A Holocene pollen record of vegetation change and human impact from Pantano de Vargas, an intra-Andean basin of Duitama, Colombia

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Abstract

Holocene environments in the intra-Andean basin of Duitama (Eastern Cordillera of Colombia, 2510 m altitude) are presented based on the 620 cm long Vargas pollen and sediment record. Seven AMS radiocarbon dates show these lake and swamp sediments represent the period from 8800 to 2610 14C yr BP. We identified seven distinct periods of environmental conditions in the basin.

From ca. 8800 to 7700 14C yr BP (9450–8300 cal yr BP) Alnus and Myrica swamp forest grew on wet soils. On the surrounding slopes Andean forest was present with Hedyosmum, Melastomataceae and Quercus. Between 7700 and 6090 14C yr BP (8300–6750 cal yr BP) open water with aquatics alternated with peat-forming Plantago meadows reflecting repeated lake levels changes. From 6090 to 5220 14C yr BP (6750–5680 cal yr BP) open water and grassy vegetation around the lake was replaced by Alnus-dominated swamp forest, suggesting lower lake levels and drier climatic conditions. Between 5220 and 4500 14C yr BP (5680–4810 cal yr BP) an alternation between Alnus-dominated swamp forest and open grassy vegetation continued, reflecting fluctuating water levels. At that time Hedyosmum, Miconia, Quercus, Solanaceae, Relbunium and Rubiaceae were common elements in the Andean forest. From about 4500 to 3680 14C yr BP (4810–3800 cal yr BP) Alnus-dominated swamp forest alternated with Myrica shrub and much organic material accumulated in the basin. Evidence of deforestation (reduction of Quercus in particular), lowering of water tables and increasing disturbance, probably by erosion (presence of Amaranthaceae/Chenopodiaceae, Borreria and Dodonaea) point to the settlement of pre-Hispanic populations. Between 3680 and 2610 14C yr BP (3800–2470 cal yr BP) there is clear evidence of crop cultivation (presence of Zea mays) and frequent fires (high amounts of charcoal in the sediments). Erosion all over the basin led to significant sediment supply and the lake gradually changed into a swamp. By this time pre-Hispanic populations had used nearly all alder wood in the catchment area. The uppermost sediments are oxidised and the last 2610 radiocarbon years of the Holocene are not represented, possibly due to agricultural activities.

Introduction

On the high plains of the central part of the Eastern Cordillera (Colombia), lake sediments have accumulated representing important natural archives which document environmental and climatic change. The high plain of Bogotá (4° N) has been studied most extensively (Van der Hammen and González, 1960a, Van der Hammen and González, 1963, Hooghiemstra, 1984, Van 't Veer and Hooghiemstra, 2000).

The high plain of Ubaté–Chichinquira (5°N), including Laguna Fúquene, documents environmental and climatic change of the last glacial cycle: core Fúquene-2 records environmental change during the last 32,000 yr (32 kyr) (van Geel and Van der Hammen, 1973), core Fúquene-7C documents with great detail the last 70 kyr (Mommersteeg, 1998, Mommersteeg and Hooghiemstra, in review) and core Fúquene-3 shows environmental change of the last 120 kyr (Van der Hammen and Hooghiemstra, 2003).

Pollen records have indicated that humans inhabited these high plains for a long time. The Tequendama and El Abra rock shelters show that early humans used these rock shelters around 12,000 14C yr BP (during the Guantiva interstadial) as temporary hunting camps (Correal et al., 1970, Correal and Van der Hammen, 1977, Schreve-Brinkman, 1978, Correal, 1990). The relatively dry period of the mid Holocene from 5000 to 3000 14C yr BP, suggests a decrease of human presence on the high plain of Bogotá (Correal, 1990). Shortly after 2500 14C yr BP the first signs of agricultural activities and ceramics are found (Cardale, 1987). In a biome-based interpretation of the Holocene development of disturbance vegetation, Marchant et al. (2001) demonstrated an increase of human occupation since 1500 14C yr BP above 2000 m altitude in the Eastern Cordillera. The pollen records of Lake Fúquene indicate possible deforestation and human impact after ca. 2000 14C yr BP (van Geel and Van der Hammen, 1973).

From the high plain of Duitama (ca. 6° N) no pollen records have been available so far. During much of the Holocene a lake occupied this high plain, gradually changing into a swamp called ‘Pantano de Vargas’ (VG). At the border of this palaeo-lake, and just outside the present town of Duitama lies the Tocogua archaeological park (Fig. 1). During archaeological excavations we collected a 620-cm core (VG-1) in the centre of the swamp area, and a 520-cm core (VG-2) from a site near the border (Fig. 1). The latter core contained insufficient pollen grains to arrive at a reliable pollen record: many samples had a pollen sum below 100 grains and several below 50 grains. Therefore, the present study is based on the centrally located core only, referred to as core Vargas (VG). The objective of this paper is to present the record of Holocene environments of the intra-Andean high plain of Duitama. Although the local archaeological record is still under study, we relate preliminary evidence of the changing presence of pre-Hispanic communities to our palaeoenvironmental record.

Section snippets

Geography, climate and vegetation

Pantano de Vargas (5°47′ N, 73°06′ W) lies at 2510 m altitude in the central part of the Province of Boyacá (Fig. 1). The surface of the sediments represent the floor of an ancient lake which received water from the Chicamocha River. The town of Duitama lies at the northeastern border of the basin and Paipa lies at its southern fringe. The palaeo-lake and swamps were drained some 50 yr ago for agricultural development (Ramírez et al., 1966). Up to the moment of draining the high plain of

Methodology

Site Vargas is located in the centre of the basin at some 5 km from Tocogua archaeological park (Fig. 1). The 620 cm long core was recovered in 25 cm increments (25 mm diameter) with a hand operated Dachnowsky Corer. Sediments were extruded into plastic bags and protected by cut PVC pipes. The core was transported to the laboratory in Amsterdam and stored in a cold room (4 °C). For pollen analysis samples of 1 cm3 were taken at 5 cm intervals. Prior to processing, one tablet of exotic Lycopodium

Stratigraphy

The following sedimentary sequence was recorded:

    0 to 50 cm

    Soft, dark grey sandy clay with some rootlets

    50 to 100 cm

    Grey clay with ferric nodules, and abundant charcoal fragments at 90–100 cm

    100 to 220 cm

    Dark grey clay; leaf fragments at 100–140 cm, almost pure peat with leaf fragments and charcoal at 140–145 cm, 165–175 cm and 210–220 cm

    220 to 280 cm

    Grey peaty clay

    280 to 515 cm

    Grey clay with organic matter

    515 to 565 cm

    Grey peaty clay

    565 to 600 cm

    Sandy clay

    600 to 625 cm

    Yellowish and grey coarse

Period from 8800 to 7700 14C yr BP (9450–8300 cal yr BP; zone VG-1)

Coarse sands at the bottom with an extrapolated age of ca. 8800 14C yr BP (9450 cal yr BP) point to an active phase of a high-energy drainage system when the basin was possibly connected to the Chicamocha River preventing sediments of older age have been conserved at the study site. Upwards, the sediments gradually change from sandy clay to peaty clay illustrating separation from the river drainage system allowing organic-rich sediments to accumulate. The presence of aquatics, such as

Discussion

This new pollen record from a yet unexplored basin shows the development of Holocene environments on the high plain of Duitama. Prior to that it seems the Chicamocha River, which today passes from the ‘alto de las panelas’ at 3500 m through the high plain of Paipa–Duitama and drains into the lower part of the Magdalena River, was a dynamic and high-energy drainage system. At ca. 8800 14C yr BP (9450 cal yr BP) the river flow apparently decreased. Changes in the sedimentary environment are

Acknowledgements

This study was carried out in the framework of the archaeological project ‘Tocogua’ during February 1994 (inspection of the site), August 1996 (coring) and November 2000 (site evaluation and presentation of first results in a poster). We thank support by COLCIENCIAS (Bogotá) and the board of the Municipality of Duitama. Thomas van der Hammen (Chia) is thanked for support and advise in the field during the first excavations at the Tocogua site. The first author thanks her parents for support to

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