Siliceous components of Pliocene-Holocene sediments of the South Atlantic

The Pliocene-Holocene sediments recovered on ODP Leg 114 from Holes 699A, 701C, and 704B are the subject of a detailed investigation to interpret changes in the Oceanographic environment of the South Atlantic in the vicinity of the Polar Front Zone (PFZ). The cores sample sediments at shallow (Hole 704B, 2532 m), intermediate (Hole 699A, 3716 m), and basinal (Hole 701C, 4647 m) depths. Sites 699 and 704 come under the influence of the Antarctic Circumpolar Current (ACC) and Circumpolar Deep Water. It is possible that the upper reaches of Antarctic Bottom Water (AABW) may also affect Hole 699A. Site 701 is influenced by AABW. Closely spaced samples were analyzed for grain-size distribution, sand fraction components, biosiliceous microfossils, organic carbon, and water content. PFZ migrations are traced using changes in bulk sedimentaccumulation rates and the abundance of the diatoms Actiniscus ssp. and Genus et species indet. 1 Fenner (1991), as well as changes in sediment grain size and composition. Diatomaceous sediments of Gilbert age in Hole 699A indicate that the PFZ was positioned over this site, but during the Gauss it migrated north, bringing in less productive Antarctic Surface Water. All cores document a very gradual southerly movement of the PFZ throughout the Matuyama (with some sharp fluctuations of the northen PFZ border over Site 704 between 1.45 and 1.83 m.y.). This regressive shift culminated in the late Matuyama. The latest Matuyama to earliest Brunhes record in Hole 699A has been removed by a hiatus lasting from 1.0 to 0.6 m.y., which was probably caused by intensification of the deep-reaching ACC. The corresponding interval in Hole 704B, the shallowest core, contains evidence of winnowing. Sharp fluctuations of large amplitude and high frequency in the lithology of the sediments from Hole 704B in the eastern South Atlantic, starting at about 0.75 m.y. and characterizing the whole Brunhes Epoch, record the rapid movement of the northern border of the PFZ over the site. These reflect strong glacial/interglacial alternations in climate. To a lesser extent, lithologic fluctuations in Hole 701C reflect the same phenomenon, whereas in Hole 699A the lithology does not vary as dramatically.

Late Miocene to Holocene planktonic foraminifers from the subantarctic South Atlantic

Late Miocene to Holocene planktonic foraminifers from the subantarctic South Atlantic were investigated for their biostratigraphic and environmental significance. The calcareous planktonic microfauna are of low diversity and are very rare or absent at sites located below the CCD. The sediments recovered from Sites 703 and 704 on the Meteor Rise at about 47°S are useful for biostratigraphic and environmental studies. In the whole sequence 16 species or varieties of planktonic foraminifers were recognized. Two species occur in the uppermost Miocene. In the Pliocene the Globorotalia puncticulata population can be used to separate the early from the late Pliocene. The Pliocene/Quaternary boundary does not appear to be well distinguished in the foraminiferal assemblage. A faunal change noted at 2.5 Ma could correspond to the development of glaciation in the Northern Hemisphere and its Antarctic counterpart. At about 5.2 Ma the first increase in polar fauna near the Meteor Rise occurs. Two other cooling periods are indicated in these sequences at about 4 and 3 Ma. Moreover, the hydrologic environment became more productive at about 2.1 Ma and close to the Brunhes/Matuyama boundary.

Distribution of tripylean radiolarians from ODP Leg 114 (Table 2)

The occurrence of tripylean radiolarians is reported for the first time from subsurface sediments of the subantarctic Atlantic Ocean. Although their occurrence is rare as well as sporadic, seven species belonging to four genera are recognized from Upper Cenozoic sediments drilled at Sites 699, 700, and 701 of ODP Leg 114 in 1987.