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Active Ice How, exactly, does warm ocean water erode an ice shelf? In a field study of an ice shelf at Pine Island, Antarctica, Stanton et al. (p. 1236 ) collected data from radar, seismic surveys, and oceanographic sensors inserted in holes bored through the ice shelf. The results show that localized, intensive melting occurs in a complex network of discreet channels that are formed on the underside of the shelf. This pattern of melting may limit the absolute rate of ice-shelf mass loss.

We use three‐dimensional (3‐D) temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid‐ocean ridge lithospheric structure. We find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along‐axis gradients in crustal temperature and a “pita‐pocket”‐shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle. In contrast, fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis. The overthickened crust produced by ridge‐hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling, creating an extremely weak lithosphere in a steady state model. We apply a simple two‐dimensional (2‐D) cyclic faulting model to across‐axis sections of temperature and rheological structure. Slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing. In contrast, fast spreading and hotspot‐affected segments are predicted to have much smaller variations in faulting styles, except in the immediate vicinity of a major transform fault. The presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness, spreading rate, and the ability of rift‐bounding normal faults to propagate along axis. For fast spreading and hotspot‐affected segments, the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma‐chamber‐supported isostatic features. This is in contrast to most slow spreading segments, where axial rift valleys are predicted to be a dominant morphological feature. For a slow spreading segment of great length, however, the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center.

[1] As part of the 1997-1998 Surface Heat Budget of the Arctic Experiment (SHEBA), a nearly yearlong record of upper ocean observations was obtained below a drifting ice camp in the Beaufort Gyre. A combination of observational and numerical modeling techniques are used to estimate heat fluxes across the under-ice ocean boundary layer. Over the Canada Basin, the upper pycnocline contained moderate heat, but strong stratification effectively insulated it from mixed layer turbulence. Average resulting heat fluxes at the base of the mixed layer (F pyc ) and at the ocean-ice interface (F 0 ) were small (0.3-1.2 and 0.2 W m À2 , respectively). Over the Chukchi Borderlands, the presence of relatively warm and salty Pacific origin water increased upper pycnocline heat content and reduced stratification, which permitted moderate F pyc and F 0 (2.1-3.7 and 3.5 W m À2 , respectively). Solar insolation was the dominant heat source during the final, summertime portion of the drift. During the heating period, F pyc was relatively small (0.4-1.5 W m À2 ) while F 0 was large (16.3 W m À2 ). The drift-averaged value of F 0 was 7.6 W m À2 . Energy budgets for the ice cover were constructed. The oceanic contribution to the budget during the portion of the drift over the Chukchi Borderlands, supported by entrainment of heat stored in the upper pycnocline, was responsible for a 15% reduction in ice growth. During the summer heating season, the F 0 estimates were larger than the latent energy changes associated with basal melting.

Based on these findings, both 10-Hz GPS and timing gates provide valid measures of 40-m V assessment compared with a radar gun. However, as error did exist between measures, the same testing protocol should be used when assessing 40-m V over time. Furthermore, in light of the above results, it is recommended that when assessing changes in GPS-derived V over time, practitioners should use the same unit for each player and perform the analysis with the same software, preferably Catapult Openfield.