Nature versus nurture: the curved spine of the galaxy cluster X-ray luminosity–temperature relation

The physical processes that define the spine of the galaxy cluster X-ray luminosity–temperature ( L – T ) relation are investigated using a large hydrodynamical simulation of the universe. This simulation models the same volume and phases as the millennium simulation and has a linear extent of  500  h ⁻¹ Mpc  . We demonstrate that mergers typically boost a cluster along but also slightly below the L – T relation. Due to this boost, we expect that all of the very brightest clusters will be near the peak of a merger. Objects from near the top of the L – T relation tend to have assembled much of their mass earlier than an average halo of similar final mass. Conversely, objects from the bottom of the relation are often experiencing an ongoing or recent merger.     

Enhanced Acoustic Backscatter Due to High Abundance of Sand Dollars,Dendraster excentricus

Abstract

Detailed acoustic surveys of benthic sediments were conducted in July 1995 and September 1998 in the vicinity of Humboldt Bay, California. During these surveys, a band of enhanced acoustic backscatter was observed offshore from the bay entrance, approximately parallel to the isobaths, in water depths ranging from 16–24 m. In order to assess the cause of the increase in backscatter levels, a more comprehensive study was conducted in August and September 1999 using 100 kHz side-scan sonar, bottom grab sampling and underwater video recording. New observations indicated that a dense population of sand dollars (Dendraster excentricus) coincided with the enhanced backscatter band. Compared to the two previous acoustic studies, the central section of the band expanded westward by 180 m and the southern section of the band shifted eastward by 160 m, possibly resulting from a change in the biological or physical factors which influence the location and breadth of sand dollars.

The relationship between high sand dollar abundance and enhanced acoustic backscatter was further verified in the near shore region off Samoa Beach California, where a dense, banded population of sand dollars was previously observed. Video footage confirmed the presence of a band of sand dollars, also nominally parallel to the isobaths, in water depths of 8–15 m. A band of enhanced backscatter coincided with the dense sand dollar population. The identification of dense aggregations of sand dollars through enhanced acoustic backscatter could lead to the use of acoustic techniques to study sand dollar distributions and abundance.     

Artificial neural network assisted Bayesian calibration of climate models

We demonstrate and validate a Bayesian approach to model calibration applicable to computationally expensive General Circulation Models (GCMs) that includes a posterior estimate of the intrinsic structural error of the model. Bayesian artificial neural networks (BANNs) are trained with output from a GCM and used as emulators of the full model to allow a computationally efficient Markov Chain Monte Carlo (MCMC) sampling of the posterior for the GCM parameters calibrated against seasonal climatologies of temperature, pressure, and humidity. We validate the methodology by calibrating to targets produced by a model run with added noise. We then demonstrate a calibration of five GCM parameters against an observational data set. The approach accounts for both parametric and structural uncertainties of the model as well as uncertainties associated with the observational calibration data. This enables the generation of statistically rigorous probabilistic forecasts for future climate states. All calibration experiments are performed with emulators trained using a maximum of one hundred model runs, in accord with typical resource restrictions imposed by computationally expensive models. We conclude by summarizing remaining issues to address in order to create a complete and validated operational methodology for objective calibration of computationally expensive models.     

Freshwater inflow and estuarine variability in the Gamtoos Estuary, South Africa

The Gamtoos is a shallow flood-tidal estuary located on the south coast of South Africa. Even though it has an extensive catchment area, dams limit runoff and mean freshwater inflow is estimated at less than 1 m³ s^?1, and the flood tidal deltas constrict and at times even close the mouth. The results presented here derive from an intensive measurement program carried out over a 3-wk period at the end of 1992, immediately after good rains in the Gamtoos catchment region. Freshwater inflow increased to more than 10 m³ s^?1, driving the salt wedge downstream and resulting in intense haloclines in the mid-estuary region. The program monitored the return to more average estuarine structures, and even though tidal exchange was restricted, marked differences occurred in stratification at neap and spring tides; tidal exchanges provided the dominant mixing forces. It is found that the shallower upper reaches of the estuary are flushed with relatively small increases in freshwater inflow, though a balance exists with the tidal exchanges through the constricted mouth. The variation in the position of the salt wedge and in the salinity stratification can have substantial implications for biota.     

Late Quaternary evolution of the Madeira Abyssal Plain,Canary Basin,NE Atlantic

The deepest part of the Canary Basin, the Madeira Abyssal Plain, receives allochthonous sediments derived from a large drainage basin which, if its subaerial continuation is included, covers an area of 3.36 times 10⁶ km². An international research effort over the last 10 years has recovered over 160 sediment cores from the plain, and the development of a high-resolution stratigraphy has enabled individual turbidites to be correlated layer by layer. Sedimentation on the Madeira Abyssal Plain during the late Quaternary is dominated by thick turbidite muds separated by thin pelagic intervals. The core density has allowed the mapping of each sedimentary unit throughout the abyssal plain, thus building up a layer by layer picture of sediment accumulation. Over the last 300 kyr, 600 km³ of turbidites compared to 60 km³ of pelagic sediments have been deposited on the plain. Sedimentary structures developed in the coarse basal facies of the larger turbidites are more complex than simple models predict due to surging flows, fluctuating flow velocities and reflection from adjacent high ground. Over the last 300 kyr, there has been a switching of entry points for turbidity currents entering the abyssal plain. From 300 ka to 200 ka, organic-rich turbidites were emplaced predominantly from the south but around 200 ka this source switched off and subsequent organic- and volcanic-rich turbidites, which included units deposited by giant, possibly hyperconcentrated flows, were emplaced from northern or eastern sources. Although restricted to the late Quaternary, the data presented provides a detailed case study of the evolution of an oceanic basin fill.     

Origin of magnetization in lunar breccias: An example of thermal overprinting

Twenty six samples from seven hand specimens, collected from the station 6 boulder at the Apollo 17 landing site, were studied magnetically. The boulder is a breccia consisting of three lithologic units distinguished by their clast population. The direction of magnetization of samples from unit B which is almost devoid of large clasts cluster fairly well after alternating field demagnetization. Samples from unit C which is characterized by abundant large clasts up to 1 m in size do not contain a uniform direction of magnetization but the distribution is not random. Based on these data we propose that the natural remanent magnetization (NRM) in these breccias is the vector sum of two magnetizations, a pre-impact magnetization and a partial thermoremanence acquired during breccia formation. The relative contribution of the two components is controlled by the thermal history of the ejecta, which in turn is determined by its clast population. Depending on the clast population, the NRM can be a total thermoremanence, a partial thermoremanence plus a pre-impact magnetization, or a pre-impact magnetization. This model of thermal overprinting might be applicable to all lunar breccias of medium and higher metamorphic grade.     

The TiO2–K2O–P2O5 diagram: A method of discriminating between oceanic and non-oceanic basalts

The TiO₂–K₂O–P₂O₅ ternary diagram is proposed as a method of discriminating between oceanic and non-oceanic (continental) basalts. This diagram is effective for non-alkaline “primitive” basalts: fractionated rocks cannot be adequately discriminated. Suitable analyses are those which have total alkalies ≤ 20% in an (Fe₂O₃ + FeO)–MgO–(Na₂O + K₂O) diagram. The proposed dividing line separates 93% of 222 ocean-floor and ocean-ridge basalts into the oceanic field and > 80% of continental basalt analyses into the non-oceanic field. Two exceptions are the Tertiary basalts of Greenland and the Deccan Traps which have oceanic affinities. “Continental” suites displaying an oceanic affinity in the TiO₂–K₂O–P₂O₅ diagram may be a result of abortive attempts to generate new sea floor. Preliminary results for dike swarms and Archean basalts suggest preponderant oceanic affinities. Alteration and metamorphism of oceanic basalts generally occasion enrichment of K₂O relative to TiO₂ and P₂O₅.     

3rd International Biodegradation Symposium

Dumping grounds in New York Bight receive very large quantities of sewage sludge. Lobsters and rock crabs collected in or near the dumping grounds sometimes show various pathological conditions of the shell and gills. In this study the histopathology of ‘shell disease’, the causative agents and its effects on respiration are discussed in connexion with a possible association with the disposal of solid wastes into the ocean.