Late Eocene-Oligocene oceanographic development at southern high latitudes, from terrigenous and biogenic particles: a comparison of Kerguelen Plateau and Maud Rise, ODP Sites 744 and 689

摘要

Detailed investigation (630 samples) of clay mineral assemblages and benthic foraminifer accumulation rates (BFAR) as a proxy for paleoproductivity has been conducted on two late Eocene-Oligocene sequences drilled at intermediate water depths of the Kerguelen Plateau and Maud Rise (Southern Ocean). Distinct changes in clay assemblages and BFAR -that are in general a factor two higher on the Maud Rise compared to the Kerguelen Plateau - unravel a step-by-step evolution of marine and continental Antarctic environments that eventually resulted in permanent ice coverage and dense cold water formation (1) late Eocene dominance of chemical weathering was associated with intermittent erosion of Antarctic soils and substrates. Main erosional events are recorded at 36 and 34 Ma, prior to the major earliest Oligocene glacial event, and are especially noticeable in the Indian sector of the Southern Ocean; (2) physical weathering progressed in continental areas adjacent to the Maud Rise and intermediate water production intensified in the Indian sector of the Southern Ocean throughout the Eocene-Oligocene transition, as the Tasmanian seaway developed. Environmental changes and a strong 600 kyr lasting peak of paleoproductivity seem closely associated with the earliest Oligocene isotope shift and the accumulation of ice-rafted detritus on the Kerguelen Plateau; (3) aridity and physical weathering progressed in East Antarctica near the Maud Rise (Site 689) at 33.2-33.15 Ma, as polar transfer of heat within the deep ocean increased slightly. This may have prevented extensive ice development during at least part of the early Oligocene; (4) increased productivity, vertical mixing and turnover at Site 744 on the Kerguelen Plateau, followed by a two-step increase of physical weathering on the adjacent continent from 32 to 31.2 Ma, suggest progressive homogenization of climates and circulation as the passageway South of Tasmania widened and deepened; (5) physical weathering reached a maximum in East Antarctic areas adjacent to the Maud Rise and a hiatus developed on the Kerguelen Plateau during the mid-Oligocene Oi2-Oi2a interval (31-26.5 Ma), in possible relation with opening processes of the modern Drake Passage and Scotia Sea and a further step in the development of oceanic circumpolar circulation; (6) near-400-kyr cycles of productivity and clay mineral composition throughout the two sequences suggest that orbital frequencies played a role in the small-scale variations of both continental and marine Antarctic environments of the late Paleogene.