The middle Miocene climate transition (MMT) is the final shift from a greenhouse climate into a modern-day-like ocean-atmospheric system with cold high latitudes and a strong meridional temperature gradient. The event is marked by the major expansion of the East Antarctic ice sheet (Mi-3; ~13.8 Ma) as indicated by a 1‰ increase in benthic foraminiferal δ18
O values. The MMT is tied to enhanced bottom water formation and the cooling of high latitude regions; however, little is known about its effect on low latitude climate and oceanography. To determine how low latitudes were affected and if high latitude cooling/Antarctic ice sheet expansion influenced equatorial upwelling and surface productivity, we examined the calcareous nannoplankton assemblage, opal weight %, and grain size of sediments from IODP Site U1338 in the eastern equatorial Pacific. These records are compared to the benthic and planktonic foraminiferal stable isotope records, XRF element data, and surface temperature proxies to reconstruct upwelling conditions, vertical stratification, and biological pumping efficiency.
Our results show a distinct shift in the nannoplankton assemblage associated with the MMT and Mi-3. At the MMT, warmer-water, oligotrophic taxa decrease in abundance and are replaced by cooler-water taxa. During the interval that is coincident with the maximum in Antarctic glaciation, there is a significant increase in small Dictyococcites species (< 3 µm). The dominance of this species has a bloom-like appearance, which in the modern ocean is indicative of high nannoplankton productivity. In addition, sedimentation rates increase during the first half of the glacial interval. This evidence suggests that equatorial productivity increased during the glaciation. Environmental changes also led to the rapid response of diatoms. These results can help us to better understand the linkage between high latitude forcing and equatorial upwelling dynamics.