High Resolution Lacustrine Sediments and Their Potential for Palaeoclimatic Reconstruction

Abstract

Annually laminated (varved) sediment records are high resolution archives of palaeoenvironmental conditions. They can provide an absolute chronology based on calendar (siderial) years and information about processes in the lake and the catchment area (e.g. through varve thickness measurements). The dominating factor controlling the depositional system is climate and anthropogenic change. The climate-varve link is considerably well understood for proglacial lakes with clastic varves: mean summer temperature controls varve thickness in continental climate regimes, whereas mean summer precipitation controls varve thickness in oceanic climate regimes. The signal in midlatitudinal organic varves is more complex. In addition to the allochthonous sediment component, autochthonous organic productivity becomes a dominant factor modifying the yearly increment. This seems not to be the case in the sediments of Lake Holzmaar (Germany) since BC 750, when vegetation was removed due to human impact making soils susceptible to increased erosion. The increase in varve thickness can be mainly related to minerogenic detritus which is regarded as a discharge proxy. A climate-varve link is elaborated through a negative correlation of varve thicknesses with mean seasonal temperatures of winter and spring during the last 30 years, when instrumental data are available. Extrapolating this relationship to the last millennium, colder winters are indicated around AD 1300, 1500, and from 1650 to 1900. Colder winters might be related to sun spot minima except for the period AD 1715 to 1900 when other forcing factors, such as increased volcanic activity, may have played a role.