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Reconstructing the size and glacial style of past ice-sheet advances requires interpreting complex glacial sedimentary facies. We use diatoms, a major component of Antarctic continental shelf deposits, to infer the physical conditions under which these deposits were emplaced. The degree of diatom fragmentation and the presence of diatoms of varying stratigraphic age in glacial sediments provide means to qualitatively gauge glacial mixing and transport. Here we report an experimentally calibrated index of diatom fragmentation that provides a simple but objective method of assessing the degree of subshearing imparted on marine glacial sedimentary deposits. By using a ring-shear device to subject diatomaceous sediment to stresses comparable to those beneath the Ross ice streams, we quantitatively assess patterns of diatom comminution resulting from compaction and from progressive shear stress. Elongate pennate diatoms are found to break disproportionately to discoid centric diatoms when subjected to shear stress; thus, a simple ratio of unbroken centric to pennate diatoms provides a reliable gauge of past shearing. Comparison of ring-shear results with a suite of previously analyzed sediments that represent a variety of glacial, glacial-marine, and hemipelagic settings of the Ross Sea and subglacial Ross Embayment demonstrates that this index can be employed for estimating relative subglacial stresses in this setting. Keywords: diatom, fragmentation, ring shear, Antarctic, Ross Sea, ice stream, till, diamicton, pennate, centric.

Slow, stable shear of granular materials in landslides, beneath glaciers, and along fault surfaces is common, despite little or no intrinsic strengthening of such materials with increasing deformation rate. Compacted, water-saturated sediments, subjected to constant stresses in a ring-shear device, sheared slowly without unstable acceleration in repeated episodes that included pore dilation during shear, attendant pore-water-pressure decline, and consequent strengthening, followed by gradual pore-pressure recovery and weakening. Time-averaged shear velocities (2-800 mm/d) depended inversely on the magnitude of pore dilation with shear and were significantly lower for fine-grained sediment than for coarse-grained sediment, owing to different rates of pore-pressure diffusion in the two materials. When sediment had dilated to its critical-state (steady) porosity and therefore could not dilate further, shear accelerated catastrophically. These data indicate that pore-pressure decreases and consequent strengthening caused by shear-induced dilation may not only suppress rapid shear of landslide debris, subglacial till, and fault gouge, but may also result in slow episodic shear at rates that depend on both material porosity and hydraulic diffusivity. Keywords: shear, porosity, pore-water pressure, landslides, till, gouge.

Results from laboratory experiments show that ice at its melting temperature regelated through dense, three-dimensional arrays of clasts abrading a rock bed. Regelation theory for an isolated, stationary array of clasts successfully approximates periods when ice separated from the bed, thermal gradients, and clast-bed contact forces, despite the relative motion of clasts and incoming heat from the bed. The process may entrain sediment at glacier beds and restrict deposition to zones where sediment in ice is concentrated, effective pressure is low, and basal melting is rapid.