Past Amazon precipitation under different Atlantic Meridional Overturning Circulation and sea surface temperature scenarios of the last deglaciation

Changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) have been suggested to affect the mean latitude of the tropical rain-belt and the intensity of the South American Monsoon System. These changes impact Amazonian precipitation patterns, which play a critical role in sustaining the most biodiverse ecosystem on Earth. Consequently, the projected decrease in AMOC strength in response to anthropogenic climate change may constitute a serious risk to the stability of the Amazon rainforest. AMOC strength was significantly reduced during most millennial-scale stadials (e.g., Heinrich Stadials (HS)) of the last glacial and deglacial periods. These stadials offer valuable opportunities to understand the impact that slowdowns of the AMOC, as well as other concurrent changes of large-scale ocean and atmospheric conditions (e.g., sea ice extent, surface winds, ice sheet height and extent, sea level), had on Amazonian precipitation. Here we reconstruct precipitation changes that occurred in the Amazon Basin from 27.4 to 3.6 cal ka BP with a focus on Heinrich Stadial 1 (HS1, 18.6–14.6 cal ka BP) and the Younger Dryas (YD, 12.9–11.7 cal ka BP, also known as Heinrich stadial 0). To achieve this, two marine sediment cores collected from the slope of the western equatorial Atlantic were radiocarbon dated and analyzed for major elemental composition. To support the interpretation of our data, we employed (i) a comprehensive evaluation of major elemental composition in suspended sediments from the major rivers of the Amazon Basin, (ii) a regional compilation of tropical South American hydroclimate records, and (iii) outputs from a transient fully-coupled climate model run covering the last 21 ka. Our results reveal different precipitation patterns in the Amazon Basin during HS1 and the YD that were related to heterogenous displacements of the Intertropical Convergence Zone (ITCZ) and heterogenous sea surface temperature (SST) anomalies in the western equatorial Atlantic and the eastern equatorial Pacific. These changes were at least in part associated to different AMOC scenarios. Furthermore, two distinct phases are discernible within HS1 (i.e., HS1a from 18.6 to 16.6 cal ka BP and HS1b from 16.6 to 14.6 cal ka BP), each being characterized by variations in the relative concentrations of Al, K, Ca, Ti, and Fe. We interpret these sediment composition changes to be related to a shift in the main precipitation locus, that moved from the central Andes (HS1a) to southeastern Amazonia (HS1b). During HS1b, the low-pressure region over the eastern equatorial Pacific (due to a stronger positive SST anomaly in the eastern equatorial Pacific relative to the western equatorial Atlantic) produces, together with a shift further south in the ITCZ position, a positive precipitation anomaly over the southern and eastern Amazon Basin. Precipitation anomalies during the YD were apparently less conspicuous, located in the eastern Amazon Basin, but we cannot exclude the possibility of major elemental signals to be affected by a significant sea level rise that increased accommodation space for terrigenous sediments on the continental shelf. The heterogeneous response of precipitation under different AMOC scenarios reinforces the importance of high-temporal resolution paleoclimate studies from the Amazon Basin and the need to spatially compartmentalize the precipitation responses to these events due to the continental dimension of the basin.