Miocene changes in the vertical structure of the Southeast Atlantic near-surface water column: Influence on the paleoproductivity

Abstract

Stable isotopes of benthic and planktonic foraminifers and faunal analyses of planktonic foraminifers from two cores in the South Atlantic reveal paleoceanographic changes during the Mid-Miocene cooling interval (~14-7Ma). At ODP site 1092 (46.4°S, 7.1°E) the benthic d18O increase (~14.1-13.2 Ma) generates extraordinary heavy values, compared to other oceanic regions. This evolution suggests the subantarctic Atlantic to be a prime site of deep water formation. Surface water cooling is indicated by the gradual advance of relatively cooler foraminiferal faunas from ~13.6Ma to 10.3Ma. Since then both faunas and surface to deep d18O gradients indicate a hydrography similar to the modern Polar Frontal Zone.ODP site 1085 (29.4°S, 14.0°E), situated in the modern Southwest African upwelling area, exhibits a shift from (sub-)tropical faunas to cool, eutrophic faunas at ~11.3Ma, while increasing surface to deep planktonic d13C gradients imply enhanced surface productivity. Since ~10.5Ma, planktonic d18O records imply cooling and reduced thermal stratification of the upper ocean, most likely reflecting an invigoration of coastal upwelling.