(Table 1) Soil density and concentration of 10Beryllium and 26Aluminium in samples from two ash deposits in the McMurdo Dry Valleys

The McMurdo Dry Valleys, Antarctica (MDV) are among the oldest landscapes on Earth, and some landforms there present an intriguing apparent contradiction such that millions of years old surface deposits maintain their meter-scale morphology despite the fact that measured erosion rates are 0.1-4 m/Ma. We analyzed the concentration of cosmic ray-produced 10Be and 26Al in quartz sands from regolith directly above and below two well-documented ash deposits in the MDV, the Arena Valley ash (40Ar/39Ar age of 4.33 Ma) and the Hart ash (K-Ar age of 3.9 Ma). Measured concentrations of 10Be and 26Al are significantly less than expected given the age of the in situ air fall ashes and are best interpreted as reflecting the degradation rate of the overlying sediments. The erosion rate of the material above the Arena Valley ash that best explains the observed isotope profiles is 3.5 ± 0.41 x 10**-5 g/cm**2/yr (~0.19 m/Ma) for the past ~4 Ma. For the Hart ash, the erosion rate is 4.8 ± 0.21 x 10**-4 g/cm**2/yr (~2.6 m/Ma) for the past ~1 Ma. The concentration profiles do not show signs of mixing, creep, or deflation caused by sublimation of ground ice. These results indicate that the slow, steady lowering of the surface without vertical mixing may allow landforms to maintain their meter-scale morphology even though they are actively eroding.

(Table 1) Grain size, sedimentation rates and accumulation rates of ODP Hole 152-919A coarse-sand ice-rafted debris

At mid- to high-latitude marine sites, ice-rafted debris (IRD) is commonly recognized as anomalously coarse-grained terrigenous material contained within a fine-grained hemipelagic or pelagic matrix (e.g., Conolly and Ewing, 1970; Ruddiman, 1977, doi:10.1130/0016-7606(1977)88<1813:LQDOIS>2.0.CO;2; Krissek, 1989, doi:10.2973/odp.proc.sr.104.114.1989; Jansen et al., 1990; Bond et al., doi:10.1038/360245a0, 1992; Krissek, 1995, doi:10.2973/odp.proc.sr.145.118.1995). The presence of such ice-rafted material is a valuable indicator of the presence of glacial ice at sea level on an adjacent continent, whereas the composition of the IRD can often be used to identify the location of the source area (e.g., Goldschmidt, 1995, doi:10.1016/0025-3227(95)00098-J). Because the amount of core recovered during Leg 163 was very limited, this shore-based, postcruise study focuses on materials recovered at a nearby site during Leg 152. In particular, this study examines sediments recovered at Site 919; these sediments were described as containing a significant ice-rafted component in the Leg 152 Initial Reports volume (Larsen, Saunders, Clift, et al., 1994, doi:10.2973/odp.proc.ir.152.1994). In this study, the sedimentary section from Site 919 has been examined with the goal of providing a detailed history of glaciations on Greenland and other landmasses adjacent to the Norwegian-Greenland Sea; this history ultimately will be calibrated using an oxygen isotope stratigraphy (Flower, 1998, doi:10.2973/odp.proc.sr.152.219.1998), although that calibration has not been completed at this time. Because ice-core studies of the Greenland Ice Sheet (GIS) have shown that the GIS changed dramatically, and in some cases extremely rapidly, during at least the last interglacial stage (GRIP Members, 1993, doi:10.1038/364203a0), a detailed IRD record from the Southeast Greenland margin should provide insight into the longer term behavior of this sensitive component of the Northern Hemisphere climate system.

Experimental investigation of water, snow and granular ice effects on ice failure processes and impact loads

A large series of laboratory ice crushing experiments was performed to investigate the effects of external boundary condition and indenter contact geometry on ice load magnitude under crushing conditions. Four boundary conditions were considered: dry cases, submerged cases, and cases with the presence of snow and granular ice material on the indenter surface. Indenter geometries were a flat plate, wedge shaped indenter, (reverse) conical indenter, and spherical indenter. These were impacted with artificially produced ice specimens of conical shape with 20° and 30° cone angles. All indenter – ice combinations were tested in dry and submerged environments at 1 mm/s and 100 mm/s indentation rates. Additional tests with the flat indentation plate were conducted at 10 mm/s impact velocity and a subset of scenarios with snow and granular ice material was evaluated. The tests were performed using a material testing system (MTS) machine located inside a cold room at an ambient temperature of - 7°C. Data acquisition comprised time, vertical force, and displacement. In several tests with the flat plate and wedge shaped indenter, supplementary information on local pressure patterns and contact area were obtained using tactile pressure sensors. All tests were recorded with a high speed video camera and still photos were taken before and after each test. Thin sections were taken of some specimens as well. Ice loads were found to strongly depend on contact condition, interrelated with pre-existing confinement and indentation rate. Submergence yielded higher forces, especially at the high indentation rate. This was very evident for the flat indentation plate and spherical indenter, and with restrictions for the wedge shaped indenter. No indication was found for the conical indenter. For the conical indenter it was concluded that the structural restriction due to the indenter geometry was dominating. The working surface for the water to act was not sufficient to influence the failure processes and associated ice loads. The presence of snow and granular ice significantly increased the forces at the low indentation rate (with the flat indentation plate) that were higher compared to submerged cases and far above the dry contact condition. Contact area measurements revealed a correlation of higher forces with a concurrent increase in actual contact area that depended on the respective boundary condition. In submergence, ice debris constitution was changed; ice extrusion, as well as crack development and propagation were impeded. Snow and granular ice seemed to provide additional material sources for establishing larger contact areas. The dry contact condition generally had the smallest real contact area, as well as the lowest forces. The comparison of nominal and measured contact areas revealed distinct deviations. The incorporation of those differences in contact process pressures-area relationships indicated that the overall process pressure was not substantially affected by the increased loads.