Mid- to late-Holocene hydroclimate and vegetation changes near the Amazon-Tapajós confluence

Abstract

The rainforests and freshwater resources of lowland Amazonia depend on precipitation from the Intertropical Convergence Zone (ITCZ) and the South American Summer Monsoon (SASM). However, our understanding of the climate and vegetation history of lowland Amazonia, particularly its response to the dynamics of the ITCZ and SASM, is limited by the lack of Holocene paleohydroclimate and paleoenvironmental records from eastern Amazonia. Here, we present biomarker-based records of climate and vegetation shifts over the past ~5600 years from Lago Caranã (2.84 S, 55.04 W, 5 m asl), Brazil. We compare the δ¹³C and δD of sedimentary leaf waxes to detailed fossil pollen and charcoal data analyzed from the same sediment core to disentangle hydroclimate and vegetation changes. Unlike some proxy records located elsewhere in the SASM domain, we observe no orbital-scale trend in reconstructed precipitation δD (δD_precip). Instead, we observe pronounced centennial-scale negative excursions in δD_precip, particularly after ~2000 years BP, likely driven by increased moisture transport from the Atlantic under warmer equatorial sea surface temperatures and a southward or expanded ITCZ. Our results indicate that stable, wet conditions persisted around the Amazon-Tapajós confluence since the mid-Holocene, suggesting that areas along the shores of Amazonian rivers and rias may be more resilient to precipitation changes relative to elsewhere within Amazonia. The n-alkane δ¹³C and chain length distributions, together with fossil pollen and charcoal data, support human-driven changes in vegetation composition and canopy structure. Overall, our leaf wax records indicate a relatively stable precipitation regime along riverbanks in eastern Amazonia since the Mid-Holocene.