Late Glacial and Holocene vegetation history and paleoclimate of the Kaibab Plateau, Arizona

Abstract

Sediment cores spanning the last 13,500 calendar years (cal yr) were obtained from two lakes (Fracas Lake, 2518 m; Bear Lake, 2778 m) on the Kaibab Plateau in northern Arizona. Pollen and plant macrofossil records indicate that before ∼12,900 cal yr B.P., high elevation landscapes of the Kaibab Plateau near Bear Lake were covered by alpine tundra, while lower elevations near Fracas Lake were occupied by Picea woodland. At ∼12,900 cal yr B.P., Picea engelmannii and Abies lasiocarpa forest expanded upward to occupy the top of the plateau. Pinus ponderosa arrived near Fracas Lake 11,000 cal yr B.P., replacing Picea forests. Since then, Pinus ponderosa forest has dominated the Fracas Lake area. Pinus ponderosa did not appear at Bear Lake until ∼9730 cal yr B.P. Mixed forests of Picea (mainly Picea pungens), Abies lasiocarpa, Pinus ponderosa, and Pseudotsuga (after ∼8000 cal yr B.P.) grew near Bear Lake for the remainder of the Holocene. Picea engelmannii populations reexpanded near Bear Lake after 4000 cal yr B.P. Charcoal records indicate that fire probably helped Pinus ponderosa to become established near Bear Lake. Climate changes on the Kaibab Plateau since the Late Glacial were inferred from lake levels and vegetation patterns. The Late Glacial (>11,000 cal yr B.P.) was cold and probably wet. The early Holocene (11,000 to ∼8000 cal yr B.P.) was cooler than today and may have been the wettest period. Fracas Lake and Bear Lake were probably deepest then. During this period, a strengthened summer monsoon brought in more moisture from the eastern Pacific Ocean and the Gulf of Mexico. During the dry and warm mid-Holocene, Fracas Lake and Bear Lake experienced water-level declines. The late Holocene was relatively wet and cool again, and aquatic plants were abundant in the two lakes. Increasing effective moisture in the late Holocene was related to decreasing summer insolation.