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

Abstract

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 δ 13C value corresponds with submerged and emergent aquatic vegetation during the late-glacial, suggesting high respired carbon input. Similarly, light δ 15N values imply that nitrogen is derived from sediment and groundwater sources. Early Holocene δ 13 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 δ 15N and δ 13C 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. δ 15N remains near 0‰, relatively close to the atmospheric value of δ 15N, 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.