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

Abstract

Groundwater-dominated wetlands contain abundant peat deposits, but few studies have addressed the potential of using the oxygen isotopic composition ( δ 18O) of fen peat cellulose to evaluate past changes in climate. Here, we investigated the relationship between δ 18O 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. Our results show a consistent offset of 27 ± 1‰ between average swale water δ 18O and cellulose δ 18O 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.