Climatic implications of annual to decadal resolution stable isotope data from calcite varves of the Piànico interglacial lake record, Southern Alps

Stable oxygen isotope analyses at annual, 2-, 5-, 10- and 20-varve sample resolutions were carried out on two selected varve intervals from the interglacial sediment record of the Piànico palaeolake. These sediments are particularly suitable for ultra-high-resolution isotope analyses on lacustrine endogenic calcite because of the exceptionally well-preserved varve structure. A bias through detrital contamination can be excluded because microscopically controlled sampling enabled selecting detritus-free samples. The studied sediment intervals comprise 352 and 88 continuous varve series formed during periods of rapid climate change at the onset and end of a marked millennial-scale cool interval during the Piànico Interglacial. The most intriguing result is a pronounced short-term oscillation in the bi-annually resolved isotope record superimposed on the general decreasing and increasing δ18O trends at the climatic transitions that is recorded at lower sample resolution. Spectral analyses of the bi-annual time series reveal periodicities indicating solar and NAO controls on the δ18O record. Multiple δ18O measurements from endogenic calcite of individual varves showed variations of up to 0.6‰, thus larger than the observed inter-annual variability and most likely explained by seasonal effects.