Phase Structure of Radar Stratigraphic Horizons Within Antarctic Firn

We have recorded reflection profiles of firn through large areas of West Antarctica and part of the East Antarctic plateau using 400 MHz short-pulse radar. The locations show accumulation rates that vary from well above to well below the vertical radar resolution. Most reflection horizons have extensive lateral continuity, and are composed of distinctive wavelets with a consistent phase polarity sequence within their successive half-cycles. We modeled these waveforms, and conclude that they arise from thin, double layers of ice over hoar, which is consistent with the standard model of firn stratification. In addition, we conclude that ice/hoar layers are extensive throughout West Antarctica and also present (although more sparsely) beneath the Antarctic Plateau.

Stratigraphic Variation Within Polar Firn Caused by Differential Accumulation and Ice Flow: Interpretation of a 400 Mhz Short-Pulse Radar Profile from West Antarctica

We investigate causes of the stratigraphic variation revealed in a 177 km, 400 MHz short-pulse radar profile of firn from West Antarctica. The profile covers 56 m depth, and its direction was close to those of the ice flow and mean wind. The average, near-surface accumulation rates calculated from the time delays of one radar horizon consistently show minima on leeward slopes and maxima on windward slopes, confirming an earlier study based on stake observations. The stratigraphic variation includes up to 30 m depth variation in individual horizons over tens of km, fold limbs that become progressively steeper with depth, and fold-hinge loci that change direction or propagate down-ice with depth over distances far less than predicted by the ice speeds. We use an accumulation rate model to show how local rate anomalies and the effect of ice speed upon a periodic variation in accumulation rate cause these phenomena, and we reproduce two key features seen in the stratigraphic variations. We conclude that the model provides an explanation of changes in spatial stratigraphy and local measures of accumulation history given the constraints of surface topography, ice and wind velocities, and a general accumulation rate for an area.

In Situ Monitoring of Free-Phase Gas Accumulation and Release in Peatlands Using Ground Penetrating Radar (GPR)

We tested a set of surface common mid-point (CMP) ground penetrating radar (GPR) surveys combined with elevation rods ( to monitor surface deformation) and gas flux measurements to investigate in-situ biogenic gas dynamics and ebullition events in a northern peatland ( raised bog). The main findings are: ( 1) changes in the two-way travel time from the surface to prominent reflectors allow estimation of average gas contents and evolution of free-phase gas (FPG); ( 2) peat surface deformation and gas flux measurements are strongly consistent with GPR estimated changes in FPG content over time; ( 3) rapid decreases in atmospheric pressure are associated with increased gas flux; and ( 4) single ebullition events can induce releases of methane much larger ( up to 192 g/m(2)) than fluxes reported by others. These results indicate that GPR is a useful tool for assessing the spatial distribution, temporal variation, and volume of biogenic gas deposits in peatlands.

Stratigraphic Continuity in 400 Mhz Short-Pulse Radar Profiles of Firn in West Antarctica

We track dated firn horizons within 400 MHz short-pulse radar profiles to find the continuous extent over which they can be used as historical benchmarks to study past accumulation rates in West Antarctica. The 30-40 cm pulse resolution compares with the accumulation rates of most areas. We tracked a particular set that varied from 30 to 90 m in depth over a distance of 600 km. The main limitations to continuity are fading at depth, pinching associated with accumulation rate differences within hills and valleys, and artificial fading caused by stacking along dips. The latter two may be overcome through multi-kilometer distances by matching the relative amplitude and spacing of several close horizons, along with their pulse forms and phases. Modeling of reflections from thin layers suggests that the - 37 to - 50 dB range of reflectivity and the pulse waveforms we observed are caused by the numerous thin ice layers observed in core stratigraphy. Constructive interference between reflections from these close, high-density layers can explain the maintenance of reflective strength throughout the depth of the firn despite the effects of compaction. The continuity suggests that these layers formed throughout West Antarctica and possibly into East Antarctica as well.

Modeling Englacial Radar Attenuation at Siple Dome, West Antarctica, Using Ice Chemistry and Temperature Data

The radar reflectivity of an ice-sheet bed is a primary measurement for discriminating between thawed and frozen beds. Uncertainty in englacial radar attenuation and its spatial variation introduces corresponding uncertainty in estimates of basal reflectivity. Radar attenuation is proportional to ice conductivity, which depends on the concentrations of acid and sea-salt chloride and the temperature of the ice. We synthesize published conductivity measurements to specify an ice-conductivity model and find that some of the dielectric properties of ice at radar frequencies are not yet well constrained. Using depth profiles of ice-core chemistry and borehole temperature and an average of the experimental values for the dielectric properties, we calculate an attenuation rate profile for Siple Dome, West Antarctica. The depth-averaged modeled attenuation rate at Siple Dome (20.0 +/- 5.7 dB km(-1)) is somewhat lower than the value derived from radar profiles (25.3 +/- 1.1 dB km(-1)). Pending more experimental data on the dielectric properties of ice, we can match the modeled and radar-derived attenuation rates by an adjustment to the value for the pure ice conductivity that is within the range of reported values. Alternatively, using the pure ice dielectric properties derived from the most extensive single data set, the modeled depth-averaged attenuation rate is 24.0 +/- 2.2 dB km(-1). This work shows how to calculate englacial radar attenuation using ice chemistry and temperature data and establishes a basis for mapping spatial variations in radar attenuation across an ice sheet.

Spatial Variability in Biogenic Gas Accumulations in Peat Soils Is Revealed By Ground Penetrating Radar (GPR)

We performed surface and borehole ground penetrating radar (GPR) tests, together with moisture probe measurements and direct gas sampling to detect areas of biogenic gas accumulation in a northern peatland. The main findings are: (1) shadow zones (signal scattering) observed in surface GPR correlate with areas of elevated CH4 and CO2 concentration; (2) high velocities in zero offset profiles and lower water content inferred from moisture probes correlate with surface GPR shadow zones; (3) zero offset profiles depict depth variable gas accumulation from 0-10% by volume; (4) strong reflectors may represent confining layers restricting upward gas migration. Our results have implications for defining the spatial distribution, volume and movement of biogenic gas in peatlands at multiple scales.