LEG 114

Subantarctic South Atlantic

Seven sites (Site 698 to Site 704) were investigated during Leg 114 which, together with Leg 113, recovered the first detailed Late Cretaceous-Cenozoic sedimentary record from the Antarctic to subantarctic regions of a single sector of the Southern Ocean. The paleoceanographic history of the South Atlantic was influenced by tectonic events which lead to drastic changes in latitudinal and vertical oceanic thermal gradients and ocean circulation patterns.

Prior to Legs 113 and 114, no detailed chronology of the evolution of the southern oceans existed. The new data has constrained models for the complicated tectonic evolution of this region. The northeast Georgia Rise is at least as old as Late Cretaceous, was once subaerially exposed, and may have a continental root. The Islas Orcadas Rise was formed during the late Cretaceous to early Paleocene, as was Meteor Rise although its volcanic history extends to the Eocene. These features obstructed deep- and intermediate-water communication between the Antarctic and South Atlantic; their separation by Eocene seafloor spreading created a deep-water gateway, allowing circulation during the mid-to-late Paleogene that was more vigorous than contemporaneous circulation at shallower sites.

Late Cretaceous faunal and floral assemblages have some low-latitude affinities and the relatively warm surface waters were well oxygenated but suboxic conditions occurred periodically in the low-energy benthic environment. Maximum warmth occurred during the early Eocene. The first evidence of increased vigor of the deep-water communication between the Weddel Sea and South Atlantic basins is represented by a hiatus generated by increased bottom currents at the end of the middle Eocene and lasting 2 to 5 m.y. along the eastern foot of the Islas Orcadas Rise. Sedimentation shows persistent bottom-current control throughout the section, commencing with the late Oligocene. Age-equivalent sediments also became more clay-rich, suggesting an increased supply of terrigenous material to the Weddel Basin from the surrounding landmasses during the Oligocene sea-level low stand. The clay was probably brought north in the benthic boundary layer of the emerging Antarctic bottom-water communication with the South Atlantic. An early-to-late- Neogene hiatus foreshadowed the northward advance of the zone of biosiliceous deposition around Antarctica. The intensification of circum-Antarctic circulation was related to the opening of the Drake Passage in the early Miocene and is recorded by hiatuses at all Leg 114 sites. In part, the missing interval appears to have been caused by multiple erosional and nondepositional events. A greater time interval is represented in the hiatuses of the shallower sites, indicating higher current velocities or longer erosion times during initial stages of the passage opening.