The Lower Palaeozoic (Upper Ordovician-Silurian) succession of North Africa contains one of the world's most prolific black shale source rocks, yet the origin of these rocks remains contentious. The black shale of the Batra Formation in Jordan was deposited at high palaeolatitude during rapid Hirnantian to early Silurian deglaciation. Here we report geological and organic geochemical results that provide evidence for an increase in photic zone primary productivity during ice melting. The decay of this organic matter through oxidative respiration resulted in euxinia, which enhanced the potential for organic matter preservation. The occurrence of isorenieratane in all samples indicates euxinia extended from the photic zone to the sediment water interface. The stratified basins and fjords of east Antarctica provide a likely modern analogue.