Paleoenvironmental changes of the last 16,000 years based on diatom and geochemical stratigraphies from the varved sediment of Holzmaar (West-Eifel Volcanic Field, Germany)

Abstract

Lacustrine environmental archives with annually laminated (varved) sediments allow a calendar-year chronology and the calculation of precise sediment accumulation rates. We present multiproxy analyses of a varved sediment record from Holzmaar (West-Eifel Volcanic Field, Germany) for the last 16,000 years providing a continuous diatom stratigraphy supported by physical and chemical variables with a centennial resolution. Patterns of diatom assemblages infer the trophic history of Holzmaar. There is a distinct variation at the Pleistocene/Lateglacial transition characterized by the replacement of Staurosira construens with Stephanodiscus minutulus, increases of Ca/Ti, total organic carbon/total nitrogen (TOC/TN) ratios and biogenic silica, together suggesting an increase in lacustrine productivity and a shift of the lake's trophic status from oligotrophic to mesotrophic. These conditions remain during the Bölling/Alleröd interstadial. During the Younger Dryas stadial, there is a decrease in organic productivity as reflected by TOC, a subtle increase in benthic and epiphytic diatoms indicating colder and dry conditions, and S. minutulus remains dominant suggesting increased winds. The Postglacial is dominated by Lindavia radiosa, S. minutulus and Nitzschia paleacea. The latter occurs between 9000 and 6000 cal yr BP together with an increase of TOC/TN and total sulphur (TS) suggesting eutrophication with periodic anoxia during the Holocene Thermal Maximum. After 2200 cal yr BP, L. radiosa and Pantocsekiella comensis are dominant suggesting warmer conditions (especially during the Medieval Climate Anomaly) and thermal stability. Increased surface runoff is reflected by higher values of Ti and magnetic susceptibility, suggesting more humidity and enhanced anthropogenic disturbance. Moreover, the appearance of Aulacoseira subarctica is related to a cold and wet period coinciding with the Little Ice Age. In addition to environmental changes, our multiproxy analyses track human impact since the Middle Neolithic.