Abstract
Climatic expressions of the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) vary regionally, with reconstructions often depicting complex spatial patterns of temperature and precipitation change. The characterisation of these spatial patterns helps advance understanding of hydroclimate variability and associated responses of human and natural systems to climate change. Many regions, including north-eastern North America, still lack well-resolved records of past hydrological change. Here, we reconstruct hydroclimatic change over the past millennium using testate amoeba-inferred peatland water table depth reconstructions obtained from fifteen peatlands across Maine, Nova Scotia, Newfoundland and Québec. Spatial comparisons of reconstructed water table depths reveal complex hydroclimatic patterns that varied over the last millennium. The records suggest a spatially divergent pattern across the region during the Medieval Climate Anomaly and the Little Ice Age. Southern peatlands were wetter during the Medieval Climate Anomaly, whilst northern and more continental sites were drier. There is no evidence at the multi-decadal sampling resolution of this study to indicate that Medieval mega-droughts recorded in the west and continental interior of North America extended to these peatlands in the north-east of the continent. Reconstructed Little Ice Age hydroclimate change was spatially variable rather than displaying a clear directional shift or latitudinal trends, which may relate to local temporary permafrost aggradation in northern sites, and reconstructed characteristics of some dry periods during the Little Ice Age are comparable with those reconstructed during the Medieval Climate Anomaly. The spatial hydroclimatic trends identified here suggest that over the last millennium, peatland moisture balance in north-eastern North America has been influenced by changes in the Polar Jet Stream, storm activities and sea surface temperatures in the North Atlantic as well as internal peatland dynamics.
Highlights
The last millennium is a key focus for palaeoclimate reconstruction and climate modelling because environmental boundary conditions are broadly comparable with the present day and detailed climate reconstructions can be obtained from wellresolved proxy data with robust chronological control
Testate amoeba analysis is a well-established proxy in peatlands that is well-suited to reconstructing past hydroclimatic variability because species assemblages predominantly respond to changes in peatland surface moisture (e.g. Woodland et al, 1998; Mitchell et al, 2008)
Analysis of the testate amoebae-based water table reconstructions developed from peatlands in north-eastern North America has identified complex hydroclimatic spatial trends that varied over the last millennium
Summary
The last millennium is a key focus for palaeoclimate reconstruction and climate modelling because environmental boundary conditions are broadly comparable with the present day and detailed climate reconstructions can be obtained from wellresolved proxy data with robust chronological control. Reconstructed MCA mean surface temperatures in the North Atlantic region were 0.9e1.4 C above the 1961e1990 reference period (Mann et al, 2009). The MCA and LIA predate the Industrial Revolution; they act as excellent case studies to further understand natural ecosystem functioning and climate variability. Both the MCA and LIA have been described as global events In order to improve our understanding of hydroclimate variability related to past climate change, the spatial coverage of palaeoclimate reconstructions along the eastern seaboard of the North Atlantic must be increased, for the Medieval period (Neukom et al, 2018)
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