Post-miocene disconformities and paleoceanography in the Atlantic sector of the Southern Ocean

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The magnetostratigraphy and biostratigraphy of 166 Islas Orcadas piston cores in the Atlantic sector of the Southern Ocean reveal disconformities in the sedimentary record which are attributed to bottom-current erosion. The hiatus frequency as a function of water depth reveals both shallow erosion by Circumpolar Deep Water (CDW) and deep erosion by Antarctic Bottom Water (AABW). The post-Miocene history of erosion by both bottom-currents is delineated by analysis of areal and temporal fluctuations of hiatus frequency. Erosion by CDW caused disconformities on the Maud Rise, Astrid Ridge, Indian—Antarctic Ridge, Scotia Sea, and Islas Orcadas Rise. The shallow disconformities increased in frequency and size from ∼ 4.0 to 1.0 Ma with two periods of distinctly widespread disconformities from 5.5 to 4.5 Ma and 2.5 to 1.5 Ma; after 1.0 Ma the frequency and extent of the hiatuses decreased. Erosion by AABW caused disconformities in the Weddell Abyssal Plain, Argentine Basin, and through a narrow conduit in the South Sandwich Basin downstream from the Bullard Fracture Zone. The deep disconformities increased in frequency and size from 4.5 to 5.0 Ma, ∼ 3.5 to 4.0 Ma and from ∼ 1.0 to 2.5 Ma; after ∼ 1.5 Ma the frequency and extent of hiatuses decreased. Fluctuations in hiatus frequency are consistent with results of a similar study in the southeast Indian Ocean and may be correlated with paleoceanographic events. Southern Ocean cooling and possible re-establishment of the West Antarctic ice sheet and fringing ice-shelves at ∼ 4.3-3.5 Ma was accompanied by increased AABW and CDW activity. The formation (expansion?) of the Northern Hemisphere ice sheets at ∼ 2.5–3.0 Ma resulted in a decrease in AABW velocity and was accompanied by increased CDW activity. Both deep and shallow bottom-currents increased in velocity during the late Matuyama Chron (∼ 2.0–1.5 Ma) as a result of Antarctic cooling. After ∼ 1.0 Ma, however, the frequency and extent of shallow and deep hiatuses was reduced, and the modern extent of hiatuses is the lowest within the post-Miocene.