Unveiling climate transformations in the northwestern Caucasus: the Middle Holocene divergence of landscape evolution.

Climate change during the Holocene (5500-4000 year BP) transformed the altitudinal zonation of the northwestern Caucasus, affecting the development and distribution of prehistoric populations, whose specialized economic system were based on dry farming and cattle breeding. This study analyzes reliably dated palaeosols buried beneath Early Bronze Age Maikop culture kurgans, comparing these findings with palaeorecords from the Eastern Mediterranean-Black Sea-Caspian Corridor. Our results demonstrate that the forest-steppe boundary in the foothills (100-1100 m a.s.l.) is highly sensitive to climatic fluctuations. Palaeosol and palaeobotanical analyses show that, where luvisols under broadleaved forests are widespread in the foothills of the northwestern Caucasus today, between 5300 and 4900 year BP, Black Soil (Chernozem) under meadow and meadow-steppe ecosystems was present. The general cooling trend between 5000 and 4000 year BP, culminating in the '4.2 ka event', shifted the boundary between the broadleaved forests and the forest-steppes by 20 km northwards (descending from 800 to 400 m a.s.l.) while the meadow steppes retreated to the adjacent plains. Such a large-scale landscape restructuring has not been recorded in either the mountainous Caucasus or the East European Plain steppes. This forest expansion triggered persistent soil transformations, driving the evolution of Chernozems into Luvisols and establishing a new altitudinal zonation. The proposed reconstruction demonstrates that the '4.2 ka event', typically associated with cooling and aridification, can trigger differential responses across adjacent landscapes. While it caused intensified droughts and xerophytization in the southern East European Plain, the same cooling on the humid slopes of the northwestern Caucasus led to glacial expansion and a shift toward wetter forest landscapes. Taking these divergent landscape responses into account is essential to understanding how ancient populations of the northwestern Caucasus responded to the climatic changes of the Holocene.