Plant trash in the basal sediments of glacial lakes

Abundant macrofossils of Picea and other forest plants, mixed with sand and other glacial material, occur at the base of the fine organic sediment of many lakes in Minnesota and adjacent areas at the margin of the Laurentide ice sheet in central North America. Such accumulations are interpreted as representing forest-floor litter washed into depressions from superglacial forest on stagnant ice. An example of a trash layer in Minnesota is described from Steel Lake, which contains not only macrofossils of Picea from the superglacial forest but also much older fragments representing forest overridden by the advancing ice. A modern analogue is on the surge moraines of the Klutlan glacier in the Canadian Rocky Mountains. Trash layers have not been described from similar dead-ice landscapes in Europe, perhaps because tundra plants rather than forest invaded the stagnant ice and left little litter. The closest example is found in Poland, where a basal peat of Allerød age is interpreted as of superglacial origin. The trash layer in the Minnesota sites is abruptly overlain by many meters of typical Holocene fine organic sediment. At the same level the Picea pollen zone changes to the Pinus zone, marking a prominent climatic change at nearly the same date as the end of the Younger Dryas cool interval, which is registered especially in areas around the North Atlantic Ocean. In Minnesota, however, the Picea/Pinus transition is time-transgressive from southwest to northeast over about 2000 years, in synchrony with the retreat of the Laurentide ice sheet.