Strontium dating and the termophilic fauna of the lower Pisco Formation (Peru) suggest a Late Miocene strengthening of the Humboldt Current

We seek to elucidate the Cenozoic history of the upwelling system off Peru-Chile, which currently forms the basis for one of the most productive fisheries in the world. To pursue this aim, we investigate the outstanding record of Eocene– Pliocene sedimentation and associated marine fossils in the East Pisco Basin of southern Peru, where the lower Miocene Chilcatay Formation is overlain by the Pisco Formation, the latter containing three allomembers, each reflecting a transgressive cycle: P0, P1 and P2, in ascending stratigraphic order. The Chilcatay Formation and the overlying middle Miocene P0 allomember (based on our recent strontium isotope stratigraphy) are comprised of lithologies (sandstones and siltstones) that suggest a limited upwelling. Furthermore, a warm-water palaeoenvironment is indicated by the thermophilic fossil assemblage in these units, including mollusks (architectonicids, cypraeids, and the genus Ficus), the only coral colony known from the basin (a rhizangiid scleractinian from P0), and the extinct snaggletooth shark Hemipristis serra. The overlying P1 and P2 allomembers are late Miocene in age and are characterized by abundant diatomites that suggest high primary productivity conditions. The present-day Peruvian coastal upwelling system results from a combination of the Peru Coastal Current and the offshore Humboldt Current. A precursor of this upwelling system emerged after the K/Pg boundary, and a proto-Humboldt Current originated only in the late Eocene. The Miocene transition in the East Pisco Basin from warm-water conditions to high-productivity and cooler conditions shows that there was a strengthening of the Humboldt Current during the late Miocene.