0.04.     

Origin and development of tafoni in Tunnel Spring Tuff,Crystal Peak,Utah, USA

Bedding‐parallel tafoni are well developed over much of the surface of the Tunnel Spring Tuff (Oligocene) exposed in 300‐m‐high Crystal Peak, an inselberg. The Tunnel Spring Tuff is a crudely stratified, non‐welded rhyolite ash‐flow tuff with > 30 per cent porosity. Clasts of Palaeozoic dolomite, limestone and quartzite make up 10 per cent of the tuff. The tafoni are remarkable because of their size (up to 20 m wide but rarely wider than 4 m), shape of the openings (spherical, arch‐like or crescent‐shaped) and abundance (up to 50 per cent of an outcrop face). They are actively forming today. Calcite is abundant (10 to 40 per cent by weight) in tafoni as an efflorescence in spalling flakes of tuff on their roofs and walls. Halite and gypsum generally make up less than 0·01 per cent of the efflorescence. The absence of corroded quartz and feldspar grains in spall fragments indicates that chemical weathering is unimportant in development of the tafoni. Calcite, aragonite, halite and gypsum dust from modern salt pans less than 20 km from Crystal Peak are potential sources of salt in the tuff, but the prevailing winds are in the wrong direction for significant amounts of these evaporite minerals to reach the inselberg. Calcite is the only evaporite mineral present in the tafoni in more than trace amounts, and this mineral is readily available within the tuff itself as a result of rock weathering. We propose that meteoric water containing carbonic acid infiltrates the tuff, dissolves carbonate clasts, and migrates to the steep flanks (>20°) of the peak through abundant megapores and micropores. There it evaporates and precipitates calcite. Crystallization pressure spalls off grains and sheets as the physical manifestation of salt weathering. The quasi‐uniform spacing of tafoni suggests that a self‐organization process is active in the water flow. Copyright © 2000 John Wiley & Sons, Ltd.