Extraprovincial cold-water marine mollusks: Evidence bearing on ocean circulation patterns off northwestern Peru during the Late Pliocene and Pleistocene

Abstract

Northwestern Peru at the latitudes of ∼4°30′S to ∼6°30′S straddles the boundary between the Panamic and Peruvian molluscan faunal provinces. At the same latitudes currents of the cold-water eastern boundary Humboldt Current System blend with warmer-water Pacific equatorial currents and countercurrents. On the coastal plain facing those currents, Pliocene and Pleistocene mollusks are abundantly preserved in the Taime Formation and Mancora, Talara, and Lobitos tablazos in sediments characteristic of nearshore, shoreface, and lagoonal environments. During the late Early Pleistocene, the number of cool-water “Peruvian” species in molluscan assemblages in those sediments abruptly declined and the number of warm-water “Panamic” species substantially increased to a level seen in modern assemblages from the same region. A case is made that the shift from a cool-water to warm-water fauna can be related to (1) a changing configuration of the coastline of northwestern Peru due to Early Pleistocene uplift of the coastal plain that forced cold-water currents to veer westward farther south than they once had, and/or (2) a warming of the Ecuador-Peru Coastal Current, the consequence of interactions between its source of water, the Equatorial Undercurrent, and the Galápagos Islands. To ascertain whether the faunal change in northwestern Peru occurred during the Mid-Pleistocene Transition (∼1 Ma) and is related to the onset of higher amplitude glacial-interglacial eustatic sea-level fluctuations and/or related to the blocking at ∼1.6 Ma of the Equatorial Undercurrent by the Galápagos Islands, will require new approaches to dating fossil mollusks from the Taime Formation and tablazos.