Late-glacial and Holocene δ15N and δ13C variation from a Kenai Peninsula, Alaska peatland

We use stable carbon and nitrogen isotopes from a Kenai Peninsula, Alaska fen spanning the late-glacial and Holocene to enhance our understanding of peatland hydrology and nutrient cycling. We find that isotopic values for carbon and nitrogen are lighter in the late-glacial and early Holocene (−28 to −34‰ and 0 to −4‰, respectively) than the remainder of the record. The light δ13C value corresponds with submerged and emergent aquatic vegetation during the late-glacial, suggesting high respired carbon input. Similarly, light δ15N values imply that nitrogen is derived from sediment and groundwater sources. Early Holocene δ13 values ranging from −30‰ to −34‰ infer high recycled carbon uptake from methanogenesis, related to the warm climate and the wet swale environment. A shift to heavier δ15N and δ13C isotopic values ∼ 9600 cal yr BP coincides with a change in the lithology to Sphagnum-dominated peat, indicative of peat accumulation above the water table. A shift to lighter values from 5500 to 4200 cal yr BP coincides with a period of glacial recession related to warmer conditions, whereas a subsequent period of heavier values is correlative with glacial advance culminating at 3000 cal yr BP, suggesting that the hydrology of this peatland is closely linked with the glacial meltwater. δ15N remains near 0‰, relatively close to the atmospheric value of δ15N, throughout the Holocene, suggesting that the fen has been supported by N-fixation. We propose that groundwater on the Kenai lowlands was fed by subglacial meltwater, and the warm early Holocene summers resulted in high glacial ablation and groundwater recharge, sustaining a high Swanson Fen water table. Cooler temperatures resulted in a decrease of glacial meltwater input at the close of the early Holocene.