Luminescence geochronology of Pleistocene slack-water deposits in the Frasassi hypogenic cave system, Italy
Authors: Montanari Alessandro , Lüthgens Christopher , Lomax Johanna , Mainiero Maurizio , Mariani Sandro , Fiebig Markus 
Published: October 2019

In this study, we discuss the results from different luminescence dating methods applied to four samples of Pleistocene slack-water sediments from the Frasassi hypogenic cave system, in the northeastern Apennines of Italy. Two samples came from a well-sorted, fine sand deposit in the Grotta Grande del Vento cave (SDS site), while two others were taken from a borehole through a clayey deposit in the adjacent Caverna del Carbone cave (CDC site). Both sites are located at an elevation of ~235 m above sea level (asl), which corresponds to ~30 m above the thalweg of the Sentino River flowing through the Frasassi Gorge outside the cave. In the Frasassi multistory cave system, the elevation of 235 ± 5 m asl corresponds to the third karst level or “floor,” the minimum age of which from speleothem U-Th dating is ca. 130 ± 15 ka. The luminescence ages for the two samples from the SDS site are in good agreement with each other within error, just like the two samples from the CDC profile. Different luminescence dating protocols were used to determine the ages for each individual sample. By applying this comparative approach, and taking the luminescence characteristics of the samples into consideration (quartz optically stimulated luminescence, different feldspar luminescence signals), the ages could be based on the most robust measurement protocol. The ages presented here were all derived from measurements using the post-infrared infrared signal of potassium-rich feldspar stimulated at a temperature of 225 °C (pIRIR225). Incomplete bleaching of the luminescence signal prior to deposition, leading to age overestimation when not detected and corrected for, was not a significant factor for the samples under investigation, because ages calculated for luminescence signals with different bleachability yielded results in agreement within error. Bleaching can therefore be assumed to have been sufficient before the samples entered the cave system. The ages determined for both sites are reliable from a methodological standpoint. The pIRIR225 luminescence dates from the SDS sand range between 129 and 101 ka and are consistent with the minimum age for the third cave floor (~235 m asl) as obtained from previous U-Th dating. In contrast, the pIRIR225 luminescence dates obtained from the clay-rich CDC deposit range from 217 to 158 ka, which is consistent with the minimum age for the fifth subhorizontal cave level when measured from the modern water table, found at ~65 m above the present river thalweg. This apparent discrepancy may be due to the fact that the present entrance of the CDC cave was incised by the river on the south side of Frasassi Gorge sometime during the Eemian interglacial period (marine isotope stage [MIS] 5e), but, being part of a hypogenic karst system in an uplifting tectonic structure, the actual third floor was preexisting, thus anteceding the river incision. On the other hand, the fifth floor of the cave system, some 30 m above the third floor, was incised sometime during the interglacial MIS 7 at around 200 ka, at a time when the saturated phreatic third floor had already been formed and thus was capable of collecting the fine suspension sediment settling from muddy river water flooding the cave.