Boreal peatland water table depth and carbon accumulation during the Holocene thermal maximum, Roman Warm Period, and Medieval Climate Anomaly

The Boreal Shield and James Bay Lowland regions of Ontario have few Holocene reconstructions specific to surface moisture despite their potential importance to documenting the geographic patterns and teleconnections of Holocene pluvials and droughts, as well as their role in the global carbon cycle. We reconstructed water table depth using preserved testate amoebae in four stratigraphic peatland cores, supplemented with a literature review to investigate the effects of the Holocene Thermal Maximum (HTM) and Medieval Climate Anomaly (MCA) on surface moisture and long-term apparent carbon accumulation (LARCA) in the regions. A 7320 calendar years before AD 1950 (yrBP) length record registered a wet shift at 4600 yrBP in concert with the HTM and a later post-2550 yrBP shift towards wetter and more stable conditions, possibly related to the Roman Warm Period. During the late-Holocene the three most southern boreal records indicated a lack of droughts during the MCA. Medieval pluvials in boreal Ontario would be consistent with North American precipitation patterns under La Niña conditions. Of the three wet-warm periods identified (HTM, 2550 yrBP, MCA), the 2550 yrBP shift had the most consistent positive influence on LARCA regionally. Although past precipitation was positively correlated with temperature in the region, recent drying may be indicative of a fundamental change in evaporation-precipitation balance.