Annual deposition signal versus lake dynamics: Microprobe analysis of Lake Van (Turkey) sediments reveals missing varves in the period 11.2-10.2 ka BP

摘要

Microprobe measurements revealed changes of elemental concentrations along thin-section profiles of laminated Lake Van sediments. Results show pronounced variations, interpreted as seasonal cycles, thus allowing a new varve count based on alternating Ca and Si peaks. Compared with previous optical counts, the new record adds 170 years and places the investigated interval at 11.2-10.2 ka BP. The microprobe data reveal unusual Mg concentrations between 11.2 and 10.7 ka BP. These are caused by diagenetic magnesite and protodolomite, thought to be indicative of a lake-level regression and evaporative conditions causing the Mg content of the lake water to rise to critical levels. The dry phase ended with a transgression that started at around 10.7 ka BP and brought about a meromictic phase which weakened the climate-induced seasonal deposition signal. Both the diagenetic mineral formation and the mixing behavior of the lake decreased the grey scale value differences of the annual deposition signal, which had led to underestimation of years when varves were counted optically. Similar processes may have occurred in the evolution of Lake Lisan which would explain the marked differences between optical laminae counting and radiocarbon dating observed by other authors.