6) Grass pollen and charred cuticle in the Amazon Basin

KEITH RICHARDS
KrA Stratigraphic, Deganwy, UK

Abstract of talk presented at Linnean Society Palynology Special Interest Group Meeting (October 2000)

GRASS POLLEN AND CHARRED CUTICLE IN PLIO-PLEISTOCENE SEDIMENTS FROM THE AMAZON BASIN

Palynological records from Quaternary and late Neogene sediments in the Amazon region invariably contain grass pollen, often in significant quantities. In the low-latitude tropics, grass pollen in the palynological record is often assumed to be an indicator of open (i.e. non-closed forest) vegetation and, therefore, of dry climatic conditions. However, several studies (e.g. Hoorn, 1994; Hooghiemstra & van der Hammen, 1998) have highlighted the likelihood that grass pollen may be sourced from a variety of  vegetation types, including swamp forest where grasses are often found as a component of floating vegetation communities. The significance of grass pollen as an indicator of past vegetation and climate is therefore open to question.

In a palynological study of late Neogene sediments from the Niger Delta, West Africa (Morley and Richards, 1993) sediments were found to contain significant quantities of black or opaque grey cuticular fragments (with stomatal cells clearly visible). These are believed to be from savannah grasses that have been charred during periods of fire. Peak abundances of this charred cuticle in the Niger delta were considered to be a product of climatic conditions sufficiently dry to permit savannah fires.

A study of similar age sediments from the Amazon basin confirms that similar charred cuticle fragments also occur, and that they are most frequent in sediments of  Late Pliocene and Early Pleistocene age. It is suggested that these charred cuticle fragments from the Amazon are also a product of  a periodically dry climate and that charred cuticle, rather than grass pollen, may be more reliable as an indicator of dry conditions. It is concluded that the Late Pliocene and Early Pleistocene periods in the Amazon basin were characterised, at least in part, by the presence of open forest and/or savannah-like vegetation in response to a periodically dry climate.

References:
Hooghiemstra, H. and van der Hammen, T. (1998)
Neogene and Quaternary development of the neotropical rain forest: the forest refugia hypothesis, and a literature overview. Earth Science Reviews, 44, 147-183

Hoorn, C. (1994)
An environmental reconstruction of the palaeo-Amazon River system (Middle-Late Miocene, NW Amazonia). Palaeogeography, Palaeoclimatology, Palaeoecology, 112, 187-238

Morley, R.J. and Richards, K. (1993)
Gramineae cuticle: a key indicator of Late Cenozoic climatic change in the Niger Delta. Review of Palaeobotany and Palynology, 77, 119-127
 

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