Geochronology of Hg pollution in italian sub-alpine lakes

The deep, large subalpine lakes (DSL), located south of the Alps between Italy and Switzerland, are regionally important economic and environmental resources. DSL share several common morphological features (e.g., narrow and elongated shapes, steep sides, considerable depths, and most often north-south orientation), although the lithological and petrographical composition of their watersheds differs markedly because of the geological complexity of the Alpine region. Most of the DSL experienced eutrophication during the 20th century and their present trophic status varies from oligotrophy (e.g., Lake Maggiore, thanks to recovery measures) to eutrophy (e.g., Lake Lugano). To assess how regional vs. local factors influence the Hg cycle in the DSL, several sediment cores covering the last 100 to 700 years (depending on core length and sedimentation rates) were collected at suitable locations of DSL. The deepest sections of each core were usually free from human influences (pre-1850) and show that background Hg levels (0.05-0.1 mg kg-1) are similar across the DSL despite differences in watersheds composition. Mercury content in the top-layers of the cores remains above background levels in all DSL and moderate to strong anthropogenic impact is observed not only in Lake Maggiore (for which a well-documented history of Hg pollution exists), but also in lakes Orta and Como. Analysis of the complete Hg profiles is being undertaken and will elucidate how Hg deposition in DSL responds to changes in trophic status and regional climate patterns.