The Neogene evolution of the Amazon Basin: reconciling continental and marine provenance records

The development of the transcontinental Amazon River connected the Andean Cordillera to the Atlantic Ocean, fundamentally reorganizing South American landscapes, sediment fluxes, and Neotropical ecosystems. However, the timing and mechanisms by which Andean-derived sediment became persistently exported to the Atlantic remain debated, partly because continental and marine archives record different components of the evolving source-to-sink system. Here, we integrate Neogene provenance records across northern South America with a new, astronomically dated neodymium-isotope record from BP-3 on the upper Amazon Fan, covering preserved intervals of the past ~24 Myr. The BP-3 record identifies the onset of sustained Andean sediment delivery at 12.2 ± 1.3 Ma, earlier than the Late Miocene onset commonly invoked. Older radiogenic excursions at the Ceará Rise document intermittent delivery of younger material but have non-unique provenance and do not independently establish persistent transcontinental integration. Nd-isotope mixing models reveal three stages of asynchronous drainage integration. Stage I (12.2–7 Ma) records embryonic transcontinental connectivity involving mixed Andean and cratonic contributions, while substantial sediment storage and recycling persisted within western Amazonian wetlands and foreland depocenters. During Stage II (7–4 Ma), increasingly radiogenic compositions and rising smectite abundance indicate enhanced erosion and recycling of retroarc foreland successions. Critically, the provenance shift at ~7 Ma preceded the nearly ninefold increase in Amazon Fan accumulation at ~4.9 Ma by approximately 2 Myr, indicating that Andean–Atlantic connectivity developed before the sediment-routing system attained high throughput. Stage III (4–0.6 Ma) is characterized by sustained high accumulation rates marking establishment of a mature transcontinental system whose shorter-term provenance variability was increasingly modulated by Plio-Pleistocene hydroclimate and sea-level change. These findings show that Amazon transcontinentalization was not a single event but a stepwise process in which tectonic reorganization established connectivity, retroarc foreland accommodation buffered sediment export, and climate increasingly modulated sediment transfer after basin-scale integration.