Tabular iceberg collisions within the coastal regime Douglas R. MACAYEAL, 1 Marianne H. OKAL, 1* Jonathan E. THOM, 2 Kelly M. BRUNT, 1 Young-Jin KIM, 1 Andrew K. BLISS 3 1 Department of Geographical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA E-mail: drm7@midway.uchicago.edu 2 Space Science and Engineering Center, University of Wisconsin–Madison, 1225 West Dayton Street, Madison, Wisconsin 53706, USA 3 Department of Geography, University of California Berkeley, 507 McCone Hall, Berkeley, California 94720-4740, USA ABSTRACT. During 2000–07, five giant icebergs (B15A, B15J, B15K, C16 and C25) adrift in the south- western Ross Sea, Antarctica, were instrumented with global positioning system (GPS) receivers and other instruments to monitor their behavior in the near-coastal environment. The measurements show that collision processes can strongly influence iceberg behavior and delay their progress in drifting to the open ocean. Collisions appear to have been a dominant control on the movement of B15A, the largest of the icebergs, during the 4-year period it gyrated within the limited confines of Ross Island, the fixed Ross Ice Shelf and grounded C16. Iceberg interactions in the near-coastal regime are largely driven by ocean tidal effects which determine the magnitude of forces generated during collision and break-up events. Estimates of forces derived from the observed drift trajectories during the iceberg-collision- induced calving of iceberg C19 from the Ross Ice Shelf, during the iceberg-induced break-off of the tip of the Drygalski Ice Tongue and the break-up of B15A provide a crude estimate of the stress scale involved in iceberg calving. Considering the total area the vertical face of new rifts created in the calving or break-up process, and not accounting for local stress amplification near rift tips, this estimated stress scale is 10 4 Pa. INTRODUCTION Since James Cook’s second voyage of discovery (1772–75), when large tabular icebergs originating from a south-polar continent were seen and initially documented (Herdman, 1959; Glaciological Society, 1962), icebergs have been a focus of scientific inquiry, attempting to understand their relationship to, and impact on, environmental and ecologi- cal systems of the high southern latitudes. Efforts to study these vagabonds of the Southern Ocean have been difficult, however, because calving of large icebergs (i.e. exceeding an area of 500 km 2 ) is episodic, unpredictable and occurs on a timescale of 10–100 years. Erratic iceberg drift also makes rendezvous with scientific parties impractical. To expand the body of surface-based observations of large tabular icebergs, we undertook a field-observation campaign in early 2001 to study a series of giant icebergs, ranging from 500 to >3000 km 2 , that had been calved from the Ross Ice Shelf since March 2000. This campaign was opportunistic, being motivated in large part by the presence of the icebergs in the vicinity of the US research base on Ross Island, Antarctica. The scientific activity consisted of occupying icebergs B15A, C16, B15J, B15K and C25 (Fig. 1) to install automatic weather stations (AWS), global positioning system (GPS) receivers and other instruments configured to monitor iceberg movement and surface conditions. The purpose of our field campaign was to determine the effects of iceberg collisions with various objects of the coastal environment, such as ice-shelf ice fronts, ice tongues, ice-free coastlines, shoals and other icebergs. Collisions in the coastal zone were of particular interest because they are believed to act as a dynamic filter that reduces the size of icebergs released from Antarctica, and that allows the impact of iceberg discharge on the ocean beyond Antarctica to have greater temporal and spatial dispersion. The initial objective of the field effort was to study iceberg B15, which calved in March 2000 from the Ross Ice Shelf. Over the first 12 months of B15’s lifetime, the iceberg was broken into nine pieces as a result of collisional contact with the Ross Ice Shelf ice front and was dispersed spatially with pieces becoming separated by >1000km. Two of the pieces that remained close to Ross Island, B15J and B15K, were incorporated into the field effort because of their proximity to the US research station. The most significant result of the work so far, and that which forms the subject of the present paper, is the under- standing of how several of the largest icebergs released into the Ross Sea when B15 calved were unexpectedly retained in the near-coastal environment for periods up to many years. B15A was trapped in the southern Ross Sea, without ever becoming grounded, from early 2000 to late 2005. B15J, a piece of B15A that broke off in response to collisional activity in late 2003, continues to be trapped in the southwestern Ross Sea. This exemplifies the nature of the effect of the coastal environment on iceberg break-up and drift: the environmental impact of the original B15 iceberg, containing 2000 km 3 of ice to eventually be discharged as meltwater, was mitigated by effects of the near-coastal environment. The observational picture presented by our data confirms that iceberg-on-ice and iceberg-on-coast collisions act as an important filter on iceberg drift, retarding it within the coastal zone. What distinguishes the present study from previous studies of iceberg motion is the coastal proximity of the instrumented icebergs studied here and importance of collisional dynamics as a control on iceberg drift in this near-coastal environment (for a review of iceberg Journal of Glaciology, Vol. 54, No. 185, 2008 * Present address: UNAVCO, 6350 Nautilus Drive, Boulder, Colorado 80301-5554, USA. 371