Collaborative Research: Using Fracture Patterns and Ice Thickness to Study the History and Dynamics of Grounding Line Migration and Shutdown of Kamb and Whillans Ice Streams

The work is in collaboration with co-PI Christina Hulbe and at Portland State University. In our previous work we were able to image, map, and interpret streak lines (flow trajectories) on the Ross Ice Shelf and demonstrate, using numerical simulations, that this pattern of flow-related features required significant changes in flow direction in the outflows of a number of ice streams on the Siple Coast of West Antarctica. In the present project, we have expanded the suite of features to include fracture patterns mapped from satellite imagery, expanded the modeling work to capture the impact of grounding and ungrounding of ice in the mouths of ice streams, and we have worked to constrain stop and start cycles recorded by the features out in the Ross Ice Shelf.

A summary of results, starting with the abstract of the paper that is out in JGR Earth Surfaces (see publications record in this DIF):

"Flow features on the surface of the Ross Ice Shelf, West Antarctica, record two episodes of ice stream stagnation and reactivation within the last 1000 years. We document these events using maps of streaklines emerging from individual ice streams made using visible band imagery, together with numerical models of ice shelf flow. Forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration. According to our analysis, Whillans Ice Stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and MacAyeal Ice Stream either stopped or slowed significantly between 800 and 700 years ago, restarting about 150 years later. Until now, ice-stream scenarios emphasized runaway retreat or stagnation on millennial time scales. Here, we identify a new scenario: century-scale stagnation and reactivation cycles, as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains. This introduces uncertainty into predictions for future sea-level withdrawls by the West Antarctic Ice Sheet, which are based in part on recent slowing of Whillans Ice Stream and the stagnant condition of Kamb Ice Stream."

This paper documents the evidence for delimiting the source regions (provenance) of ice now in the shelf; for bracketing the possible discharge scenarios that could have produced the patterns captured in the shelf, and shows that it is difficult to reproduce these patterns with a model without stopping and restarting individual streams. The modeling facilitated by Co-PI Hulbe’s system has allowed us to cover a range of possible scenarios, and in the process to learn about other aspects of the behavior of this system. The flow variability recorded in the ice shelf provides an important reference history against which current changes can be put in context.

Some other findings (some of them in this paper, some from the last year of the project):

Major, rapid, bi-directional translations of the grounding line have occurred, are common in the recent past, and are very likely to be going on now. This conclusion is required by the changes in flow and crevassing recorded in the ice shelf. It is a direct consequence of changing ice stream discharge across a low-slope grounding zone – small changes in ice thickness can produce large horizontal changes in grounding line location. Although there is limited knowledge of the cavity shape under the Ross Ice Shelf, the gently sloping floor downstream of the Siple Coast ice streams is fairly well constrained. Thickness variations out on the Ross Ice Shelf from ICESAT data (courtesy of Christopher Shuman) show strong gradients along and across flow that should decay rapidly over time; their presence suggests recent ungrounding in several areas. Early model work by MacAyeal on the thickness signatures to be expected on an ice shelf from an ice stream shutdown are large only local to the grounding line – and we in fact see a combination of thickness changes and flow provenance that reveals the impact of changes in the mouths of ice streams, but decays farther out on the shelf.

Ice stream restarting is required by the lack of Whillans ice stream ice out in the ice shelf; this corresponds in estimated time with changes upstream of Engelhardt Ice Ridge (Scambos and Conway). The time available for restarting is significantly shorter than the millennium-length period required to substantially thicken the ice and change either the basal thermal gradient or basal shear stress, which has been the prevailing model of how streams may restart. This strongly points to more variable parts of the system as possible mechanisms – basal water being the most likely. Basal water movement is very common on Whillans, Bindschadler and MacAyeal ice streams based on time series of ICESat laser altimeter data, which show meters of vertical elevation change over multiple months for a series of lakes (Fricker et al). This type of water flow could restart a stopped stream, if it could be effective in the right places. The implications of this for possible field measurements that would document the process will require more work. The complex layers of dirty and clean ice seen at the base of the Caltech borehole on Kamb ice stream may reflect intermittent freeze-on and restarting of flow in this area.

The seemingly disparate conclusions of the work of Catania et al, and Catania and Hulbe, from surface-based radar profiling of the past margins and floating regions in the lower reaches of Kamb ice stream, and Anandakrishnan, Bindschadler, and more recently Weins about the twice-daily episodic stick-slip motion on the ice plain of Whillans ice stream can be connected based on an analysis of the detailed topography of these regions. Many of the conclusions of Catania et al about the recent history of the ice at the mouth of Kamb ice stream are borne out by this comparison, though relating the features produced by basal melting at the edges of Siple Dome and Engelhardt ice rise to old grounding lines is problematic given the amount of vertical relief along the length of these features today. The nucleation points of the slip events, centered around Ice Raft A in the ice plain of Whillans ice stream is characterized by local rises which can reach 10+ meters but are features only a kilometer across. These are likely produced when ice flows across a bed feature that inhibits sliding. On longer time scales, this detailed topography provides a quantitative picture of the flow variations that are also shown in the modeling of ice stream start/stop behavior and grounding line migration by Hulbe at Portland State University; further analysis of the combination of these data should yield additional insight.