Question

During the abrupt cooling of the Younger Dryas (circa 12,900 to 11,700 years ago), the dynamics of the southern margin of the Laurentide Ice Sheet underwent significant restructuring. Traditional glaciological models attributed the episodic accelerations of the ice sheet's ice streams predominantly to thermal insulation from thick overlying ice, which raised basal temperatures to the melting point. However, recent geochemical analysis of proglacial lake sediment cores has revealed that basal shear stress was modulated primarily by subglacial hydrological networks rather than thermal regime alone. Researchers examining trace element concentrations—specifically the ratio of manganese to calcium ($Mn/Ca$) in benthic foraminifera tests—found distinct episodic peaks corresponding to transient pulses of highly oxygenated, subglacial meltwater. These pulses coincided with periods of rapid ice-bed decoupling, during which pressurized basal water reduced effective overburden pressure. Notably, while previous hypotheses assumed that meltwater conduits maintained continuous, low-pressure drainage that stabilized the ice mass, the $Mn/Ca$ proxy evidence demonstrates that subglacial drainage operated via an inefficient, distributed network of cavities. This distributed configuration periodically trapped subglacial water under high hydrostatic pressure, thereby initiating transient ice-surging events prior to the eventual formation of channelized tunnel valleys that relieved the pressure.

According to the passage, the geochemical evidence derived from manganese-to-calcium ($Mn/Ca$) ratios in benthic foraminifera indicates which of the following regarding subglacial drainage during the Younger Dryas?