Identifying Possible Preferential Flow Paths and Biochemical Reactions Between Surface Water and Deep Ground Water through Stable Isotopic Analysis In a Large Circumboreal Peatland

Two synoptic samplings, 10 years apart, of the stable isotopes of pore water and surface water in two different bog – fen complexes in the Glacial Lake Agassiz Peatlands (MN) show markedly different sources for recharge. Different isotopic signatures in fen water imply large-scale preferential flow paths, yet not well understood in the glacial deposits under the peat and isotopic differences in bog pore waters suggest changes in rates of methanogensis in the deep peat. The δ¹⁸O of Red Lake II and Lost River bog waters ranges from - 10.2 to - 12.7 and - 11.70 to - 12.7 %o respectively while the δD content ranges from - 71.6 to -89.4 and - 83.2 to -90.1 %o respectively. Deuterium enrichment due to methanogenesis affects bog pore water more than evaporative enrichment. In 1998, deuterium was enriched in Red Lake II bog water by +11%o relative to the local meteoric water line (LMWL), whereas in 2007, the deuterium enrichment was ~ 4%o less enriched than in 1998. In contrast, the δ¹⁸O values of bog pore water remained similar. The difference in deuterium enrichment suggests that the rate of methanogenesis may have decreased over the past decade.

Evaporation of surface water in both Red Lake II and Lost River fens enriches both deuterium and ¹⁸O at an shallow acute angle to the LMWL, and this isotopic shift surprisingly persists throughout the fen peat profiles to ~3 meters deep. This evaporative signal cannot be related to surface evaporation alone, but must reflect evaporation at another source that discharges to the fen from below via groundwater. Samples of water collected at Upper Red Lake and small lakes on a beach ridge located ~ 10 km from the sampled fen site show evaporative enrichment similar to that of the peat pore water, and these may be the recharge areas for the fens.