Stable isotopes of water show deep seasonal recharge in northern bogs and fens

Ground water recharge is assumed to occur primarily at raised bog crests in northern peatlands, which are globally significant terrestrial carbon reservoirs. We synoptically surveyed vertical profiles of peat pore water δ¹⁸O and δ²H from a range of bog and fen landforms across the Glacial Lake Agassiz Peatlands, northern Minnesota. Contrary to our expectations, we find that local-scale recharge penetrates to not only the basal peat at topographically high bog crests but also transitional Sphagnum lawns and low-lying fen water tracks. Surface landscape characteristics appear to control the isotopic composition of the deeper pore waters (depths ≥0.5m), which are partitioned into discrete ranges of δ¹⁸O on the basis of landform type (mean±standard deviation for bog crests=-11.9±0.4‰, lawns=-10.6±0.1‰, fen water tracks=-8.8±1.0‰). Fen water tracks have a shallow free-water surface that is seasonally enriched by isotope fractionating evaporation, fingerprinting recharge to underlying pore waters at depths ≥3m. Isotope mass balance calculations indicate on average 12% of the waters we sampled from the basal peat of the fen water tracks was lost to surface evaporation, which occurred prior to advection and dispersion into the underlying formation. These new data provide direct support for the hypothesis that methane production in deeper peat strata is fuelled by the downward transport of labile carbon substrates from the surface of northern peat basins.