Cryogenic cave minerals recorded 1889 CE melt event in northeast Greenland

Abstract. The investigation of cryogenic cave minerals (CCMs) has developed in recent decades to be a particularly valuable proxy for palaeo-permafrost reconstruction. Due to difficulties, however, in obtaining reliable chronologies with the so-called “fine” form of these minerals, such studies have thus far utilised the “coarse” form. In this study, we successfully investigate the northernmost-known deposit of fine-grained cryogenic cave minerals (CCMs), which are situated in Cove Cave (Greenlandic translation: Eqik Qaarusussuaq), a low-elevation permafrost cave in northeast Greenland (80° N). The Cove Cave CCMs display a complex mineralogy that consists of fine-grained cryogenic cave carbonates (CCCfine) as well as sulphate minerals (gypsum, eugsterite, mirabilite, and löweite). In comparison to CCCfine from the mid-latitudes, positive δ13C values (7.0 to 11.4 ‰) recorded in Cove Cave are similar. In contrast, Cove Cave CCCfine δ18O values are ca. 8 to 16 ‰ lower. Furthermore, despite previous CCCfine dating efforts being unsuccessful, here we demonstrate that precise dating is possible with both isochron-based 230Th/U dating and 14C dating if the dead carbon fraction is reliably known. The dating result (65 ± 17 a BP; 1885 ± 17 CE) shows that the CCMs formed during the late Little Ice Age, a time interval characterised by cold temperatures and abundant permafrost in northeast Greenland, making water infiltration into Cove Cave dependent on water amount and latent heat. We relate the CCM formation to a combination of black carbon deposition and anomalously high temperatures, which occurred over a few days, in the summer of 1889 CE. Such extreme conditions led to widespread melting over large areas of the Greenland ice sheet. We propose that the anomalous (weather) conditions of 1889 CE also affected northeast Greenland, where the enhanced melting of a local ice cap resulted in water entering the cave and rapidly freezing. While CCCfine and gypsum likely precipitated concurrently with freezing, the origin of the other sulphate minerals might not be purely cryogenic but could be linked to subsequent sublimation of this ice accumulation in the very dry cave environment.