New results on the ages of star clusters in region B of M82

The post-starburst region B in M82 and its massive star cluster component have been the focus of multiple studies, with reports that there is a large population of coeval clusters of age ～1 Gyr, which were created with a Gaussian initial mass distribution. This is in disagreement with other studies of young star clusters, which invariably find a featureless power-law mass distribution. Here, we present Gemini-North optical spectra of seven star clusters in M82-B and show that their ages are all between 10 and 300 Myr (a factor of 3–100 younger than previous photometric results) and that their extinctions range between near zero and 4 mag (A _V ). Using new HST ACS-HRC U-band observations we age date an additional ～30 clusters whose ages/extinctions agree well with those determined from spectroscopy. Completeness tests show that the reported ‘turn-over’ in the luminosity/mass distributions is most likely an artefact, due to the resolved nature of the clusters. We also show that the radial velocities of the clusters are inconsistent with them belonging to a bound region.     

The effect of spacecraft radiation sources on electron moments from the Cassini CAPS electron spectrometer

Data from the Cassini plasma spectrometer (CAPS) electron spectrometer (ELS) have been found to be contaminated with an energy-independent background count rate which has been associated with radiation sources on Cassini. In this paper we describe this background radiation and quantitatively assess its impact on numerically integrated electron moments. The general properties of such a background and its effects on numerical moments are derived. The properties of the ELS background are described and a model for the background presented. A model to generate synthetic ELS spectra is presented and used to evaluate the density and temperature of pure noise and then extended to include ambient distributions. It is shown that the presence of noise produces a saturation of the electron density and temperature at quasi-constant values when the instrument is at background, but that these noise level moments are dependent on the floating spacecraft potential and the orientation of the ELS instrument with respect to the spacecraft. When the ambient distribution has a poor signal-to-noise ratio (SNR) the noise determines the density and temperature; however, as the SNR increases (increasing primarily with density) the density and temperature tend to those of the ambient distribution. It is also shown that these noise effects produce highly artificial density-temperature inverse correlations. A method to subtract this noise is presented and shown to correct for the presence of the noise. Simulated error estimates for the density and temperature are also presented. The analysis described in this paper not only applies to weak background noise, but also to more significant penetrating backgrounds such as those in radiation belt regions of planetary magnetospheres.     

Quarter diurnal shelf resonances and tidal bed stress in the English Channel

The mean sea level and mean bed stress due to tidal co-oscillations in the presence of quadratic friction is examined analytically and numerically. In some idealised situations under conditions of near M₄ shelf resonances it is shown that phase relationships tend to exist between the M₂ tidal currents and the M₄ tidal currents which combine to give maximum currents in the flood or ebb direction. In the presence of quadratic friction these effects result in a mean bed stress and affect mean sea level. It is shown that these idealised responses are in part relevant to the sea level differences and sand transport paths due to tides around the British Isles.     

Channel bed steps and pool shapes along Soda Creek, Three Sisters Wilderness, Oregon

Field study of bedrock step–pool systems along the upper reaches of Soda Creek in the Three Sisters Wilderness of Oregon shows strong correlation between several form variables (shape) and channel slope. Although step height and step length showed no regular spacing and variable correlation with channel slope, length to height ratios demonstrated strong negative correlations: steep slopes (20% to 80%) featured greater step height and shorter pool lengths than did flatter channel slopes. Correlations between step height to length ratios and channel slope varied between three lithologies. Explained variations ranged from 0.984 for the oldest channel steps developed in basalt, to 0.982 for steps of intermediate age developed in andesite, to 0.964 for the youngest steps developed in dacite. Sample size was 57, 40, and 33, respectively. The frequency of pool shape classes did not vary by lithology, but specific shape classes developed under differing slope conditions by rock type. All pool classes have adjusted (developed) their form to maximize resistance to flow H/L/S, and they have done so in remarkably uniform fashion.     

Regional application of an index of estuarine biotic integrity based on fish communities

We applied an index of estuarine biotic integrity (EBI) to 36 sites in 16 estuaries on Cape Cod and in Buzzards Bay, Massachusetts, U.S. Two estuaries were sampled in 6 years, from 1988–1999 (Waquoit and Buttermilk Bays), and a total of 14 others in Buzzards Bay were sampled in 1993, 1996, and 1998. Habitats at each site were classified as either low or medium quality by density and biomass of submerged rooted vegetation (eelgrass). The EBI and its metrics (fish abundance, biomass, total species, species dominance, life history, and proportion by life zone) were successful in classifying habitat quality. Greatest success and least bias of the EBI and its metrics in classifying habitat quality occurred when eelgrass habitats were least degraded. The EBI tracked habitat degradation over time in Waquoit and Buttermilk Bays. Average EBI values in medium-quality habitats of Buzzards Bay estuaries during 1996 and 1998 were less than expected based on earlier EBI values from Waquoit and Buttermilk Bays, suggesting that many of these sites are in transition from medium to low quality. Our results indicate that the EBI is sensitive to habitat quality change, and further suggest that low-quality habitats may approach a stable fish community structure that is well reflected by the EBI. The relationship of the EBI to an independent measure of water quality demonstrated inherent time lags between the degradation and improvement of water quality, fish habitat, and response of the fish community.     

Development and validation of an estuarine biotic integrity index

We tested hypotheses about how estuarine fish assemblages respond to habitat degradation and then integrated these responses into an overall index, the Estuarine Biotic Integrity Index (EBI), which summarized observed changes. Fish assemblages (based on trawl catches) and habitat quality were measured monthly or biweekly at nine sites in two estuaries from March 1988 to June 1990. Submerged aquatic vegetation habitats were classified as low or medium quality based on year-round measurements of chemical and physical characteristics (phytoplankton blooms; macroalgae; dissolved oxygen; nutrients; dredged channels). We tested 15 metrics and selected 8 for inclusion in the EBI: total number of species, dominance, fish abundance (number or biomass), number of nursery species, number of estuarine spawning species, number of resident species, proportion of benthic-associated fishes, and proportion abnormal or diseased. Fish assemblages in low-quality sites had lower number of species, density, biomass, and dominance compared with medium-quality sites. Fish abundance peaked in July and August, and was lowest in January to March. The seasonal cycle in low-quality sites was damped compared with medium-quality sites. Abundances of fishes using estuaries as a spawning and nursery area and of benthic species were lower in low-quality sites compared to medium-quality sites. The individual metrics and the overall index correlated with habitat degradation. The EBI based on biomass did not do better than the EBI based on number, indicating that the extra effort to obtain biomass may not be warranted. We suggest the EBI is a useful indicator of estuarine ecosystem status because it reflects the relationship between anthropogenic alterations in estuarine ecosystems and the status of higher trophic levels.     

Fossil spreading center and faults within the Panama Fracture Zone

The north/south-trending Panama Fracture Zone forms the present eastern boundary of the Cocos Plate, with the interplate motion being right-lateral strike-slip. This fracture zone is composed of at least four linear troughs some hundreds of kilometers in length. Separate active or historic faults undoubtedly coincide with each trough. The greatest sediment fill is found in the easternmost trough. Surface and basement depths of the western trough are generally greater than those of the other three; the western trough contains the least sediment, and is most continually linear. Morphology and sediments suggest that the principal locus of strike-slip movement within the fracture zone probably migrated incrementally westward from one fault-trough to another. From north to south, the fracture zone apparently narrows from the continental intersection to approximately 5°30N, and again widens from about 5°N to at least 3°N. Residual E/W-trending magnetic anomalies are centered between two of the four troughs; sea floor spreading in a north-south direction is interpreted to have occurred between 5°30N and 7°N from 4.5 m.y. ago to 2 m.y. ago, with the symmetric center roughly coinciding with a rift valley at 6°10N, 82°30W.     

The last lavas erupted during the main phase of the Siberian flood volcanic province: results from experimental petrology

The final lavas of the Siberian flood basalts are a ∼1,000 m thick section of meimechites, high-alkali, high-titanium, hydrous lavas that contrast sharply with the tholeiites that precede them. This paper presents a phase equilibrium study indicating that a candidate primary meimechite magma with 1 wt% water originated at ∼5.5 GPa and 1,700°C, both hotter and shallower than other estimates for melting beneath continental lithosphere. The experiments also suggest that a higher volatile content was involved in meimechite source genesis. Both the absence of orthopyroxene in any experiment and the close field association with carbonatites suggest that the meimechite source region may have been metasomatized with a CO₂-rich fluid. A small additional quantity of CO₂ and water would move magma origination to ∼1,550–1,600°C.