Hydroclimatic variability and basin morphology control terrestrialization in glacial kettles

Background/Question/Methods

Introductory ecology and wetland science textbooks typically portray terrestrialization of glacial kettles as a gradual process of floating peatland encroachment over open water.  Under this autogenic model, peatland expansion rates are climate-independent and spatiotemporal patterns of peatland development are uninfluenced by the underlying basin morphology or the location of the basin within the broader landscape.  However, the validity of this model has been questioned before.  We recently demonstrated a highly episodic developmental history in a western Pennsylvania kettle peatland and linked pulses of rapid peatland expansion to severe paleodroughts.  We hypothesize that water level fluctuations and basin morphology control spatiotemporal patterns of peatland development in glacial kettles and that most of these systems have experienced episodic peat accumulation histories during the Holocene.  We developed two conceptual models of peatland development in glacial kettles to explore how interactions between hydroclimatic variability and geomorphology could result in episodic peatland expansion in these systems.

Results/Conclusions

Here, we present preliminary results from a paleoecological study aimed at testing our conceptual models in three northern Wisconsin kettle peatlands.  In each peatland, we established transects of tightly spaced sediment coring locations, collected sediment cores that captured the contact between peat and lake sediments at each site, and performed both loss-on-ignition and plant macrofossil analyses along the length of each sediment core.  Aboveground plant macrofossils were radiocarbon dated from the lake-peat contact in each core and used to estimate the timing of peatland establishment at each coring location.  Data suggest that peatland expanded episodically in each basin and that large areas of similar basin depth were synchronously invaded by peatland around 5000, 3000, 2000 and 1000 calendar years before present.  The timing of these expansion episodes corresponds with times of rapid climate change documented in regional paleoclimate records.  Our results suggest that kettle ecosystems may be vulnerable to abrupt state-shifts resulting from anticipated changes in mid-latitude hydroclimatic variability, with major implications for long-term carbon cycling in glaciated regions.