AMOC-driven shifts in Amazon sediment sources since the Last Glacial Maximum

As the largest drainage in the world, the Amazon River basin shows intricate and only partially known responses to hydroclimate changes linked to atmospheric reorganization and/or the strength of the Atlantic Meridional Overturning Circulation (AMOC). Many of the hydroclimatic reconstructions for the region were obtained from speleothems, often representing limited local characteristics. Thus, tracking the source of siliciclastic sediments deposited off northeastern South America is particularly well suited to understanding how precipitation in different sectors of the basin may have responded to distinct climate and ocean circulation states. Here we present a high-resolution multi-proxy approach to determine the provenance of the sediments deposited off the Amazon River mouth since the Last Glacial Maximum (LGM) using radiogenic Nd isotopes on clay-size detrital fractions, bulk-sediment major elemental ratios (e.g., Fe/K, Fe/Ca, and Al/K), and quartz optically stimulated luminescence (OSL) sensitivity. We applied these proxies to marine sediment core GL-1251 (1°04.1' N, 45°48.0' W, 2.596 m water depth), the most proximal core to the Amazon River mouth ever studied, making it an excellent archive to address this subject. The new data presented here shows that during the (i) LGM, the (ii) Bølling–Allerød and the (iii) Younger Dryas (YD), deposition at our core site was dominated by sediments transported directly by the Amazon River. During these periods, Andean material dominated the siliciclastic fraction, and fluvial sediment discharge into our core site was favored by relatively low sea level. Our εNd data suggest an abrupt increase in the contribution of the Solimões catchment (northern Central Andes) at the expense of the Madeira catchment (southern Central Andes) during the late YD. However, we identify two periods during which cratonic sources dominated the siliciclastic fraction. The first and most prominent occurred during Heinrich stadial 1 (HS1), and the second during the early to mid-Holocene. During HS1, we argue that, despite enhanced Amazon River freshwater discharge caused by increased precipitation over the Amazon basin, relatively few Andean-derived sediments were deposited at the GL-1251 site. This could be explained by a reduction in the strength of the North Brazilian Current (NBC) towards the northwest, which, in turn, depends on the control of the AMOC. In contrast, the massive intensification of precipitation over eastern Amazon and northeastern Brazil substantially increased cratonic sediment input from catchments draining the Brazilian Shield, resulting in high sedimentation rates. During the early Holocene, we propose that sea-level rise was accompanied by predominant transport of the Amazon sediment plume in the northwestern portion of the Amazon shelf, allowing sustained sediment input from rivers draining the Brazilian Shield at the site of GL-1251. Overall, our data indicate markedly changing precipitation patterns over tropical South America since the LGM, which affected the source of siliciclastic sediments deposited on the northeastern continental margin of South America and possibly imply direct linkage with abrupt changes in the strength of AMOC.