Extensive wetland development in mid-latitude North America during the Bølling–Allerød

Abstract

Palaeoecological reconstructions of the American Midwest during the last deglaciation suggest the expansion of parkland biomes lacking modern analogues. Despite their spatial extent and persistence over several millennia, the landscape configuration and environmental drivers for the ‘no analogue’ biomes remain speculative. Here we use regression analysis linking settlement-era forest composition and wetland extent to identify specific trees and forest units strongly indicative of high wetland prevalence. We then recompile a regional pollen time series to show transient increases in these flood-tolerant trees, with prominent peaks during the Bolling–Allerod interstadial (~14.6–12.8 thousand years ago). Taken together, the pollen records, and analyses of settlement-era forest composition and wetland prevalence, suggest the establishment of extensive deglacial wetlands in the American Midwest (40–60% of land cover). This extensive, yet transient, wetland was possibly supported by southward discharge of Laurentide Ice Sheet meltwater during the Bolling–Allerod. The timing of this wetland expansion and its mid-latitude location have implications for deglacial methane source dynamics; our estimate of ~11 Tg yr−1 of methane is comparable to the northern source enhancement modelled from ice-core records. Regional decline of the Laurentide Ice Sheet meltwater discharge at the onset of the Younger Dryas (~12.8–11.7 thousand years ago) explains in part why these no-analogue wetland-rich parklands sharply declined, weakening this potential methane source. Widespread, temporary wetlands in the American Midwest were likely a major cause of methane emission peaks during the last deglaciation, according to an analysis of regional pollen records combined with hydrological modelling.