Evidence of Holocene surface and near-surface palaeofires in karst caves and soils

Black deposits covering speleothems, cave walls and floors in Postojna Cave and Predjama Cave (SW Slovenia), and charcoal found in soil outside the Postojna Cave, were studied in order to establish their origin and age. SEM/EDS analysis of these deposits from both caves indicated that they are charred plant remains, very similar to oxidised charcoal in soil. Stable carbon isotope delta C-13 values of -29.41%. (Crna Jama, P1), -26.9% (Predjama), and -25.1%. (charcoal in soil) confirm their origin from biomass burning. Radiocarbon dating from Postojna Cave (Crna Jama (P1) 8394 +/- 35 cal yr B.P. and Pisani Roy 8235 +/- 25 cal yr B.P.) suggests that the oldest analysed black deposits formed in the Early Holocene (Mesolithic). The age is consistent with soil charcoal found at 1 m depth in the surrounding area. Black deposits from Predjama were dated to 4095 +/- 25 cal yr B.P. (Eneolithic) and were caused by burning fires at the cave entrance, as supported by archaeological remains. Prehistoric (2375 +/- 20 cal yr B.P.) charcoal from the upper part of the soil profile (0.5 to 0.6 m) and historic (1240 +/- 29 cal yr B.P. or 745 cal yr CE) black deposits from Postojna Cave (Crna Jama, P2) are related to forest fires, which are most likely human-induced. We interpret the oldest dated black deposits in Postojna Cave as traces of Early Holocene natural forest fires, since fire activity inside Crna Jama and Pisani Roy is not supported by archaeological remains. Favourable cave entrance morphology and microclimate allowed smoke to enter the cave during the winter period and deposit on speleothems. But an anthropogenic cause of forest fires cannot be excluded, since there are well-documented Mesolithic archaeological cave sites located close to the area. The study shows that black deposits on speleothems in caves are an important palaeoenvironmental record and proxy for Holocene climate. (C) 2017 Elsevier B.V. All rights reserved.