14C constraints to the Holocene aggradation of three major tributary valleys of the Tiber River in Rome
Description
By means of core data collected in a dedicatedly performed 35 m deep borehole (CAF-S2), seven 14C ages constraints to sediment aggradation, and new chronostratigraphic information from previously unpublished and new borehole stratigraphic logs, we reconstruct the post-glacial aggradation of the alluvial successions occurring in three tributary valleys draining the left hydrographic basin of the Tiber River in central Rome: Caffarella, Murcia and Grottaperfetta valleys. We combine the new information with that achieved in the major Tiber Valley by previous work providing evidence for a widespread marked increase in the water transport capacity within the entire catchment, highlighted by the sudden deposition of gravel with diameter of the pebbles ≤2 cm, occurred 5200 through 4000 yr BP. In the Tiber Valley, this gravel horizon is emplaced at the bottom of a fining-upwards aggradational succession that, 4000 through 2800 yr BP, re-filled a deep paleo-incision excavated in the former 5500 yr BP alluvial plain, suggesting that a lowering of the base level is the cause of the increase in capacity of transport within the Tiber catchment. In contrast, a corresponding phase of non-deposition is observed in the lowest stretches of the investigated tributary valleys, concomitant with the accumulation of a 4 m thick fine gravel horizon in the higher portion of the Caffarella Valley. Such framework is interpreted as the result of retrograde erosion from the Tiber mouth towards the inland catchments, which didn't affect the tributary valleys due to the short lasting of the sea-level fluctuation. While no evidence for a significant sea-level fall 5200 through 4000 yr BP occurs in the global records, this time span is broadly coinciding with the "4.2 ka Event" which is considered a global cooling and drying period lasting 4.3 through 3.8 ka. Therefore, we propose that the observed "Mid-Holocene transgressive phase" affecting the Tiber catchment 5.2 - 3.8 ka represents the local evidence of the 4.2 ka Event, possibly extending back in time its beginning, and we suggest that further investigation is necessary in order to verify whether any significant sea-level fluctuation is associated to this climate event.