Early Oligocene diatoms on the Ceara Rise and the Cenozoic evolution of biogenic silica accumulation in the low-latitude Atlantic

Abstract

Pronounced biogenic silica accumulation has taken place in the Ceara Rise region during two episodes of the Cenozoic: the early Oligocene (33.5-30.5 Ma) and the early Miocene (23.0-15.5 Ma). The Paleogene deposits are dominated by well preserved assemblages of diatoms and radiolarians representing the early Oligocene Coscinodiscus excavatus diatom zone. The rich lower Miocene biosiliceous deposits are dominated by radiolarians and siliceous sponge spicules and deprived of diatoms. Apart from these two conspicuous biosiliceous intervals, siliceous microfossils are rare or absent in the upper Paleocene to Holocene sediments recovered from the Ceara Rise. The Ceara Rise data point to a very modest silica accumulation in the late Paleocene through late Eocene period in the western equatorial Atlantic. Accumulation of biogenic silica increased during the early Oligocene in a possible response to the glo bal cooling. Following a decline in the late early Oligocene, an observed early Miocene increase in biosiliceous sediments is consistent with the widespread early Miocene biosiliceous productivity increase recorded in other parts of the low-latitude Atlantic. A simple plot of the distribution of biosiliceous sediments from the Ceara Rise and from previously reported occurrences in the low-latitude Atlantic reveals that the early Oligocene and early Miocene periods of increased biosiliceous sedimentation in the Ceara Rise region are representative of the entire low-latitude region of the Atlantic Ocean. The data further suggest that accumulation of biogenic silica terminated across most of the low-latitude Atlantic near the end of early Miocene time, possibly in response to the late early Miocene “silica shift” from the Atlantic Ocean to the Pacific.