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Science Papers
New ice-core measurements suggest that soot influenced recent Arctic climate change.
Read the latest in Richard's recent Science Express paper.
rba6@psu.edu. Published Online August 9, 2007 Science DOI: 10.1126/science.1147470
Richard B. Alley 1.
1 Department of Geosciences, Pennsylvania State University, University Park, PA, USA.
Sediment stabilizes grounding lines, Scott was wrong, and ice shelves matter. 
We report on the discovery of a grounding-line sedimentary wedge ("till delta") deposited by Whillans Ice Stream, West Antarctica. Our novel observation is that grounding-line deposition serves to thicken the ice and stabilize the position of the grounding line. The grounding line is well above floatation and will tend to remain in the same location even though sea level changes (until sea level rises high enough to overcome the influence of the wedge). Further, our observation demonstrates the occurrence of rapid subglacial erosion, sediment transport by distributed subglacial till deformation, and grounding-line sedimentation, with important implications for ice dynamics, for numerical modeling of ice flow, and for interpretation of the sedimentation record.
sak@essc.psu.edu. Published Online March 1, 2007. Science DOI: 10.1126/science.1138393
Sridhar Anandakrishnan 1, Ginny A. Catania 2, Richard B. Alley 1, Huw J. Horgan 1.
1 Department of Geosciences, Pennsylvania State University, University Park, PA, USA. 2 University of Texas at Austin, Austin, TX, USA.
Sedimentation filling space beneath ice shelves helps stabilize ice sheets against retreat in response to rise in relative sea level of at least several meters. Recent Antarctic changes thus cannot be attributed to sea-level rise, strengthening prior interpretations that warming has driven ice-sheet mass loss. Large sea level rise, such as the 100 m at the end of the last ice age, may overwhelm the stabilizing feedback from sedimentation, but smaller sea-level changes are unlikely to have synchronized behavior of ice sheets in the past.
rba6@psu.edu. Published Online March 1, 2007. Science DOI: 10.1126/science.1138396
Richard B. Alley 1, Sridhar Anandakrishnan 1, Todd K. Dupont 2, Byron R. Parizek 3, David Pollard 1
1 Department of Geosciences and Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA. 2 Department of Geosciences and Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA; Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL 60607, USA. 3 Department of Geosciences and Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA; Department of Physics, The College of New Jersey, Ewing, NJ 08628, USA.
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Development of a wirelessly-linked mesh of geophones.
PSICE in the news
Join Don, Huw, Leo, and Sridhar while they unravel the mystery of what lies more than a mile below the South Pole. Click here and here
to watch movies about their 2006/07 season in Antarctica. Use the following link to get to an audio diary from this season.
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Tidal Influences on Ice Stream Motion
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 1. Sketch map of the Siple Coast region of West Antarctica. Inset is a map of Antarctica with the Greenwich meridian (and Africa) straight up (at "12 o'clock"); S. America is off the map at "10 o'clock" off the end of the Antarctic Peninsula; New Zealand is at "6 o'clock"; Australia and India are at around "5 o'clock" and "4 o'clock".
The main map shows the so-called "ice streams" of West Antarctica. The old names of these enormous glaciers are shown: ice streams A to E. The new names are Mercer, Whillans, Kamb, Bindschadler, and MacAyeal, respectively. These ice streams flow from the interior of West Antarctica (the left of the main map) to the floating Ross Ice shelf (lower right of the map). The ice streams are thick (approximately 1 km), rapidly-flowing (up to 1000 meters per year) masses of ice that are embedded within slower-flower ridges of ice. Much research over the past few decades has focused on why these ice streams exist; why they form; what controls their position and flow speed; how they will respond to ongoing climate change such as changing sea level or changing temperature or changing snowfall. These ice streams are particularly important because of their deep penetration into the West Antarctic Ice Sheet and because of the large volume of ice that they move.
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 2. The flow speed of these glaciers can be surprisingly non-steady. Recent work by Penn State researchers and colleagues at NASA has shown that the flow speed of Whillans Ice Stream (the former ice stream B) can vary enormously over the course of a single day. This plot shows the position of a stake stuck into the snow on Whillans Ice Stream. Over the course of a few days it does the following: it stays at the same position for most of the day (Indicated by the red circle) and then suddenly "lurches forward" in a short time (identified by the black circle). The ice stream does this in a very regular pattern that is driven by the ocean tide (the blue curve). This surprising behavior is poorly understood and was entirely unexpected from our prior models of glacier flow. New models are being developed at PSU and elsewhere to try and explain this behavior.
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Ice Shelf Temperature Movie
Click here to view a Quicktime movie of ice sheet thickness, grounding line position, and surface elevation as a function of temperature. |
Age Dating Western US Moraines
 Estimates of the age of the outer moraine in a representative glaciated drainage in the western United States (Lake Fork River, Uinta Mountains, Utah). Part c shows the cosmogenic exposure dates obtained from the last-glacial moraines in that drainage. The solid curve was constructed by summing the probability density functions (bell curves) of all dates from the outer moraine in that drainage; the dotted curve includes all dates from the moraines inset into the outer moraine. The arrow indicates an abnormally young exposure date (an outlier) from the outer moraine. Parts a and b show statistical estimates of the age of the outer moraine calculated from these exposure dates, with and without the outlier. The mean, weighted mean, oldest date, and youngest date are calculated from the dates on the outer moraine only; the maximum likelihood estimator incorporates all of the dates shown. The maximum likelihood estimator is more sensitive to outliers than the mean or weighted mean, but less sensitive than the oldest date and youngest date estimators. Data in part c from Laabs, Ph. D. dissertation, University of Wisconsin, 2004; figure from Applegate, Granger and Alley (in preparation).
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Modeling the Effect of Grounding-Zone Sediment Wedges on the Stability of Ice Streams to Sea Level Rise.
David Pollard(1), Sridhar Anandakrishnan(1), Richard B. Alley(1), Todd Dupont(2), Byron Parizek(3)1 Penn State Ice and Climate Exploration Center, Pennsylvania State University; 2 Earth and Environmental Sciences, University of Illinois at Chicago; 3 Department of Physics, The College of New Jersey, Ewing.
A till deposition wedge has recently been discovered at the modern grounding line of Whillans Ice Stream, West Antarctica (Anandakrishnan et al., submitted to Science). Similar widespread grounding-line wedges are observed on the Ross Sea continental shelf with fluting on their upstream flanks, formed during the last deglaciation (Mosola and Anderson, QSR, 2006). Both observations suggest that grounding-line wedges (i) are able to survive and support overriding ice during and after their formation, (ii) are formed with their upstream flanks behind the grounding line so raising the ice, and (iii) may interact with ice dynamics so as to stabilize grounding-line retreat against future sea level rise.
This stabilization is investigated using three separate coupled ice-sheet/stream/shelf models applied to idealized flowline segments spanning the grounding line of an active ice stream. All models include both grounded and floating ice flow, and the grounding line is free to migrate. Although the models differ in their treatments of ice-flow physics, their basic results agree well with each other. Simulations with flat bedrock are compared to those with prescribed sediment wedges of roughly the same size as that observed at Whillans (~10 to 30 m in height and ~10 to 30 km in length), both for steady-state conditions and with sea level slowly rising. The wedge causes an increase in surface ice elevations above and upstream of the wedge, and the ice thickness-over-flotation increases by ~5 to 10 m just upstream of the new grounding line. This prevents or at least delays grounding line retreat for future sea level rise of up to several meters. The figure shows snapshots from a time-varying experiment over several thousand years with a linear rise in sea level, in which the grounding line retreats over the whole domain over the course of the experiment (solid lines=with wedge, dashed lines=no wedge). As shown by the grounding-line positions (km) versus time in the lower panel, the presence of a wedge causes a ~1500 year pause in grounding-line retreat, during which the grounding line is "stuck" and slowly creeps up the downstream side of the wedge.
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A COUPLED ICE-SHEET/ICE-SHELF/SEDIMENT MODEL APPLIED TO A MARINE-MARGIN FLOWLINE: PATTERNS OF SEDIMENT DEPOSITS
Poster presented at Fall AGU 2006.
A broad history of Cenozoic Antarctic ice sheet variations since its first major growth ~34 Ma has been deduced from deep-sea-core records. Important questions remain, however, such as numerous large-amplitude ?18O fluctuations that seem to require the unlikely melting and re-growth of the entire ice sheet (~75 m sea level, Pekar et al., 2006).
There is a potential wealth of information on ice variations in Cenozoic marine sediment deposits on Antarctic continental shelves (Anderson, 1999; Escutia et al., 2005; many others). Much of these sediments have been deposited in glaciomarine or sub-ice environments, as the ice sheet grounding line has advanced and receded over parts of the continental shelf.
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Slope Analysis of Siple Coast, Antarctica
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 Figure 1. Surface gradient map of a portion of West Antarctica (see inset map). The study area encompasses the Siple Coast, where the West Antarctic Ice Sheet flows into the Ross Ice Shelf. The inset map displays the Moderate Resolution Imaging Spectroradiometer (MODIS) Mosaic of Antarctica (MOA) image map [Haran et al., 2005, NSIDC digital media].
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 Figure 2. Validation of the high magnitude slope methodology. Left: A portion of the grounding line of Whillans Ice Stream (see Figure 1 for location). Crosses denote strand cracks observed on the surface, diamonds denote GPS array deployment, lines denote the high slope portions identified on individual ICESat tracks (gray) and kinematic GPS tracks (black), large circles denote the midpoints of these high slope regions. Also shown as small black circles are the InSAR grounding line picks of Gray et al. [2002]. Right: Vertical component of motion for 24 hours of the GPS grid deployment, the location of which is shown in A (diamonds) and to the bottom right. Note that station one is held fixed in processing.
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 Figure 3. Grounding line picks overlain on MODIS MOA imagery (with Lambertian radiance applied). The grounding lines from this study (black), Gray et al [2002] and Shabtaie and Bentley [1987] shown, as are this study's slope magnitude picks from ERS-1 data. Areas of low confidence stated by Shabtaie and Bentley [1987]are shown with greater spacing between circles. Letters A through E correspond to features discussed in the text [Horgan and Anandakrishnan, 2006].
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Seismicity Associated with Flow of Antarctic Glaciers
contributed by Lucas Zoet, Sridhar Anandakrishnan1, and Douglas Wiens
 Seismicity in the Transantarctic Mountain Range is higher than previously thought. Events are too small in size to be observed on the global seismology network, one reason why the levels of seismicity in these areas was previously thought to be much lower.
The remoteness of these locations also makes it difficult for continuous large scale monitoring of the areas adding to the difficulty in detecting smaller events. There may be an association between the tidal cycle and the seismicity in zone one (see poster for details). Further analysis of this region over different seasons is necessary to make a final conclusion on tidal cycle link.
Click the image to the right to get an 11x8.5" printable version of the poster. To get the full size (25MB) click here.
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Greenland Ice Sheet Mass Balance
 contributed by R.Alley
Recently published estimates of Greenland mass balance. Warming is occurring, and the ice is being lost. In the past, warming has casued ice loss (Eemain, mid-Holocene).
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