Cryoconite: The dark biological secret of the cryosphere

Cryoconite is granular sediment found on glacier surfaces comprising both mineral and biological material. Due to its dark colour, cryoconite efficiently absorbs solar radiation and ‘drills’ quasi-circular holes up to tens of centimetres deep into glacier ice surface. These are known as cryoconite holes, and are very common features on ablating ice surfaces worldwide, although cryoconite can also be found in other environments such as stream deposits and scattered on ice surfaces (Hodson et al., 2008). Despite having been recognised as important glaciological and biological entities in the nineteenth century (e.g. Agassiz, 1846; Nordenskiöld, 1870; Nansen, 1882) cryoconite and cryoconite holes still remain poorly understood. They are thought to be ”ice cold hotspots” of biodiversity and microbial activity on glacier surfaces (Edwards et al. 2012) and are regularly examined; however, studies have been invariably reductionist in approach and based upon assumptions of discrete habitat boundaries and simple thermodynamics established by early polar explorers over a century ago. We appraise the literature on cryoconite for the first time, with the aim of evaluating current knowledge and suggesting areas for future research. Our review describes the properties of cryoconite and the environment in which it is usually found, summarises current knowledge of cryoconite biology and discusses its wider significance. We generally focus upon cryoconite in the Arctic in summer, with Antarctic and alpine settings examined individually. We conclude by comparing the current state-of-the-science with that at the turn of the twentieth century, and suggest