Peatlands are widely distributed in Alaska and contain valuable archives of past environmental variability. In the past few decades, a range of proxies has been used to infer past changes in surface-moisture conditions from oligotrophic peat stratigraphy, including testate amoebae and the stable isotope composition of peat-forming vegetation. However, calibration and validation of surface-moisture proxies in different regions and peatland types is critical to successful application and interpretation. Testate amoebae are a group of moisture-sensitive protozoa that produce decay-resistant shells, and community composition has been successfully used as a proxy for water-table depth. However, only limited calibration work has been undertaken in Alaska. The δ13C composition of Sphagnum has also recently been used as a proxy for past surface-moisture conditions, although little quantitative validation of this proxy has occurred. In this study, testate amoebae, the δ13C composition of Sphagnum, and environmental conditions (e.g., pH, water-table depth) were examined at 127 sites within 12 peatlands in south-central Alaska to assess the potential of testate amoebae and δ13C as surface-moisture proxies in the region. Non-metric multidimensional scaling (NMS) was used to describe patterns of variability in the testate amoeba data and assess potential controls on community structure, and transfer functions for water-table depth were developed using several models.
Results/Conclusions
NMS results indicated that water-table depth and pH were both strongly correlated with the composition of testate amoeba communities, consistent with previous studies. Cross-validation of the transfer functions demonstrated that water-table depth could be inferred from testate amoeba communities with a mean error of 8 cm. However, the δ13C of Sphagnum was found to be a relatively poor indicator of water-table depth (r2=0.12), suggesting caution in the interpretation of downcore δ13C records. Possible reasons for the poor relationship between δ13C and water-table depth include variable carbon sources, such as CO2 released by methanotrophic bacteria, and the influence of temperature and light regimes on moss physiology. The testate amoeba calibration dataset developed in this study will be widely applicable to paleoenvironmental studies of oligotrophic peatlands in Alaska and provides baseline data on the ecology of testate amoebae in this understudied region.
