Dating of upwelling archives in the eastern South Pacific: a multiproxy approach in the Late Neogene Bahía Inglesa Formation, North-Central Chile

In modern oceans, upwelling processes are responsible for high biological productivity and low sea surface temperatures at coastal zones. Upwelling may have intensified during the late Neogene in the eastern South Pacific due to the strengthening of the Humboldt Current System. Records of Neogene coastal upwelling are preserved in outcrops along the coast of north-central Chile (~ 26°S to 28°S) as diatomaceous mudstone deposits of the Neogene Bahía Inglesa Formation. To place such records in a broader paleoceanographic context, however, their stratigraphic assessment still needs refinements. Our work presents a multiproxy dataset to provide a stratigraphic framework for the Bahía Inglesa Formation at Quebrada Tiburón (27°42' S, 70°59' W), one of the southernmost outcrops of diatomaceous mudstone. Our approach is based on tephrochronometry, strontium isotope chronology (mollusk shells ⁸⁷Sr/⁸⁶Sr), and calcareous nannoplankton, diatom, and planktonic foraminifera biostratigraphy. Zircon crystals separated from a volcanic ash layer at the base of the sequence were analyzed by laser ablation ICP-MS for U-Pb dating. The youngest cluster of five concordant zircon crystallization ages indicates a tephra deposition after 8.68 ± 0.15 Ma. The ⁸⁷Sr/⁸⁶Sr analyses were performed on an oyster and a pectinid from sandstones underlying the diatomaceous mudstone using high-precision MC-ICP-MS measurements. The corrected and adjusted ⁸⁷Sr/⁸⁶Sr ratios resulted in 8.12 ± 0.40 Ma and 6.10 ± 0.25 Ma ages. The microfossil biostratigraphy was based on First (FAD) and Last Appearance (LAD) datums of biostratigraphic markers from the diatomaceous mudstone. The presence of mainly Miocene diatoms (e.g., Actinocyclus ingens, Cavitatus joseanus, and Nitzschia fossilis) and the planktonic foraminifera Neogloboquadrina acostaensis (sinistral) indicate a Tortonian age for the base of the diatomaceous mudstone. The Messinian-Zanclean boundary was identified in the middle interval of the mudstone by the disappearance of the calcareous nannoplankton species Calcidiscus pataecus and the appearance of Helicosphaera sellii and Umbilicosphaera sibogae. This interpretation is supported by the continuity and limit of the diatoms Actinocyclus ellipticus, Azpeitia nodulifer, and Coscinodiscus plicatus overlapping with Hemidiscus cuneiformis. A Zanclean age was attributed to the upper part of the mudstone sequence due to the co-occurrence of the calcareous nannoplankton species Reticulofenestra pseudoumbilicus and Sphenolithus moriformis, the diatoms Actinocyclus ellipticus, the co-occurrence of the diatoms Nitzschia fossilis and Shionodiscus oestrupii, and the planktonic foraminifera Globoconella miotumida and Sphaeroidinellopsis seminulina. The following sandstones contain Pliocene mollusks. Although inconsistencies between biostratigraphic data of taxa from different microfossil groups were observed (likely due to the lack of a local biozonation appropriate for the upwelling context), our dataset suggests a late Tortonian to Zanclean (8.68 to 3.5 Ma) age constraint for the succession and late Messinian to Zanclean (6.09 to 3.5 Ma) age for the diatomaceous mudstone. Further studies will apply our stratigraphic constraints for paleoenvironmental reconstructions. This research is part of the CRC 1211 “Earth-Evolution at the dry limit” project, funded by the German Research Foundation (DFG).