Neogene and Quaternary paleoenvironmental history at DSDP Leg 71 Holes

The Neogene and Quaternary sedimentary record of Leg 71 and previously drilled sequences from the Southern Ocean reveal evidence of a major late Miocene change of oceanic and glacial conditions in the southern high latitudes during paleomagnetic Chron 9. The characteristics of late Miocene sedimentation and in particular the study of erosional patterns and ice-rafted debris suggest the following conclusions. 1) In the late Miocene, the Polar Front first migrated to the northern latitudes of the Southern Ocean and surface water temperatures became similar to those of today. 2) Extensive ice shelves or ice tongues were not present along the Antarctic margin until late Chron 9 (about 9.0 Ma). 3) Before Chron 9, West Antarctica was occupied by an archipelago and the West Antarctic Sea. 4) Extensive ice shelves formed in the West Antarctic region, eventually coalescing and thickening to form the grounded West Antarctic ice sheet by Chron 9. 5) The newly formed West Antarctic ice sheet was probably unstable and frequently became an ungrounded ice shelf, thus accounting for the scarcity of late Miocene ice-rafted debris. 6) Extensive erosion or nondeposition of sediment was probably the result of increased Antarctic Bottom Water (AABW) formation in the West Antarctic region during the initial formation of extensive West Antarctic ice shelves and during periods when the West Antarctic ice sheet was ungrounded. 7) In the Southwest Atlantic, AABW velocity waned during the latest Miocene. During the late Gilbert Chron a major and permanent change occurred in the pattern of ice-rafting to the South Atlantic, and after 4.35 Ma the increased IRD accumulation rate and frequency of major episodes of IRD accumulation suggest increased stability of the West Antarctic ice sheet. In addition, radiolarian faunas of Hole 514 record at least eight migrations of the Polar Front to the north of the site during the past 4.07 m.y. An apparent increase in the frequency of Polar Front migrations occurred about 2.7-2.6 Ma, possibly in response to oceanic change induced by fluctuations in glacial conditions of the Northern Hemisphere.

Mineralogy and diagenesis: Their effect on acoustic and electrical properties of pelagic clays at DSDP Leg 86 Holes

Analysis of pelagic clay samples from Sites 576, 578, and 581 shows that physical, acoustic, and electrical trends with increasing burial depth are related to mineralogical and diagenetic changes. The properties of interest are bulk density (roo), porosity (phi), compressional-wave velocity (Vp) and velocity anisotropy (Ap), and electrical resistivity (Ro) and resistivity anisotropy (Ar). In general, as demonstrated in particular for the brown pelagic clay, the increase in roo, Vp, Ro, and to a lesser extent Ap and Ar with increasing depth is primarily caused by decreasing phi (and water content) as a result of compaction. The mineralogy and chemistry of the pelagic clays vary as a function of burial depth at all three sites. These variations are interpreted to reflect changes in the relative importance of detrital and diagenetic components. Mineralogical and chemical variations, however, play minor roles in determining variations in acoustic and electrical properties of the clays with increasing burial depth.