A wavefront tilt correction servo for the Sydney University Stellar Interferometer

The tilt correction servo for the Sydney University Stellar Interferometer (SUSI) consists of a 'pyramid' detector and piezo-electrically controlled tilt mirrors. The system measures image position and re-centers it with a sample frequency of 1000 Hz thereby holding the two beams of the interferometer parallel with a standard deviation of 0.164 ± 0.025 arcseconds. With an aperture size of 0.06 metres this implies less than a 2% loss in the visibility measurements made by SUSI. The servo has been used to magnitude 6.5 stars and is predicted to have a limiting magnitude of 7.5 and possibly as high as 8.5. The system not only corrects for the tilt introduced by the atmosphere but will supply a good estimate of seeing conditions using the same optical path through the atmosphere as the visibility measurements of the interferometer.     

Inferring autogenically induced depositional discontinuities from observations on experimental deltaic shoreline trajectories

Palaeo shoreline is a commonly used proxy for palaeo sea level, but only if deposition is continuous and constant will shoreline trajectory T(l) completely capture sea‐level time‐series E(t). Artificial deltas were generated in the Eurotank flume facility under stepwise tectonic subsidence, periodic sea‐level fluctuation and two periodic water‐discharge scenarios, one in‐phase and the other out‐of‐phase with sea level. Independent input variables tectonic subsidence Y, sea level E and water discharge Q (controlling sediment supply S) were varied and dependent output variable shoreline trajectory T was monitored. These experiments confirm that deposition is discontinuous even for continuous sediment supply, and this hinders the inference of sea‐level curve from shoreline trajectory. These results justify the here‐developed methodology for converting shoreline trajectory from the space domain to the time domain, thereby improving the accuracy of the inferred sea‐level curve.     

Startle response of captive North Sea fish species to underwater tones between 0.1 and 64 kHz

World-wide, underwater background noise levels are increasing due to anthropogenic activities. Little is known about the effects of anthropogenic noise on marine fish, and information is needed to predict any negative effects. Behavioural startle response thresholds were determined for eight marine fish species, held in a large tank, to tones of 0.1-64 kHz. Response threshold levels varied per frequency within and between species. For sea bass, the 50% reaction threshold occurred for signals of 0.1-0.7 kHz, for thicklip mullet 0.4-0.7 kHz, for pout 0.1-0.25 kHz, for horse mackerel 0.1-2 kHz and for Atlantic herring 4 kHz. For cod, pollack and eel, no 50% reaction thresholds were reached. Reaction threshold levels increased from approximately 100 dB (re 1 microPa, rms) at 0.1 kHz to approximately 160 dB at 0.7 kHz. The 50% reaction thresholds did not run parallel to the hearing curves. This shows that fish species react very differently to sound, and that generalisations about the effects of sound on fish should be made with care. As well as on the spectrum and level of anthropogenic sounds, the reactions of fish probably depend on the context (e.g. location, temperature, physiological state, age, body size, and school size).     

The impact of aluminium sacrificial anodes on the marine environment: A case study

Cathodic protection using Al sacrificial anodes is a method of corrosion control that is often applied to submerged metallic structures. This technique leads to the progressive dissolution of the sacrificial anode and the metal(s) constituting the anode are inevitably released into the surrounding aquatic environment with potentially detrimental effects. The chemistry of sediments and waters from a French harbour where Al sacrificial anodes are deployed is examined in this study. In order to assess the potential influence of Al dissolution from the anode on the aquatic environment, total Al concentrations in water and sediment were determined and corresponding Al enrichment factors were calculated. Single and sequential extractions were also undertaken in order to evaluate the mobility of Al in the sediments. Results showed that whilst anodic dissolution does not significantly increase the concentration of Al in the water, both enrichment and an increase in the mobility of Al were evident in sediments sampled from the vicinity of the sacrificial anodes.     

Electrical conductivity of synthetic iron-bearing olivine

The electrical conduction in synthetic, dry polycrystalline, iron-bearing olivine (Fo₉₀) was investigated as a first-order approach to the electrical conductivity in the upper mantle. This fundamental study is of great importance to better understand the charge-transport mechanisms seen in olivine. Conduction processes in synthetic samples are not influenced by a complex geological history in contrast to conductivity in natural olivine. The experiments show that the apparent activation energy for conductivity for Fo₉₀ is 230 kJ mol⁻¹. In currently accepted defect modeling, natural and synthetic olivine requires a mechanism involving small polaron formation (Fe^· _Mg and magnesium vacancies (V _Mg) as the dominant diffusing species to explain a fO₂^1/6 relation to electrical conduction. Here, Fo₉₀ shows no contribution of small polarons to conductivity at temperatures between 1,000 and 1,200°C and almost no dependence on fO₂. Instead, under reducing conditions magnesium vacancies (and electrons) appear to be the major charge carriers.     

The effects of Martian near surface conditions on the photochemistry of amino acids

In order to understand the complex multi-parameter system of destruction of organic material on the surface of Mars, step-by-step laboratory simulations of processes occurring on the surface of Mars are necessary. This paper describes the measured effects of two parameters, a CO₂ atmosphere and low temperature, on the destruction rate of amino acids when irradiated with Mars-like ultraviolet light (UV). The results show that the presence of a 7 mbar CO₂ atmosphere does not affect the destruction rate of glycine, and that cooling the sample to 210 K (average Mars temperature) lowers the destruction rate by a factor of 7. The decrease in the destruction rate of glycine by cooling the sample is thought to be predominantly caused by the slower reaction kinetics. When these results are scaled to Martian lighting conditions, cold thin films of glycine are assumed to have half-lives of 250 h under noontime peak illumination. It has been hypothesised that the absence of detectable native organic material in the Martian regolith points to the presence of oxidising agents. Some of these agents might form via the interaction of UV with compounds in the atmosphere. Water, although a trace component of Mars’ atmosphere, is suggested to be a significant source of oxidising species. However, gaseous CO₂ or adsorbed H₂O layers do not influence the photodestruction of amino acids significantly in the absence of reactive soil. Other mechanisms such as chemical processes in the Martian regolith need to be effective for rapid organic destruction.     

Nephelometer derived and directly measured aerosol optical depth of the atmospheric boundary layer

The aerosol optical depth of the atmospheric boundary layer was determined both from direct solar irradiance measurements and from vertical extrapolation of ground-based nephelometry, during a period with cloudless skies and high aerosol mass loadings in the Netherlands. The vertical profile of the aerosol was obtained from lidar measurements. From humidity controlled nephelometry at the ground and humidity profiles from soundings, the scattering aerosol extinction as a function of height was assessed. Integration of the extinction over the aerosol layer gave the aerosol optical depth of the atmospheric boundary layer. This optical depth at the narrow band of the nephelometer was translated to a spectrally integrated value, assuming an Angstrom wavelength exponent of 1.5, a typical value for The Netherlands.It was found that scattering by the boundary layer aerosol contributed on average 80% to the total atmospheric aerosol optical depth. The uncertainty in this value is estimated to be of the order of 13%. Ammonium nitrate dominated the light scattering. This is an anthropogenic aerosol component.The radiative forcing caused by the light scattering of the anthropogenic aerosol was calculated assuming an upward scattered fraction of 0.3. An average value of − 12 W m ⁻² was found (with an estimated uncertainty of 20%). This corresponds to an absolute increase in the planetary albedo of 0.03, which is equivalent to a 15% increase in the local planetary albedo of 0.2.