The last ca 2000 years palaeolimnology of Lake Candia (N. Italy): Inorganic geochemistry, fossil pigments and temperature time-series analyses

Abstract

The palaeoenvironmental history of Lake Candia, a small, shallow, eutrophic lake in Northern Italy, is described for the last ca 2000 years. Sediment samples from a sediment core collected in autumn 1995 were analysed for a range of palaeolimnological indicators, which included the principal algal and sulphur photosynthetic bacterial pigments, as well as magnetic susceptibility, organic matter, carbonates, organic carbon, total nitrogen, total sulphur and various forms of phosphorus. An accurate sediment chronology was determined using 210Pb, 137Cs and 14C. The results show that throughout a first, long phase of the history of this period (from ca AD 100 to 1830; zone 1) the sediments have an organic carbon content of ca 10% d.w. and low concentrations of algal pigments, suggesting a moderately productive environment. Sedimentary carotenoids unique to anaerobic photosynthetic bacteria indicate a seasonally hypolimnetic anoxia during the whole ca 2000 year period. Clear effects of climate changes on lake productivity were inferred from the carotenoid, β-carotene, okenone and organic carbon estimates. Values were higher in the warm periods before AD ca 660 and during the so-called Little Optimum of the Medieval Warm Epoch (AD ca 1100-1300), and lower during cold moist periods, such as the main phase of the Little Ice Age (AD ca 1550-1700). After AD ca 1830 (zone 2), anthropogenic impacts resulted in a sharp increase in lake trophic state, leading first to a decoupling of the trophic state from natural (climate) variability, and then to "cultural" eutrophication. The onset of this latter process in the Turin area has been set around 1830, when a sharp increase of sedimentary sulphur concentration took place.