The grounding zone of the Ross Ice Shelf, West Antarctica, from ice-penetrating radar

As ice streams flow into the Ross Ice Shelf, West Antarctica, their bed coupling transitions from weak to transient to zero as the ice goes afloat. Here we explore the nature of the bed across these crucial grounding zones using ice-penetrating radar. We collected several ground-based 2 MHz radar transects across the grounding zones of Whillans and Kamb Ice Streams and inferred bed-reflectivity changes from in situ measurements of depth-averaged dielectric attenuation, made possible by the observation of both primary and multiple bed echoes. We find no significant change in the bed reflectivity across either grounding zone. Combined with reflectivity modeling, this observation suggests that a persistent layer of subglacial water (>∼0.2 m) is widespread several kilometers upstream of the grounding zone. Our results are consistent with previous inferences of gradual grounding zones across this sector of the Ross Ice Shelf from airborne radar and satellite altimetry. Separately, the only clear bed-reflectivity change that we observed occurs ∼40 km downstream of the Kamb Ice Stream grounding zone, which we attribute to the onset of marine ice accretion onto the base of the ice shelf. This onset is much nearer to the grounding zone than previously predicted.

Morphological Transition of Epitaxial Rhodium (111)

ABSTRACTWe report a global morphological transition in the nucleation and growth of epitaxial Rh (111). The transition occurs near 600 K, about 1/4 of the Rh melting temperature, and is signaled by a change in the shape of the surface features from fingered to compact. The transition appears to be related to a change in the critical nucleation size from 1 to 2 atoms. On both sides of the transition, there lies a regime of persistent layer-by-layer growth indicated by a minimum in surface roughness and by the presence of RHEED oscillations. The general surface features exhibit well defined length scales that are not self-affine, and as growth proceeds they increase in size following a power-law dependence on film thickness with a temperature-independent exponent of 0.33 ± 0.03. The results suggest a general pathway to the layer-by-layer growth of close-packed metals.

The Relationship of Midwater Trawl Catches to Sound Scattering Layers off the Coast of Northern British Columbia

In 1964 and 1965 three midwater trawl cruises off the continental slope near the Queen Charlotte Islands were made to study the relationship of the catches to sound scattering layers.During the daytime four layers were distinguished, not all necessarily present at one time. In summer the most intense and persistent layer lay at 220–230 m; others were at 330–400, 185, and 90 m. In fall the main and deep layers were shallower. Vertical migration occurred at dusk and dawn. The main layer frequently split into three parts with different vertical migration patterns. At dawn a layer gradually appeared at about 90 m, intensified rapidly, and descended to the daytime level.Catches show fishes are associated with the sound scattering layers. The largest day catches were from the main layer. At night fish did not appear to be concentrated to the same extent relative to the layers. The largest night catches were usually smaller than the largest day catches and shallower.Myctophids of eight species, four common, made up 80% of the catch, chauliodontids 6%, argentinids 4%, melamphids, rockfish, and larval flatfish 2% each. The remaining 2% comprised 27 species in 21 families. Depth distribution and vertical migration varied considerably with the species.Myctophids with gas-filled swim bladders were apparently dominant in the main layers whereas species without swim bladders — stomiatoids, argentinids, and melamphids — or myctophids with fat-filled swim bladders were dominant below the main layer and in the deep layer. Fish with no swim bladders or with fat-filled ones may make longer vertical migrations than those with gas-filled bladders.