Seasonal Amazonian rainfall variation in the Miocene Climate Optimum

Modern and fossil freshwater bivalves from north-eastern Peru are investigated to reconstruct seasonal rainfall patterns in Miocene Amazonia. Oxygen isotope variation in incremental growth bands of fossil bivalves reflects past hydrological conditions in the Miocene Climate Optimum (MCO), when the world was warmer than today. A calibration experiment was conducted on a modern bivalve. Modern river dwelling Triplodon corrugatus shows large amplitudinal changes in δ18O, which mirror the seasonal variation in rainfall as a result of the annual migration cycle of the Inter Tropical Convergence Zone (ITCZ). Growth incremental oxygen isotope records of Miocene Amazonian Diplodon aff. longulus bivalves show strikingly similar patterns. This suggests that the seasonal migration of the ITCZ and the intensity of the hydrological cycle in the MCO were comparable to today. The implications are that humid climate conditions sufficient to sustain a rainforest ecosystem already existed ∼16 Ma ago.