The sedimentary filling of small depressions on Central Apennine till deposits: A contribution to the understanding of the post-glacial environmental variations

Abstract

The moraines of the last glacial maximum preserved on the Central Apennine massifs include many small basins (from tens to hundreds of square meters wide) draining in closed depressions. The bottom of such depressions are formed by deposits made of aeolian, colluvial and lacustrine sediments, tephra layers and soils. The sediments have been studied and interpreted in order to understand the Lateglacial and Holocene environmental variations. The small depressions were found at altitudes including between 1600 and 2100 m, in places far from the roads: therefore the sediments were sampled by means of an auger. The correlation between the sediments studied on the different massifs has been obtained using radiocarbon datings and some stratigraphic markers: a loess formed mainly by quartz, two tephra layers (the Neapolitan Yellow Tuff, dated ca. 12,000 years BP, produced by an eruption of the Phlegrean Fields volcano, and the "Duchessa" tephra, dated ca. 4000 years BP, of still unknown origin) and a volcanic minerals-rich layer (Meta layer: perhaps another tephra) covering soils dated ca. 3400-3500 years BP The sediment and soils, filling the small depressions, showed evidence of some environmental variations during the late Upper Pleistocene and Holocene. Some of such variations are a consequence of the climatic changes due to the glacial - interglacial transition, involving changes in temperature and precipitations. In particular, aeolian reworking of the Neapolitan Yellow Tuff occurred during the Younger Dryas. The Holocene, until approximately 4000 years BP, seems to have been characterized by morphological stability; beginning from that date until the present, at least three phases of instability took place, dated after 4000÷4300, 3400÷3500 and 1600 years BP. The morphological stability of the first part of the Holocene is testified by the presence of a soil: its development could have also been supported by the presence of silt horizons (a tephra layer and aeolian and colluvial sediments deriving from its reworking) that have remarkablly supported the expansion of the vegetation cover. In some places, one can observe that the environmental variations, pointed out by the sediments, seem to have been more frequent during the late Holocene than in the period between the Last Glacial Maximum and the fall of the Neapolitan Yellow Tuff tephra, dated ca. 12,000 years ago. As a matter of fact, before the fall of the tephra layer, the glacial till grain size and permeability did not allow for superficial running water: into the small basins the morphological stability prevailed, but the coarse grain size of the till inhibited the development of the soils; apart from the thin layer of loess, any sedimentation occurred in the basins, while, in other environments, during the same period, the climatic changes produced strong sedimentary variations. The silty tephra, penetrating between the glacial drift and covering it, could have reduced considerably the superficial permeability. This supported either the development of soils, or, in periods of reduction of the vegetation cover, the water runoff and, therefore, the erosion of the soils. Taking into consideration the Holocene environmental variations, one can observe that their number is greater during the period after 4000 years BP than in the previous one: that is due either to the infuence of climatic variations, recorded also in other environments, or, probably, to the anthropic impact on some of the studied places.