The 14C dates and 13C/ 12C ratios of soil organic matter as a means of studying the past vegetation in intertropical regions: Examples from Colombia (South America)

Abstract

The 14C ages and 13C/ 12C values were obtained on soil-organic matter of various soils sampled in the Guasca Valley adjacent to the high plain of the Sabana de Bogota, in the Colombian Cordillera Oriental. A soil distribution is observed as a function of climatic and altitudinal changes: in the lowest parts of the valley, planosols are found from the bottom of the valley to 2700–2800 m. At altitudes up to 2900 m, brunizems and ferrisols occur. Above 3000 m andic soils are almost everywhere present in more humid mountain zones. At present, the landscape is widely open, without any natural forest. Nevertheless, it was proved by pollen-analytical data that at altitudes up to 3000 m the Andean forest replaced a paramo formation 13,000 yr ago. To conciliate the facts with the known evolution of the vegetal population, a first hypothesis consists of admitting that open landscapes were due to a recent anthropogenic destruction of the forest. The forest-degradation hypothesis was tested by determination of the 13C/ 12C ratio because the trees have a δ 13C of −25% to −28% (influence of plants with C 3 photosynthesis pathway), whereas herbs characteristic of open vegetation, such as tropical grasses, may have a δ 13C of −23% to −16% (due to a C 4 photosynthetic cycle). The organic matter dated at 13,000–30,000 yr has δ 13C close to −25% in the andic soil zone. This indicates that at elevations of around 3000 m a cold paramo vegetation was probable. For organic matter dated at 12,000-6000 yr, δ 13C shows a variation from −23 to −21% that does not indicate forest conditions. It is uncertain that a forest stage occurred in the past. For the planosols, brunizem and ferrisol, organic matter dated at 13,000–20,000 yr has a δ 13C (−23%) compatible with the paramo vegetational phase occurring at that time. For organic matter dated at 9500 yr and less, δ 13C varies from −20% to −17%. These values do not confirm the evidence of forest conditions but rather the existence of open vegetation typified by a mixture of C 3 plants (shrubs and rare trees) and tropical Graminaea. Thus, the 13C values prove that the valley has known drier and warmer influences that prevented the forest from developing during the Holocene. To explain the origin of the open landscapes of the valley, the hypothesis of recent human deforestation must be discarded. Moreover, the valley appears as a persistent dry enclave in the oriental Cordillera.