Pollen Influx to Laminated Sediments: A Pollen Diagram from Northeastern Minnesota

Abstract

A pollen diagram is presented from Lake of the Clouds, Lake County, Minnesota, in the Boundary Waters Canoe Area. Of the 5.6 m of sediment analyzed, almost 5 m are laminated. The 9,349 laminations counted, composed of alternating dark organic and light layers, are annual varves which provide a time scale for study of vegetational history and calculation of pollen influx. The pollen concentration in each sample was determined by a technique involving the addition of microspheres. The pollen influx was calculated from the pollen concentration and varve count. Pollen concentration was very low in the herb pollen zone of late—glacial time. During postglacial time the pollen influx was generally 2—4 x 104 grains cm‐2 years‐1. The pollen diagram has been divided into six zones. Zone 1 records tundra—like vegetations; most of the pollen rain, which was sparse, was probably blown in from a great distance. Zone 2 records spruce forest, with some pine and thermophilous deciduous trees, succeeded in zone 3 about 9,200 years ago by forests dominated by jack or red pine or both. With the start of zone 4, dated about 8,300 years ago, alder migrated into the area, followed about 7,000 years ago by white pine, concurrently with the maximum eastward extension of prairie in the state. In the period represented by zone 5, commencing about 6,500 years ago, white pine was very important, spruce was at a minimum, and the climate was probably relatively dry. Environmental changes brought about increases of spruce and cedar and a decline of pine in zone 6, commencing about 3,000 years ago. The diagram is truncated at 200 years or more before the present.