In this study, a sediment sequence from Lake Skottenesjön, southern Sweden was investigated to reconstruct the effect of forestry and land-use on erosion rates and delivery of organic and minerogenic matter to the lake. Catchment-scale vegetation changes during the last 1000 years were reconstructed quantitatively using pollen analysis and the Landscape Reconstruction Algorithm (LRA). Organic matter in the lake sediments was analyzed using lignin phenols and bulk carbon stable isotopes. Changes in the delivery of minerogenic matter were analyzed using X-ray fluorescence scanning.
Variations in pollen assemblages, lignin phenol concentration and other geochemical proxies show that deforestation, agricultural activities and other human impacts on the lake catchment modified the organic and inorganic matter deposition in the lake. Between ca. AD 1000 and ca. AD 1350, the local land-use was characterized by small-scale agricultural activities associated with the medieval expansion. During this period, the woodland cover was dominated by deciduous trees, as revealed by both pollen composition and high values in the ratio between syringyl and vanillyl phenols (S/V). Increased deposition of terrestrial organic matter was indicated by the high concentration of lignin phenols. Around AD 1350, much of the farmland was abandoned and coniferous woodland cover increased as revealed by the pollen composition. A progressive decline in the lignin phenol concentration after ca. AD 1350 reflects a reduction of terrestrial organic matter input to the lake, probably as a direct effect of increased forestation of the catchment. After ca. AD 1650, both cultivated crops and pasture & meadows expanded in the catchment as seen in pollen assemblages. The increased land-use indicated by the pollen data is in agreement with the population increase documented in the study area between ca. AD 1700 and ca. AD 1850, but locally the human impact remained high for some time after the regional peak in population. Substantial increases in lithogenic elements (K, Ti, Rb) together with lower TOC content and higher C/N ratios indicate enhanced soil erosion from ca. AD 1650 to ca. AD 1950. Elevated δ13C values during this period reflect an increase in lake productivity that can be attributed to higher nutrient input associated with intensive soil erosion. S/V ratios increased in concert with an increase in deciduous trees and a distinct drop in coniferous woodland cover due to timber exploitation. Cinnamyl to vanillyl phenol ratios (C/V) increased together with an increase in grassland cover. The flux of lignin phenols increased drastically reflecting a substantial elevation of terrestrial organic matter input as a consequence of intensive human activities. This study illustrates that the combination of pollen and lignin phenols is useful for investigating past changes in the delivery of organic matter in response to the anthropogenic impact on lake catchments.
- Multidisciplinär geovetenskap