The Amazon in the Late Quaternary

For this post I read a paper titled “Late Quaternary Vegetation Dynamics in the Southern Amazon Basin Inferred from Carbon Isotopes in Soil Organic Mattter”, by Freitas et al., which analyzes, as the title states, carbon isotopes of soil organic matter (SOM) extracted from a 200km transect of a region in Brazil that is composed of savannas surrounded by forests. The objective of this study is to determine what kind of vegetation was previously present in this location.

 Fig. 1 Schematic diagram of the vegetation distribution and sample sites. Single lines, forest; double lines, savanna; arrows, sites of sample collection. Sample sites are identified by the distance along BR 319, from km 0 (Porto Velho) to km 200 (Humait´a).

However, I do need to point something out about this article before I go on. It was published in 2001, and for this reason the authors still entertain the hypothesis that, as our very own Mackay puts it “during intervals of full glacial conditions, increased aridity resulted in the Amazon rainforest being restricted to isolated pockets, surrounded by expanded savannah environments”. However, as Mackay’s chapter goes on to explain, this hypothesis is at odds with palaeoecological evidence, which suggests that forests made it through time periods of full glacial conditions.

Fig. 2 The ±13C range of the C3 and C4 plant species collected at: (a) km 46 (f); (b) km 142 (f); and (c) km 188 (s). The three sites have some common plant species.

The study does not lose its relevance because of this, I merely point it out as the authors mentioned this debate in the paper, and it no longer seems to be much of a debate. This study nonetheless still has valid results is that the geographical area being analyzed has well-defined natural boundaries between savanna and forest. The carbon isotopes showed that at least in this location, there were shifts between forest and savanna vegetation. From 17,000 and 9000 ¹⁴C yr B.P. the entire area was a forest, then from 9000 to 3000 ¹⁴C yr B.P. it was a savanna, and then after 3000 ¹⁴C yr B.P. there was forest expansion once again to the present mixed state. The authors also propose that the forest-dominated periods probably correlated with a wetter climate, and a drier one for the savanna-dominated time frame. They also recognize that while these results cannot say much about the changes in regional vegetation change due to past climate change, it does indicate that the Amazon rainforest has receded and expanded in at least some areas.

Sources: De Freitas, H. A., Pessenda, L. C. R., Aravena, R., Gouveia, S. E. M., De Souza Ribeiro, A. & Boulet, R. 2001 Late Quaternary vegetation dynamics in the southern Amazon Basin inferred from carbon isotopes in soil organic matter. Quatern. Res. 55, 39–46.

Mackay, A. W. (2009) - An introduction to Late Glacial-Holocene environments. In S. T. Turkey (Edt.), Holocene Extinctions, Oxford University Press, Oxford, 1-15.