Observations of tidal melt and vertical strain at the Filchner‐Ronne Ice Shelf, Antarctica

The Filchner‐Ronne Ice Shelf experiences strong tidal forcing known to displace portions of the ice shelf by several meters over a tidal cycle. These large periodic displacements may cause significant variation of the ice shelf vertical strain. Further, tidal currents in the ice shelf cavity may be responsible for basal melt variations. We deployed autonomous phase‐sensitive radio‐echo sounders at 17 locations across the ice shelf and measured basal motion and internal vertical ice motion at sufficiently short intervals to allow the resolution of all significant tidal constituents. Basal melt estimates with this surface‐based technique rely on accurate estimation of vertical strain changes in the ice shelf. We present a method that can separate the vertical strain changes from the total thickness changes at tidal time scales, yielding a tidal basal melt estimate. The method was used to identify vertical strain and basal melt variations at the predominant semi‐diurnal M2 tidal constituent. At most sites the tidal vertical strain was depth‐independent. Tidal deformation at four sites was controlled by local effects causing elastic bending. Significant tidal melt was observed to occur at six locations and upper bounds on the tidal melt amplitude were derived for the remaining sites. Finally, we show that observations of basal melt spectra, specifically at tidal frequencies and their multiples, can provide constraints on the hydrographic conditions near the ice base, such as the non‐tidal background ocean flow

The nature of ice intermittently accreted at the base of Ronne Ice Shelf, Antarctica, assessed using phase‐sensitive radar

In-situ phase-sensitive radar measurements from the Ronne Ice Shelf (RIS) reveal evidence of intermittent basal accretion periods at several sites that are melting in the long-term mean. Periods when ice is accreted at the ice-shelf base coincide with a decrease in the amplitude of the basal return of up to 4 dB. To quantify basal accretion we constrain simultaneously the dielectric constant, electrical conductivity, and thickness of the accreted ice. We do this by exploring the sensitivity of the received basal echo strength and phase to different transmit frequencies using the radar data in combination with a simple model. Along the western RIS we detect episodic basal accretion events leading to ice accumulation at a rate equivalent to 1-3 mm of meteoric ice per day. The inferred accumulation rates and electromagnetic properties of the accreted ice imply that these events are caused primarily by the deposition of frazil ice crystals. Our findings offer the possibility of monitoring and studying the evolution of boundaries between ice-shelf basal melting and accretion regimes using remote observations, collected from the ice-shelf surface

Strong tidal variations in ice flow observed across the entire Ronne Ice Shelf and adjoining ice streams

We present a compilation of GPS time series, including those for previously unpublished sites, showing that flow across the entire Ronne Ice Shelf and its adjoining ice streams is strongly affected by ocean tides. Previous observations have shown strong horizontal diurnal and semidiurnal motion of the ice shelf, and surface flow speeds of Rutford Ice Stream (RIS) are known to vary with a fortnightly (Msf) periodicity. Our new data set shows that the Msf flow modulation, first observed on RIS, is also found on Evans, Talutis, Institute, and Foundation ice streams, i.e. on all ice streams for which data are available. The amplitude of the Msf signal increases downstream of grounding lines, reaching up to 20 % of mean flow speeds where ice streams feed into the main ice shelf. Upstream of ice stream grounding lines, decay length scales are relatively uniform for all ice streams but the speed at which the Msf signal propagates upstream shows more variation. Observations and modelling of tidal variations in ice flow can help constrain crucial parameters that determine the rate and extent of potential ice mass loss from Antarctica. Given that the Msf modulation in ice flow is readily observed across the entire region at distances of up to 80 km upstream of grounding lines, but is not completely reproduced in any existing numerical model, this new data set suggests a pressing need to identify the missing processes responsible for its generation and propagation. The new GPS data set is publicly available through the UK Polar Data Centre at http://doi.org/10.5285/4fe11286-0e53-4a03-854c-a79a44d1e35

Using Multiple Sources of Knowledge to Investigate Northern Environmental Change: Regional Ecological Impacts of a Storm Surge in the Outer Mackenzie Delta, N.W.T.

Field data, remote sensing, and Inuvialuit knowledge were synthesized to document regional ecological change in the outer Mackenzie Delta and to explore the timing, causes, and implications of this phenomenon. In September 1999, a large magnitude storm surge inundated low-lying areas of the outer Mackenzie Delta. The storm was among the most intense on record and resulted in the highest water levels ever measured at the delta front. Synthesis of scientific and Inuvialuit knowledge indicates that flooding during the 1999 storm surge increased soil salinity and caused widespread vegetation death. Vegetation cover was significantly reduced in areas affected by the surge and was inversely related to soil salinity. Change detection analysis, using remotely sensed imagery bracketing the 1999 storm event, indicates severe impacts on at least 13 200 ha of terrestrial vegetation in the outer delta. Inuvialuit knowledge identifying the 1999 surge as anomalous is corroborated by geochemical profiles of permafrost and by a recently published paleo-environmental study, which indicates that storm surge impacts of this magnitude have not previously occurred during the last millennium. Almost a decade after the 1999 storm surge event, ecological recovery has been minimal. This broad-scale vegetation change is likely to have significant implications for wildlife and must be considered in regional ecosystem planning and in the assessment and monitoring of the cumulative impacts of development. Our investigations show that Inuvialuit were aware of the 1999 storm surge and the environmental impacts several years before the scientific and regulatory communities recognized their significance. This study highlights the need for multidisciplinary and locally informed approaches to identifying and understanding Arctic environmental change.La synthèse des données d’exploitation et de télédétection de même que des connaissances des Inuvialuit a été effectuée afin de répertorier les changements écologiques enregistrés dans la région extérieure du delta du Mackenzie et d’explorer la temporisation, les causes et les incidences de ce phénomène. En septembre 1999, une onde de tempête de grande magnitude a inondé les zones de faible élévation de l’extérieur du delta du Mackenzie. Il s’agit de la tempête la plus intense à n’avoir jamais été enregistrée, ce qui s’est traduit par les niveaux d’eau les plus élevés à n’avoir jamais été mesurés à la hauteur du delta. La synthèse des données scientifiques et des connaissances des Inuvialuit nous montre que l’inondation de 1999 a eu pour effet d’augmenter la salinité du sol et a entraîné la mort de la végétation à grande échelle. La couverture végétale a été réduite considérablement dans les zones visées par l’onde et était inversement reliée à la salinité du sol. L’analyse des détections de changement effectuée au moyen de l’imagerie télédétectée dans le cas de la tempête de 1999 laisse entrevoir de fortes incidences sur au moins 13 200 hectares de végétation terrestre dans l’extérieur du delta. Les connaissances des Inuvialuit, qui affirment que l’onde de 1999 était anormale, sont corroborées par les profils géochimiques du pergélisol ainsi que par une étude paléoenvironnementale qui indique que des incidences de cette ampleur découlant d’une onde de tempête ne se sont pas produites à un autre moment donné du dernier millénaire. Près d’une décennie après l’onde de tempête de 1999, le rétablissement écologique était minime. Ce changement de végétation à grande échelle aura vraisemblablement d’importantes incidences sur la faune et doit entrer en considération dans la planification de l’écosystème régional ainsi que dans l’évaluation et la surveillance des incidences cumulatives des travaux d’aménagement et de mise en valeur. Nos enquêtes nous ont permis de constater que les Inuvialuit étaient conscients des incidences environnementales de l’onde de tempête de 1999 plusieurs années avant que les scientifiques et le personnel s’occupant de la réglementation ne reconnaissent leur importance. Cette étude fait ressortir la nécessité d’avoir des méthodes multidisciplinaires et de faire appel aux gens de la région pour déterminer et comprendre les changements environnementaux dans l’Arctique

Velocity-selective sublevel resonance of atoms with an array of current-carrying wires

Resonance transitions between the Zeeman sublevels of optically-polarized Rb atoms traveling through a spatially periodic magnetic field are investigated in a radio-frequency (rf) range of sub-MHz. The atomic motion induces the resonance when the Zeeman splitting is equal to the frequency at which the moving atoms feel the magnetic field oscillating. Additional temporal oscillation of the spatially periodic field splits a motion-induced resonance peak into two by an amount of this oscillation frequency. At higher oscillation frequencies, it is more suitable to consider that the resonance is mainly driven by the temporal field oscillation, with its velocity-dependence or Doppler shift caused by the atomic motion through the periodic field. A theoretical description of motion-induced resonance is also given, with emphasis on the translational energy change associated with the internal transition.Comment: 7 pages, 3 figures, final versio

A ground-based radar for measuring vertical strain rates and time-varying basal melt rates in ice sheets and shelves

The ApRES (autonomous phase-sensitive radio-echo sounder) instrument is a robust, lightweight and relatively inexpensive radar that has been designed to allow long-term, unattended monitoring of ice-shelf and ice-sheet thinning. We describe the instrument and demonstrate its capabilities and limitations by presenting results from three trial campaigns conducted in different Antarctic settings. Two campaigns were ice sheet-based – Pine Island Glacier and Dome C – and one was conducted on the Ross Ice Shelf. The ice-shelf site demonstrates the ability of the instrument to collect a time series of basal melt rates; the two grounded ice applications show the potential to recover profiles of vertical strain rate and also demonstrate some of the limitations of the present system

A new bathymetry for the southeastern Filchner-Ronne Ice Shelf: implications for modern oceanographic processes and glacial history

The Filchner‐Ronne Ice Shelf, the ocean cavity beneath it and the Weddell Sea that bounds it, form an important part of the global climate system by modulating ice discharge from the Antarctic Ice Sheet and producing cold dense water masses that feed the global thermohaline circulation. A prerequisite for modeling the ice sheet and oceanographic processes within the cavity is an accurate knowledge of the sub‐ice‐sheet bedrock elevation, but beneath the ice shelf where airborne radar cannot penetrate, bathymetric data are sparse. This paper presents new seismic point measurements of cavity geometry from a particularly poorly sampled region south of Berkner Island that connects the Filchner and Ronne ice shelves. An updated bathymetric grid formed by combining the new data with existing datasets reveals several new features. In particular, a sill running between Berkner Island and the mainland could alter ocean circulation within the cavity and change our understanding of paleo‐ice‐stream flow in the region. Also revealed are deep troughs near the grounding lines of Foundation and Support Force ice streams, which provide access for seawater with melting potential. Running an ocean tidal model with the new bathymetry reveals large differences in tidal current velocities, both within the new gridded region and further afield, potentially affecting sub‐ice‐shelf melt rates

Updated Nucleosynthesis Constraints on Unstable Relic Particles

We revisit the upper limits on the abundance of unstable massive relic particles provided by the success of Big-Bang Nucleosynthesis calculations. We use the cosmic microwave background data to constrain the baryon-to-photon ratio, and incorporate an extensively updated compilation of cross sections into a new calculation of the network of reactions induced by electromagnetic showers that create and destroy the light elements deuterium, he3, he4, li6 and li7. We derive analytic approximations that complement and check the full numerical calculations. Considerations of the abundances of he4 and li6 exclude exceptional regions of parameter space that would otherwise have been permitted by deuterium alone. We illustrate our results by applying them to massive gravitinos. If they weigh ~100 GeV, their primordial abundance should have been below about 10^{-13} of the total entropy. This would imply an upper limit on the reheating temperature of a few times 10^7 GeV, which could be a potential difficulty for some models of inflation. We discuss possible ways of evading this problem.Comment: 40 pages LaTeX, 18 eps figure

Measurement of the xx- and Q2Q^2-Dependence of the Asymmetry A1A_1 on the Nucleon

We report results for the virtual photon asymmetry $A_1$ on the nucleon from new Jefferson Lab measurements. The experiment, which used the CEBAF Large Acceptance Spectrometer and longitudinally polarized proton ($^{15}$NH$_3$) and deuteron ($^{15}$ND$_3$) targets, collected data with a longitudinally polarized electron beam at energies between 1.6 GeV and 5.7 GeV. In the present paper, we concentrate on our results for $A_1(x,Q^2)$ and the related ratio $g_1/F_1(x,Q^2)$ in the resonance and the deep inelastic regions for our lowest and highest beam energies, covering a range in momentum transfer $Q^2$ from 0.05 to 5.0 GeV$^2$ and in final-state invariant mass $W$ up to about 3 GeV. Our data show detailed structure in the resonance region, which leads to a strong $Q^2$--dependence of $A_1(x,Q^2)$ for $W$ below 2 GeV. At higher $W$, a smooth approach to the scaling limit, established by earlier experiments, can be seen, but $A_1(x,Q^2)$ is not strictly $Q^2$--independent. We add significantly to the world data set at high $x$, up to $x = 0.6$. Our data exceed the SU(6)-symmetric quark model expectation for both the proton and the deuteron while being consistent with a negative $d$-quark polarization up to our highest $x$. This data setshould improve next-to-leading order (NLO) pQCD fits of the parton polarization distributions.Comment: 7 pages LaTeX, 5 figure