COS 300-6 - Paleoecological view on tropical forest tipping point in NW Amazonia

Abstract: It has been proposed that Amazonia is approaching a critical thermal tipping point that if passed would result in forest dieback and a massive loss of biodiversity. Paleoecology can provide insights into such vulnerability by documenting the fate of rainforest the last time Earth was c. 2 ^oC warmer than today - during the last interglacial c. 130,000 -115,000 years ago. We report fossil pollen, charcoal and sediment chemistry (micro-XRF) findings from a new lake record from the Hill of Six Lakes in northern Amazonia. This setting at c. 350 m elevation receives c. 3000 mm precipitation annually, with a short dry season from July to October. Nevertheless, the hill is edaphically dry and supports a modern forest that has small stature and relatively low biomass compared with a typical lowland forest, making it potentially sensitive to further drying. Using a combination of radiometric dating and orbital tuning of the XRF data we derive a chronology and show that this 6 m long core spans at least the last c. 150,000 years. We show that the lands adjacent to Lake Verde2 maintained a dense tropical forest cover under both last interglacial and the subsequent glacial. Vegetation composition shifted from a dominance by Moraceae and Alchornea in the interglacial to an assemblage rich in Melastomataceae and Myrtaceae during the glacials. Charcoal consistent with local fire was not recorded in this system. An absence of Poaceae pollen was consistent with dense forest cover and lack of openness throughout the record. Taxa adapted to cooler climates (Ilex, Podocarpaceae) were recorded in very low abundances during the glacial and the last deglaciation suggesting the invasion of the lowlands by taxa from higher altitudes and is consistent with the previously reported glacial cooling of c. 5 ^oC. The abundance of taxa in the families Asteraceae, Melastomataceae, and Myrtaceae would suggest somewhat drier glacial climates, but evidently not enough to cause the forest cover to be broken. In the current interglacial, increased abundances of Cecropia and Moraceae and a decrease in cool-adapted taxa suggests warmer and moister climates after c. 13,000 years ago. Our data do not support the imminent loss of forest in northwestern Amazonia as a result of projected climate change.