Late‐glacial climatic oscillations as recorded in Swiss lake sediments

Abstract

Regional pollen assemblage zones for the late‐glacial period of the Swiss Plateau are introduced and defined. They include four major zones (Artemisia, Juniperus—Hippophaë, Betula, Pinus PAZ) with several subzones. Pollen and oxygen‐isotope analyses on lacustrine sediments from several lakes in the area reveal four distinct phases of climatic oscillation in the time period of 13 000‐9500 yr BP. The first oscillation, termed the Aegelsee fluctuation, occurs shortly before 12 000 yr BP and varve counts suggest its duration was ca. 100 yr. It is characterised by a short decrease in the oxygen isotopes as well as a short increase in NAP associated with a depression in birch pollen values. The second oscillation, which occurs in the δ 18 O record shortly before the deposition of the Laacher See Tephra (ca. 11 000 yr BP), is termed the Gerzensee fluctuation. It occurs during a pine‐dominated phase and its vegetational effects cannot be determined palynologically. The most prominent regressive phase is the Younger Dryas biozone (ca. 10 700‐10 000 yr BP) characterised by an increase in heliophilous NAP and low δ 18 O values. The Younger Dryas biozone can often be subdivided palynologically into two parts: a first part rich in grasses and juniper and a second part with higher Filipendula and birch values. During the Preboreal biozone another distinct oscillation is evidenced only in the oxygen isotope ratios. Comparison of the Swiss oxygen isotope profiles with the Greenland Dye 3 record suggests that not only the three major shifts in the δ 18 O curves but also the minor ones are closely comparable, suggesting some common climatic control.