Last millennium palaeoenvironmental changes from a Baltic bog (Poland) inferred from stable isotopes, pollen, plant macrofossils and testate amoebae

The Baltic coast of Northern Poland is a region of considerable potential interest for paleoclimatic studies because this region is under the influence of both oceanic and continental climates and that these two influences have likely changed through time. Also unlike many more intensively studied regions of Europe human impact was more limited until the 19th century. We present a multiproxy high-resolution record from Stążki mire, an ombrotrophic mire located in northern Poland 35 km from the Baltic sea coast. We combined four proxies from the peat: testate amoebae, stable isotopes (δ13C) of Sphagnum stems, pollen, and plant macrofossils supported by dendroecological data to assess the palaeoenvironmental changes that took place during the last millennium. The general aim of this study was to test if a range of proxies (testate amoebae, pollen, plant macrofossils and tree rings) could allow inferring a climatic signal from a peat monolith despite the considerable anthropogenic impact on the Pomerania landscape since the Early Medieval period. Combining internal (e.g. macrofossils and testate amoebae) and external (pollen) signals is key to distinguishing human from climatic signals in palaeoenvironmental studies of peatlands. Our more specific aims were to: a) reconstruct the last millennium palaeoenvironment in a Baltic raised bog and its surroundings, b) identify the major wet-dry shifts and correlate those events with climate change and human impact, c) assess the resilience of the Baltic bog ecosystem following human impact. We noted two disturbance periods in the history of the mire. The two major disturbances in the history of the mire were dry periods [(I) 1100-1500 AD and (II) 1650-1900 (2005) AD]. These are reflected in all indicators and can be interpreted as the mixture of climate impact and human activities. However, the pollen record shows little evidence of human indicators during the first dry shift. The second disturbance can be related to local peat exploitation of the mire and then in the 20th century with limited artificial drainage. From 1500 AD all proxies show that the water table abruptly rose in the mire. The beginning of this shift occurred during the Little Ice Age and therefore it is likely a climatic signal. Alternatively, this wet shift could have been interpreted as a re-wetting mire surface after peat burning, but this interpretation is rejected because there is no evidence of major local fire in the macrofossil record. The macrofossil data shows that Sphagnum fuscum dominated the pristine mire vegetation but then declined and finally disappeared. This pattern is comparable with the timing of extinction of Sphagnum austinii in Western Europe. This study illustrated the value of palynological analysis of peat archives (especially for last millennium) to assess the magnitude of anthropogenic land-use changes. For example the comparison of Fagus (beech) and Carpinus (hornbeam) history with the palaeohydrological record allowed us to exclude climate change as the most important factor for the decline and expansion of these tree species. This study is the first direct comparison of testate amoeba and stable isotope data.