Turbulent mixing in a stratified estuarine tidal channel: Hikapu Reach,Pelorus Sound,New Zealand

Channel constrictions within an estuary can influence overall estuary-sea exchange of salt or suspended/dissolved material. The exchange is modulated by turbulent mixing through its effect on density stratification. Here we quantify turbulent mixing in Hikapu Reach, an estuarine channel in the Marlborough Sounds, New Zealand. The focus is on a period of relatively low freshwater input but where density stratification still persists throughout the tidal cycle, although the strength of stratification and its vertical structure vary substantially. The density stratification increases through the ebb tide, and decreases through the flood tide. During the spring tides observed here, ebb tidal flow speeds reached 0.7?m?s^?1 and the buoyancy frequency squared was in the range 10^?5 to 10^?3?s^?2. Turbulence parameters were estimated using both shear microstructure and velocimeter-derived inertial dissipation which compared favourably. The rate of dissipation of turbulent kinetic energy reached 1?×?10^?6?m²?s^?3 late in the ebb tide, and estimates of the gradient Richardson number (the ratio of stability to shear) fell as low as 0.1 (i.e. unstable) although the results show that bottom-boundary driven turbulence can dominate for periods. The implication, based on scaling, is that the mixing within the channel does not homogenise the water column within a tidal cycle. Scaling, developed to characterise the tidal advection relative to the channel length, shows how riverine-driven buoyancy fluxes can pass through the tidal channel section and the stratification can remain partially intact.     

Composition of 298 Baptistina: Implications for the K/T impactor link

Abstract— Bottke et al. (2007) suggested that the breakup of the Baptistina asteroid family (BAF) 160⁺³⁰/_‐20 Myr ago produced an “asteroid shower” that increased by a factor of 2–3 the impact flux of kilometer‐sized and larger asteroids striking the Earth over the last ?120 Myr. This result led them to propose that the impactor that produced the Cretaceous/Tertiary (K/T) mass extinction event 65 Myr ago also may have come from the BAF. This putative link was based both on collisional/dynamical modeling work and on physical evidence. For the latter, the available broadband color and spectroscopic data on BAF members indicate many are likely to be dark, low albedo asteroids. This is consistent with the carbonaceous chondrite‐like nature of a 65 Myr old fossil meteorite (Kyte 1998) and with chromium from K/T boundary sediments with an isotopic signature similar to that from CM2 carbonaceous chondrites. To test elements of this scenario, we obtained near‐IR and thermal IR spectroscopic data of asteroid 298 Baptistina using the NASA IRTF in order to determine surface mineralogy and estimate its albedo. We found that the asteroid has moderately strong absorption features due to the presence of olivine and pyroxene, and a moderately high albedo (?20%). These combined properties strongly suggest that the asteroid is more like an S‐type rather than Xc‐type (Mothé‐Diniz et al. 2005). This weakens the case for 298 Baptistina being a CM2 carbonaceous chondrite and its link to the K/T impactor. We also observed several bright (V Mag. ≤16.8) BAF members to determine their composition.     

Accessory phases in aubrites: spectral properties and implications for asteroid 44 Nysa

The reflectance spectra of meteoritic metal, meteoritic troilite and the CR carbonaceous chondrite EET87770 have been measured in order to investigate the causes of the spectral differences between the surface of the E-class asteroid 44 Nysa and the opaque free fraction of the Happy Canyon aubrite meteorite. The data indicate that the spectral differences require the presence on Nysa's surface of a small amount of a spectrally red sloped material, of which metal and troilite are the most reasonable candidates, and a material possessing absorption bands near 0.9m and 1.8m. A material similar to the carbonaceous chondrite inclusions found in some aubrites can provide a match to the 0.9m feature and perhaps the 1.8m feature. The required abundances of these components depends on whether they are areally distributed or intimately mixed with an enstatite rich material. Based on the petrologic associations seen in aubrites and a series of simulated mineral mixtures, an intimate mixture of 69–92% enstatite and 1–11% metal + troilite and an areal component of 7–20% carbonaceous chondrite type material can provide a reasonable match to the 0.3–2.6m spectrum of Nysa.     

Seasonal variations of the clear-sky greenhouse effect: the role of changes in atmospheric temperatures and humidities

We present an analysis of the factors which control the seasonal variations of the clear-sky greenhouse effect, based on satellite observations and radiative transfer simulations. The satellite observations include the radiation budget at the top of the atmosphere from the Earth Radiation Budget Experiment and the total column moisture content derived from the Special Sensor Microwave/Imager. The simulations were performed with the SAMSON system described in an earlier paper, using atmospheric temperatures and humidities from operational analyses produced by the European Centre for Medium Range Weather Forecasts. At low latitudes, the magnitude of the clear-sky greenhouse effect is dominated by the strong thermodynamic link between the total column moisture content of the atmosphere and sea surface temperatures, with minimal seasonal variations. In contrast, at middle to high latitudes there are strong seasonal variations, the clear-sky greenhouse effect being largest in winter and smallest in summer. These variations cannot be explained by the seasonal cycle in the total column moisture content, as this is largest in summer and smallest in winter. The variations are controlled instead by the seasonal changes in atmospheric temperatures. The colder atmosphere in winter enhances the temperature differential between the atmosphere and the sea surface, leading to a larger greenhouse effect despite the lower moisture contents. The magnitude of the clear-sky greenhouse effect is thus controlled by atmospheric humidity at low latitudes, but by atmospheric temperature at middle and high latitudes. These controls are illustrated by results from sensitivity experiments with SAMSON and are interpreted in terms of a simple model.     

Spectrophotometry (0.33 to 1.07 μm) of 433 Eros and compositional implications

The spectral reflectance (0.33–1.07 μm) for the asteroid 433 Eros was determined as a function of rotational phase during January 28–30, and February 15, 1975. Interpretation of absorption features suggests Eros is composed of an undifferentiated assemblange of moderate to high temperature minerals (iron, pyroxene, and olivine, but no carbon). H-type ordinary chondrites are such assemblages, but it would be premature to conclude that Eros is like an H chondrite meteorite in composition until a better understanding is reached of possible physical differences between laboratory powders and asteroid regoliths for metal-bearing assemblages. There are no large-scale major compositional variations on the different sides of Eros.