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Observations of the sungrazing comet C/2012 S1 (ISON) were carried out usingthe Atacama Large Millimeter/submillimeter Array (ALMA) at a heliocentricdistance of 0.58-0.54 AU (pre-perihelion) on 2013 November 16-17. Temporallyresolved measurements of the coma distributions of HNC, CH$_3$OH, H$_2$CO anddust were obtained over the course of about an hour on each day. During theperiod UT 10:10-11:00 on Nov. 16, the comet displayed a remarkable drop inactivity, manifested as a $>42$% decline in the molecular line and continuumfluxes. The H$_2$CO observations are consistent with an abrupt, $\approx50$%reduction in the cometary gas production rate soon after the start of ourobservations. On Nov. 17, the total observed fluxes remained relativelyconstant during a similar period, but strong variations in the morphology ofthe HNC distribution were detected as a function of time, indicative of aclumpy, intermittent outflow for this species. Our observations suggest that atleast part of the detected HNC originated from degradation of nitrogen-richorganic refractory material, released intermittently from confined regions ofthe nucleus. By contrast, the distributions of CH$_3$OH and H$_2$CO during theNov. 17 observations were relatively uniform, consistent with isotropic outflowand stable activity levels for these species. These results highlight a largedegree of variability in the production of gas and dust from comet ISON duringits pre-perihelion outburst, consistent with repeated disruption of the nucleusinterspersed with periods of relative quiescence.

ALMA mapping of rapid gas and dust variations in comet C/2012 S1 (ISON): new insights into the origin of cometary HNC