X-ray CT annually laminated lake sediments calcite precipitation catchment dynamics dating dropstones end-members endogenic varves environmental monitoring eutrophication flux rate freshwater GDGTs geochronology grain-size hypolimnetic hypoxia oscillations hypoxia lake sediments long-term ecology mast meromixis novel technology methodology oxygen deficiency pigments pollen analysis pollen traps sediment flux sedimentation snow avalanche tree-rings ultra-high resolution