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Origin of Organic Matter Leading to Peat Formation in the Southeastern Guayana Uplands and Highlands

2011; Springer Nature; Linguagem: Inglês

10.1007/978-3-642-20138-7_8

2196-971X

Ernesto Medina, Elvira Cuevas, O. Huber,

Botany, Ecology, and Taxonomy Studies

Peat samples from a wide range of sites in the Guayana uplands and highlands of southeastern Venezuela have been dated using the 14C technique. The simultaneous determination of the natural abundance of 13C in these samples (δ13C values) allowed the separation of four types of peat deposits related to the photosynthetic type of the predominant vegetation (C3 or C4), the proportion of woody components, and differential leaching during the decomposition processes. The peats of the Auyán-tepui highlands (Auyán-tepui and Chimantá massif, 1,860–2,150 m a.s.l.) have the more negative δ13C values (−26‰ at the peat surface), indicating that they derive mainly from C3 woody plants. The peats of the Churí highlands (2,250 m a.s.l.) derive from C3 herbaceous plants, with δ13C values about 1.7‰ higher than the former. The Guaiquinima group included peats from the Guaiquinima tepui and several sites in the Gran Sabana (Guayana uplands, 900–1,350 m a.s.l.) with an average δ13C value of −21.8‰ that indicates a mixture of organic matter from C3 and C4 plants. Finally, the peat of the Divina Pastora site (Guayana uplands, 800 m a.s.l.) derives exclusively from C4 plants (−18.2‰). The δ13C values of all groups decreased slightly with age (0.1–0.4‰ per 1,000 years), resulting from the increase in resistant organic matter (lignins), relatively impoverished in 13C during the decomposition process. Plant cover composition in the area appears to have been stable during the last 6,000 years. More detailed sampling in the Gran Sabana and at other Guayana sites is needed to detect possible changes in vegetation composition between 10,000 and 6,000 years bp.