Question

In glaciological studies of the Antarctic ice sheet, subglacial hydrologic systems—the networks of liquid water beneath kilometers of glacial ice—exert a profound control on basal sliding velocities and overall ice-sheet stability. Historically, glaciologists modeled basal water drainage predominantly through the framework of discrete, channelized conduits known as R-channels (Röthlisberger channels). These conduits form within the basal ice itself as frictional heat generated by turbulent water flow melts the channel walls faster than the immense overburden pressure of the overlying ice can close them through plastic creep. Because the cross-sectional capacity of R-channels increases nonlinearly with discharge, these discrete conduits exhibit an inverse pressure-discharge relationship: an increase in meltwater volume actually lowers the internal hydraulic water pressure, drawing water out of the surrounding basal interface and thereby increasing frictional traction between the ice and the underlying bedrock.

However, subsequent in situ borehole measurements and radar-sounding surveys beneath several major Antarctic ice streams revealed that subglacial water does not always organize into discrete R-channels. Over soft, unconsolidated till—the porous sedimentary layer common beneath fast-flowing ice streams—R-channels are mechanically unstable. In such geological settings, subglacial drainage operates via linked-cavity networks or broad, distributed water sheets flowing across the sediment interface. Unlike R-channels, distributed drainage systems exhibit a direct pressure-discharge relationship: surges in water input overwhelm the shallow conduits, driving up basal pore-water pressure. This elevated pressure reduces the effective normal stress at the bed, which partially decouples the ice from the substrate and accelerates glacial streaming.

A further complication arises where subglacial water encounters deep bedrock depressions, forming subglacial lakes. In these hydrological basins, geothermal heat flux continually melts the ice base, but as the water circulates and ascends along the sloping ice ceiling toward regions of lower overburden pressure and lower ice thickness, it encounters temperatures below the local pressure-melting point. Consequently, basal freeze-on (accretion) occurs, incorporating subglacial water into the bottom of the moving ice sheet as distinct, crystal-coarse accretion ice. Glaciologists have discovered that this accretion process physically strips water from the basal drainage system while simultaneously preserving microbiological samples from the subglacial lake in the ice column above.

According to the passage, which one of the following is an effect of an increase in meltwater volume within discrete R-channels?