ABSTRACT
Antarctic ice streams are associated with pressurized subglacial meltwater but the role this water plays in the dynamics of the streams is not known. To address this, we present a model of subglacial water flow below ice sheets, and particularly below ice streams. The base-level flow is fed by subglacial melting and is presumed to take the form of a rough-bedded film, in which the ice is supported by larger clasts, but there is a millimetric water film which submerges the smaller particles. A model for the film is given by two coupled partial differential equations, representing mass conservation of water and ice closure. We assume that there is no sediment transport and solve for water film depth and effective pressure. This is coupled to a vertically integrated, higher order model for ice-sheet dynamics. If there is a sufficiently small amount of meltwater produced (e.g. if ice flux is low), the distributed film and ice sheet are stable, whereas for larger amounts of melt the ice-water system can become unstable, and ice streams form spontaneously as a consequence. We show that this can be explained in terms of a multi-valued sliding law, which arises from a simplified, one-dimensional analysis of the coupled model.
ABSTRACT
We studied the induction of hepatic alkaline phosphatase by colchicine, a drug which impairs bile flow. By analogy to bile duct obstruction, where retained bile acids induce alkaline phosphatase, we hypothesized that the mechanism of induction involves the drug's cholestatic property and subsequent intrahepatic retention of bile acids. After administering colchicine to bile-fistula rats, we monitored bile flow. In colchicine-treated rats which did not develop cholestasis, we varied the transhepatic flux of taurocholate 15-fold over the physiologic range in order to study the role of intrahepatic bile acids. Induction of alkaline phosphatase was observed when cholestasis was absent and the bile acid flux was 5% of normal. The magnitude of enzyme induction was not augmented by increasing the transhepatic flux of taurocholate. The results demonstrate that colchicine induces hepatic alkaline phosphatase by a mechanism which operates independently of cholestasis and is not modified by the level of bile acid in the liver.
