Insolation-Driven Changes in Aridity Within the Amazon Basin Over the Last 40,000 Years

Annual precipitation over the Amazon Basin is thought to be strongly linked to the average latitudinal position of the Inter-Tropical Convergence Zone (ITCZ). A more southerly ITCZ is considered to bring moisture to the Basin via the humid northeasterly trade winds, drawn in from the tropical North Atlantic. When the ITCZ is constrained further to the north these trades are restricted, and so the Basin should become more arid. Past changes in Amazon Basin hydrology therefore have the potential to monitor shifts in the palaeo-latitude of the ITCZ over northern South America. However, great debate surrounds the Pleistocene moisture history of the Amazon Basin largely due to the paucity of reliable, uninterrupted, regionally-representative proxy records back through the last glacial maximum (LGM). As a result, reconstructions are often highly-localised and based on qualitative indicators of change. On the other hand, material collected from the Amazon Fan (ODP Site 942) has allowed us to examine an average effective moisture signal from the whole of the Amazon Basin for the last 40 ka within a single sedimentary sequence. Quantitative reconstructions of effective moisture based upon δ ¹⁸O analyses of planktonic foraminifera, suggest a significant reduction in Amazon River outflow during both the LGM and Lateglacial (to ∼60% and ∼55% of modern flow, respectively), becoming increasingly moister toward the modern day. This trend is similar to other records from South America, including the Cariaco Basin, and correlates well with insolation records implying the ITCZ as a driver. The signal also displays centennial and millennial-scale variability which are most likely climate-driven, and Heinrich Events are apparent as more arid periods within the record. We provide further evidence for glacial-stage aridity vs. Holocene humidity through a quantified reconstruction of the fire history of the Amazon Basin, where biomass burning-specific biomarkers are of coincident increased abundance throughout the more arid periods.