engleski

Late Holocene landuse change record from Vrana Lake catchment on Cres island (Norhern Adriatic, Croatia)

Lake sediment cores, coluvial and floodplain soils from closed karstic lake catchment of Vrana Lake (Vransko jezero, Cres Island, Northern Adriatic) show a record of intensive late Holocene land-use change. Geochemistry and Pb/Sc ratios as well as Pb isotope ratios together with the mineralogical composition of sediment and soils were used to reconstruct Late Holocene events in the catchment. The area of the Vrana Lake catchment basin is 33 km2, and the lake has a surface of 5.75 km2. An average annual water level of the Lake is about 13 m above sea level and the bottom of the swallow hole type of depression is at 61.3 m below the mean sea level. The flat part of the lake floor has a depth of 50 to 52 m (39-42 m below sea level). The present deep water lake es¬ tablished at about 8.5 ky BP (about 9.6 cal. ky BP, Schmidt et al., 2000) which corresponds to an pluvial period, which was detected at the same time for the whole Adriatic basin (Oldfield et al., 2003) and the Holocene sea level rise (Surić et al., 2005). Due to intensive loss of soil cover the famous Italian traveler Fortis (1771) described the area of the Vrana Lake catchment as „ Arabia petrea“ . Cores (1 m long) obtained from the lake floor at the depth of 50 m covered a time span of the last 5000 years and soil cores of 5.5 m the time span of the last 8500 years. The increased influx of soil dust indicated by sediment geochemistry was caused by soil erosion after forest clearing in the lake catchment, that started before the Roman times and peaked between 1400 and 1650 AD. Fluctuations in water levels of this closed karst lake catchment based on fossil pollen records were within the range of +/- 4 m of the average 20th century lake level (+12m). The deepest lake sediment core intervals from the central part of the lake have Pb/Sc ratios of 2.4 which lie within crustal value ranges and have high radiogenic values (206Pb/207Pb ratio of 1.24). These values correspond to Pleistocene loess ratio, which ranges from 1.228 to 1.245 in. The Pb/Sc ratios gradually increase to > 3.5 with a peak value of 5.8 (206Pb/207Pb ratio 1.210) in the core interval which corresponds to the Roman Pb mining peak encountered in other parts of Europe as well as lead concentrations and the206Pb/207Pb ratios slightly higher indicating the post Roman Pb mining decline (400-900 AD). As a consequence of the arrival of Croat grazing farmers after 700 AD there is change in land use (increased erosion) during which the Pb/Sc ratios slightly drop (3.1) and 206Pb/207Pb ratios increase but do not reach the paleosoil values. The influx of catchment soil material causes a gradual decline of CaCO3 from 65% to 25% in lake sediments. Due to the unfavorable climatic conditions and a thin soil cover on the carbonate bedrock, most of soil cover in the catchment was lost to erosion and deposited in the lake within approximately 200 years.

Karst lake; landuse; climate change; lead isotopes; Vrana Lake

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