Holocene palaeohydrology and environmental history in the South Harghita Mountains, Romania

Abstract

In this study we present the results of a multi-proxy investigation carried out on the sediment of a
crater-lake - Saint Ana (Szent Anna-tó) - located in the Harghita Mts in the middle range of the East
Carpathians in Romania. Pollen, plant macrofossil, siliceous algae and trace element analyses were used level changes was based on the diatom and plant macrofossil records. According to these, the water
depth showed considerable fluctuation during the Holocene, in most cases occurring coincidentally with
the terrestrial vegetation changes. It is significant that, low water depths characterised the early
Holocene until ca. 8800 cal yr, when the open water became overgrown with vegetation and the lake
turned into a Sphagnum bog. From ca. 7050 cal yr BP the water depth grew gradually and shallow water
conditions prevailed until ca. 5550 cal yr BP, when a marked increase was detected and this was
accompanied by increasing productivity. The highest lake-levels were found between 2700 and 700 cal
yr BE The geochemical record points to the thermal stratification of the lake in this period. The early
Holocene terrestrial vegetation of the area was characterised by Ulmus - Fraxinus forests that were
replaced by Corylus at ca. 8800 cal yr BP This was followed by the re-expansion of Fraxinus, Quercus and
Tilia and the spread of Carpinus betulus from ca. 7500 cal yr BP The formation of a forest belt dominated
by Carpinus betulus was dated to 5500 cal yr BP while the two-step expansion of Fagus sylvatica took place
between 3700 and 3300 cal yr BE From 3300 cal yr BP Fagus sylvatica became the dominant canopy tree in
the forests around the lake. Joint interpretation of the terrestrial pollen and plant macrofossil records also
highlights the difference between the lakeshore and hillslope vegetation throughout the Holocene. The
persistence of the lakeshore Picea abies forest belt can be explained by the cooler and moister microclimate
of the basin, leading to thermal inversion. The geochemical record shows high Fe and Mn concentrations
between 8150-6600 cal yr BP; from which reducing soil conditions and an oxygen rich sedimentary
environment have been inferred. Following the increase in lake-level at 7050 cal yr BE a gradually
increasing amount of P was deposited in the lake pointing to increasing productivity.