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

Abstract

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, Bâlea (2034 m a.s.l.), to reconstruct sub-century scale Holocene climate variability and environmental change. Chironomid-inferred average T_July 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), T_July 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 precipitation 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 productivity. The dominant taxon of the early- and mid-Holocene assemblages, Micropsectra radialis-type, disappeared at 2750 cal yr BP. We explained this faunal change with lake substrate change and proved the bias of the inferred increase in T_July after 2750 cal yr BP.