Global warming during the LGIT initiated very widespread and intense thermokarst activity in the icy permafrost lowlands of the Arctic and sub-Arctic. The warming occurred in two short bursts, the first at ~14,500 calibrated year BP — the start of the B0lling-Aller0d warm period (Greenland Interstadial 1e); and the second at ~11,500 cal year BP — the start of the Holocene (Bjorck et al. 1998). Evidence for LGIT thermokarst activity is preserved in thermokarst basin fills, relict active layers and ice-wedge pseudomorphs in Alaska, Canada and Siberia (McCulloch and Hopkins 1966; Rampton 1974, 1988; Tomirdiaro 1982; Burn et al. 1986; Burn 1997; Romanovskii et al. 2000; Walter et al. 2007). Interestingly, evidence for more muted thermokarst activity in northwest Europe — horizons of ice-wedge pseudomorphs, thick involuted layers and deformed brecciated bedrock (Vandenberghe and Van Den Broek 1982; Vandenberghe and Pissart 1993; Murton et al. 2003) — suggests that regional thermokarst commenced before the LGIT, probably when climate warmed in Greenland Interstadial 2 (21,800-21,200 cal year BP).
The penultimate glacial-to-interglacial transition at ~130,000 year BP also triggered regional thermokarst activity. Climate warming at that time initiated deep and rapid thaw of ice-rich permafrost in treeless terrain in the Yukon-Tanana upland, east-central Alaska. Meltwater from thawing ground ice thermally eroded gulleys, triggering block slumping and producing an irregular thermokarst terrain. Subsequently a protective cover of boreal forest developed above the thermokarst features during the Last Interglaciation (LIG), remains of which are preserved as the Eva Interglaciation Forest Bed (Pewe et al. 1997). During the LIG, the summer climate of the Arctic was markedly warmer than during the twentieth century or the late Holocene, providing a potential analogue for future global warming (Otto-Bliesner et al. 2006).
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