A geologic perspective on the Miocene evolution of the Antarctic Circumpolar Current system

Abstract

The dominant geologic feature of pelagic sediment accumulation patterns at intermediate water depths in the Southern Ocean is the sequential northward migration and replacement of a calcareous ooze sedimentary facies by a siliceous ooze facies, following the Late Oligocene opening of the Drake Passage and the subsequent initiation of the Antarctic Circumpolar Current. Closely spaced whole-core measurements of bulk density are evaluated for four sites drilled in the Southern Ocean during Ocean Drilling Program (ODP) legs 113 (sites 689 and 690) and 120 (sites 747 and 751). These sites are all located on submarine rises in open-ocean (pelagic) depositional environments south of the Polar Front, a major oceanographie and sedimentologic boundary. High-resolution profiles of bulk density are used to correlate between adjacent drill holes on the Maud Rise, where multiple overlapping sedimentary sequences were recovered using the advanced hydraulic piston core barrel (APC). Stratigraphie age models have been applied to the bulk-density data to provide a temporal framework for a discussion of the sedimentologic basis for paleoceanographic interpretations and also to constrain future acoustic research studies. The Early to Middle Miocene intervals at sites 689 and 690 are marked by distinct sequences of meter scale, alternating carbonate and siliceous units separated by several proposed stratigraphie hiatuses. At least six dissolution/productivity events are identified in the Middle to Late Miocene on the Maud Rise, and others are noted at the ODP sites on the Kerguelen Plateau. The downhole profiles are used to identify the “density signature” and position of sediment facies changes and proposed stratigraphie hiatuses in sediment cores of Late Oligocene to Late Miocene age, based on the reversals, inflections and discontinuities in the downhole bulk-density trends. An analysis of the observed bulk-density trends at these sites is combined with a review of other tectonic, sedimentologic, oceanographie and modeling studies to develop a hypothesis for explaining the observed sediment accumulation patterns at intermediate water depths of the Southern Ocean.