Early Pliocene coastal upwelling off central Chile (Coquimbo Formation, Tongoy)

Diatom-rich sediments exposed along the coastline of northern and central Chile are prominent archives of coastal upwelling properties in the Neogene. Unlocking these archives is fundamental to evaluate the potential significance of changing sea surface temperatures for landward moisture transport and palaeoclimatic fluctuations in the Atacama Desert. One of the southernmost occurrences of Neogene diatom-rich sediments is known from a series of quebradas, i.e. seaward ravines, on the Tongoy Pensinsula (30.3°S). The diatomaceous muds, previously correlated with the Middle to Upper Miocene, have been deposited in the Tongoy Paleobay. Here we present new bio- and magnetostratigraphic data from a 3.5 m-thick section of diatomaceous muds in the Quebrada Las Salinas together with a preliminary assessment of the depositional environment.

While calcareous microfossils are absent, the sample material is rich in silicious microfossils. Diatoms are the predominant group, while silicoflagellates, ebridians, radiolarians, and sponge spicules contribute to a lesser extent. Co-occurrences of the diatom species Nitzschia fossilis and Shionodiscus oestrupii constrain the diatomaceous muds to an age < 5.7-5.9 Ma. Together with normal magnetic polarity at the base of the section, the lowermost age limit is further refined to an Early Pliocene age < 5.23 Ma (base of the Thvera subchron). Occurrences of the diatom species Rouxia californica in the upper part of the diatomaceous muds suggest an upper age limit within the Zanclean. The results demonstrate for the first time that diatomaceous muds in the Tongoy Paleobay extend well into the Pliocene. Comparison with previous studies suggests that diatom-rich deposits in the study area may be highly diachronous within and between quebradas.

The microfossil assemblages suggest that coastal upwelling was prevalent in the Tongoy Paleobay during the Early Pliocene. Diatom assemblages are dominated by planktonic and meroplanktonic taxa that prefer cold, nutrient-rich surface waters of coastal upwelling such as Chaetoceros and its resting spores, Thalassionema nitzschoides, Coscinodiscus spp., and Actinoptychus senarius. Cold surface waters are also indicated by high abundances of Distephanus spp. in the silicoflagellate assemblages. Benthic taxa are very rare, suggesting water depths of 100-150 meters. The Early Pliocene diatom assemblages more closely resemble those of today’s upwelling in Eastern equatorial Pacific and off the northern Peruvian continental margin than off central Chile. This may suggest a range of surface temperatures at least comparable to or even warmer than off central Chile today, and significantly enhanced primary productivity. This preliminary assessment will be further evaluated through detailed statistical analysis.

This study is part of the CRC 1211 “Earth-Evolution at the dry limit” project, funded by the Deutsche Forschungsgemeinschaft (DFG).