In episode 6 Goodsir begins feeding Jacko the canned food and recording his observations. Later on Jacko goes crazy and ends up killing herself. Goodsir examines the corpse and discovers Jacko died of lead poisoning.
Some were oversealed and the lead dripped inside and gave lead poisoning to the food, production designer Jonathan McKinstry said. Over the course of the series, that conjecture becomes less and less of a hypothesis and more of a reality for the various men aboard both the Terror and the Erebus.