A 1.8 Million Year History of Amazonian Biomes

During the Pleistocene, long-term trends in global climate were controlled by orbital cycles leading to high amplitude glacial/interglacial variability. The impact of Pleistocene climate cooling on Amazonian vegetation is largely unknown, mainly because no continuous record from lowland Amazonia extends far beyond the last glacial stage. Here we present a history of Amazonian biomes for the last 1,800 kyr based on palynological data, biome reconstructions and biodiversity metrics from a marine sediment core continuously archiving sediments off the Amazon River.Palynological data from site CDH-79, located atop a seamount, indicates that tropical rainforests dominated the Amazonian lowlands during last 1,800 kyr, exchanging with warm-temperate rainforests and tropical seasonal forests. Between 1,800 and 1,000 ka, various tropical rainforest biomes were present in the Amazon drainage basin together with more extensive riparian wetlands. In the early Mid-Pleistocene Transition (ca. 1,250 to 700 ka), tropical rainforest expansion occurred during the relatively warm Marine Isotope Stages 33 and 31 (ca. 1.11 to 1.06 Ma), before both forests and wetlands contracted until ca. 800 ka. Between 800 and 400 ka, low diversity of palynological assemblages allows little interpretation on the Amazonian vegetation. A strong synchronicity between changes in vegetation and glacial-interglacial cycles was established around 400 ka. Interglacial vegetation after 400 ka was dominated by lowland tropical rainforest during warmer temperatures and higher CO 2 . During glacial stages, tropical seasonal forests have expanded presumably in eastern Amazonia. Nevertheless, forested vegetation persisted during periods of cooler temperatures and lower CO 2 . Our study provides no evidence supporting a significant expansion of savanna or steppe vegetation within the Amazonian lowlands. However, changes in the rainforest composition in some parts of the basin indicate a higher proportion of deciduous elements within the rainforest, suggesting less humid conditions and/or greater seasonality of precipitation. At the same time, evergreen rainforests persisted in other parts of the large Amazonian realm. These findings confirm the susceptibility of tropical lowland vegetation to changes in CO 2 and climate, with most suitable conditions for tropical rainforests occurring during the warmest stages of the Mid Pleistocene Transition and during the interglacial stages of the past 400 kyr.