Background/Question/Methods In kettlelake ecosystems, marginal floating peat mats influence lake structure and function by altering chemical properties of the water column.  An autogenic model is typically used to explain the initiation and lateral expansion of floating peat mats in these ecosytems.  This model suggests gradual lateral encroachment of peatlands over open water, and implies similarly gradual changes within the remnant lake ecosystem.  However, empirical evidence to support these assumptions is surprisingly limited, and some evidence suggests that floating peatlands may develop rapidly.  We tested the autogenic model of peat mat initiation and expansion by collecting a dense array of sediment cores in two kettle peatlands in the Great Lakes region.  Sites included Titus Bog in northwestern Pennsylvania and Fallison Bog in northern Wisconsin.  Plant macrofossil analysis, loss-on-ignition, sediment stratigraphy, and extensive ¹⁴C dating were used to estimate the spatial and temporal patterns of peat mat establishment and expansion at each site. Spatiotemporal patterns were compared to predictions of the autogenic model and proxy records of hydroclimate variability. 
Results/Conclusions Results provide strong evidence that initiation and lateral expansion of peat mats in glacial kettles is episodic and triggered by hydroclimatic variability, in stark contrast to the autogenic model.  For example, at Titus Bog the floating peat mat established along the southwestern margins of the basin 900 – 700 cal BP.  This initial establishment phase was followed by a period of minimal change in peat mat extent until 400 – 350 cal BP, when the mat expanded rapidly throughout the rest of the basin.  Proxy hydroclimate records from nearby tree-ring and sediment archives indicate that peat-mat establishment and expansion events coincided with periods of extreme hydroclimate variability, particularly widespread multidecadal droughts.  Data from Fallison Bog indicate a similarly episodic developmental history, with peat expanding throughout much of the basin at ~5000 and ~3000 cal BP. A smaller expansion event may have occurred ~1000 cal BP, but the present pond margin has not changed significantly in 1000 years.  Comparison to regional paleoclimate records suggests that, similar to the patterns at Titus Bog, hydroclimate fluctuations also triggered peat-mat expansion at Fallison Bog.  Given anticipated changes in the frequency and intensity of extreme climate events, kettlehole-lake ecosystems may be vulnerable to rapid climate-induced state shifts in the coming decades, altering the physiochemical conditions and ecological functions of these aquatic ecosystems.