Biological records of the late Pleistocene and Holocene environmental changes from two italian crater lake sediments: results from an european interdisciplinary research project (PALICLAS)

Abstract

: In the framework of the EU funded research project Palaeoenvironmental Analysis of Italian Crater Lake and Adriatic Sea Sediments (PALICLAS), several 8.5 to 14 m long cores were collected from two neighbouring crater lakes, Albano and Nemi (Central Italy) in 1994. The results reported in the present paper refer to three cores: Two from Lago di Albano (PALB 94-1E and PALB 94-6B; water depth of 70 m and 30 m, respectively) and one from Lago di Nemi (PNEM 94-1B; water depth of 30 m). This paper summarises the results from the study of algal and sulphur photosynthetic pigments, diatom, chrysophyte cyst, ostracod and cladocera fossil remains. Based on AMS calibrated radiocarbon dates, core correlations, tephrochronology and pollen, the boundaries between Glacial/Late Glacial and Late Glacial/ Holocene were established. The core in Lago di Abano collected at -70 m from the surface, spans the longest record (ca. 30 ka BP). The core from the shallowest site in Lago di Albano is unique because, except for the uppermost ca. 8.5 cm, it represents pre-Holocene deposit only (i.e. the isotope stage 2). The core from Lago di Nemi pertains only to the Holocene (except for a few centimetres at the bottom of the core). At the base of the cores from Lago di Albano, a thick volcanoclastic deposit, dated ca. 25±5 ka BP in PALB 94-1E, was present. Two other tephra layers (the Etna and Avellino tephras) were detected within the cores of both lakes. The sediments deposited during the glacial period commonly contain low concentrations of biological remains. However, several periods of relatively high productivity (mesotrophic conditions) were observed from 25-30 ka BP to ca. 17 ka BP (tephrochronology). These rapid, sharp, probably related to climatic fluctuations (cycle of ca. 200-500 years), as observed in all the biological record stratigraphies, are in phase with the physical and chemical trends. Ostracod valves were found only in sediments deposited during the glacial period, when there were high carbonate concentrations and oxygenated bottom conditions. A dramatic, oscillating increase in biological activity occurred in the early, mid-Holocene. High concentrations of carotenoids from sulphur photosynthetic bacteria indicate that these sediments accumulated in anoxic bottom waters. This high level of eutrophication, which is likely to be related to a warmer climate, was also identified by other independent parameters, such as high content of organic matter, diatoms and cladocera concentrations and indicator species (e.g. small forms of Stephanodiscus). In the mid-Holocene, a sudden increase in minerogenic clastic input, accompanied both by a sharp decline in pigment concentrations in biota densities and by a change in diatom and cladocera assemblages, is here interpreted as the first discernible impact of human activity in the catchment (i.e. deforestation). The close link between catchment surface processes and in-lake productivity is shown by the clear relationship between forest clearance and the cladocera assemblages from lakes Albano and Nemi, which shifted from Daphnia to Bosmina. Taken together, pigments and fossil organisms allow us to make a detailed reconstruction of the palaeoenvironment of Lago di Albano, which includes productivity, variations in the lake water level and climate. Additional detailed information about the Holocene was obtained from Lago di Nemi, where very high and fluctuating concentrations of pigments from algal and sulphur photosynthetic bacteria were detected from near the base of the core to about 5 m (6 ka BP), supporting the palaeoclimatic-related evolution reconstructed for Lago di Albano.