Paleoclimate and changing composition of the Paleogene-Neogene shallow molluscan assemblages of Patagonia

Abstract

A detailed compilation and updated taxonomy of the shallow molluscan assemblages of the Paleogene-Neogene of Patagonia was performed. Together with the Sr-chronostratigraphic framework developed during the last decade, five assemblages of Danian, late middle Eocene, latest Oligocene-early Miocene, middle Miocene and late Miocene ages were recognized. Temporal variation in the composition of these assemblages reflects the successive transient hyperthermal events superimposed to the global cooling trend in the sea surface temperatures during Cenozoic times. Tropical or warmest subtropical temperatures in the region during the Danian and late middle Eocene times coincide respectively with the Dan-C2 and the Latest Danian Event, and the Middle Eocene Climatic Optimum events. Tropical taxa, represented by Tethyan and warm-Cosmopolitan genera constitute 27.6% and 31.3% of the fauna respectively. The declining participation of Paleogene warm-genera, comprising just 15.3% of the fauna, coupled with the massive appearance of the subtropical Neogene Southern elements, reveals a decrease in sea surface temperatures during the latest Oligocene-early Miocene interval until the establishment of warm-temperate conditions in the area. Although lower than during the Paleogene, temperatures should have been higher than today, in coincidence with the late Oligocene warming and the multiple warming events in the early Miocene. The middle Miocene records the first large occurrence of Caribbean taxa that will be continue being important components of the fauna during the late Miocene. The contribution of warm-taxa to the faunas rose from 15.3% in the early Miocene to 23% and 22.8% in the middle and late Miocene respectively. Both faunas point to the development of subtropical conditions in the area and match with the record of three new warming peaks. One hyperthermal event is recognized at the Middle Miocene Climatic Optimum, and the other two increments coincide with the transient peaks occurring at ~11 Ma and ~9 Ma. Paleogene-Neogene heritage in modern faunas of the region is rather poor and comprises 36% of fossil survivor genera. Taxa that managed to survive are mainly Cosmopolitan, Neogene Southern and Endemic genera that mostly had appeared in the early Miocene and adapted to Recent cool and temperate-cool conditions of the Southwesten Atlantic Ocean.