Late Pleistocene charcoal-rich sediments in the Puerto Rico Trench, possible remnants of gigantic wildfires in North-Eastern South America

Abstract

An 18,000 km² area of the Guyana Shield of South America, known as the Gran Sabana, is characterized by savannah vegetation that contrasts strongly with surrounding rain forests. Its origin has been linked to multiple episodes of forest fires. In this paper, we report a deposit encountered in two piston cores sampled during the CASEIS marine cruise, at 6000 m-depth at the southern entrance of the Puerto Rico Trench. The existence of this deposit call into question our understanding of the evolution of the Gran Sabana. We sampled its upper ∼60 cm, which comprises leaves and wood fragments, seeds, and charcoal, intermixed with siliciclastic sediment of igneous-metamorphic continental provenance. Radiocarbon dates of the vegetal fragments and charcoal range between 30 and 23 kyr BP. We propose that these deep ocean charcoal-rich sediments, located 2500 km offshore from the Orinoco Delta, may be remnants of gigantic forest fires of the Guyana Shield. We infer that this material was eroded during an extreme regional rainfall event, transported down rivers during one or more episodes to the Orinoco delta, and then travelled offshore via a deep turbiditic submarine system flowing on the Atlantic seafloor. It finally reached the Puerto Rico Trench, forming what we term, the Baracuda Trench Debrite. While published paleoclimatic analyses of lacustrine sediments have suggested that the Gran Sabana originated during episodes of wildfire ∼12.5 kyr BP ago, radiocarbon dating of Baracuda Trench Debrite suggests the occurrence of earlier fires in this region, leading us to re-evaluate the age of the Gran Sabana. These fires occurred during the low glacial maximum (LGM) and were likely promoted by climate change.