Testate amoebae in an ombrotrophic bog reconcile equivocal palaeoclimate reconstructions for the Western Carpathians

Abstract

Knowledge about climate changes is crucial for understanding past and current anthropogenic ecosystem changes, but individual palaeoclimate proxies involve different habitat-dependent confounding factors. Continuously ombrotrophic bogs are an excellent system for palaeoclimate reconstructions because their functioning depends tightly on water table depth (WTD), which varies exclusively with the balance between precipitation and temperature. Here, we investigate a 7000-year-long record from the Puścizna Wielka bog (Poland). We focused on developing the first palaeoclimatological reconstruction for the Western Carpathians, which together includes testate amoebae (TA), the isotopic signal in Sphagnum stems (δ¹³C and δ¹⁸O), and plant macrofossils (PM), the latter enabling to trace micro-topographical changes and assess if the consistent ombrotrophic conditions needed for unbiased reconstruction were met. Both the regional (TA_REG, PM) and continental (TA_EU) calibration training sets were used for quantitative WTD reconstructions. Our WTD reconstructions were compared with the downscaled CCSM3 simulations. Pollen data provided information on the surrounding vegetation. Reconstructed pH and species composition of PM confirmed continual ombrotrophy. At the Middle-Late Holocene boundary (ca 4300–4200 cal yr BP), the TA-rich lawn phase with Sphagnum magellanicum and Eriophorum vaginatum replaced the initial hollow-like phase with Scheuchzeria, Sphagnum sec. Cuspidata and Archerella flavum. The climate driver of this change (decreased humidity) is also suggested by a 100–200 years delayed increase in fir pollen at the expense of spruce. For the last ca 230 years, δ¹³C and TA suggest that the bog had experienced unprecedented drought. The bog was dominated by Sphagnum rubellum-dwarf shrub hummocks with Alabasta militaris, Assulina muscorum and Hyalosphenia elegans at this time. Apart from these major changes, we identified several other events that are otherwise documented only by individual studies across Europe for the Late Holocene, especially for the Little Ice Age. Six distinctly wet and five distinctly dry periods were consistent among at least two proxies, with a conspicuously warm and dry climate at the beginning of the Bronze Age, allowing human colonisation of previously harsh mountain areas. The consistency with CCSM3 was the highest for TA_REG-inferred WTD and lowest for δ¹⁸O. The temperature and precipitation of the warmest quarter were the most influential explanatory variables in multiple regression. Additively to CCSM3, isotopes in Sphagnum stems explained some WTD variation reconstructed by TA_EU and PM, suggesting a more critical role of the microtopographic position for these proxies. Our results suggest that TA reconcile multiple equivocal reconstructions, reveal subtle fluctuations only occasionally reported by other proxies, and are less affected by climate-independent microtopography. However, the inclusion of PM in palaeoclimatic studies is essential to verify the continuous ombrotrophy of recent bogs.