Thomas van der Hammen

The El Abra Stadial, a Younger Dryas equivalent in Colombia

Abstract A survey of radiocarbon-dated palynological, stratigraphical and glacio-morphological evidence from Colombia is presented showing a cold climatic fluctuation most likely between 10,900 and 10,100 BP, immediately preceding the initiation of the Holocene. This fluctuation, called the El Abra stadial, is characterized by lower temperatures and less precipitation, and occurs after the Late-Glacial Guantiva interstadial. Average annual temperature during the Guantiva interstadial may have been 2°C lower than today, and during the El Abra stadial from 4° to 6°C lower than today. The upper forest line during the El Abra stadial was some 400 to 500 m lower than during the Guantiva interstadial and some 600 to 800 m lower than today. The Guantiva interstadial-El Abra stadial-early Holocene sequence is radiocarbon dated by some 45 dates and evidence is shown from 14 areas, mostly located at high elevations (2000–4000 m) in the Eastern and Central Cordillera and in the Sierra Nevada de Santa Marta, but some from the tropical low-lands. Although more precise dating of the boundaries is desirable, we interpret the Guantiva-El Abra climatic fluctuations as equivalents in time and climatic phenomena to the Allerod-Younger Dryas sequence in Europe.

Mid- to Late-Holocene pollen-based biome reconstructions for Colombia

Abstract The assignment of Colombian pollen data to biomes allows the data to be synthesised at 10 ‘time windows’ from the present-day to 6000 radiocarbon years before present (BP). The modern reconstructed biomes are compared to a map of modern potential vegetation to check the applicability of the method and the a priori assignment of pollen taxa to plant functional types and ultimately biomes. The reconstructed modern biomes are successful in describing the composition and distribution of modern vegetation. In particular, altitudinal variations in vegetation within the northern Andean Cordilleras are well described. At 6000 BP the biomes are mainly characteristic of warmer environmental conditions relative to those of the present-day. This trend continues until between 4000 and 3000 BP when there is a shift to more mesic vegetation that is thought to equate to an increase in precipitation levels. The period between 2500 and 1000 BP represents little or no change in biome assignment and is interpreted as a period of environmental stability. The influence attributed to human-induced impact on the vegetation is recorded from 5000 BP, but is particularly important from 2000 BP. The extent of this impact increases over the Late-Holocene period, and is recorded at increasingly high altitudes. Despite these changes, a number of sites do not change their biome assignment throughout the analysis. This asynchronous vegetation response is discussed within the context of site location, non-linear response of vegetation to Late-Holocene environmental change, regionally differential signals, localised human impact and methodological artefacts.

Late quaternary vegetation history and paleoecology of Laguna Pedro Palo (subandean forest belt, Eastern Cordillera, Colombia)

Abstract Laguna Pedro Palo is situated on a saddle at ca. 2000 m altitude in the subandean forest belt on the western slope of the Eastern Cordillera of Colombia. Three pollen records from the site show the vegetation history and paleoclimatological sequence since ca. 13, 000 BP. The pollen records were correlated on the basis of radiocarbon-dated horizons (11,950 ± 100, 11,380 ± 130, 10,380 ± 90, and 10,280 ± 90 yr BP) and palynostratigraphy. Almost no palynological data are available from the subandean forest belt during glacial conditions. Most important is the direct evidence that the upper Andean forest line shifted to below 2000 m alt. during glacial conditions, providing evidence of a glacial forest line depression of ca. 1400 m and aninferred temperature depression of ca. 8°C. During the early Late Glacial (or Pleniglacial-Late Glacial transition) under dry climatic conditions, (sub)andean (semi-)open dry vegetation was locally in contact with dry paramo vegetation and a proper Andean forest belt was apparently absent. Open shrubparamo and grassparamo vegetation surrounded the lake at 2000 m alt. Subsequently patches of Andean forest reached closer to the area, indicating gradually less dry conditions shortly before 12,000 yr BP. During the Guantiva interstadial (ca. 12,000–11,000 yr BP) a well-developed Andean forest belt occurred from ca. 1800 to 2800 m alt. consisting of mainly Quercus forest and Weinmmannia forest. Climatic conditions were cool temperate and more humid than before the Guantiva interstadial. Alnus carr vegetation occurred around the lake. During the next El Abra stadial (ca. 11,000–10,150 yr BP) a cooler climate (−3°C) is indicated by alowering of the upper Andean forest line by ca. 500 m from its position during the Guantiva interstadial. Carr vegetation of Alnus diminished and disappeared at the end of the El Abra stadial, an approximate time-equivalent of the Younger Dryas event. Subsequently the upper Andean and subandean forest lines reached Holocene elevations, ca. 3300 and 2400 m, respectively. First a massive increase of Cecropia, a pioneer tree, and shortly later Acalypha and Alchornea dominated the pollen spectra of the subandean forest belt. In the Andean forest belt Quercus and Weinmannia forest types dominated with important contributions of Hedyosmum, Ilex, Juglans, Miconia, Urticaceae and Brunellia. A Late Holocene pollen record shows a phase showing human disturbance of the forest belt. Agricultural activity is indicated by relative high percentages of Gramineae and Chenopodiaceae followed by a possible forest succession phase in which Cecropia played a major part. Finally a “post-conquest” phase (after ca. 1500 AD) is registered, shown by relative high percentage of Gramineae and Rumex acetosella.

Chronostratigraphy and correlation of the Pliocene and Quaternary of Colombia

Abstract Names for the chronostratigraphic units of the Pliocene and Quaternary of Colombia are defined and formalized based on studies of outcrops and deep bore holes in the area of the high plain of Bogota. Chronostratigraphic, biostratigraphic and lithostratigraphic sequences have been correlated as far as possible. A tentative correlation with the marine oxygen isotope stratigraphy is indicated. Pliocene chronostratigraphic units are provisionally characterized by abundance/first appearance in the pollen record of specific floral elements: Mauritia zone (Tequendamian: ca. 5.5-4.2 Ma), Hedyosmum zone (Facatativanian: ca. 4.2-3.5 Ma), Myrica zone (Sisganian: ca. 3.5-3.1 Ma), Borreria zone (Chocontanian: ca. 3.1-2.7 Ma and Engativanian: ca. 2.7-2.4/2.2 Ma), although precise boundaries cannot yet be established. The Engativanian is a transitional phase, characterized by a continuous temperature decrease in successive cold episodes, and may include the latest part of the Pliocene and the earliest part of the Quaternary. Quaternary chronostratigraphic units are precisely limited by levels of pronounced climatic change; these boundaries are near to the first appearance/abundance of Juglans-Plantago (Funzanian: ca. 2.4/2.2-1.0 Ma), Alnus (Fuchanian: ca. 1000-660 ka and Cotanian: ca. 660-335 ka), and Quercus (Subanian: ca. 335-128 ka, Bacatanian: ca. 128-73 ka, Fuquenian: ca. 73-10 ka, and the Holocene). The relationship with the officially defined position of the base of the Quaternary, at ca. 1.80 Ma, is shown. The last glaciation in Colombia is named the Cocuy glaciation.

The Plio-Pleistocene climatic record of the tropical Andes

Sequences of lake sediments on the high plains of the Colombian Cordillera Oriental (at an altitude of c. 2500 m) represent some 5 or 6 Ma. The last glacial–interglacial cycle is well documented in the sediments of Lake Fuquene; very long sequences (up to 350-m long cores) were collected in the high plain of Bogota, representing more than 3.5 Ma. Pollen diagrams are now available, covering that time and representing the longest known continuous terrestrial record available. They were dated using potassium-argon and fission track methods on material from intercalated volcanic ash layers. The pollen diagrams can be interpreted in terms of shifting vegetation zones, and these in turn enable us to draw approximate temperature and humidity curves. The temperature curve for the last 3.5 Ma can be correlated with deep-sea curves (oxygen isotopes, temperature). An important change in rhythm and intensity seems to take place at approximately 3.1 Ma and a cold period that should correspond in time with the Pre-Tiglian of Western Europe is conspicuous.

Preceramic Sequences in the El Abra Rock-Shelters, Colombia

A series of crude stone artifacts, characterized by a trimming mainly of the working edge of a single face of a chert flake or a fragment of a nodule, was excavated in the El Abra rock-shelters, north of Bogot�, Colombia. The tools indicate a cultural complex distinct from others that have been described for the Paleo-Indian of South America.

Vegetation disturbance and human population in Colombia – a regional reconstruction

Palaeoecologists using pollen to map vegetation since the last ice age have noted numerous changes – which they feel increasingly obliged to blame on humans. These changes, such as deforestation or the dominance of certain plants, may happen suddenly or take place over thousands of years. The authors study the pollen record in Colombia, identify plants diagnostic of cultivation or disturbed ground (“degraded vegetation”) and use them to map human activities by proxy. They show how the people move and the landscape changes between 5000 BP and the present day, from the coast inland, and from the lowlands up into the Andes.