I laghi effimeri postglaciali del Massiccio de La Meta (Parco Nazionale d'Abruzzo, Italia centrale): segnalazione e significato paleoclimatico

Abstract

Closed basins, which contained ephemeral lakes have been identified on La Meta massif (National Park of Abruzzo) in central Italy. These basins are delimited by morainic ridges and arches dating to the phases of glaciers retreat during the Upper Pleistocene Last Glacial Maximum. Study and dating of the postglacial ephemeral lakes' sediments allowed for the reconstruction of hydrological balance variations, and in turn, of the effects of the principal climatic phases of the Late Glacial and Holocene on the environment. The sediments of the basins were correlated through-lithostratigraphic evidence, two 14C dates and two tephra levels. It was concluded that the time following the retreat of the principal glaciers can be divided into at least three periods, i) The first, oldest period corresponds to the Late Glacial and includes probably the first part of the Holocene. It is characterized by the deposition of sediments of primarily detritic origin, because of the limited morphological stability caused also by scarcity of vegetation. The precipitations-evaporation difference (P-E) in the catchement area was positive given that low temperature limited evaporation, ii) The intermediate period coincides with the first part of the Holocene, or, more likely, with the period between ca. 7000 and 5000 years B.P., and is characterized by the sedimentation of deposits of primarily chemical origin favoiured by the high evaporation caused by high temperatures, morphological stability and abundance of vegetation. P-E difference in the catchment area was barely positive, iii) The third, youngest period coincides with the second part of the Holocene, and lasted until historic times. It is characterized by the deposition of sediments of primarily detritic origin, because of the decreased morphological stability as caused by the vegetation cover reduction. P-E difference in the catchement area was positive. At present the mean annual precipitation equals 1600-1700 mm and the ephemeral lakes are dry. Soils cover their bottom areas, and the basins are characterized by morphological stability. P-E difference in the catchement area is negative. Independently of the cause of the present P-E difference negative value, we can say that such an environmental situation was not documented for other periods of the Holocene. The negative P-E may simply depend upon the recent increase of the vegetation cover, which reduces percolation towards the basins, but may also be indicative of a temperature increase or modification in rainfall regime.