Palynomorph assemblages evidence for river reorganization 8.5 million years ago in Southeast Asia

Abstract

River reorganization occurs frequently in Southeast Asia, due to the rugged topography and heavy precipitation. However, accurately constraining the age of past reorganization events is difficult. Here we apply a novel palynological approach to evaluate how rivers evolved during the late Cenozoic (~12–0 Ma), based on three deep-sea drilling cores from the South China Sea (SCS): International Ocean Discovery Program (IODP) Sites U1499 in the north and U1433 in the central SCS, and Ocean Drilling Program (ODP) Site 1143 in the south. A rapid, dramatic runoff reorganization event at 8.5 Ma is identified in the cores by a sharp decrease in terrestrial pollen and freshwater algae input in the northern SCS, coeval with a contrasting increase in the South SCS, which is supported by contemporaneous changes in neodymium isotopes composition (εNd), and sediment rates and budgets across the SCS. We postulate that the reorganization of rivers was the main dynamic driver of rapid changes in palynomorph assemblages, which took place against a background of tectonic and climatic changes.