Peat porewaters have contrasting geochemical fingerprints for groundwater recharge and discharge due to matrix diffusion in a large, northern bog-fen complex

Although northern peatlands represent a globally significant reservoir for carbon, considerable uncertainty exists concerning solute transport systems within large (>1000 km(2)) peat deposits. We therefore delineated geochemical gradients linked to groundwater recharge and discharge along a 6 km transect within the 1200 km(2) Red Lake Peatland of northwestern Minnesota. We used ratios of Ca/Mg and Sr-87/Sr-86 to distinguish discharge of calcareous groundwater (similar to 1.4 and 0.7155, respectively) to the peat land from the mineral substratum along a topographic gradient from a bog crest downslope to an internal fen water track and bog islands. In contrast, the stable isotopes of the porewaters (delta O-18 from -12.8 parts per thousand to -7.8 parts per thousand) show that the active pore-spaces in these peat profiles have been flushed by recharge from the near-surface peat. We hypothesize that back-diffusion of groundwater-derived solutes from the peat matrix to active pore-spaces has allowed the geochemical signal from paleo-hydrogeologic discharge to persist into the current regime of dilute recharge. This effect has not been observed previously on the landform-scale and has important implications for carbon cycling in peatlands. (C) 2016 Elsevier B.V. All rights reserved.