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

摘要

Groundwater-dominated wetlands contain abundant peat deposits, but few studies have addressed the potential of using the oxygen isotopic composition (delta(18)O) of fen peat cellulose to evaluate past changes in climate. Here, we investigated the relationship between delta(18)O values of pond water and those of moss cellulose from species inhabiting different microenvironments 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 parts per thousand between average swale water delta(18)O and cellulose delta(18)O values for hollow Sphagnum species, reflecting the fractionation associated with cellulose biosynthesis. When Sphagnum specics from hummocks are included, the offset shows a larger variability (27 +/- 3 parts per thousand), pointing to evaporative effects affecting hummock species. This evaporative effect is more pronounced in emerged mosses, which show the largest difference (> 30 parts per thousand) 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.