Bacteria as paleoenvironmental proxies: the study of a cave Pleistocene profile

Abstract. Caves are well-known archives that preserve valuable information about the past, relevant for reconstructing past climates and environments. We sampled sediments from a 480 cm deep profile, and 16S ribosomal ribonucleic acid (rRNA) gene-based metabarcoding analyses were undertaken that complemented lithological logging, sedimentology, and optically stimulated luminescence (OSL) dating. These analyses revealed different sedimentation conditions along the profile with various water inputs. The OSL age of the sediments places the profile between 74.7 ± 12.3 to 56 ± 8 ka (base to top). However, the more recent Last Glacial Maximum (LGM) paleofloods might have occurred in the upper and lower passages of the cave. Bacterial compositions changed with depth, from soil bacteria (present in the upper part of the sediment profile) to thermophilic/sulfurous bacteria (abundant in the deeper samples of the profile). Considering the thermophilic bacteria, we could only assume their origin from a surface of hot sulfurous springs, old thermal springs, or sapropel sediments.