engleski

Geochemical record of Holecene land use change and anthropogenic Pb deposition in a Northern Adriatic karst lake (Lake Vrana, Cres Island):

The geochemistry of Holocene sediments from Vrana Lake (Cres Island, Northern Adriatic) were used to trace historic land-use change. A total of 13 cores 1m long were obtained from the lake floor at the depth of 50m. The core intervals from 86 to 70 cm have Pb/Sc ratios of 2.4 in which lie within crustal value range has high radiogenic values (206Pb/207Pb ratio of 1.24). These values correspond to pre-Holocene palaeosoil ratio, which ranges from 1.228 to 1.245. The Pb/Sc ratios gradually increase to > 3.5 with a peak value of 5.8 in the interval from 56 to 58 cm (206Pb/207Pb ratio 1.210). The high lead anomaly corresponds to the Roman Pb mining peak encountered in other parts of Europe. The interval from 47 to 50 cm has lower 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 in the 700 AD there is change in land use 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%. During the dated interval from 40 cm to 18 cm (1400 to 1600 AD which corresponds to the early period of the Little Ice Age -LIA) the Pb/Sc ratios range from 4 to 6. Due to the unfavourable climatic conditions and a thin soil cover on a carbonate bedrock most of soil cover in the catchment was lost to erosion and deposited in the lake within approx. 200 years. The highest Pb/Sc ratios are found above 18 cm and range from 7 to 8 and the amount of calculated anthropogenic Pb reached 70% of the total Pb concentration. The 206Pb/207Pb ratios in this section range from 1.175 to 1.181, corresponds to the topsoil range from the highland karst hinterland. The top 10 cm interval shows a gradual upward increase of carbonate content and carbonate sedimentation has now almost reached 50% of the pre-erosion period (MWP). Although dramatic changes in lake sediment composition occurred due to deforestation during the MWP and soil erosion in the early stages of the LIA period, the sediments nevertheless contain a continuous history both background anthropogenic inputs of Pb as observed in other more remote regions of Europe during the past three millennia.

karst lake; little ice age; lead isotopes; landuse; antrophogenic impact

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