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Palaeoceanographic Issues |
1 Marine Geosciences Research Group, Department of Earth Sciences, University of Wales College of Cardiff, PO Box 914, Cardiff CF1 3YE, UK
2 Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth, Devon PL4 8AA, UK
3 Department of Earth Sciences, University of California Santa Cruz, Santa Cruz, California, CA 95064, USA
Tethyan Outflow Water (TOW), characterized by high benthic
High
Indian Ocean TOW was restricted laterally by the Chagos—Lacadive Ridge, and vertically (1.3 to 3 km) by overlying intermediate water, characterized by the occurrence of coarse-walled bolivinids, and underlying deep water characterized by high abundances (>10%) of Nuttallides umboniferus. The coarse-walled bolivinid and ‘N. umboniferus’ faunas were associated with lower temperature, salinity and higher oxygen concentrations. Tethyan outflow to the northwest Indian Ocean ended at c. 14.5 Ma, probably in response to the closure of a gateway, and resulted in a two-component deep water mass structure. The coeval increase in the relative abundances of N. umboniferus at Indian Ocean Sites 237 (c. 2 km) and 710 (c. 3.5 km), at c. 13 Ma, probably records the impact of this change on the nature of Indian Ocean deep water at depths between 2 and 4 km.
The termination of the Atlantic and, to a lesser extent, Indian Ocean TOW contributed to global cooling and to the expansion of the Antarctic Ice Sheet by limiting the meridional heat transport.
13C values, was a component of the early Miocene deep water mass of the Atlantic (c. 20 and c. 13 Ma) and Indian (c. 20 and c. 14.5 Ma) Oceans. Fluctuations in temperature, salinity and quantity of dissolved oxygen coincided with high abundances of infaunal, smooth-walled bolivinids within the Atlantic and Indian Ocean TOW between 19.5 and 16.5 Ma. O values recorded in the Atlantic water mass may have originated, in part, from evaporation and the formation of warm, saline water within the Tethys. The southward flowing Atlantic TOW at depths of 2–4.5 km enhanced meridional heat transport. It was overlain by Northern Component Water (NCW) with a lower C signal. TOW and NCW were replaced by southward flowing North Atlantic Deep Water at c. 13 Ma when cold dense Norwegian—Greenland Sea water flowed across the Greenland—Scotland Ridge.
