ALBERT

Description: At Ocean Drilling Program (ODP) Site 1090 (lat 42°54.8'S, long 8°54.0'E) located in a water depth of 3702 m on the Agulhas Ridge in the sub-Antarctic South Atlantic, ~300 m of middle Eocene to middle Miocene sediments were recovered with the advanced piston corer (APC) and the extended core barrel (XCB). U-channel samples from the 70-230 meters composite depth (mcd) interval provide a magnetic polarity stratigraphy that is extended to 380 mcd by shipboard whole-core and discrete sample data. The magnetostratigraphy can be interpreted by the fit of the polarity-zone pattern to the geomagnetic polarity time scale (GPTS) augmented by isotope data and bioevents with documented correlation to the GPTS. Three normal-polarity subchrons (C5Dr.1n, C7Ar.1n, and C13r.1n), not included in the standard GPTS, are recorded at Site 1090. The base of the sampled section is correlated to C19n (middle Eocene), although the interpretation is unclear beyond C17r. The top of the sampled section is correlated to C5Cn (late early Miocene), although, in the uppermost 10 m of the sampled section, a foraminifer (Globorotalia sphericomiozea) usually associated with the Messinian and early Pliocene has been identified. 87Sr/86Sr, d13C, and d18O values measured on foraminifera, including the d18O and d13C shifts close to the Eocene/Oligocene boundary, support the correlation to the GPTS. For the interval spanning the Oligocene/Miocene boundary, benthic d13C, d18O, and 87Sr/86Sr records from Site 1090 can be correlated to isotope records from ODP Site 929 (Ceara Rise), providing support for the recently-published Oligocene/Miocene boundary age (22.92 Ma) of Shackleton et al.

Description: Today the deep western boundary current (DWBC) east of New Zealand is the most important route for deep water entering the Pacific Ocean. Large-scale changes in deep water circulation patterns are thought to have been associated with the development of the East Antarctic Ice Sheet (EAIS) close to the main source of bottom water for the DWBC. Here we reconstruct the changing speed of the southwest Pacific DWBC during the middle Miocene from ~15.5-12.5 Ma, a period of significant global ice accumulation associated with EAIS growth. Sortable silt mean grain sizes from Ocean Drilling Program Site 1123 reveal variability in the speed of the Pacific inflow on the timescale of the 41 kyr orbital obliquity cycle. Similar orbital period flow changes have recently been demonstrated for the Pleistocene epoch. Collectively, these observations suggest that a strong coupling between changes in the speed of the deep Pacific inflow and high-latitude climate forcing may have been a persistent feature of the global thermohaline circulation system for at least the past 15 Myr. Furthermore, long-term changes in flow speed suggest an intensification of the DWBC under an inferred increase in Southern Component Water production. This occurred at the same time as decreasing Tethyan outflow and major EAIS growth between ~15.5 and 13.5 Ma. These results provide evidence that a major component of the deep thermohaline circulation was associated with the middle Miocene growth of the EAIS and support the view that this time interval represents an important step in the development of the Neogene icehouse climate.