Holocene peat accumulation rate, hiatus events, and hummock development affected by climate, tephra deposition, and geomorphic setting

Peatland development is affected by changing climates, disturbances such as fire, and autogenic processes. Peatlands may be susceptible to periods of peat loss or reduced accumulation resulting in hiatuses in the peat stratigraphy. At eight minerotrophic peatlands in isolated topographic depressions in western Washington State, peat accumulation rates were reconstructed using radiocarbon-dated continuous peat cores. Age-depth models, fit to 7 to 11 radiocarbon dates and known tephra ages per peatland, were built using a Bayesian approach that constrains the range of potential peat accumulation rates and specifies potential hiatus events at abrupt changes in organic soil composition, tephras, and abrupt changes in the bulk-density profile. The among-site mean long-term rate of organic matter accumulation varied from 30 to 40 g/m 2 /year during the Late Holocene but was much lower, 18–22 g/m 2 /year, between 9500 and 6500 year before present. Fifteen modeled hiatuses encompassed 22% of the period in the age depths models and were more common during 9500 to 6500 year before present. During this period, hiatuses suggest that 50% of the potential peat accumulation is missing. Compared to nearby pollen and charcoal records, the hiatal intervals were likely the result of peat loss from a lower water table, fire, or burial by 4–7 cm of Mazama tephra. Sites with small watershed:peatland area ratios had the largest hiatuses and small peatlands in kettle depressions had the smallest hiatuses, consistent with water-table sensitivity to climate change. Charcoal and pollen stratigraphy from hummocks at three sites indicate that fires during the last century occurred near the peat core sites with little impact on organic matter accumulation. The variability in peat accumulation, among sites and over time, suggests site-dependent climatic sensitivity of these biologically and hydrologically significant sites.