Paleoclimatic implications of the relationship between oxygen isotope ratios of moss cellulose and source water in wetlands of Lake Superior

Groundwater-dominated wetlands contain abundant peat deposits, but few studies have addressed the potential of using the oxygen isotopic composition (δ18O) of fen peat cellulose to evaluate past changes in climate. Here, we investigated the relationship between δ18O values of pond water and those of moss cellulose from species inhabiting different micro-environments in three different swales near Lake Superior, United States. Isotope results over two growing seasons indicate a negligible effect of evaporation on the studied fens and the strong control of groundwater on the isotopic composition of swale water. Isotopic values for groundwater show a small seasonal variability and their low values suggest the influence of snowmelt. sults show a consistent offset of 27 ± 1‰ between average swale water δ18O and cellulose δ18O values for hollow Sphagnum species, reflecting the fractionation associated with cellulose biosynthesis. When Sphagnum species from hummocks are included, the offset shows a larger variability (27 ± 3‰), pointing to evaporative effects affecting hummock species. This evaporative effect is more pronounced in emerged mosses, which show the largest difference (> 30‰) relative to their source water. The smaller variability associated with hollow species points to the possibility of performing species-specific analyses from peat cellulose for improved paleoclimatic reconstructions.