Variations of 18O/16O in plants from temperate peat bogs (Switzerland):: implications for paleoclimatic studies

Despite the great potential of peat bogs as climatic archives, to date only few studies have focused on the climatic controls on cellulose isotopic composition in modern bog plants. This study attempts to calibrate plant-climate relationships by sampling a set of modern plant species (both vascular plants and mosses) and bog surface waters along an altitude transect in Switzerland. Isotopic analyses of water samples show that the delta(18)O-values of surface bog waters follow the trend of precipitation despite significant scatter in the data set. Detailed sampling of surface waters within one bog shows that delta(18)O-values vary widely and are closely related to the micro-topography of the bog surface. More enriched O-18/O-16 ratios in water samples collected from small raised hummocks than the ones collected from hollows are documented in both horizontal and vertical profiles. A delta(18)O-deltaD plot indicates that the process leading to the isotopic enrichment of the uppermost surface waters is evaporation, greater above Sphagnum covered hummocks than above open pools. To investigate the implications of such high variability of source water for plant alpha-cellulose delta(18)O-values, a detailed study of both surface water and alpha-cellulose delta(18)O-values within one site is conducted. The large delta(18)O variability observed in surface waters is found to be considerably smoothed in a-cellulose (by a factor of 5-10 depending on the plant species). This indicates that the water used by plant photosynthetic processes reflects the isotopic composition of the average annual precipitation. This points to a source water level for plants of a few decimeters where the variations are smaller than at the air-water interface. The response of the alpha-cellulose delta(18)O to the environmental gradient along the altitude transect varies considerably from species to species. For most of the species studied, the delta(18)O-values decrease with altitude, following the trends of delta(18)O-values in precipitation and in surface water samples. Some species, the cotton sedge Eriophorum vaginatum and the moss Sphagnum capillifolium, show statistically significant delta(18)O relationships to an altitude of -1.8 parts per thousand /km and -2.9 parts per thousand /km respectively. However, some other plant species, Calluna vulgaris, Vaccinium uliginosum, Andromeda poliflora, Carex pauciflora, Sphagnum cuspidatum and Sphagnum magellanicum, do not, or only partially, reflect changes in climatic parameters associated with an altitude increase. Furthermore, changes in relative humidity, which are not correlated with altitude, are found to explain a large part of the variability in delta(18)O-values for the sedge Carex pauciflora and the moss Sphagnum cuspidatum. Therefore, this study confirms the importance of species-specific studies when interpreting O-18/O-16 ratios of macrofossils along a fossil peat sequence as a record of past climate changes. Our study allows to extend the mechanistic model that isotopically links source water and cellulose to the physiological specificities of sedges and mosses. A comparison of the modeled and calculated net biological fractionation factors for Eriophorum vaginatum and Sphagnum capillifolium reveals that these two species appear to have a more homogeneous leaf reservoir than trees. (C) 2002 Elsevier Science B.V. All rights reserved.