The temporal and spatial complexity of carbonate deposition at Romincka forest cupola spring-fed fen (Central Europe) during the Holocene

Calcium carbonate records from alkaline fens provide valuable insights into past climate fluctuations. Yet, the climate is not the sole factor controlling tufa deposition. The present study aims to determine the drivers of postglacial tufa deposition at the exceptionally well-developed cupola alkaline fen located in the north-eastern Poland. Tufa deposition time frames and conditions are reconstructed using radiocarbon dating, geochemical, including loss on ignition and δ13C and δ18O, plant macrofossils, and mollusc analyses of two sediment sequences taken from the top and slope of the fen cupola. The exceptional in young-glacial area continuous Holocene-long history of CaCO3 precipitation was supported by a strong supply of artesian waters. The locally high endogenic heat flow density accelerated permafrost degradation and warranted the early start of artesian water circulation and tufa deposition ca. 11650 cal yr BP. Deposition of interlayered peat and tufa continued until ca. 4700 cal yr BP when the increasing height of the cupola disabled water outflow at the fen top, and peat with scattered CaCO3 started to accumulate. Seepages on the fen slopes supported deposition of interlayered peat and tufa on lower elevated parts of the mound until ca. 2300 cal yr BP. Colder and/or dryer episodes weakened the predominately intensive CaCO3 precipitation mainly at the top of the mound. Vegetation favoured tufa deposition (i) after the development of the soil, which enhanced chemical denudation of carbonate detritus from glacial deposits, (ii) during times of decreased evapotranspiration when the forest cover on the surrounding land diminished. Tufa deposition at the cupola top was limited between ca. 8450 and 4700 cal yr BP when trees entered the fen surface. The human impact on tufa deposition was likely limited except for the melioration of the surrounding area ca. 50 cal yr BP, after which CaCO3 precipitation eventually stopped.