Early to Middle Miocene Deep Ocean Palaeoceanography of the Ceara Rise, Western Atlantic Ocean

• Main Author: Tisserant, Paul Marc Aime
• Other Authors: Faculty of Science
• Format: Thesis
• Language: English
• Published: 2003
• Subjects: Antarctic; The Antarctic; East Antarctic Ice Sheet; Mid-Atlantic Ridge; Antarc*; Ice Sheet
• Online Access: http://hdl.handle.net/10026.2/1963

The early to middle Miocene palaeoceanography of the ODP Site 926B, Ceara Rise of the equatorial western Atlantic Ocean was investigated with respect to palaeoenvironmental conditions and the presence of significant numbers of smooth-walled bolivinids that marked the High Abundance Event that indicated the presence of Tethyan Outflow Water in the early Miocene. Prior to 15 Ma, the generally warmer deep ocean waters of the Ceara Rise were strongly affected by a number of temporary pulses of increased Southern Component Water (SCW) related to periodic temporary increases in the growth of the Antarctic ice sheet. These pulses brought water with high 5^®0 values that was cold (0.6 - 0.8*'C), well-oxygenated and chemically corrosive to the site and led to an increase in dissolution rates that in turn led to a decrease in diversity. The temporary pulse of Southern Component Water between 19 and 17.6 Ma led to a sharp increase in productivity and subsequently increased the organic carbon flux to the ocean floor of the Ceara Rise. After 15 Ma, a prolonged bout of East Antarctic ice sheet expansion lead to a sustained period of Antarctic Bottom Water, the modem analogue of Southern Component Water, production. This cold, well-oxygenated water flowed up to affect the Ceara Rise and led to increased productivity and subsequent increased organic carbon flux to the ocean floor. This long term increase in productivity led to the equally long term increases in the relative abundances of species sensitive to increased productivity such as E. exigua and G. subglobosa. Productivity and subsequent organic carbon flux to the ocean floor did not return to the low pre-14 Ma levels after the 12.8 Ma peak, but remained at higher levels until at least 8.84 Ma. This was the time of the middle Miocene faunal event and is marked by G. subglobosa's rise to dominance of the benthic foraminiferal assemblage. The Monterey Carbon Excursion of Vincent & Berger (1985) is cleariy found to have started at 16.79 Ma and continued through to c.13.4 Ma at the Ceara Rise. The major factor controlling the abundance and distribution of benthic foraminifera in the deep ocean of the Ceara rise was the input of food, in the form of organic carbon, which was itself affected by the input of oxygen-rich Southern Component Water/Antarctic Bottom Water. The High Abundance Bolivinid event is not thought to have occurred at the Ceara Rise. Whilst some of the conditions required for this event, such as high rates of organic carbon flux to the ocean floor, seem to have been present, some where absent. The increased oxygen conditions associated with the Miocene glaciation event at around the time of the increased BFARs (19 and 17.6 Ma) may have prevented the dominance of smooth-walled Bolivinids from taking place at this location. The Mid Atlantic Ridge could well have acted as a topographic restriction upon the westward expansion of Tow in the Atlantic Ocean.