Holocene summer temperature variability in the Southern Carpathians: possible North Atlantic Jet forcing and high-altitude sensitivity

The Southern Carpathians, a pivotal yet understudied transition zone between Atlantic and continental climate regimes, lack high-resolution summer temperature reconstructions critical for deciphering North Atlantic Oscillation (NAO) and North Atlantic Jet (NAJ)-driven Holocene climate variability. In this multi-proxy study, we use subfossil chironomids, pollen, loss-on-ignition, magnetic susceptibility, as well as major and trace element analyses from the sediments of a high alpine lake, B?alea (2034 m a.s.l.), to reconstruct sub-century scale Ho- locene climate variability and environmental change. Chironomid-inferred average TJuly at the lake is 9.3 ?C today. The lowest temperatures around 6 ?C were found in the Lateglacial (~13,600 cal yr BP). In the Holocene (since 8330 cal yr BP), TJuly ranged from 7.5 to 10.4 ? C with cooler periods between 5500 and 4800 cal yr BP (~8.9 ?C) and around 4200 cal yr BP (~9 ?C). After 4200 cal yr BP, ln(Rb/K) ratios suggested increasing pre- cipitation that likely affected the sediment composition of the lake. Our findings support the NAJ’s summer impact in Eastern Central Europe by showing a ~0.5–1 ?C multi-decadal summer cooling in periods with a more northerly position of the summer NAJ. Chironomid assemblage composition change at 4200 cal yr BP involved the increase of Micropsectra insignilobus-type and Heterotrissocladius marcidus-type, while Micropsectra radialis- type declined. In addition, the overall chironomid population size declined pointing to decreasing lake pro- ductivity. The dominant taxon of the early- and mid-Holocene assemblages, Micropsectra radialis-type, dis- appeared at 2750 cal yr BP. We explained this faunal change with lake substrate change and proved the bias of the inferred increase in TJuly after 2750 cal yr BP.