Speleothem record of Holocene natural and anthropogenic environmental changes in continental Croatia

The Holocene is on global multimillenial scale generally regarded as a relative stable in hydroclimate terms, but at the finer scale, it is characterized by frequent changes both in temperature and precipitation. Based on stable isotope (δ13C and δ18O) records of two coeval stalagmites from Nova Grgosova Cave (Croatia), we identified several centennial to millennial hydroclimate oscillations. During the 9.2-8.8 ka period local environmental conditions were improved in terms of vegetation dynamics, while the following 8.2 ka event was characterized by change in seasonal distribution of precipitation with somewhat drier autumn/winter seasons. The most remarkable excursion in δ13C series occurred around 7.4 ka within the Holocene Climate Optimum (HCO), due to reduced vegetation and soil microbial activity in response to enhanced spring/summer aridity, and likely also reduced autumn/winter precipitation. Regional signal of aridity during the 4.2 ka event is relatively small in magnitude, and must be confirmed in other coeval speleothems. Since the lowlands along the rivers further to east were preferred for human settlement over hilly region of Nova Grgosova Cave, Neolithic agricultural revolution and associated anthropogenic deforestation left no trace in studied speleothems, supporting hydroclimate causes of isotopic variations rather than anthropogenic. However, potential record of the short-term (centennial) climatic variability during the last millennium has been overprinted by the human intervention into natural forest landscape related to the mining activity. Namely, increased copper production required more arable land for increased population, mine timber (supporting pillars), as well as wood for smelting, which all lead to intensified deforestation and consequently masked δ13C signal of concurrent rapid climate changes such as those of Medieval Warm Period and Little Ice Age.