Lake Holzmaar is a young quaternary crater lake located in the Westeifel, Germany. Sediments trapped within the lake are varved for almost the last 22,500 years (Zolitschka, 1990; Brauer, 1994). Investigations of the Holocene and the Late-Glacial section (the last 14,000 years) revealed sedimentological changes which can be related to the paleoclimatic development and to human impacts. Stable carbon isotope analyses carried out with high resolution (2-40 years/sample) can be used as a tool to distinguish between autochthonous and allochthonous carbon sources, as the latter are characterised by less negative d13C-values. The round and steep shoreline of the volcanic lake provides primary production as main source of sedimentary organic carbon. Terrestrial carbon input is unimportant during the major part of the Holocene. Therefore isotopic fractionation of the phytoplankton can be used to reconstruct paleoclimate by analysing bulk organic matter.
The Holocene section allows a differentiation between three limnological and climatic stages: Between 14,000 cal. BP and 9,700 cal. BP d13Corg-values fluctuate around -30 with large and rapid changes mainly caused by input of allochthonous material. In the time interval from 9,700 cal. BP to 2,700 cal. BP negative d13Corg-values are typical, showing a smooth temperature-correlated record. The climatic optimum is indicated for the time between 8,000 cal. BP and 6,000 cal. BP. In the youngest part (< 2,700 cal. BP) of the Holocene record human influences became more important. These events and phases of high allochthonous sedimentation lead to higher d13Corg-values comparable to the signals resulting by natural increase of allochthonous deposition during the Late-Glacial and Early Holocene.
Brauer, A., Documenta naturae 85, 1-210 (1994).
Zolitschka, B., Documenta naturae 60, 1-241 (1990).