Microfossils provide evidence of environmental changes during the Eocene-Oligocene transition in northwestern South America.

Marine and terrestrial microfossils respond to environmental and climatic changes during the upper Eocene to lower Oligocene in the onshore ANH-SJ-1 well in NW South America. The interval consists of mudstones and calcareous mudstones sedimented in lower to middle bathyal depths. During the upper Eocene, the warm calcareous nannofossil taxa dominated the assemblage. In contrast, over the early Oligocene, we recorded an increased abundance of cold-water taxa (e.g., Braarudosphaera spp., Clausicoccus subdistichus, and Isthmolithus recurvus). This shift is accompanied by pulses of eutrophic taxa (e.g., Braarudosphaera bigelowii, C. altus, Chiasmolithus spp., Ciclycargolithus floridanus, and Zygrhabilithus bijugatus), indicating fertilization in the surface waters.

The marine palynomorphs record (dinoflagellates and foraminiferal organic linings) showed the highest abundance over the upper Eocene. Conversely, terrestrial palynomorphs (spores, angiosperms, and fungi) increased at the beginning of the early Oligocene. Reducing marine palynomorphs and incrementing in continental ones does not mean a shallowing since benthic foraminifera do not show bathymetric variations. Therefore, we suppose that the increase in continental palynomorphs input is due to the increase in fluvial discharges, which became influential in NW South America.

An abrupt change in oxygenation rates, diversity, and wall type of benthic foraminifera was recorded at the Eocene-Oligocene boundary. Thus, we documented a dominance of agglutinated foraminifera and low diversity during the Eocene and increased calcareous foraminifera and high diversity during the Oligocene. A rise in the oxygenation index and the collapse of epifaunal microhabitats was also observed.

These biotic variations in NW South America are interpreted as an ecosystem response to climatic changes during the Eocene-Oligocene transition (EOT). According to the turnover on benthic foraminifera, deep environments pass from corrosive bottom waters in the late Eocene to a deepening of the carbonate compensation depth (CCD) in the early Oligocene. This change coincided with the global CCD variations due to the EOT. The increased cold-water calcareous nannofossils in the lower Oligocene agree with the cold water's expansion to the tropical regions during the EOT. The increased abundance of nannofossils and terrestrial palynomorphs in surface waters, indicative of substantial inputs from phytoplankton and phytodetritus, suggested a general increase in export production. This could have triggered fluctuations in food availability for benthic foraminifera.

The biotic response of microfossils to environmental changes coincides with climatic changes during the EOT, suggesting that tropical ecosystems in NW South America were affected by decreasing global temperatures and a new climatic regime.