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USGS Open-File Report 94-023

Paleoclimatic Conditions Around 3 Million Years BP: Pollen Evidence From Colombia

Henry Hooghiemstra
Amsterdam University, The Netherlands

Introduction

In the Eastern Cordillera of Colombia, the high plain of Bogotá (ca. 25x40 km in size) represents the bottom of a former lake that occupied a subsiding intermontane basin. Pollen records from deep bore holes represent the period from the Late Pliocene to latest Pleistocene. Climatic changes are well documented by the pollen rain that is conserved in the slowly accumulating lake sediments of the Bogotá basin. The tropical Andes especially seem to be in a favorable position to register climatic change. A change in climatic conditions results mainly in a vertical shift of vegetation belts over the mountain slopes (Fig. 1). The different vegetation belts stay in the vicinity and are registered continuously by their intercepted pollen. The sediments of the Bogotá basin (2550 m alt.) accumulated in an elevation that lies halfway between the highest position of the upper forest line during interglacial conditions (c. 1800 m alt.), rendering the Bogotá sediments a sensitive recorder of paleoclimatic change in northern Latin America.
Figure 1. Altitudinal distribution of vegetation belts in the Eastern Cordillera of Colombia at present and during the last glacial maximum.
This figure is available as a GIF, PICT, or TIFF (line-art) image.
Vertical shifts of the vegetation belts are mainly related to changes in temperature and form the basic mechanism of registration of climatic change (after Van der Hammen, 1974).

Pollen Data And Time Control

A new 586 m long core Funza II (Fig. 2) was recovered from the sediments of the high plain of Bogotá. Results of the palynological analysis of 430-540 m core depth, representing the estimated time interval 3.2-2.36 Ma, with sample distances of 100 cm are presented (Fig. 3). The interval 586-540 m of the sediment core was poor in pollen recovery or barren. Time control of the Funza II core is based on zircon fission-track datings of intercalated volcanic ash horizons. In addition, the upper part of pollen record Funza II could be correlated in detail with pollen record Funza I; the latter has been graphically correlated with the oxygen isotope record of ODP Site 677 (E. Pacific). It is expected that the pollen record can be correlated with the deep-sea record up to about oxygen isotope stage 110 when samples are available at 20 cm distance (Hooghiemstra, in prep.)

Several phases in the evolutionary history of the montane forests and paramo vegetation in the Eastern Cordillera of Colombia can be recognized. Late Pliocene and Pleistocene climatic change and the closure of the Panamanian Isthmus had a substantial influence on the development of these montane ecosystems.

Figure 2. Map of Colombia and the high plain of Bogotá‡in the Eastern Cordillera, indicating the geographical setting of the sites of the long continental pollen records near the village of Funza.
This figure is available as a GIF, PICT, or TIFF (line-art) image.
Figure 3. Pollen percentage diagram of the 540-430 m interval of the Funza II core (Eastern Cordillera, Colombia, 2550m altitude).
This figure is available as a GIF, PICT, or TIFF (line-art) image.
The core was analyzed with 100cm sampling distance. Records of selected pollen taxa are presented. The pollen zones are provisional and will be defined more precisely after the present time resolution of ca. 6000 years has been increased (Hooghiemstra, in prep.). Time control of the lower part of the Funza II core is provided by zircon fission-track dating of an intercalated volcanic ash horizon at 506m core depth (2.74 ± 0.63 Ma; Andriessen et al., submitted). (The upper part of the Funza pollen record is correlated with the ODP Site 677 oxygen isotope record (Shackleton and Hooghiemstra in Andriessen et al., 1993). Note the absence of Alnus and Quercus in this part of the record (Northern Hemisphere taxa which arrived in the Eastern Cordillera during the middle Pleistocene time after having passed the Panamanian land bridge).

Paleoclimatic Conditions At Ca. 3 Ma

The interval 540-465 m core depth (3.2-2.7 Ma) shows warm climatic conditions and pollen spectra have no late Quaternary analogs. The basin had just started to accumulate lacustrine and river sediments, after a period in which sediment only accumulated in the perifere valleys. The upper limit of the subandean forest belt was situated at some 500 m lower elevation than today. In the Andean forest belt Podocarpus rich forest, Hedyosmum-Weinmannia forest (a precursor of the modern Weinmannietum), and Vallea-Miconia forest, respectively, were the main constituents with increasing elevation. Hypericum and Myrica played an important part in the timberline dwarf forests, which possibly constituted a substantial transitional zone from the early Andean forest belt (upper montane forest belt) to the open grassparamo belt. The contribution of herbs to the paramo vegetation, dominated by Gramineae and Compositae, seems less diverse than during the late Quaternary. The late Pliocene (upper) Andean forests were more open than during the middle and late Quaternary, as heliophytic elements, such as Borreria, were abundant (Fig. 3). The composition of forests on the high plain was dynamic: arboreal taxa with pioneer qualities (Dodonaea, Eugenia) and other taxa (Symplocos, llex) constituted seemingly at irregular intervals azonal forests in the basin. The upper forest line oscillated most of the time from 2800 to 3600 m elevation. The average annual temperature on the high plain was ca. 22.5-16.5°C.

Using composite pollen records Van der Hammen also documented warmer conditions in sediments from the area of the high plain of Bogotá during Pliocene time (Van der Hammen et al., 1973). Recently additional evidence for warmer late Pliocene climatic conditions was collected on the basis of plant macrofossils from this area (Wijninga and Kuhry, submitted). Although these fragments of pollen records most probably evidence phases of the final upheaval of the Eastern Cordillera, which has a 'cooling' effect on the pollen records, the continuous Funza II pollen record does evidence climatic change. It is expected that part of the variability documented in the Funza II summary diagram (Fig. 4) is associated with the initial stage of development of the large tectonic sedimentary basin of Bogotá.

Figure 4. Summary pollen diagram of Funza II
This figure is available as a GIF, PICT, or TIFF (line-art) image.
The figure shows the 540-430 m interval with pollen zones and tentative correlation with the deep-sea oxygen isotope stratigraphy (Ages of d18O stages after Imbrie et al., 1984).

Potential Use Of Pollen Data As Modern/Future Analogs

The late Pliocene (3.2-2.8 Ma interval) pollen spectra differ significantly from the late Quaternary ones. It is inferred that the late Pliocene Andean forests were of a more open character (e.g. presence of Borreria, originally a savanna element) whereas important elements of the present-day forest belt (Alnus and Quercus) were absent. Estimation of the altitudinal position of the upper forest line, therefore, is less accurate than during more recent intervals of the pollen record. However, accumulating evidence and better understanding of these late Pliocene ecosystems will enable to provide quantitative paleoclimatic estimates concerning temperature ranges and climatic variability (Hooghiemstra, in prep.).

References

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