Daniel Pringle

Thermal conductivity of landfast Antarctic and Arctic sea ice

[1] We present final results from a program to measure the thermal conductivity of sea ice with in situ thermistor arrays using an amended analysis of new and previously reported ice temperatures. Results from landfast first-year (FY) ice near Barrow, Alaska, and McMurdo Sound, Antarctica, are consistent with predictions from effective-medium models but 10–15% higher than values from the parameterization currently used in most sea ice models. We observe no previously reported anomalous near-surface reduction, which is now understood to have been an artifact, nor a convective enhancement to the heat flow, although our analysis is limited to temperatures below � 5C at which brine percolation is restricted. Results for landfast multiyear (MY) ice in McMurdo Sound are also consistent with effective-medium predictions, and emphasize the density dependence. We compare these and historical measurements with effective-medium predictions and the representation commonly used in sea ice models, developed originally for MY Arctic ice. We propose an alternative expression derived from effective-medium models, appropriate for both MY and FY ice that is consistent with ��

Depth and seasonal variations in the thermal properties of Antarctic Dry Valley permafrost from temperature time series analysis

[1] We present the first dedicated study of the thermal properties of perennially frozen, ice-cemented, subsurface Dry Valley permafrost. From time series analysis of 14 months’ temperature measurements, we resolve depth and seasonal variations in the thermal properties at two nearby sites at Table Mountain with different origin, composition, and polygonal ground patterning. We determine apparent thermal diffusivity (ATD) profiles directly from thermistor array measurements at 13.5-cm-depth intervals and 4-hour time intervals in the top 2 m. We treat the system as purely conductive year round due to the cold temperatures and compare the performance of several common analysis schemes with a graphical finite difference method that we present in detail. This comparison is facilitated by one site showing strong depth variations including an abrupt twofold increase in ATD across a sharp compositional boundary. We characterize the composition of the inhomogeneous ground from recovered cores and estimate an ice-fractiondependent heat capacity in the range C = 1.7 ± 0.1 to 1.8 ± 0.1 MJ m � 3 � C � 1 . We calculate apparent thermal conductivity profiles that correlate very well with the core compositions. The conductivity generally lies in the range 2.5 ± 0.5 W m � 1 � C � 1 but is as high as 4.1 ± 0.4 W m � 1 � C � 1 for a quartose Sirius sandstone unit at one site. The seasonal variation in the ATD is consistent with its expected temperature dependence. INDEX TERMS: 3299 Mathematical Geophysics: General or miscellaneous; 5134 Physical Properties of Rocks: Thermal properties; 5194 Physical Properties of Rocks: Instruments and techniques; KEYWORDS: Dry Valleys, Antarctica, thermal properties of frozen ground, permafrost

New five-circle κ diffractometer for reference-beam diffraction studies

A compact five-circle κ-geometry diffractometer has been designed and implemented at CHESS for automated reference-beam X-ray diffraction (RBD) experiments. The details of the diffractometer design are presented, along with its geometry calculations, and its alignment and control algorithm. An outline of the overall RBD experimental procedure has been developed based on this κ diffractometer. Measured RBD interference profiles from a lysozyme crystal demonstrate that efficient triplet-phase data collection is possible using this new device in a modified crystallography oscillation setup.

Magnetovariational soundings across the South Island of New Zealand: difference induction arrows and the Southern Alps conductor

Abstract Magnetovariational (MV) data from 14 sites on a transect of the South Island of New Zealand are presented. The induction arrows clearly show the effects of induction in the surrounding oceans. Comparison with the responses of an analogue model of the effect of the oceans on induction in the New Zealand region indicates that the induction arrows also contain information on anomalous on-land conductivity structure. The use of difference induction arrows to remove the effect of induction in the oceans from the observed responses is validated using both two- and three-dimensional numerical models pertinent to the New Zealand situation. Difference induction arrows calculated by subtracting the analogue model results from the observed data show a clear reversal, in both real and quadrature arrows, across the South Island. Two-dimensional numerical modeling of the projections of the difference arrows on to the transect shows that the response is consistent with the existence beneath the Southern Alps of an anomalously conducting region previously identified by magnetotelluric (MT) sounding. Use of the difference arrow technique is seen to be valid over a large period band because of the reduction in the coast effect response range as a result of the island situation.

Solving the crystallographic phase problem with reference-beam diffraction

By using a reference-beam diffraction data-collection technique, it is possible to directly measure a large number of relative phases of Bragg reflections on an area detector in a typical protein crystallography experiment. The technique, being developed at Cornell, incorporates the principle of three-beam diffraction into the most common method of data collection, i.e., the oscillating-crystal method, and allows recordings of many phase-sensitive three-beam interference profiles simultaneously. Recent advances include a dedicated five-circle κ diffractometer and new data acquisition and analysis algorithms. Experimental results on a protein crystal are presented and the strategies of using the measured phases for solving crystal structures are discussed.

Cross-borehole resistivity tomography of Arctic and Antarctic sea ice

Abstract As an inhomogeneous mixture of pure ice, brine, air and solid salts the physical properties of sea ice depend on its highly temperature-dependent microstructure. Understanding the microstructure and the way it responds to variations in temperature and salinity is crucial in developing an improved understanding of the interaction between sea ice and the environment. However, measurements monitoring the internal structure of sea ice are difficult to obtain without disturbing its natural state. We have recently developed an application of cross-borehole d.c. resistivity tomography to make in situ measurements that resolve the anisotropic resistivity structure of first-year sea ice. We present results from measurements made in 2008 off Barrow, Alaska, and in 2009 off Ross Island, Antarctica. the sea ice in these two regions forms in different environments: at Barrow, relatively quiescent conditions typically lead to a predominance of columnar ice, while more turbulent conditions and underwater ice formation in McMurdo Sound tend to produce a larger component of frazil or platelet ice. Interpretation of the resistivity measurements carried out in association with temperature and salinity data collected simultaneously allows both observation of the temporal evolution of the ice structure and, in the case of the Antarctic measurements, the identification of different ice types.

Attracting, retaining, and engaging early career scientists

This young scientists event was organized to engage younger scientists with the International Union of Geodesy and Geophysics (IUGG) and to provide a specific forum to express their views at the General Assembly. It comprised a panel discussion chaired by Kate Heal and with three young geosciences panelists (Masaki Hayashi, University of Calgary, Canada; Kalachand Sain, National Geophysical Research Institute, Hyderabad, India; and Simona Stefanescu, National Meteorological Administration, Bucharest). The group, which had identified several topics relevant to young geoscientists, presented their views in open discussion session. Thirty IUGG conference attendees were present.

Phasing Protein Structures By Reference‐Beam X‐Ray Diffraction

Reference beam X‐ray diffraction has been developed as an alternative method for obtaining phase information from protein single crystals. It is based upon the standard oscillating‐crystal diffraction technique and allows a large number of triplet phases to be measured simultaneously on an area detector. These triplet phases can then be used to help compute the 3 dimensional electron density map i.e. the structure of the protein.

Phase-sensitive data collection in x-ray crystallography using reference-beam diffraction

We present recent theoretical and instrumentation developments in phase-sensitive x-ray crystallography using a reference- beam data collection technique. The technique is based on the principle of three-beam or multiple-beam interference, but unlike the conventional method where interference profiles are measured one at a time, the reference-beam diffraction technique integrates three-beam diffraction into the popular oscillation method of data collection, and allows parallel recording of many three-beam interference profiles on an area detector simultaneously. Complete interference profiles are measured by taking multiple oscillation exposures while stepping through the rocking curve of the reference reflection. Recent developments include a portable five-circle kappa diffractometer designed to be a part of an automated reference-beam oscillation setup, and a simplified three-beam diffraction theory using a distorted-wave approach that can be used in reference-beam data analyses to extract the reflection phases.