Quaternary and upper-pliocene glaciations and forest development in the tropical andes: Evidence from a long high-resolution pollen record from the sedimentary basin of Bogotá, Colombia

Abstract

Pollen analysis of 1230 samples from a 357 m long core of lake sediments from the sedimentary basin of Bogotá, Eastern Cordillera, Colombia (alt. 2550 m above sea level) has provided a high-resolution record of changes in the Andean vegetation belts and long-range climatic fluctuations during the last 3.5 Ma. Direct time-control is provided by K/Ar-datings and fission-track datings of 14 intercalated volcanic ash layers. The resulting pollen diagram has been divided into 55 pollen zones, representing 27 complete major climatic cycles (glacial-interglacial cycles). A tentative correlation with the marine oxygen isotope chronology and the European stratigraphical climatic subdivision could be established. The altitudinal position of the forest line, which corresponds with the 9.5°C isotherm, shifted between minimal c. 1700–1800 m and maximal c. 3400–3700 m alt. (at present situated at c. 3100 m alt.) which rendered the altitudinal position of the high plain of Bogotá (the dry bed of a former lake) at 2550 m a sensitive recorder of temperature changes. The estimated fluctuations in mean annual temperature at the level of the high plain of Bogotá are from <6°C to > 15°C. A distinct change in the nature of the climatic cycles occurs around 2.5 Ma B.P.: the interval of 3.5-2.5 Ma B.P. shows high frequency-low amplitude climatic oscillations, whereas the last 2.5 Ma is characterized by low frequency-high amplitude climatic oscillations (with a superposed high frequency oscillation). It is assumed that these oscillations correspond mainly with the variations in orbital eccentricity and obliquity. The major constituents, making up the Andean forest, have been subjected to considerable changes in the course of time. In the early (Pliocene) forests Vallea and Myrica are prominent elements. Weinmannia was almost absent. The role of Polylepis in the timberline forests increased markedly around 3.1 Ma B.P. and also migrated by adaptation from the upper Andean forest line to higher elevations. The closure of the Isthmus of Panama during the Pliocene enabled northern-hemisphere elements to migrate into South America. Alnus reached the area of Bogotá around 2.7 Ma B.P. and dramatically changed the vegetation, especially on the flats around the lake of Bogotá where Alnus swamp forests replaced Myrica carr. Around 2.9 Ma B.P. Weinmannia became an important constituent of the upper Andean forest, but slightly later slope forests of Alnus replaced parts of the area which had previously been covered by Weinmannia forest. The marked role of Vallea in the Andean forest diminished definitively after 2.5 Ma B.P. Quercus migrated more recently to South America and reached the area of Bogotá 1 Ma B.P. It changed for the last time drastically the aspect of the Andean forest. All palynologically recognizable elements of the modern Andean forest were present from now on and the subsequent adaptation and speciation of Quercus and many other elements must have mainly taken place at the species level.