Effects of land-use change on deposition and composition of organic matter in Frickenhauser See, northern Bavaria, Germany.

Characterization of bulk organic matter (OM) from lacustrine sediments of Frickenhauser See (northern Bavaria, Germany) reveals considerable variation during the last two millennia. Atomic C/N ratios and total organic carbon (TOC) content are positively correlated with arboreal pollen percentages which are used as an indicator of land-use intensity. Despite possible alterations of OM during early diagenesis, differences between three major lithological units are large enough to be interpreted as human impact on the sedimentation of OM in the lake. Sediments deposited prior to deforestation in the catchment area (unit C) are characterized by mean C/N ratios of 14.5, indicating a mixed composition of organic matter derived from lacustrine and terrestrial sources (forest litter). During a period of intensive soil erosion (ca. 1000 AD until 1870 AD; unit B), low C/N ratios of around 7.7 suggest that the relative proportion of forest litter decreased in favour of the lacustrine component and soil OM. Terrestrial plant detritus is only transported into the lake through numerous turbidite events. Deforestation and agriculture also coincide with a decreasing TOC content from an average value of 10.7% to 1.5%. This decrease is explained through a dilution effect due to the high input of minerogenic matter. Stable carbon isotopes indicate eutrophication of the lake due to the mobilization of nutrients through soil erosion. Starting around 1870 AD, organic-rich sapropelic sediments are deposited (unit A). A decline of turbidite events and increased wind shelter from trees due to reduced land-use intensity led to meromictic conditions. Consequently, negative excursions in the delta13C isotope record at the onset of unit A probably reflect methanogenesis under permanently anoxic conditions.