Diatom and geochemical evidence of mid- to late Holocene climatic changes at Lake Holzmaar, West-Eifel (Germany)

Abstract

Changes in diatom assemblages, geochemistry and stable carbon isotope composition of bulk organic matter in a varved sediment section from Lake Holzmaar, West-Eifel (Germany) were used to infer mid- to late Holocene climatic changes and human impact from 6340 to 1470 cal yr BP. Shifts in total lacustrine primary production were derived from δ 13C org values. Changes in non-siliceous algae biomass productivity were deduced from accumulation rates of organic matter. Diatom valve accumulation rates and biogenic opal accumulation rates were used as proxies for diatom productivity and for siliceous phytoplankton abundance, respectively. Based on weighted averaging of relative diatom abundance in subsamples, qualitative changes in past trophic status were inferred. Accumulation rates of minerogenic material and the TOC/TN molar ratio permitted the deduction of past changes in allochthonous matter influx. Diatom assemblage composition showed statistically significant changes around 5130, 3660 and 2900 cal yr BP, whereas planktonic diatom productivity was subject to distinct shifts around 5900, 5130, 4500, 3600, 2660 and approximately 1630 cal yr BP, and siliceous phytoplankton biomass changed ca. 6100, 4500, 2660, 1900 and 1700 cal yr BP. Causally determined changes in diatom assemblage composition and productivity, for example, due to shifts in nutrient ratios and/or concentrations invoked by changes in lake circulation patterns, precipitation rates and/or erosion rates are discussed. Glacier fluctuations and lake level changes reported from other study sites in western Europe support the finding that reconstructed lacustrine processes are at least indirectly linked to climate. Palynological and archaeological evidence, however, has shown a strong superimposition of lacustrine processes with local/regional anthropogenic activity since the Iron Age.