Vegetational history of Carbury Bog (Co. Kildare, Ireland) during the last 850 years and a test of the temperature indicator value of 2H/1H measurements of peat samples in relation to historical sources and meteorological data



A palynological study of contiguous samples of the upper 128 cm of Carbury Bog, Co. Kildare, Ireland, showed details of the fluctuating human impact on the vegetation during the last c. 850 years. During the period of c. 1130–1440 A.D. the landscape around Carbury Bog was still dominated by forest. During the second half of the 15th century there was a considerable reduction of the forested area, reflected in the pollen diagram. The commercial exploitation of the forest during the 17th century resulted in almost complete deforestation. The pollen records for the 18th century vegetation show a rise in the curves of Pinus, Fraxinus and Fagus as a consequence of planting of these trees.

Apart from pollen many other palynomorphs (‘Types’, a.o. fungal spores and rhizopods) were recorded. The spectrum of Types appeared to be comparable with the Type spectrum of continental raised bogs. The results of the macrofossil analysis in combination with the indicator values of some Types were used in a detailed reconstruction of the local vegetation succession and the changes in hydrological and trophic conditions. The disappearance of Sphagnum imbricatum during the late medieval period possibly was caused by the increased influx of dust, as a consequence of man's impact on the environment around the bog. The recent importance of S. magellanicum as a peat forming species is possibly the effect of an increased atmospheric deposition of nitrate.

Parallel to the analysis of micro- and macrofossils a sample series from the same monolith was analysed for the ratio of the stable isotopes 2H and 1H. Theoretically the 2H/1H ratio in cellulose of peat-forming plants should be indicative for palaeotemperatures, but, apart from a temperature dependent signal, this ratio is also strongly related to specific characteristics in the isotopic behaviour of different peat forming species. The overall fractionation is a complex function of climatic and plant-physiological parameters.

In the present study the results of the 2H/1H measurements were compared with Lamb's palaeoclimatic data derived from historical sources and meteorology. The 2H/1H ratios of Carbury Bog appeared to be negatively correlated with the medieval climatic optimum, the Little Ice Age and the subsequent phase of warming climate. There is an obvious relationship between the 2H/1H ratio and the local succession of peat forming taxa and, as a consequence, with strictly local hydrological and trophic conditions. The present study shows that a climatic signal in the 2H/1H ratio of peat samples is completely dominated by the direct effects of the spectrum of plants. In addition, the variation in the duration of the growing season in different climatic periods may have had a considerable influence on the 2H/1H ratio of the peat-forming plants.