High resolution OSL dating back to MIS 5e in the central Sea of Okhotsk

Marine sediments contain important archives of past ocean and climate changes, but at high latitudes the absence of carbonate has prevented the construction of accurate chronological models. We have begun a study to (1) determine the accuracy of luminescence ages in deep-sea marine sediments, e.g. by comparison with marine oxygen isotope stratigraphy where possible, (2) describe changes in sedimentation rate through time, and (3) test whether it is possible to date back to marine isotope stage 5e (MIS 5e). We show here that optical dating of fine grains of quartz from the central Sea of Okhotsk is able to provide an accurate and precise chronology for the reconstruction of the palaeoceanic and palaeoclimatic environment at our site. The upper 6.5 m of the 18.42 m long core MR0604-PC07A is believed, based on its magnetic susceptibility and the oxygen isotope (δ¹⁸O) records to contain the last ～150 ka. Forty OSL samples were taken from this upper part of the core. The single-aliquot regenerative-dose (SAR) procedure is used for equivalent dose (D_e) determination. The luminescence characteristics of fine-grained quartz (4–11 μm) extracted from the core are described. The OSL signal is dominated by the fast component and a dose recovery test shows that we can accurately measure a known dose given in the laboratory prior to any heat treatment. Dose rates were determined using high-resolution gamma spectrometry, and vary between 0.4 and 1.6 Gy/ka. The OSL ages from this section lie between ～140 ka and ～15 ka and are in very good agreement with the δ¹⁸O stratigraphy up to MIS 5e. A clear change in sedimentation rate is identified: between ～139 and 110 ka, the sedimentation rate was ～0.09 m/ka, but then from ～110 to 15 ka, the sedimentation rate decreases to a constant value of ～0.04 m/ka. Our data confirm that OSL dating using widely distributed fine-grain quartz has great potential for dating deep-sea sediments. Because luminescence methods use clastic materials, they do not depend on the presence of biogenic carbonate. As a result it is now likely that we can establish a chronology in regions of the ocean that were previously undatable.     

Space use by Rhabdosargus holubi in a southern African estuary,with emphasis on fish movements and ecosystem connectivity

Twenty-one juvenile Cape stumpnose Rhabdosargus holubi (140–190 mm fork length) were tagged with internal acoustic transmitters in the lower, middle and upper reaches of the Kowie Estuary, South Africa. The movements of each fish were continually monitored from October 2014 to February 2015 using 22 stationary data-logging acoustic receivers situated at approximately equidistant intervals along the length of the estuary (21 km). Juvenile R. holubi spent the greatest proportion of time within the estuarine environment (83%), with the sea (16%) and riverine (1%) environments used to a much lesser extent. Within the estuarine environment, tagged individuals showed high levels of residency and fidelity to their capture and release sites; however, the degree of residency was dependent on the position of the release site, with batches in the upper and lower reaches exhibiting different space-use patterns. When larger juvenile R. holubi migrated back to the sea for the next phase of their life cycle, they generally did not return to the estuarine environment, thus indicating a permanent ontogenetic shift in habitat use with the onset of sexual maturity. This contribution to our understanding of the ecology of a ubiquitous estuarine fish further highlights the importance of estuarine habitats as nursery areas that require effective management.     

A subregional-scale method to assess aquifer vulnerability to pesticides

A method to predict aquifer vulnerability to pesticide contamination at the subregional scale was developed. The assessment method was designed to incorporate relevant hydrologic and pesticide-transport information and to use generally available data. The method assumes steady-state advection of pesticides in the vadose zone, including sorption and biological decay. The solution is presented as a vulnerability index (VI) that increases as the aquifer vulnerability increases. The hydrologic input data for the VI model are obtained from the soil survey geographic database. Pesticides were grouped into three leachability classes using a leachability ratio (half-life divided by organic carbon partition coefficient). Pesticide transformation is assumed to occur in the surface layer. The influence of vertical transport in the remainder of the vadose zone has been incorporated by applying a multiplying factor to the VI that varies with depth to ground water. Hydraulic conductivity is used as a surrogate for soil-water velocity for practical purposes. The performance of the VI model is evaluated using ground water data from Weld County, Colorado. The model is demonstrated to be successful at predicting relative vulnerability, defined as the magnitude of pesticide concentration and percent of wells in a unit that exhibit a pesticide detection. Areas of low, medium, and high vulnerability are assigned. Results indicate that the vulnerability classifications are most dependent on the leachability ratio, hydraulic conductivity, and organic carbon content.     

Direct measurement and matched-field inversion approaches to arrayshape estimation

Accurate knowledge of array shape is essential for carrying out full wavefield (matched-field) processing. Direct approaches to array element localization (AEL) include both nonacoustic (tilt-heading sensors) and acoustic (high-frequency, transponder-based navigation) methods. The low-frequency signature emitted from a distant source also can be used in an inversion approach to determine array shape. The focus of this paper is on a comparison of the array shape results from these three different methods using data from a 120-m aperture vertical array deployed during SWellEx-3 (Shallow Water evaluation cell Experiment 3). Located 2 m above the shallowest array element was a self-recording package equipped with depth, tilt, and direction-of-tilt sensors, thereby permitting AEL to be performed non-acoustically. Direct AEL also was performed acoustically by making use of transponder pings (in the vicinity of 12 kHz) received by high-frequency hydrophones spaced every 7.5 m along the vertical array. In addition to these direct approaches, AEL was carried out using an inversion technique where matched-field processing was performed on a multitone (50-200 Hz), acoustic source at various ranges and azimuths from the array. As shown, the time-evolving array shape estimates generated by all three AEL methods provide a consistent picture of array motion throughout the 6-h period analyzed     

The reducing transformation and Apocentric Librators

We propose a canonical transformation reducing the averaged planar planetary problem near resonance to a one degree of freedom problem when the perturbation is truncated at the first order in the eccentricities.This reducing transformation leads to a very simple explanation of the puzzling behaviour of the Apocentric Librators, a class of asteroids identified by Franklinet al. (1975).An exploration of the phase space of the average problem with the use of the mapping technique shows that the alternation of two libration mechanism is a common feature for initial conditions near, but not inside, the deep resonance region.     

The reversal of the solar polar magnetic fields

It is a basic feature of the Babcock-Leighton model of the solar cycle that the polar field reversal is due to the diffusive decay and poleward drift of the active region fields. The flux from follower regions moves preferentially polewards in each hemisphere, where it cancels with, and then replaces, the previously existing polar fields. A number of workers have attempted to model this process by numerical solutions of the flux transport equation, which include the surface effects of supergranule diffusion, differential rotation and meridional flow, with conflicting results.Here we describe recent changes in the polar fields using synoptic magnetic data provided by the Mount Wilson Observatory, and compare them with simulations using the flux transport equation and based on the observed fields for Carrington rotation 1815. These changes include a part-reversal of the north polar field. It is shown that the evolution of the polar fields cannot be reproduced accurately by simulations of the diffusion and poleward drift of the emerging active regions at sunspot latitudes.Histograms of the distribution of the field intensities derived from the daily magnetograms obtained at the Kitt Peak Station of the National Solar Observatory provide independent evidence that flux is emerging at high latitudes and that this flux makes a contribution to the evolution of these patterns. This implies the presence of some form of sub-surface dynamo action at high latitudes.On leave from the School of Mathematics, University of Sydney.     

Viscous boundary layer between the solar wind and cometary plasmas

The interaction between the solar wind and cometary ionospheres downstream from the subsolar region is modeled in terms of viscous MHD flow theory. Calculations of the flow stremalines within the mixing region indicate that, as a result of viscous action, both the solar wind particles and the cometary material should be gradually directed towards the interior of the plasma wake to reinforce the formation of a type 1 tail. This behavior supports the notion that a transverse force acting on cometary plasma particles is actually responsible for the collapse of tail ray structures as suggested by Öpik (1964), Wurm (1968, 1975) and Wurm and Mammano (1972).     

The dynamics of tadpole and horseshoe orbits: II. The coorbital satellites of saturn

The recently discovered coorbital satellites of Saturn, 1980S1 and 1980S3, are shown to be librating in horseshoe orbits. By considering the effects of tangential forces on the semimajor axes of the satellite orbits, we derive an accurate relation between the sum of the satellite masses and (a) their minimum angular separation, (b) the variation of their angular separation with time and (c) the libration period. Observations of (b) and (c) are the most practical methods of determining the satellite masses. The orbits of the coorbital satellites of Dione and Tethys are discussed. We demonstrate the possibility of calculating a new value for the mass of Dione and we show that one of the coorbital satellites of Tethys could be moving in a horseshoe orbit even though another satellite is librating in a tadpole orbit about the leading Lagrangian equilibrium point L₄. The origin of coorbital satellites and the stability of their orbits are discussed.