Changing Southern Ocean Oxygenation at Intermediate Water Depths during the Transition Eocene-Oligocene Ice Sheet Development: Bulk Sediment Elemental data from IODP Site U1553, New Zealand.

The Eocene-Oligocene (E-O) transition was a vital period in Earth's history 35-30 Ma where the planet transitioned from a warm to a cold environment and was an important period of continental Antarctic ice sheet growth. This ice sheet growth may have influenced oxygenation in the interior of the world's oceans at the time. A cooling Southern Ocean may have contained more oxygen, which would heavily influence biogeochemical processes and ocean productivity. However, the development of the proto-Antarctic Circumpolar Current would also impact Southern Ocean productivity.

Here, we investigate the upper sediments of IODP Site U1553 from 26-36 Ma. The 581.16 m long sediment core collected south of New Zealand (52°13.44'S:166°11.49'E) at 1221 m water depth, remained at intermediate water depths through the Oligocene to the early Paleogene. The depth of this site makes it an ideal candidate for identifying possible oxygenation changes in Southern Ocean subsurface waters. Elemental analysis of bulk sediments by Q-ICP-MS at the Michigan Elemental Analysis Lab was completed at ~1.5 m intervals. Redox-sensitive elements are compared with elements associated with biogenic sediment to identify changes in porewater oxygenation in the sediment record.

At ~25 mbsf high concentrations of redox-sensitive metals may be associated with a modern redox front identified in the shipboard chemical analysis of porewaters. Therefore we do not consider the first lower oxygen event at around 27 Ma to be the result of changes in ocean circulation. An earlier increase in redox-sensitive metals at ~170 mbsf (~32.5 Ma) occurred shortly after Oi-1 and the expansion of the Antarctic Ice Sheet and could therefore be associated with a change in Southern Ocean paleoceanography. Higher Barium concentrations during this interval suggest a potential increase in oxidant demand from increased productivity during this interval.