Mid- to late Holocene lake ecosystem response to catchment and climatic changes - A detailed varve analysis of Lake Holzmaar (Germany)

A varved sediment record of Lake Holzmaar, Germany spanning ca. 4120 years (mid- to late Holocene) was investigated with the aim to reconstruct environmental and indirectly climate change. Summer ('light') and winter ('dark') laminae were identified according to their predominantly autochthonous and allochthonous structure, respectively. Algal sublayers within the light laminae were subject to marked changes in thickness and in algal species composition. Diatoms were the dominant algal class throughout the record. However, chrysophytes played a substantial role in varve formation between ca. 5300 and 3660 cal yr BP. Autecological characteristics of the dominant algal species and algal succession allowed to infer shifts of the prevailing water circulation type and relative changes in nutrient concentration and ratios (phosphorus and silica). Based on the combination of all results, the palaeolimnological development was reconstructed and split into three distinct lake periods (6340-5300 cal yr BP; 5300-3660 cal yr BP; 3660-2220 cal yr BP). The palaeolimnological characteristics in turn allowed to make suggestions upon the palaeoclimatological development in consideration of relevant studies from other sites in Western Europe. With the beginning of the Iron Age (ca. 2660 cal yr BP), climatic interpretation remains unclear though, because anthropogenic activity sharply increased.