Natural and anthropogenic changes in a lake-forest system in Bukovina (Romania) since 1340 AD documented by sedimentary organic geochemistry (C, N and n-alkanes)

摘要

Lake Bolatau-Feredeu is a small (surface: 0.3 ha; catchment area: 31 ha) landslide-dammed lake in Bukovina (Eastern Carpathians, Romania). Elemental concentration, stable isotope composition of carbon and nitrogen, and n-alkane composition of the saturated hydrocarbon fraction of the organic material were analyzed along the sediment profile from Lake Bolatau-Feredeu covering the past 700 years. The ranges of delta C-13 values (- 30.5 to - 26.6 parts per thousand) and C/N atomic ratios (10.5 and 15.4) placed the organic material of the recent Bolatau-Feredeu sediment between the C3 Land Plants and the lacustrine algae. The n-alkane proxies provide better distinction between organic matter inputs. Detected major shifts in the n-alkane composition and the C and N concentrations along with the stable isotope compositions pointed towards three distinct stages in the environmental history of the lake-catchment system. The proxy information indicate a substantial landscape change characterized by deforestation and an increase in open pastureland with herbaceous vegetation from similar to 1820. The C/N, delta N-15, and delta C-13 values showed the highest variability probably linked to a variable lacustrine environment and decreased productivity in the catchment between similar to 1640 and 1760. It can be assumed that the extended periods of lake ice cover during the cold decades experienced at the turn of the 17th -18th centuries resulted in diminished biological productivity both in the lake and its catchment. Finally, afforestation and the decline in lake productivity have been reconstructed for the period of - 1470-1560.