The upper Quaternary sedimentary sequence at the Rieti Basin (central Italy): a record of sedimentation response to climatic changes

Lithostratigraphy, chronology and depositional environments of sixteen Holocene alluvial fill sediment cores from the southern part of the Conca of Rieti (central Italy) was established by sedimentological, palaeontological and 14C analyses. The study area experienced a lacustrine (or palustrine) conditions and the water level was effectively controlled by a natural damming of the valley due to intermitted travertine deposition at Le Marmore site. Apart from cores S10, S13, S14 and S8 which, at their bottom, cored sediments referable to ?Upper Pliocene and Middle Pleistocene which are not the object of this paper, as a rule the drilled sediments can be referred, bottom to top, to the following four episodes: interval a, corresponding to freshwater lacustrine/palustrine environment affected by alluvial fan progradations from the edges of the valley and active by ?Middle/?Late Pleistocene to early Holocene; in this interval, several hiatuses are supposed but are not evidenced by the bad state of preservation of the sediment cores; interval b, from 361 to 371–376 m a.s.l., recording a decrease of the water level, bracketed between the 14C ages of 6000 and 2700 yr B.P., which produced the formation of distinct shallow ponds/bogs, rich in aquatic vegetation; interval c, from 381 to 384 m a.s.l., indicating a further rise of the water level, recorded by sediments containing abundant molluscs and ostracods; interval d, the upper part of the cores, records an almost continuous drying up the area as shown by abundant hydrophilic land molluscs; and interval e, the top of all sediment cores, representing the present-day soil. It has been found that the water level fluctuations occurring in the ancient Conca are chronologically consistent with the postglacial climate pattern. It is further noticed that the episodes of water highstand correspond to the Boreal–Atlantic and Subatlantic climatic ameliorations, respectively, and that the water level dropped during the global cooling episodes recurred from ca. 6000 to 2100 yr B.P. The resulting palaeoenvironmental history agrees with that previously reported for the northern reach of the Conca of Rieti, thus suggesting a common setting for the whole area.