Slant Column Measurements of O3 and NO2 During the NDSC Intercomparison of Zenith-Sky UV-Visible Spectrometers in June 1996

In June 1996, 16 UV-visible sensors from 11 institutes measured spectra of the zenith sky for more than 10 days. Spectra were analysed in real-time to determine slant column amounts of O3 and NO2. Spectra of Hg lamps and lasers were measured, and the amount of NO2 in a cell was determined by each spectrometer. Some spectra were re-analysed after obvious errors were found. Slant columns were compared in two ways: by examining regression analyses against comparison instruments over the whole range of solar zenith angles; and by taking fractional differences from a comparison instrument at solar zenith angles between 85° and 91°. Regression identified which pairs of instruments were most consistent, and so which could be used as universal comparison instruments. For O3, regression slopes for the whole campaign agreed within 5% for most instruments despite the use of different cross-sections and wavelength intervals, whereas similar agreement was only achieved for NO2 when the same cross-sections and wavelength intervals were used and only one half-day's data was analysed. Mean fractional differences in NO2 from a comparison instrument fall within ±7% (1-sigma) for most instruments, with standard deviations of the mean differences averaging 4.5%. Mean differences in O3 fall within ±2.5% (1- sigma) for most instruments, with standard deviations of the mean differences averaging 2%. Measurements of NO2 in the cell had similar agreement to measurements of NO2 in the atmosphere, but for some instruments measurements with cell and atmosphere relative to a comparison instrument disagreed by more than the error bars.     

Marine seismic wavefield measurement to remove sea-surface multiples

We propose a new method for removing sea-surface multiples from marine seismic reflection data in which, in essence, the reflection response of the earth, referred to a plane just above the sea-floor, is computed as the ratio of the plane-wave components of the upgoing wave and the downgoing wave. Using source measurements of the wavefield made during data acquisition, three problems associated with earlier work are solved: (i) the method accommodates source arrays, rather than point sources; (ii) the incident field is removed without simultaneously removing part of the scattered field; and (iii) the minimum-energy criterion to find a wavelet is eliminated. Pressure measurements are made in a horizontal plane in the water. The source can be a conventional array of airguns, but must have both in-line and cross-line symmetry, and its wavefield must be measured and be repeatable from shot to shot. The problem is formulated for multiple shots in a two-dimensional configuration for each receiver, and for multiple receivers in a two-dimensional configuration for each shot. The scattered field is obtained from the measurements by subtracting the incident field, known from measurements at the source. The scattered field response to a single incident plane wave at a single receiver is obtained by transforming the common-receiver gather to the frequency–wavenumber domain, and a single component of this response is obtained by Fourier transforming over all receiver coordinates. Each scattered field component is separated into an upgoing wave and a downgoing wave using the zero-pressure condition at the water-surface. The upgoing wave may then be expressed as a reflection coefficient multiplied by the incident downgoing wave plus a sum of scattered downgoing plane waves, each multiplied by the corresponding reflection coefficient. Keeping the upgoing scattered wave fixed, and using all possible incident plane waves for a given frequency, yields a set of linear simultaneous equations for the reflection coefficients which are solved for each plane wave and for each frequency. To create the shot records that would have been measured if the sea-surface had been absent, each reflection coefficient is multiplied by complex amplitude and phase factors, for source and receiver terms, before the five-dimensional Fourier transformation back to the space–time domain.     

Spatial and temporal changes in estuarine water quality during a post-flood hypoxic event

A major fish kill occurred in the Richmond River estuary in January 2008 due to oxygen depletion following extensive overbank flooding. This paper examines spatial and temporal changes in the chemistry of main channel waters, thereby identifying the primary sources of deoxygenating water. Over 40 km of the mid- to lower estuary main channel was deoxygenated within seven days of the flood peak. Hypoxia was confined to downstream of the confluences with mid-estuary backswamp basins and occurred during the later phase of the flood recession. Water chemistry at key locations in the estuary indicated elevated concentrations of redox sensitive species associated with acid sulfate soils (ASS) during the hypoxic period. Peak concentrations of Fe²⁺ up to 18.2 μmol L⁻¹, dissolved Mn up to 4.3 μmol L⁻¹, chemical oxygen demand (COD) up to 2052 μmol L⁻¹, dissolved organic carbon (DOC) up to 960 μmol L⁻¹ and elemental S⁰ up to 4.7 μmol L⁻¹ were found in the backswamp discharge confluences and mid-estuary main channel locations. The geochemical signature of main channel floodwaters identifies anaerobic decomposition of floodplain vegetation in ASS backswamps as a primary process leading to generation of hypoxic waters. The transport of these hypoxic floodwaters to the estuary has been accelerated and prolonged by extensive floodplain drainage, thereby enhancing the magnitude and duration of estuarine deoxygenation.     

The Bothnian coupled oroclines of the Svecofennian Orogen: a Palaeoproterozoic terrane wreck

The accretion of magmatic arcs gives rise to elongate, linear orogens and is a key process in forming new continental crust. Many Precambrian continents are, however, presently equidimensional or have large areas without any clear linearity, such as the central part of the Palaeoproterozoic Svecofennian Orogen (1.92–1.77 Ga). One way of forming an equidimensional continental domain is by buckling of a linear orogen about vertical axes of rotation into one or more coupled oroclines. Here, we reinterpret existing data and demonstrate the occurrence of coupled Bothnian oroclines in the Svecofennian Orogen. Palinspastic restoration of the southern and northern Bothnian oroclines brings a 1000‐km‐long segment of the Svecofennian Orogen into an originally linear, NW‐striking geometry that restores the lithological belts, metamorphic zones and structural vergences to a common direction, and which indicates that the orogen consists of a SW‐facing arc, which has been shortened along NE‐verging folds and thrust faults.     

Orbital separation amplification in fragile binaries with evolved components

The secular stellar mass loss causes an amplification of the orbital separation in fragile, common proper motion, binary systems with separations of the order of 1000 A.U. In these systems, companions evolve as two independent coeval stars as they experience negligible mutual tidal interactions or mass transfer. We present models for how post-main sequence mass-loss statistically distorts the frequency distribution of separations in fragile binaries. These models demonstrate the expected increase in orbital separation resulting from stellar mass-loss, as well as a perturbation of associated orbital parameters. Comparisons between our models and observations resulting from the Luyten survey of wide visual binaries, specifically those containing MS and white-dwarf pairs, demonstrate a good agreement between the calculated and the observed angular separation distribution functions.