Distinguishing climatic from direct anthropogenic influences during the past 400�years in varved sediments from Lake Holzmaar (Eifel, Germany)

Abstract

A 336-year floating varve chronology from Lake Holzmaar (Eifel, Western-Germany) covering the recent period has been established by microfacies analysis of thin sections. This sequence terminates 23 cm below the core top. In the top 23 cm, the varves are disturbed. By means of linear regression, the varve sequence was dated to the period AD 1607–1942. The influences of climatic variability and anthropogenic activities in the lake’s catchment (e.g., forestry, agriculture) on lithology, fabric, and microfossil content of the varve sublaminae could be discriminated by applying statistical analyses (ordination and clustering) to the combination of the sublaminae in the varves and their thickness. Four clusters are obtained. Cluster 1 indicates cold springs, and shorter, cooler summers reflected primarily in below-average varve thickness (VT) for two stable phases: from AD 1650–1700 (during the Maunder Minimum) and from AD 1750–1785. Cluster 2 indicates years with conditions transitional to that indicated by cluster 1, characterized by vigorous and prolonged spring circulation with massive blooms of the nordic-alpine Aulacoseira subarctica. The samples assigned to Cluster 3 and Cluster 4 show the imprint of anthropogenic influences. Cluster 3 (AD 1795–1815 and AD 1825–1885) is characterized by above-average VT due to high detritus input throughout the year. The increased soil erosion can be linked to anthropogenic deforestation as a consequence of the production increase of the Eifelian iron industry at the end of the 18th century. This input dampens the climatic signal of a colder Dalton Minimum, which is reflected in a short drop in VT centered around AD 1810. At about AD 1885, Cluster 4 conditions, characterized by increased nutrient concentrations, low detritus input, and longer periods of stable summer stratification, become the stable state in Lake Holzmaar. They indicate the response of the lake to natural reforestation and the use of artificial fertilizers in the catchment, which began, according to historical records, in the 1850s in the Eifel region. The prolonged, stable summer stratification periods may be the first indication of the modern warming trend. A drop in VT centered around AD 1890 and recurring cluster-1 conditions may indicate the Damon Minimum.