Spontaneous self-combustion of organic-rich lateglacial lake sediments after freeze-drying

We report and investigate for the first time spontaneous self-combustion of freeze-dried lacustrine sediments immediately after ventilation of the vacuum freeze dryer chamber. The smouldering and flameless combustion lasted for approximately 10–20 min and reached temperatures of 357 °C. Self-combustion mainly occurred in aluminium boxes containing sediment bars taken for thin section preparation. About 40 % of these samples were affected, most of them originated from the basal approximately 3-m-thick finely laminated lateglacial sediment interval. The combustion process caused disintegration of siderite to iron oxides (hematite and magnetite) and burning of organic matter to pyrogenic carbon leading to a lowering of total inorganic and organic carbon contents to 1 %. The total sulphur content of one combusted bulk sample did not change, but the alteration of sulphur contents in different sediment components suggests a redistribution of sulphur within the sediment. We assume that the self-combustion process was initiated by exothermic oxidation reactions, which were favoured by a combination of factors including the presence of abundant fine-grained iron sulphides in the organic-rich sediments. Self-combustion could be prevented by ventilating the vacuum chamber after freeze-drying with N2.