Natural variability of the Amazonian hydroclimate over the last two glacial cycles (220,000 years).

The Amazon basin is one of the most influential hydroclimatic systems on the planet, modulating the water cycle and energy balance of the tropical regions. The long-term stability of the Amazon rainforest is closely linked to regional hydroclimate, as shifts in rainfall amount and seasonality can drive substantial ecological transformations across the basin. Understanding how the Amazon system has naturally responded to past oceanic and atmospheric forcings is crucial, and paleoclimate records provide information to investigate the mechanisms governing Amazonian hydroclimate variability through time. Yet, paleoclimate archives that allow us to explore its variability beyond the last 50 ka are limited. The objective of this study is to characterize orbital- and millennial-scale hydroclimatic changes within the Amazon basin over the last 220,000 years through high-resolution X-ray fluorescence (XRF) analysis on marine sediment cores recovered from the northern margin of French Guyana during the AMARYLLIS–AMAGAS II cruise, specifically at stations S6 and S7 where a composite record was produced for each station by combining cores MD23-3652Q/53 and MD23-3655Q/56, respectively.

XRF results of S6 cores, which have an age model, allowed us to associate geochemical changes with Marine Isotopic Stages (MIS 1–7) and Heinrich Stadials of the last 60 ka. High values of Fe/Ca and Al/K log-ratios are observed during Heinrich Stadials (HS1–H6), indicating increased input of terrigenous vs. biogenic material, consistent with enhanced fluvial discharge, and an enhanced contribution of chemically weathered material from the Amazon basin. Elevated ln(Fe/K) and ln(Al/K) ratios specifically suggest a stronger contribution from lowland, highly leached soils and enhanced precipitation-driven weathering within the basin, rather than changes in sediment provenance. These patterns suggest globally wetter conditions over the Amazon Basin during HS, in agreement with the documented southward shift of the Intertropical Convergence Zone (ITCZ) and strengthening of the South American monsoon. During interglacial periods such as MIS 5e and MIS 1, higher sea levels likely reduced the continental influence on sedimentation at the core sites, enhancing the relative contribution of marine carbonates. This is reflected by lower ln(Fe/Ca) and ln(Fe/K) ratios, together with higher ln(Sr/Ca) values, which indicate a decline in terrigenous input and a stronger oceanic influence. During glacial stages (MIS 6, 4 and 2), the combination of high ln(Fe/Ca) and an increased ln(Al/K), denotes intensified fluvial supply and stronger chemical weathering under humid conditions, despite lowered sea level.

S7 cores, although lacking an age model, allow for a qualitative comparison due to their geographic proximity to S6. The general trends in Fe/Ca and Al/K log-ratios are consistent with those of S6, suggesting that S7 cores record the same regional signal of Amazonian fluvial variability, modulated by the tropical hydroclimatic regime. These preliminary results demonstrate that XRF records from S6 and S7 cores constitute an exceptional archive for evaluating the interaction between the Amazonian hydroclimatic system and North Atlantic forcings, indicating that during Heinrich Stadials, a southward migration of the ITCZ and intensified tropical rainfall enhanced Amazonian river discharge and continental runoff.