Carter, S. P., D. D. Blankenship, D. A. Young, and J. W. Holt, Identifying the sources and distribution of subglacial water in radar sounding data, J. Glaciology, (in press), 2009
Carter, S. P., D. D. Blankenship, D. A. Young, M. E. Peters, J. W. Holt, and M. J. Siegert, Dynamic distributed drainage implied by the flow evolution of the 1996-1998 Adventure Trench subglacial lake discharge, Earth Planet. Sci. Lett., 283, 24-37, 2009, doi:10.1016/j.epsl.2009.03.019, 
The transport of subglacial water beneath the East Antarctic Ice Sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities, and ultimately global climate. Periodic outbursts are a critical mechanism in this process. Recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km3 traveling some 260 km along the ice-bed interface of the Adventure Subglacial Trench between 1996 and 1998 (Wingham et al., 2006. Rapid discharge connects Antarctic subglacial lakes. Nature 440, 1033–1036). Using radar echo sounding data from the Adventure Subglacial Trench region in conjunction with the previously reported satellite observations, along with some basic modeling, we calculate a mass budget and infer a flow mechanism for the 1996–1998 event. The volume released from the source lake exceeded the volume received by the destination lakes by ~ 1.1 km3. This discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward. The downstream release of water from the destination lakes continued until at least 2003, several years after the 1998 cessation of surface subsidence at the source lake. By 2003 a total of 1.5 km3 or nearly 75% of the water released by the source lake had traveled downstream from the destination lakes. The temporal evolution of discharge from the outlet can be simulated with the classic ice-walled semicircular channel model, if and only if the retreat of the source lake shoreline is taken into account. Further downstream, the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable. Along these reaches mechanical lifting of the ice roof and/or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge. A distributed system is also consistent with the 3-month delay between water release at the source lake and water arrival at the destination lake. Observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system. Ultimately the presence of large subglacial lakes along the flow path of the 1996–1998 Adventure Subglacial Trench flow path delayed the arrival of water to points downstream by approximately 12 months.
Joughin, I., S. Tulacyzk, J. L. Bamber, D. D. Blankenship, J. W. Holt, T. A. Scambos, and D. G. Vaughan, Basal conditions for Pine Island and Thwaites Glaciers, West Antarctica, determined using satellite and airborne data
, J. Glaciology, 55, 245-257, 2009, doi:10.3189/002214309788608705,
We use models constrained by remotely sensed data from Pine Island and Thwaites Glaciers, West Antarctica, to infer basal properties that are difficult to observe directly. The results indicate strong basal melting in areas upstream of the grounding lines of both glaciers, where the ice flow is fast and the basal shear stress is large. Farther inland, we find that both glaciers have 'mixed' bed conditions, with extensive areas of both bedrock and weak till. In particular, there are weak areas along much of Pine Island Glacier's main trunk that could prove unstable if it retreats past the band of strong bed just above its current grounding line. In agreement with earlier studies, our forward ice-stream model shows a strong sensitivity to small perturbations in the grounding line position. These results also reveal a large sensitivity to the assumed bed (sliding or deforming) model, with non-linear sliding laws producing substantially greater dynamic response than earlier simulations that assume a linear-viscous till rheology. Finally, comparison indicates that our results using a plastic bed are compatible with the limited observational constraints and theoretical work that suggests an upper bound exists on maximum basal shear stress.
Blankenship, D. D., D. A. Young, W. B. Moore, and J. C. Moore, Radar imaging of Europa's subsurface properties and processes: The view from Earth, in EUROPA, edited by Pappalardo, R. T., W. B. McKinnon, and K. Khurana, Univ. Arizona Press, (in press), 2008
Chen, J. L., C. R. Wilson, B. D. Tapley, D. D. Blankenship, and D. A. Young, Antarctic regional ice loss rates from GRACE, Earth Planet. Sci. Lett., 266, 140-148, 2008, doi:10.1016/j.epsl.2007.10.057,
Using recent improved time-variable gravity solutions from the Gravity Recovery and Climate Experiment (GRACE), we estimate rates of Antarctic ice mass change for the period January 2003 through September 2006. Combined improvements in data and filtering techniques allow observation of ice loss in the northern Antarctic Peninsula (AP) and along the coast of the west and central Amundsen Sea Embayment (ASE) in West Antarctica. There is also evidence of ice loss along the coast near the Stancomb–Wills (STA) and Jutulstraumen (JUT) glaciers in Queen Maud Land. Apparent rates are adjusted for influences of limited spatial resolution, filtering, and estimated postglacial rebound (PGR) to obtain ice loss rates for the northern AP, coastal ASE, and STA/JUT of − 28.8 ± 7.9, − 81 ± 17, and − 16.7 ± 9.7 km3/yr, respectively. This is the first estimate for the northern AP from satellite gravity data. The ASE estimate (− 81 ± 17 km3/yr) is consistent with a previous value (− 77 ± 14 km3/yr) using an earlier GRACE data release. These results indicate significant improvement in GRACE data quality, increased spatial resolution, and applicability of GRACE data to a wider class of problems than previously possible.
Diehl, T. M., J. W. Holt, D. D. Blankenship, D. A. Young, T. A. Jordan, and F. Ferraccioli, First airborne gravity results over the Thwaites Glacier catchment, West Antarctica, Geochem., Geophys., Geosyst., 9, Q04011, 2008, doi:10.1029/2007GC001878,
Recent satellite observations of Thwaites Glacier in the Amundsen Sea Embayment, West Antarctica, have shown that the glacier is changing rapidly. The causes of its dynamic behavior are uncertain but are of concern because this glacier has the most negative mass balance of all Antarctic glaciers. To better understand Thwaites Glacier's subglacial setting, we conducted a multi-instrumented aerogeophysical survey of its catchment and present here the first gravity results. We employed a new gravimeter, and it performed well despite extreme conditions and an unusual survey design. The unleveled free-air gravity anomalies have a 2.3 mGal RMS error and a 9 km spatial resolution. Despite slightly higher than standard noise levels, the free-air anomalies correlate well with radar-derived subglacial topography. The new airborne gravity data assist in interpreting radar-identified bedrock features and are an ideal basis for future studies of subglacial geology and its control on the dynamics of Thwaites Glacier.
Filina, I. Y., D. D. Blankenship, M. Thoma, V. V. Lukin, V. N. Masolov, and M. K. Sen, New 3D bathymetry and sediment distribution in Lake Vostok: Implication for pre-glacial origin and numerical modeling of the internal processes within the lake, Earth Planet. Sci. Lett., 276, 106-114, 2008, doi:10.1016/j.epsl.2008.09.012 ,
A new distribution of water and unconsolidated sediments in subglacial Lake Vostok, East Antarctica was developed via inversion of airborne gravity data constrained by 60 seismic soundings. A model was developed for host rock with a density of 2550 kg/m3 that was inferred from prior 2D modeling. Our 3D bathymetry model of Lake Vostok corresponds better with seismic data (RMS of 125 m) than two previous models based on the same gravity dataset. The good match in both water and sediment thicknesses between the gravity model and seismic measurements confirms two major facts about Lake Vostok: (1) the lake is hosted by sedimentary rocks, and (2) the bottom of the lake is covered with a layer of unconsolidated sediments that does not exceed 300 m in the southern basin and thickens almost to 400 m in the northern basin. Our new bathymetry model suggests much shallower water thicknesses (up to twice the previous estimates) in the middle and northern parts of the lake, while the water layer is thicker in the southern basin. Numerical modeling of the internal processes in the lake reveals the relevance of our new bathymetry model to the basal mass balance. A significant decrease in transport is observed in the shallower northern basin, as well as a decrease of 33% in the turbulent kinetic energy. However, only minor differences were observed in the distribution of the calculated freezing and melting zones compared to previous models. Estimates for the sedimentation rates for six possible mechanisms were made. Possible sedimentation mechanisms are: (1) fluvial and periglacial, i.e. those that are active prior to the establishment of a large subglacial lake; (2) deposition due to overlying ice sheet, including melting out of the ice, as well as bulldozering by the overriding ice; and (3) suspended sediments from subglacial water flow including those deposited by periodical subglacial outbursts. The estimates for these mechanisms show that unconsolidated sediments of the observed thickness are most consistent with a lake that existed before glaciation.
Peters, L. E., S. Anandakrishnan, C. W. Holland, H. J. Horgan, D. D. Blankenship, and D. G. Voigt, Seismic detection of a subglacial lake near the South Pole, Antarctica, Geophys. Res. Lett., 35, L23501, 2008, doi:10.1029/2008GL035704,
Seismic reflection data are analyzed to verify radar identification of a subglaical lake near the geographic South Pole. The seismic amplitude variation with offset (AVO) technique is applied to confirm the presence of extensive free water, and seismic imaging of the subsurface constrains lake depth and deeper subglacial structure in the region. This lake is at least 4.2 km wide (and likely as much as 10 km in diameter), is up to 32 ± 10 m deep, and occupies a basin of thick sedimentary strata. These results imply that extensive water storage is occurring in the South Pole region. The proximity of this lake to the Amundsen - Scott South Pole Station makes research drilling to sample the lake and underlying sediments feasible and supportable.
Young, D. A., S. D. Kempf, D. D. Blankenship, J. W. Holt, and D. L. Morse, New airborne laser altimetry over the Thwaites glacier catchment, West Antarctica, Geochem., Geophys., Geosyst., 9, Q06006, 2008, doi:10.1029/2007GC001935,
A new airborne altimetry data set collected over Thwaites Glacier, one of Antarctica's most active ice streams, demonstrates the improvement in publicly available digital elevation models (DEMs) of the Antarctic ice sheet. The airborne altimetry comprises 35,000 line km sampled at 20 m along track. The full data set has a relative error of ±20 cm; a reference subset has an error of ±8 cm. These data are offset from ICESat observations by +20 cm. We find that a recently released ICESat DEM provides a good model of the surface of Thwaites Glacier, despite cloud cover and wide track spacings. However, the ICESat DEM's accuracy is an order of magnitude less than that of the ICESat profile data. Our airborne data will serve as an additional temporal reference for the evolution of Thwaites Glacier's surface as well as aid the construction of future high-resolution DEMs.
Bingham, R. G., M. J. Siegert, D. A. Young, and D. D. Blankenship, Organized flow from the South Pole to the Filchner-Ronne ice shelf: An assessment of balance velocities in interior East Antartica using radio echo sounding data, J. Geophys. Res., 112, F03S26, 2007, doi:10.1029/2006JF000556,
Ice flow through central Antarctica has the potential to transmit accumulation changes from deep-interior East Antarctica rapidly to the shelf, but it is poorly constrained owing to a dearth of ice-velocity observations. We use parameters derived from airborne radio echo sounding (RES) data to examine the onset, areal extent, and englacial conditions of an organized flow network (tributaries feeding an ice stream) draining from the South Pole to the Filchner-Ronne Ice Shelf. We classified RES flight tracks covering the region according to whether englacial stratigraphy was disrupted (i.e., internal layers diverged significantly from the surface and bed echoes) or undisrupted (i.e., internal layers closely parallel surface and basal topography), and we calculated subglacial roughness along basal reflectors. Where satellite-measured surface ice-flow speeds are available (covering 39% of the study region), regions of fast and tributary flow correspond with RES flight tracks that exhibit more disrupted internal layers and smoother subglacial topography than their counterparts in regions of slow flow. This suggests that disrupted internal layering and smooth subglacial topography identified from RES profiles can be treated as indicators of past or present enhanced-flow tributaries where neither satellite nor ground-based ice-flow measurements are available. We therefore use these RES-derived parameters to assess the balance-flux-modeled steady state flow regime between the South Pole and Filchner-Ronne Ice Shelf. The RES analysis confirms that an organized flow network drains a wide region around the South Pole into the Filchner-Ronne Ice Shelf. However, the spatial extent of this network, as delineated by the RES data, diverges from that predicted by currently available balance-flux models.
Carter, S. P., D. D. Blankenship, M. E. Peters, D. A. Young, J. W. Holt, and D. L. Morse, Radar-based subglacial lake classification in Antarctica, Geochem., Geophys., Geosyst., 8, Q03016, 2007, doi:10.1029/2006GC001408,
Subglacial lakes in East Antarctica can be separated into four categories specified by radar reflection properties. Definite lakes are brighter than their surroundings by at least 2 dB (relatively bright) and both are consistently reflective (specular) and have a reflection coefficient greater than −10 dB (absolutely bright). Dim lakes are relatively bright and specular but not absolutely bright, indicating nonsteady ice dynamics. Fuzzy lakes are both relatively and absolutely bright, but not specular, and may indicate saturated sediments or “swamps.” Indistinct lakes are absolutely bright and specular but no brighter than their surroundings. Lakes themselves and the different classes of lakes are not arranged randomly throughout Antarctica but are clustered around ice divides, ice stream onsets, and prominent bedrock troughs, with each cluster demonstrating a different characteristic lake classification distribution. The lake classification algorithm expands on previous studies and demonstrates a novel way to characterize ice-water interactions in East Antarctica.
Chen, J. L., C. R. Wilson, B. D. Tapley, D. D. Blankenship, and E. R. Ivins, Patagonia icefield melting observed by gravity recovery and climate experiment (GRACE), Geophys. Res. Lett., 34, L22501, 2007, doi:10.1029/2007GL031871,
Using recently released reprocessed gravity solutions from the Gravity Recovery and Climate Experiment (GRACE), we estimate the ice loss rate for the Patagonia Icefield (PIF) of South America, for the period April 2002 through December 2006. After postglacial rebound and hydrological effects are corrected, the estimated rate is −27.9 ± 11 km3/year, equivalent to an average loss of ∼−1.6 m/year ice thickness change if evenly distributed over the entire PIF area. The estimated contribution to global sea level rise is 0.078 ± 0.031 mm/year. This is an independent confirmation of relatively large melting rate estimates from earlier studies employing topographic and cartographic data.
Filina, I. Y., V. V. Lukin, V. N. Masolov, and D. D. Blankenship, Unconsolidated sediments at the bottom of Lake Vostok from seismic data, in Antarctica: A Keystone in a Changing World, Online Proc. 10th ISAES, edited by A. K. Cooper and C. R. Raymond et al., USGS Open-File-Report 2007-1047, 2007, doi:10.3133/OF2007-1047.srp031
Golynsky, A., D. D. Blankenship, M. Chiappini, D. Damaske, F. Ferraccioli, C. A. Finn, D. Golynsky, A. Gocharov, T. Ishihara, S. Ivanov, W. Jokat, H.-R. Kim, M. Konig, V. N. Masolov, Y. Nogi, M. Sand, M. Studinger, R. Von Frese, and the ADMAP Working Group, New magnetic anomaly map of East Antarctica and surrounding regions, Antarctica: A Keystone in a Changing World, Online Proc. 10th ISAES, edited by A.K. Cooper and C.R. Raymond et al., USGS Open-File-Report 2007-1047, 2007, doi:10.3133.of2007-1047.srp050
Holt, J. W., D. D. Blankenship, F. Ferraccioli, D. G. Vaughan, D. A. Young, S. D. Kempf, and T. M. Diehl, New aeromagnetic results from the Thwaites glacier catchment, West Antarctica, in Antarctica: A Keystone in a Changing World Proc. Tenth Int. Symp. Antarctic Earth Sciences, edited by A. K. Cooper, C. R. Raymond et al, USGS Open-File Rept. 2007-1047, abstract 153, 3 p., 2007
Peters, M. E., D. D. Blankenship, D. E. Smith, J. W. Holt, and S. D. Kempf, The distribution and classification of bottom crevasses from radar sounding of a large tabular iceberg, IEEE Geoscience and Remote Sensing Lett., 4, 142-146, 2007, doi:10.1109/LGRS.2006.887057,
Bottom crevasses at the base of an iceberg or ice shelf are identified in radar sounding observations from their long echo tails. In November 2001, a radar sounding survey was conducted over iceberg B15A, which calved off from the Ross Ice Shelf, Antarctica, in March 2000. Pervasive basal cracking was observed, and the distribution of bottom crevasses along the flight lines is presented. The echo tails were quantitatively analyzed using a physically based model for backscattering from bottom crevasses. The identified crevasses are classified as either major water-filled crevasses or incipient/freezing crevasses, and estimates for crevasse heights are given
Peters, M. E., D. D. Blankenship, S. P. Carter, S. D. Kempf, D. A. Young, and J. W. Holt, Along-track focusing of airborne radar sounding data from West Antarctica for improving basal reflection analysis and layer detection, IEEE Geoscience and Remote Sensing Lett., 45, 2729-2736, 2007, doi:10.1109/TGRS.2007.897416,
This paper presents focused synthetic aperture radar (SAR) processing of airborne radar sounding data acquired with the High-Capability Radar Sounder system at 60 MHz. The motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking. The processing and reflection analyses are applied to data from Kamb Ice Stream, West Antarctica. The SAR processor correlates the radar data with reference echoes from subsurface point targets. The references are 1-D responses limited by the pulse nadir footprint or 2-D responses that include echo tails. Unfocused SAR and incoherent integration are included for comparison. Echoes are accurately preserved from along-track slopes up to about 0.5deg for unfocused SAR, 3deg for 1-D correlations, and 10deg for 2-D correlations. The noise/clutter levels increase from unfocused SAR to 1-D and 2-D correlations, but additional gain compensates at the basal interface. The basal echo signal-to-noise ratio improvement is typically about 5 dB, and up to 10 dB for 2-D correlations in rough regions. The increased noise degrades the clarity of internal layers in the 2-D correlations, but detection of layers with slopes greater than 3deg is improved. Reflection coefficients are computed for basal water detection, and the results are compared for the different processing methods. There is a significant increase in the detected water from unfocused SAR to 1-D correlations, indicating that substantial basal water exists on moderately sloped interfaces. Very little additional water is detected from the 2-D correlations. The results from incoherent integration are close to the focused SAR results, but the noise/clutter levels are much greater.
Vaughan, D. G., J. W. Holt, and D. D. Blankenship, West Antarctic links to sea level estimation, Eos, Trans. Amer. Geophys. Un., 88, 485-486, 2007, doi:10.1029/2007EO460001,
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Chen, J. L., C. R. Wilson, D. D. Blankenship, and B. D. Tapley, Antarctic mass rates from GRACE, Geophys. Res. Lett., 33, L11502, 2006, doi:10.1029/2006GL026369,
We estimate mass trends over Antarctica using gravity variations observed by the Gravity Recovery and Climate Experiment (GRACE) satellite mission during its first 3.5 years (April 2002–November 2005). An image of surface mass trends is constructed from 1° × 1° pixels over the entire continent, and shows two prominent features, a region of mass loss along the coast of West Antarctica, and one of accumulation in East Antarctica. After adjusting for bias due to smoothing and to GRACE's limited spatial resolution, and removing post glacial rebound (PGR) effects, the rate in West Antarctica is −77 ± 14 km3/year, similar to a recent estimate of ice mass loss from satellite altimetry and remote sensing data. The prominent East Antarctic feature in the Enderby Land region has a rate of +80 ± 16 km3/year. Published snow/ice mass rates from remote sensing measurements indicate approximate ice mass balance in this region, suggesting that this feature is either from unquantified snow accumulation in this region or more likely due to unmodeled PGR.
Filina, I. Y., D. D. Blankenship, L. Roy, M. K. Sen, T. G. Richter, and J. W. Holt, Inversion of airborne gravity data acquired over subglacial lakes in East Antarctica, Antarctica: Contributions to Global Earth Sciences, edited by D. K. Futterer, D. Damaske, G. Kleinschmidt,H. Miller, and F. Tessensohn, Springer-Verlag, Heidelberg, Germany, 129-134, 2006
Holt, J. W., M. E. Peters, S. D. Kempf, D. L. Morse, and D. D. Blankenship, Echo source discrimination in single-pass radar sounding data from the dry valleys, Antarctica: Implications for the orbital sounding of Mars, J. Geophys. Res., 111, E06S24, 2006, doi:10.1029/2005JE002525,
The interpretation of radar sounding data from Mars where significant topographic relief occurs will require echo source discrimination to avoid the misinterpretation of surface echoes as arising from the subsurface. This can be accomplished through the identification of all radar returns from the surface in order to positively identify subsurface echoes. We have developed general techniques for this using airborne radar data from the Dry Valleys of Antarctica. These data were collected in a single pass, including Taylor Glacier, ice-covered Lake Bonney, and an ice-free area of Taylor Valley. The pulsed radar (52.5–67.5 MHz) was coherently recorded. Our echo discrimination techniques included a radar simulator using a digital elevation model (DEM) to predict the location and shape of surface echoes in the radar data. Real and simulated echo strengths were used to calculate a signal-to-clutter ratio. This was complemented by the cross-track migration of radar echoes onto the surface. These migrated echoes were superimposed on the DEM and imagery in order to correlate with surface features. Using these techniques enabled us to identify a number of echoes in the radar data as arising from the surface and to identify subsurface echoes, including a continuous reflector under the main trunk of Taylor Glacier and multiple reflectors beneath the terminus of Taylor Glacier. Surface-based radar confirms the thickness of the glacier at three crossing points. The results illustrate the importance of using complementary techniques, the usefulness of a DEM, and the limitations of single-pass radar sounding data.
Holt, J. W., T. G. Richter, S. D. Kempf, D. L. Morse, and D. D. Blankenship, Airborne gravity over Lake Vostok and adjacent highlands, East Antarctia, Geochem., Geophys., Geosyst., 7, Q11012, 2006, doi:10.1029/2005GC001177,
Lake Vostok and a 1200 km transect were the targets of aerogeophysical surveys in East Antarctica during the austral summer of 2000/2001. The measurement of gravity anomalies for geologic studies was the primary goal. A total of 24,459 line-km of data were acquired. Favorable weather, aircraft navigation, and instrument performance contributed to excellent data quality. Multiple carrier-phase GPS solutions to determine aircraft-induced accelerations were available for each flight. Raw gravity and GPS position solutions were initially filtered to compensate for hardware filtering within the gravity meter. Filtering of remaining high-frequency noise was accomplished with a spatial, moving average smoother. Due to upward continuation effects imposed by the ice cover, the theoretically estimated minimum resolvable gravity feature size for the Lake Vostok survey is 8 km, consistent with an analysis of power spectra comparing the gravity signal to noise calculated from geographically repeated lines. Comparison of gravity results with subice topography indicates that the gravity data are sensitive to real features including the existence of major crustal structures. Repeated lines and crossovers were analyzed to estimate uncertainties for the Lake Vostok data set, with both of these repeatability measures indicating relative accuracy in the 2 mGal range for the unleveled data and 1 mGal after leveling.
Holt, J. W., D. D. Blankenship, D. L. Morse, D. A. Young, M. E. Peters, S. D. Kempf, T. G. Richter, D. G. Vaughan, and H. F. J. Corr, New boundary conditions for the West Antarctic Ice Sheet: Subglacial topography of the Thwaites and Smith glacier catchments, Geophys. Res. Lett., 33, L09502, 2006, doi:10.1029/2005GL025561,
Airborne radar sounding over the Thwaites Glacier (TG) catchment and its surroundings provides the first comprehensive view of subglacial topography in this dynamic part of the West Antarctic Ice Sheet (WAIS) and reveals that TG is underlain by a single, broad basin fed by a dendritic pattern of valleys, while Smith Glacier lies within an extremely deep, narrow trench. Subglacial topography in the TG catchment slopes inland from a broad, low-relief coastal sill to the thickest ice of the WAIS and makes deep connections to both Pine Island Glacier and the Ross Sea Embayment enabling dynamic interactions across the WAIS during deglaciation. Simple isostatic rebound modeling shows that most of this landscape would be submarine after deglaciation, aside from an island chain near the present-day Ross-Amundsen ice divide. The lack of topographic confinement along TG's eastern margin implies that it may continue to widen in response to grounding line retreat.
Holt, J. W., M. E. Peters, D. L. Morse, D. D. Blankenship, L. E. Lindzey, J. L. Kavanaugh, and K. M. Kuffey, Identifying and characterizing subsurface echoes in airborne radar sounding data from a high-clutter environment in Taylor Valley, Antarctica, Proc. 11th Int. Conf. Ground Penetrating Radar, 5, 2006
Lawver, L. A., D. D. Blankenship, and L. M. Gahagan, East Antarctica: Onshore-offshore uncertainties for cryosphere evolution, Terra Antartica, 12, 177-186, 2006
Vaughan, D. G., H. F. J. Corr, F. Ferraccioli, N. Frearson, A. O'Hare, D. Mach, J. W. Holt, D. D. Blankenship, D. L. Morse, and D. A. Young, New boundary conditions for the West Antarctic ice sheet: Subglacial topography beneath Pine Island glacier, Geophys. Res. Lett., 33, L09501, 2006, doi:10.1029/2005GL025588,
Predictions about future changes in the Amundsen Sea sector of the West Antarctic ice sheet (WAIS) have been hampered by poorly known subglacial topography. Extensive airborne survey has allowed us to derive improved subglacial topography for the Pine Island Glacier basin. The trunk of this glacier lies in a narrow, 250-km long, 500-m deep sub-glacial trough, suggesting a long-lived and constrained ice stream. Two tributaries lie in similar troughs, others lie in less defined, shallower troughs. The lower basin of the glacier is surrounded by bedrock, which, after deglaciation and isostatic rebound, could rise above sea level. This feature would impede ice-sheet collapse initiated near the grounding line of this glacier, and prevent its progress into the deepest portions of WAIS. The inland-slope of the bed beneath the trunk of the glacier, however, confirms potential instability of the lower basin, containing sufficient ice to raise global sea by ∼24 cm.
Davis, M. B., and D. D. Blankenship, Geology of the Scott-Reedy Glaciers area, southern Transantarctic Mountains, Antarctica, Geol. Soc. Amer., Maps and Charts, MCH093, 2005
Peters, M. E., D. D. Blankenship, and D. L. Morse, Analysis techniques for coherent airborne radar sounding: Application to West Antarctic ice streams, J. Geophys. Res., 110, B06303, 2005, doi:10.1029/2004jb003222,
Analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment. Echoes from a general interface consist of both specularly reflected and diffusely scattered contributions. Specular reflection results from smooth uniform interfaces, whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media. This article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data. Reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of Whillans Ice Stream and Ice Stream C, West Antarctica. Additionally, reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over Ice Stream C. Analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material. Echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates. Comparison of low- and high-resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and/or scattering interface. Combining the results from these independent analyses provides classification of the subglacial environment. Classified regions include smooth seawater, smooth saturated sediments, accreted marine ice, rough bottom crevasses, mixed conditions with partial subglacial water, and dry frozen conditions.
Roy, L., M. K. Sen, D. D. Blankenship, P. L. Stoffa, and T. G. Richter, Inversion and uncertainty estimation of gravity data using simulated annealing: An application over Lake Vostok, East Antarctica, Geophysics, 70, J1-J12, 2005, doi:10.1190/1.1852777,
Interpretation of gravity data warrants uncertainty estimation because of its inherent nonuniqueness. Although the uncertainties in model parameters cannot be completely reduced, they can aid in the meaningful interpretation of results. Here we have employed a simulated annealing (SA)–based technique in the inversion of gravity data to derive multilayered earth models consisting of two and three dimensional bodies. In our approach, we assume that the density contrast is known, and we solve for the coordinates or shapes of the causative bodies, resulting in a nonlinear inverse problem. We attempt to sample the model space extensively so as to estimate several equally likely models. We then use all the models sampled by SA to construct an approximate, marginal posterior probability density function (PPD) in model space and several orders of moments. The correlation matrix clearly shows the interdependence of different model parameters and the corresponding trade-offs. Such correlation plots are used to study the effect of a priori information in reducing the uncertainty in the solutions. We also investigate the use of derivative information to obtain better depth resolution and to reduce underlying uncertainties. We applied the technique on two synthetic data sets and an airborne-gravity data set collected over Lake Vostok, East Antarctica, for which a priori constraints were derived from available seismic and radar profiles. The inversion results produced depths of the lake in the survey area along with the thickness of sediments. The resulting uncertainties are interpreted in terms of the experimental geometry and data error.
Siegert, M. J., S. P. Carter, I. Tabacco, S. Popov, and D. D. Blankenship, A revised inventory of subglacial lakes, Antarctic Sci., 17, 453-460, 2005, doi:10.1017/S0954102005002889,
The locations and details of 145 Antarctic subglacial lakes are presented. The inventory is based on a former catalogue of lake-type features, which has been subsequently reanalysed, and on the results from three additional datasets. The first is from Italian radio-echo sounding (RES) of the Dome C region of East Antarctica, from which 14 new lakes are identified. These data also show that, in a number of occasions, multiple lake-type reflectors thought previously to be individual lakes are in fact reflections from the same relatively large lake. This reduces the former total of lake-type reflectors by six, but also adds a significant level of information to these particular lakes. The second dataset is from a Russian survey of the Dome A and Dome F regions of East Antarctica, which provides evidence of 18 new lakes and extends the coverage of the inventory considerably. The third dataset comprises three airborne RES surveys undertaken by the US in East Antarctica over the last five years, from which forty three new lakes have been identified. Reference to information on Lake Vostok, from Italian and US surveys taken in the last few years, is now included.
Behrendt, J. C., D. D. Blankenship, D. L. Morse, and R. E. Bell, Shallow-source aeromagnetic anomalies observed over the West Antarctic Ice Sheet compared with coincident bed topography from radar ice sounding: New evidence for glacial 'removal' of subglacially erupted late Cenozoic rift-related volcanic edifices, Global and Planetary Change, 42, 177-193, 2004, doi:10.1016/j.gloplacha.2003.10.006,
Aeromagnetic and radar ice sounding results from the 1991–1997 Central West Antarctica (CWA) aerogeophysical survey over part of the West Antarctic Ice Sheet (WAIS) and subglacial area of the volcanically active West Antarctic rift system have enabled detailed examination of specific anomaly sources. These anomalies, previously interpreted as caused by late Cenozoic subglacial volcanic centers, are compared to newly available glacial bed-elevation data from the radar ice sounding compilation of the entire area of the aeromagnetic survey to test this hypothesis in detail. We examined about 1000 shallow-source magnetic anomalies for bedrock topographic expression. Using very conservative criteria, we found over 400 specific anomalies which correlate with bed topography directly beneath each anomaly. We interpret these anomalies as indicative of the relative abundance of volcanic anomalies having shallow magnetic sources. Of course, deeper source magnetic anomalies are present, but these have longer wavelengths, lower gradients and mostly lower amplitudes from those caused by the highly magnetic late Cenozoic volcanic centers.
The great bulk of these >400 (40–1200-nT) anomaly sources at the base of the ice have low bed relief (60–600 m, with about 80%<200 m). We interpret this relief as an indication of residual topography after glacial removal of volcanic edifices comprising hyaloclastite, pillow breccia and other volcanic debris erupted into the moving ice during volcanism since the initiation of the WAIS >10 million years ago. Eighteen of the anomalies examined, about half concentrated in the area of the WAIS divide, have high-topographic expression (as great as 400 m above sea level) and high bed relief (up to 1500 m). All of these high-topography anomaly sources at the base of the ice would isostatically rebound to elevations above sea level were the ice removed. We interpret these 18 anomaly sources as evidence of subaerial eruption of volcanoes whose topography was protected from erosion by competent volcanic flows similar to prominent volcanic peaks that are exposed above the surface of the WAIS. Further, we infer these volcanoes as possibly erupted at a time when the WAIS was absent. In contrast, at the other extreme, there are a number of shallow-source, volcanic appearing magnetic anomalies overlying the very smooth bed topography in the survey area beneath Ice Stream D (Bindshadler Ice Stream); the glacial bed probably comprises a very thin layer of unconsolidated sediments (till). Probably, the volcanic edifices here were removed at a more rapid rate because of fast glacial flow. A few of the very shallow-source “volcanic” anomalies overlie the ice shelf just downstream of the grounding line of Ice Stream D, suggesting a causal relationship, if the volcanism is recent.
Siegert, M. J., B. Welch, D. L. Morse, A. Vieli, D. D. Blankenship, I. Joughin, E. C. King, G. J.-M. C. Leysinger Vieli, A. J. Payne, and R. Jacobel, Ice flow direction change in interior West Antarctica, Science, 305, 1948-1951, 2004, doi:10.1126/science.1101072,
Upstream of Byrd Station (West Antarctica), ice-penetrating radar data reveal a distinctive fold structure within the ice, in which isochronous layers are unusually deep. The fold has an axis more than 50 kilometers long, which is aligned up to 45° to the ice flow direction. Although explanations for the fold's formation under the present flow are problematic, it can be explained if flow was parallel to the fold axis 1500 years ago. This flow change may be associated with ice stream alterations nearer the margin. If this is true, central West Antarctica may respond to future alterations more than previously thought.
Studinger, M., R. E. Bell, W. R. Buck, G. D. Karner, and D. D. Blankenship, Sub-ice geology inland of the Trnasantarctic Mountains in light of new aerogeophysical data, Earth Planet. Sci. Lett., 220, 391-408, 2004, doi:10.1016/S0012-821X(04)00066-4,
The Transantarctic Mountains are a major geologic boundary that bisects the Antarctic continent, separating the low-lying, tectonically active terrains of West Antarctica from the East Antarctic craton. A new comprehensive aerogeophysical data set, extending 1150 km from the Ross Sea into the interior of East Antarctica provides insights into the complex structure inland of the Transantarctic Mountains. Geophysical maps, compiled from 21 000 km of gravity, magnetic and subglacial topography data, outline the boundaries of several geologic and tectonic segments within the survey area. The coherent pattern in magnetic data and mesa topography suggests a subglacial extent of the Transantarctic Mountains 400–500 km inland the last exposed rock outcrops. We estimate the maximum thickness of a potential sediment infill in the Wilkes Subglacial Basin to be less than 1 km, based on gravity modeling and source depth estimates from magnetic data. The coherent nature of the potential field and topography data, together with the northwest–southeast trends, define the Adventure Subglacial Trench and the Resolution Subglacial Highlands as a tectonic unit. The crustal structure and the strong similarity of the observed gravity with fold-and-thrust belts suggest a compressional scenario for the origin of the Adventure Subglacial Trench and the Resolution Subglacial Highlands. The complexity and apparent structural control of the Wilkes Subglacial Basin raise the issue of what influence pre-existing structures may have played in the formation of the Transantarctic Mountains system. The previous hypothesis of a thermal boundary beneath the mountains is difficult to reconcile with our new gravity data. The apparent difficulties to match our new data with certain key aspects of previous models suggests that a reassessment of the existing uplift models is necessary. We have modeled the prominent gravity anomaly over the Transantarctic Mountains with thicker crust.
Tsoflias, G., J.-P. Van Gestel, P. L. Stoffa, D. D. Blankenship, and M. K. Sen, Vertical fracture detection by exploiting the polarization properties of ground-penetrating radar signals, Geophysics, 69, 803-810, 2004, doi:10.1190/1.1759466,
Vertically oriented thin fractures are not always detected by conventional single-polarization reflection profiling ground-penetrating radar (GPR) techniques. We study the polarization properties of EM wavefields and suggest multipolarization acquisition surveying to detect the location and azimuth of vertically oriented fractures. We employ analytical solutions, 3D finite-difference time-domain modeling, and field measurements of multipolarization GPR data to investigate EM wave transmission through fractured geologic formations. For surface-based multipolarization GPR measurements across vertical fractures, we observe a phase lead when the incident electric-field component is oriented perpendicular to the plane of the fracture. This observation is consistent for nonmagnetic geologic environments and allows the determination of vertical fracture location and azimuth based on the presence of a phase difference and a phase lead relationship between varying polarization GPR data.
Studinger, M., R. E. Bell, G. D. Karner, A. A. Tikku, J. W. Holt, D. L. Morse, T. G. Richter, S. D. Kempf, M. E. Peters, D. D. Blankenship, R. E. Sweeney, and V. Rystrom, Ice cover, landscape setting, and geological framework of Lake Vostok, East Antarctica, Earth Planet. Sci. Lett., 205, 195-210, 2003, doi:10.1016/S0012-821X(02)01041-5,
Lake Vostok, located beneath more than 4 km of ice in the middle of East Antarctica, is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment. Melting and freezing at the base of the ice sheet, which slowly flows across the lake, controls the flux of water, biota and sediment particles through the lake. The influx of thermal energy, however, is limited to contributions from below. Thus the geological origin of Lake Vostok is a critical boundary condition for the subglacial ecosystem. We present the first comprehensive maps of ice surface, ice thickness and subglacial topography around Lake Vostok. The ice flow across the lake and the landscape setting are closely linked to the geological origin of Lake Vostok. Our data show that Lake Vostok is located along a major geological boundary. Magnetic and gravity data are distinct east and west of the lake, as is the roughness of the subglacial topography. The physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake’s circulation. Lake Vostok is a tectonically controlled subglacial lake. The tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small, but significant amounts of thermal energy into the lake.
Behrendt, J. C., D. D. Blankenship, D. L. Morse, and R. E. Bell, Removal of subglacially erupted edifices beneath the divide of the West Anatarctic Ice Sheet interpreted from aeromagnetic and radar ice sounding surveys, Royal Soc. New Zealand Bull., 35, 579-587, 2002
Behrendt, J. C., D. D. Blankenship, D. L. Morse, C. A. Finn, and R. E. Bell, Subglacial volcanic features beneath the West Antarctic ice sheet interpreted from aeromagnetic and radar ice sounding, in Volcano-Ice Interaction on Earth and Mars, edited by J. L. Smellie and M. G. Chapman, Geol. Soc. London Spec. Publ., 202, 337-355, 2002
Morse, D. L., D. D. Blankenship, E. D. Waddington, and T. A. Neumann, A site for deep ice coring in West Antarctica: Results from aerogeophysical surveys and thermo-kinetic modeling, Ann. Glaciology, 35, 36-44, 2002, doi:10.3189/172756402781816636,
The U.S. Science Plan for Deep Ice Coring in West Antarctica calls for two ice cores to be collected. The first of these cores, from Siple Dome, was completed during the 1997/98 field season.The second core is to be collected from a site near the divide that separates ice flowing to the Ross Sea and to the Amundsen Sea. Using high-resolution, grid-based aerogeophysical surveys of the Ross/Amundsen ice-divide region, we identify seven candidate sites and assess their suitability for deep coring.Weapply ice-flow and temperature calculations to predict time-scales and annual-layer resolution, and to assess the potential for basal melting for several selected sites.Weconclude that basal melting is likely for sites with very thick ice, as was observed at the Byrd core site. Nevertheless, these sites are most attractive for coring since they promise recovery of a long climate record with comparatively high time resolution during the last glacial period.
Price, S. F., R. A. Bindschadler, C. L. Hulbe, and D. D. Blankenship, Force balance along an inland tributary and onset to ice stream D, West Antarctica, J. Glaciology, 48, 20-30, 2002, doi:10.3189/172756502781831539,
The transition from inland- to streaming-style ice flow near to and upstream from the onset to Ice Stream D, West Antarctica, is investigated using the force- balance technique. Basal drag provides the majority of the flow resistance over the study area but is substantially modified by non-local stress gradients. Lateral drag increases with distance downstream, balancing ~ 50-100% of the driving stress at the onset. Longitudinal stress gradients (LSG) are also found to be significant, an observation that distinguishes ice flow in this region from the inland- and streaming-flow regimes that bound it, in which LSG are usually negligible. LSG decrease the spatial variability in basal drag and sliding speed and increase the area of the bed over which frictional melting occurs. Overall, LSG decrease the resistive influence of basal stress concentrations and increase the spatial uniformity of basal sliding. These observations suggest that streaming flow develops as an integrated response to the physical interaction between the ice and its bed over an extended region upstream from the onset, rather than being solely due to changes in basal characteristics at the onset. An implication is that non-steady-flow behavior upstream from the onset may ultimately propagate downstream and result in non-steady behavior at the onset.
Studinger, M., R. E. Bell, C. A. Finn, and D. D. Blankenship, Mesozoic and Cenozoic extensional tectonics of the West Antarctic rift system from high-resolution airborne geophysical mapping, Royal Soc. New Zealand Bull., 35, 563-569, 2002
Blankenship, D. D., D. L. Morse, C. A. Finn, R. E. Bell, M. E. Peters, S. D. Kempf, S. M. Hodge, M. Studinger, J. C. Behrendt, and J. M. Brozena, Geologic controls, on the initiation of rapid basal motion for West Antarctic ice streams: A geophysical perspective including new airborne radar sounding and laser altimetry results, in The West Antarctic Ice Sheet, Behavior and Environment, edited by R. B. Alley and R. A. Bindschadler, Amer. Geophys. Un., Antarctic Research Series, 77, 105-121, 2001
Holt, J. W., T. G. Richter, S. D. Kempf, D. L. Morse, and D. D. Blankenship, Airborne gravity over Lake Vostok and advancent highlands of East Antarctica, KIS 2001: Proc. Int. Symp. Kinematic Systems in Geodesy, Geomatics and Navigation, 576-585, 2001
Studinger, M., R. E. Bell, D. D. Blankenship, C. A. Finn, R. A. Arko, D. L. Morse, and I. Joughin, Subglacial sediments: A geological template for ice flow in West Antarctica, Geophys. Res. Lett., 28, 3493-3496, 2001, doi:10.1029/2000GL011788,
We use aerogeophysical data to estimate the distribution of marine subglacial sediments and fault‐bounded sedimentary basins beneath the West Antarctic Ice Sheet (WAIS). We find that significant ice flow occurs exclusively in regions covered by subglacial sediments. The onsets and lateral margins of ice streams coincide with the limit of marine sediments. Lateral margins are also consistently linked with fault‐bounded basins. We predict that the inland migration of ice streams B and C 1 towards the ice divide outside the region covered by marine or rift sediments is unlikely. The subglacial geology has the potential to modulate the dynamic evolution of the ice streams and the WAIS.
Bell, R. E., V. A. Childers, R. A. Arko, D. D. Blankenship, and J. M. Brozena, Airborne gravity and precise positioning for geologic applications, J. Geophys. Res., 104, 15281-15292, 1999,
Airborne gravimetry has become an important geophysical tool primarily because of advancements in methodology and instrumentation made in the past decade. Airborne gravity is especially useful when measured in conjunction with other geophysical data, such as magnetics, radar, and laser altimetry. The aerogeophysical survey over the West Antarctic ice sheet described in this paper is one such interdisciplinary study. This paper outlines in detail the instrumentation, survey and data processing methodology employed to perform airborne gravimetry from the multiinstrumented Twin Otter aircraft. Precise positioning from carrier-phase Global Positioning System (GPS) observations are combined with measurements of acceleration made by the gravity meter in the aircraft to obtain the free-air gravity anomaly measurement at aircraft altitude. GPS data are processed using the Kinematic and Rapid Static (KARS) software program, and aircraft vertical acceleration and corrections for gravity data reduction are calculated from the GPS position solution. Accuracies for the free-air anomaly are determined from crossover analysis after significant editing (2.98 mGal rms) and from a repeat track (1.39 mGal rms). The aerogeophysical survey covered a 300,000 km2 region in West Antarctica over the course of five field seasons. The gravity data from the West Antarctic survey reveal the major geologic structures of the West Antarctic rift system, including the Whitmore Mountains, the Byrd Subglacial Basin, the Sinuous Ridge, the Ross Embayment, and Siple Dome. These measurements, in conjunction with magnetics and ice-penetrating radar, provide the information required to reveal the tectonic fabric and history of this important region.
Sweeney, R. E., C. A. Finn, D. D. Blankenship, R. E. Bell, and J. C. Behrendt, Central West Antarctica Aeromagnetic Data: A Web Site for Distribution of Data and Maps, Open File Rept. 99-420, U. S. Geol. Surv., 1999
Thorsteinsson, T., E. D. Waddington, K. C. Taylor, R. B. Alley, and D. D. Blankenship, Strain-rate enhancement at Dye 3, Greenland, J. Glaciology, 45, 338-345, 1999
Anandakrishnan, S., D. D. Blankenship, R. B. Alley, and P. L. Stoffa, Influence of subgalcial geology on the position of a West Antarctic ice stream from seismic observations, Nature, 394, 62-65, 1998, doi:10.1038/27889,
Ice streams drain much of the interior West Antarctic Ice Sheet and buffer the main ice reservoir from oceanic influences1,2. The slow-flowing interior feeds the floating Ross Ice Shelf with ice via fast-flowing ice streams3 that are believed to modulate sea-level change through their control of inland ice storage. Understanding ice-stream behaviour, and predicting the response to climate change4, requires a better knowledge of the subglacial geology5,6. It is known that a thawed ice-bed and high-pressure basal water are necessary, but not sufficient, conditions to cause ice streaming7,8. Moreover, it has been hypothesized that a soft sedimentary bed is also required, because of its intrinsic low frictional resistance to flow9, and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10,11, or to bury rough features and smooth the bed for sliding. Here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill. The ice stream is within the basin and the ice outside the basin is slow-flowing. The basin fill presents an order-of-magnitude lower frictional resistance to ice flow than the subglacial material outside the basin. We conclude that the ice stream position is dependent on subglacial geology.
Behrendt, J. C., C. A. Finn, D. D. Blankenship, and R. E. Bell, Aeromagnetic evidence for a volcanic caldera(?) complex beneath the divide of the West Antarctic Ice Sheet, Geophys. Res. Lett., 25, 4385-4388, 1998, doi:10.1029/1998GL900101,
A 1995–96 aeromagnetic survey over part of the Sinuous Ridge (SR) beneath the West Antarctic Ice Sheet (WAIS) divide shows a 70‐km diameter circular pattern of 400–1200‐nT anomalies suggesting one of the largest volcanic caldera(?) complexes on earth. Radar‐ice‐sounding (RIS) shows the northern part of this pattern overlies the SR, and extends south over the Bentley Subglacial Trench (BST). Modeled sources of all but one the caldera(?) anomalies are at the base of <1–2‐km thick ice and their volcanic edifices have been glacially removed. The exception is a 700‐m high, 15‐km wide "volcano" producing an 800‐nT anomaly over the BST. “Intrusion” of this “volcano” beneath 3 km of ice probably resulted in pillow basalt rather than easily removed hyaloclastite erupted beneath thinner ice. The background area (−300 to −500‐nT) surrounding the caldera(?) is possibly caused by a shallow Curie isotherm. We suggest uplift of the SR forced the advance of the WAIS.
Bell, R. E., D. D. Blankenship, C. A. Finn, D. L. Morse, T. A. Scambos, J. M. Brozena, and S. M. Hodge, Influence of subglacial geology on the onset of a West Antarctic ice stream from aerogeophysical observations, Nature, 394, 58-62, 1998, doi:10.1038/27883,
Marine ice-sheet collapse can contribute to rapid sea-level rise1. Today, the West Antarctic Ice Sheet contains an amount of ice equivalent to approximately six metres of sea-level rise, but most of the ice is in the slowly moving interior reservoir. A relatively small fraction of the ice sheet comprises several rapidly flowing ice streams which drain the ice to the sea. The evolution of this drainage system almost certainly governs the process of ice-sheet collapse2, 3, 4, 5. The thick and slow-moving interior ice reservoir is generally fixed to the underlying bedrock while the ice streams glide over lubricated beds at velocities of up to several hundred metres per year. The source of the basal lubricant — a water-saturated till6,7 overlain by a water system8 — may be linked to the underlying geology. The West Antarctic Ice Sheet rests over a geologically complex region characterized by thin crust, high heat flows, active volcanism and sedimentary basins9, 10, 11, 12, 13, 14, 15, 16. Here we use aerogeophysical measurements to constrain the geological setting of the onset of an active West Antarctic ice stream. The onset coincides with a sediment-filled basin incised by a steep-sided valley. This observation supports the suggestion5,17 that ice-stream dynamics — and therefore the response of the West Antarctice Ice Sheet to changes in climate — are strongly modulated by the underlying geology.
Sen, V., P. L. Stoffa, I. W. D. Dalziel, D. D. Blankenship, A. M. Smith, and S. Anandakrishnan, Seismic surveys of central West Antarctica: Data and processing examples from the ANTALITH field tests (1994-1995), Terra Antartica, 5, 761-772, 1998
Behrendt, J. C., R. Saltus, D. Damaske, A. McCafferty, C. A. Finn, D. D. Blankenship, and R. E. Bell, Patterns of Late Cenozoic volcanic and tectonic activity in the West Antarctic rift system revealed by aeromagnetic surveys, Tectonics, 15, 660-676, 1996,
Aeromagnetic surveys, spaced ≤5 km, over widely separated areas of the largely ice- and sea-covered West Antarctic rift system, reveal similar patterns of 100- to 1700-nT, shallow-source magnetic anomalies interpreted as evidence of extensive late Cenozoic volcanism. We use the aeromagnetic data to extend the volcanic rift interpretation over West Antarctica starting with anomalies over (1) exposures of highly magnetic, late Cenozoic volcanic rocks several kilometers thick in the McMurdo-Ross Island area and elsewhere; continuing through (2) volcanoes and subvolcanic intrusions directly beneath the Ross Sea continental shelf defined by marine magnetic and seismic reflection data and aeromagnetic data and (3) volcanic structures interpreted beneath the Ross Ice Shelf partly controlled by seismic reflection determinations of seafloor depth to (4) an area of similar magnetic pattern over the West Antarctic Ice Sheet (400 km from the nearest exposed volcanic rock), where interpretations of late Cenozoic volcanic rocks at the base of the ice are controlled in part by radar ice sounding. North trending magnetic rift fabric in the Ross Sea-Ross Ice Shelf and Corridor Aerogeophysics of the Southeast Ross Transect Zone (CASERTZ) areas, revealed by the aeromagnetic surveys, is probably a reactivation of older rift trends (late Mesozoic?) and is superimposed on still older crosscutting structural trends revealed by magnetic terrace maps calculated from horizontal gradient of pseudogravity. Long-wavelength (∼ 100-km wide) magnetic terraces from sources within the subvolcanic basement cross the detailed survey areas. One of these extends across the Ross Sea survey from the front of the Transantarctic Mountains with an east-southeast trend crossing the north trending rift fabric. The Ross Sea-Ross Ice Shelf survey area is characterized by highly magnetic northern and southern zones which are separated by magnetically defined faults from a more moderately magnetic central zone. Aeromagnetic data in the south delineate the Ross fault of unknown age. The extension of the southern Central Basin south of the Ross fault is associated with an 825-nT magnetic anomaly over the Ross Ice Shelf requiring inferred late Cenozoic volcanic rock essentially at the seafloor at its south end, as shown by magnetic models. Models show that the thickness of magnetic volcanic rocks beneath Hut Point Peninsula at McMurdo Station is probably <2 km. The detailed surveys, combined with data from > 100,000 km of widely spaced aeromagnetic profiles, led to the interpretation of the mostly subglacial West Antarctic flood basalts(?) or their subglacially erupted and intruded equivalent. The volume of the exposed volcanos is small in contrast to the much greater volume (> 106 km3) of late Cenozoic magmatic rock remaining at volcanic centers beneath the continental shelf, Ross Ice Shelf and West Antarctic Ice Sheet. We suggest as an alternative or supplemental explanation to the previously proposed mantle plume hypothesis for the late Cenozoic volcanism significantly greater lower lithosphere (mantle) stretching resulting in greater decompression melting than the limited Cenozoic crustal extension allows. However, this implies a space problem that is not obviously resolved, because the Antarctic Plate is essentially surrounded by spreading centers.
Behrendt, J. C., D. D. Blankenship, D. Damaske, and A. K. Cooper, Glacial removal of Late Cenozoic subglacially emplaced volcanic edifices by the West Antarctic Ice Sheet, Geology, 23, 1111-1114, 1995, doi:10.1130/0091-7613(1995)023<1111:GROLCS>2.3.CO;2,
Local maxima of the horizontal gradient of pseudogravity from closely spaced aeromagnetic surveys over the Ross Sea, northwestern Ross Ice Shelf, and the West Antarctic ice sheet, reveal a linear magnetic rift fabric and numerous subcircular, high-amplitude anomalies. Most of these anomalies have sources that probably resulted from late Cenozoic volcanism. Some of these volcanic structures penetrate the Neogene sediments beneath the deglaciated continental shelf and are present at the base of the present grounded ice sheet and beneath the ice shelf. Geophysical data indicate two or three youthful volcanic edifices at widely separated areas beneath the sea and ice cover in the West Antarctic rift system. In contrast, we suggest glacial removal of edifices of volcanic sources of many more anomalies. Magnetic models, controlled by marine seismic reflection and radar ice-sounding data, allow us to infer that glacial removal of the associated late Cenozoic volcanic edifices (probably debris, comprising pillow breccias, and hyaloclastites) has occurred essentially concomitantly with their subglacial eruption. “Removal” of unconsolidated volcanic debris erupted beneath the ice is probably a more appropriate term than “erosion,” given its fragmented, ice-contact origin. The exposed volcanoes may have been protected from erosion by the surrounding ice sheet because of more competent rock or high elevation above the ice sheet. Glacial removal may be the general case; exposed late Cenozoic volcanic peaks and outcrops, consisting primarily of flows, which erupted during Antarctic glacial conditions since (Approx.)30 Ma, may be the exceptions. The volume of the exposed volcanoes is small in contrast to the much greater volume (>106 km3) of late Cenozoic magmatic rock remaining at volcanic centers beneath the continental shelf, Ross Ice Shelf, and West Antarctic ice sheet.
Behrendt, J. C., D. D. Blankenship, C. A. Finn, R. E. Bell, R. E. Sweeney, S. M. Hodge, and J. M. Brozena, CASERTZ aeromagnetic data reveal Late Cenozoic flood basalts(?) in the West Antarctic rift system, Geology, 22, 527-530, 1994, doi:10.1130/0091-7613(1994)022<0527:CADRLC>2.3.CO;2,
The late Cenozoic volcanic and tectonic activity of the enigmatic West Antarctic rift system, the least understood of the great active continental rifts, has been suggested to be plume driven. In 1991-1992, as part of the CASERTZ (Corridor Aerogeophysics of the Southeast Ross Transect Zone) program, an ∼25000 km aeromagnetic survey over the ice-covered Byrd subglacial basin shows magnetic "texture" critical to interpretations of the underlying extended volcanic terrane. The aeromagnetic data reveal numerous semicircular anomalies ∼100-1100 nT in amplitude, interpreted as having volcanic sources at the base of the ice sheet; they are concentrated along north-trending magnetic lineations interpreted as rift fabric. Models constrained by coincident radar ice soundings indicate highly magnetic sources, with a probable high remanent magnetization in the present field direction, strongly suggesting a late Cenozoic age. Magnetic anomalies over exposed late Cenozoic volcanic rocks along part of the rift shoulder and in coastal Marie Byrd Land are similar in form and amplitude. The CASERTZ aeromagnetic results, combined with >100 000 km of widely spaced aeromagnetic profiles, indicate at least 106 km3 of probable late Cenozoic volcanic rock (flood basalt?) in the West Antarctic rift beneath the ice sheet and Ross Ice Shelf. Comparison with other plumes in active rift areas (e.g., Yellowstone and East Africa) indicates that this volume estimate lies in the range of magma generation found in these other low-extension continental rifts.
Bell, R. E., B. J. Coakley, D. D. Blankenship, S. M. Hodge, J. M. Brozena, and J. Jarvis, Airborn gravity from a light aircraft: CASERTZ 1990-91, in Recent Progress in Antarctic Earth Science, edited by Yoshida et al., 571-577, 1993
Blankenship, D. D., R. E. Bell, S. M. Hodge, J. M. Brozena, J. C. Behrendt, and C. A. Finn, Active volcanism beneath the West Antarctic Ice Sheet and implications for ice-sheet stability, Nature, 361, 526-529, 1993, doi:10.1038/361526a0,
IT is widely understood that the collapse of the West Antarctic ice sheet (WAIS) would cause a global sea level rise of 6 m, yet there continues to be considerable debate about the detailed response of this ice sheet to climate changel–3. Because its bed is grounded well below sea level, the stability of the WAIS may depend on geologically controlled conditions at the base which are independent of climate. In particular, heat supplied to the base of the ice sheet could increase basal melting and thereby trigger ice streaming, by providing the water for a lubricating basal layer of till on which ice streams are thought to slide4,5. Ice streams act to protect the reservoir of slowly moving inland ice from exposure to oceanic degradation, thus enhancing ice-sheet stability. Here we present aerogeophysical evidence for active volcanism and associated elevated heat flow beneath the WAIS near the critical region where ice streaming begins. If this heat flow is indeed controlling ice-stream formation, then penetration of ocean waters inland of the thin hot crust of the active portion of the West Antarctic rift system could lead to the disappearance of ice streams, and possibly trigger a collapse of the inland ice reservoir.
Blankenship, D. D., and R. E. Bell, Delving into the West Antarctic Ice Sheet, Geotimes, 38 (8), 12-15, 1993
Rooney, S. T., D. D. Blankenship, R. B. Alley, and C. R. Bentley, Seismic reflection profiling of a sediment-filled graben beneath ice stream B, West Antarctica, in Geological Evolution of Antarctica. edited by M. R. A. Thompson, J. A. Crame, and J. W. Thomson, Cambridge, 261-265, 1993
