Water Stable Isotopes of Peatland Catchments: Toward a Better Understanding of Water Budgets in the Marcell Experimental Forest, MN

Abstract:

An important yet less-studied subset of catchment hydrology is peatland hydrology. Peatlands are globally abundant, with biogeochemical feedbacks that increase water cycle complexity and therefore, necessitate an understanding of the unique hydrologic processes in these catchments. Additionally, changes to peatland water budgets can have cascading effects for plants, microorganisms, and carbon accumulation. Analyses of stable isotopes are commonly applied as a tracer in catchment studies; however, until recently this method has been under-utilized in peatland hydrology. With a long history of observation by the USDA Forest Service, the Marcell Experimental Forest (MEF) in northern Minnesota provides a natural laboratory of peatland (ombrotrophic bog) catchments well-suited for a stable isotope approach. Here we present, a time-series of stable isotopes (H and O) of natural waters collected from three bogs at the MEF since 2008. We sampled water from the outlet streams, precipitation, snowpack, bog and lagg porewater, surface and subsurface runoff, as well as nearby upland soil and groundwater. From summer to winter, the precipitation δ¹⁸O ranged from -3.8 to -28.0 ‰ and δD from -15.3 to -216 ‰. Snowpack, runoff, and near-surface porewaters are similar to precipitation of the respective seasons. Groundwater fluctuates less, with a year-round median δ¹⁸O -12.5 ± 0.5 ‰ and δD -81 ± 2‰ and d-excess values that possibly indicate winter snowmelt dominates annual recharge. Bog porewaters at different depths (0-3 m) appear homogenous in spring, then become seasonally stratified. By summer surface porewaters (0-0.5m) are enriched 2-4‰ (for δ¹⁸O) and divergent from our Local Meteoric Water Line (LMWL), possibly a signature of surface evaporative enrichment. In contrast, the deeper bog porewaters (>1m) are more similar to groundwater and remain relatively unchanged compared to near-surface. This may indicate the long residence time of deeper bog porewater. The outlet stream appears to be a mass-balance of porewater, precipitation, and groundwater with proportions from each respective source varying throughout the year. These characterizations, time-series, and patterns of fractionation establish important baselines that can inform future studies at the MEF and in peatland catchments worldwide.