Recent permafrost dynamics in a subarctic floodplain associated with changing water levels, Quebec, Canada

We have reconstructed from tree-rings the dynamics of mineral frost mounds on the floodplain of a subarctic river (Riviere Boniface, northern Quebec) likely associated with snow precipitation and temperature changes during th e past centuries. Due to their peculiar location in the river bed, we have postulated that the inception and decay of frost mounds (thermokarst ponds) were associated with snow-controlled water levels of the river. The period s of establishment, growth, and mortality of spruce around and in two therm okarst ponds on the shore zone were identified. The oldest tree-ring dates show that permafrost mounds formed during a period of water lowering in the 17th century, likely in a long-enduring sequence of low waters initiated a round cal AD 1150 (910 yr BP), which persisted for at least 200 vr until th e beginning of the 20th century. Two main periods of spruce mortality were identified, in the late 19th century and early 20th century and in the 1950 s-1960s, which corresponded to a rising river level probably due to greater snow precipitation. The patterns of spruce establishment and mortality wer e strikingly similar in the studied sites. The first spruce to establish we re all located along the pond's edges and they were also the first ones to die; most of the youngest spruce established later in the central part of e ach feature, i.e., in the early to mid-1850s to 1910, and died during the 1 950s-1960s. The sequence of events reported here suggests an important lowe ring of the Riviere Boniface during the first part of the Little Ice Age (e nd of the 16th century-17th century) which was drier and cold. During the s econd part of the Little Ice Age (mainly 19th century), greater precipitati on occurred and climaxed in the 20th century when climate warmed and the ri ver stage reached its maximum level. Changes in snow precipitation were pro bably more instrumental than temperature changes in the rise and fall of pe rmafrost landforms because of the snowypack's direct influence on the soil thermal regime and river level during snowmelt.