No glacial refugia in the Amazon Basin?

Tropical rainforest in Africa and South America is the most diverse biome on the planet, both as regards plants and animals.  One view of how such luxuriance arose is that the forests have blanketed the humid tropics for as long as 50 or 60 million years, and the fact that they encompass a huge variety of environments created by different levels in the dominant and diverse vegetation.  Thousands of niches and the interactions between organisms that exploit them during lengthy stasis inevitably drives rapid evolution towards all kinds of specialisation.  The other view is that rainforests are by no means static over millions of years, but climate shifts have caused them to retreat and advance, perhaps hundreds of times during the Cenozoic.  Amazonia in particular shows surprising variation in diversity, some patches being far more biologically rich than others, and having regionally distinct assemblages of plants and animals.  This theory suggests that climatic stress, probably drying associated with globally cool episodes, resulted in rainforest shrinking to “refugia”.  In them, populations of plants and animals shrank, thereby reducing the gene pool and giving greater chance for evolution by natural selection; different in different refuge areas.

Tropical soils are continually reworked and their highly oxidising nature destroys organic remains.  So no record of its development exists in rainforest.  However, wind and rivers transport spores, pollen and other biomarkers to seafloor sediments, where a complete record of fluctuations in biomass and diversity becomes preserved.  A test of the popular refugia hypothesis is therefore to analyse organic matter in continuous cores taken from offshore sediment.  Known fluctuations in global climate, from the oxygen isotope record should be matched by changes in the record of terrestrial biomarkers carried to the sea.  Cores from the deep-sea sediment fan off the mouth of the Amazon potentially provide such a test (Kastner, T.P. & Goñi, M.A. 2003.  Constancy in the vegetation of the Amazon Basin during the late Pleistocene: Evidence from the organic matter composition of Amazon deep sea fan sediments.  Geology, v. 31, p. 291-294). Kastner and Goñi, from the University of South Carolina, examined phenols and organic acids in the cores, which can discriminate between grassy plants and trees that would have dominated savannah and rainforest, whose relative cover of the Amazon basin should have changed, according to the refugia hypothesis, as climate shifted from globally cool-dry to warm-humid..  Although their record only spans the last glacial cycle since 70 ka, they detected no significant change in the proportion of grasses and trees in the Amazon catchment.  Moreover, the biomarkers remained similar to those carried by the Amazon today, right through the last glacial maximum, when drying of the tropics would have been most likely to have driven a shrinkage of rainforest area.  It seems unlikely that forest refugia developed during one of the most extreme climate shifts in the last 55 Ma.  Global climate fluctuations were considerably less before 1 million years ago, when the current round of 100 ka cycles began.  So there is little reason to doubt that the Amazon rainforest has had a more or less constant area for much of the Cenozoic.  The same cannot be said for those in Africa and SE Asia, partly because there are no useful data from offshore sediments, but also because those regions have experienced changing topography due to major tectonic activity, whereas eastern South America has remained stable.  To conclude, as the authors do, that the data signify no great fluctuation in rainfall is not so certain.

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