History of upwelling in the Tropical Eastern Pacific and the paleogeography of the Isthmus of Panama

Today there is a tight-knit relationship between the elevation of the Central American Isthmus and the oceanographic conditions of the Tropical Eastern Pacific. Where the elevation drops below 500 m low-level wind jets pass seasonally from the Atlantic to the Pacific driving coastal upwelling in the Tropical Eastern Pacific. This paper determines if seasonal upwelling was present in five Pliocene and Pleistocene fossiliferous sites on the Pacific coast of the Burica region of the Isthmus of Panama using two independent approaches that compare bryozoan morphology and whole community composition of fossiliferous localities with material from upwelling and non-upwelling modern localities. No definitive evidence of seasonal upwelling exists in the Pliocene, implying non-analogous oceanographic conditions because of continued interoceanic connection prior to the closure of the Isthmus of Panama. Conversely, data from three mid-Pleistocene sites reveal robust evidence of strong seasonal upwelling suggesting that the elevation of the Isthmus must have been sufficiently low to permit wind-jets to form. A low-elevation Isthmus of Panama may have persisted until as recently as the mid-Pleistocene.