A mid‐Holocene environmental change in Amazonian savannas

Abstract

AbstractAim The main goal of this study was to investigate how climate and human activities may have influenced ecotonal areas of disjoint savannas within Brazilian Amazonia.Location Eastern Brazilian Amazonia, Amapá State.Methods The fossil pollen and charcoal records of two lakes in Amapá (Marcio and Tapera) were used to provide a Holocene palaeoecological history of eastern Amazonian savannas. Detrended correspondence analysis was used to enhance the patterns of sample distribution along the sediment core.Results Even though sedimentary hiatuses were recognized in the sediment cores from both lakes, a marked change in vegetation from closed forests with swamp elements to open flooded savanna at c. 5000 yr bp was evident from the pollen record. Charcoal analysis revealed a pattern of increased accumulation of charred particles coincident with the establishment of savanna vegetation, suggesting higher fire frequency near the lakes. Because the timing of the sedimentary hiatus overlapped with the highest Holocene sea level, which would have increased the local water table preventing the lakes from drying out, we infer that both lakes used to depend heavily on flood waters, and the sedimentary gap was caused by reduced discharge from the Amazon River, due to a dry period in the Andes, when precipitation levels markedly decreased between 8000 and 5000 yr bp. The lack of Andean pollen (probably river transported) in the sediment record after this event and the existence of similar records near the study site make this interpretation more appealing. The resumption of sedimentation in Lake Marcio, contemporaneous with falling sea level and increasingly wet conditions in the Andes after 5000 yr bp, indicates that Holocene sea‐level variation did not play an important role in maintaining lake levels.Main conclusions The study site recorded long‐term occupation by pre‐Columbian peoples. However, it is still unclear whether these disjoint savannas have an anthropogenic origin. Even though locally dry environmental conditions were inferred from both records, there is no evidence of a mid‐Holocene dry climate in eastern Amazonia. Instead, the Amapá record indicates a connection between Andean climate and eastern Amazonia, demonstrating the need for a better understanding of the impacts and magnitude of climate changes.