Palaeoenvironmental conditions during the deposition of the Plio-Pleistocene sedimentary sequence of the Canoa Formation, central Ecuador: A stable isotope study

摘要

Stable isotope measurements were carried out on fossil fish otoliths, mollusc shells, benthic foraminifera and shark teeth collected from the sedimentary sequence of the Canoa Formation, coastal central Ecuador. The age of this sequence is not well defined, ranging from Late Pliocene to Early/Middle Pleistocene. The aim of this work was the reconstruction of the palaeoenvironmental conditions during the deposition of the Canoa Formation and the check of the consistency of these conditions with the hypothesis suggested by previous authors of a north-south diffusion of temperate-cold fish species from California, following the closure of the Panamic Seaway. The oxygen isotope results obtained from otoliths and molluscs reflect temperate to temperate-cold environmental conditions during the deposition of the lower section of the Canoa sequence. This situation in a coastal basin at equatorial latitude is in sharp contrast with the offshore tropical conditions suggested by the isotopic measurements carried out on fossil shark teeth (open ocean species) from the same sequence. The existence of coastal upwelling cells can reasonably explain the very homogeneous and temperate-cold conditions dominating the lower section of the Canoa sequence. The isotopic results obtained from the upper section of the sequence show an evolution towards warmer conditions or lighter delta O-18 values of seawater and, upwards, environmental conditions that seem to approach the modern conditions, which involve considerably higher water temperatures during the first semester of the year (influence of the Equatorial counter-current) and relatively low temperatures during the second semester [influence of the Humboldt (Peru) current]. The suggested palaeoenvironmental conditions are in good agreement with the hypothesis of a north-south diffusion of temperate-cold fish species from California, following the closure of the Panamic Seaway.