Occurrence of hiatuses PH and NH 1 through NH 7 in DSDP holes

Miocene paleoceanographic evolution exhibits major changes resulting from the opening and closing of passages, the subsequent changes in oceanic circulation, and development of major Antarctic glaciation. The consequences and timing of these events can be observed in variations in the distribution of deep-sea hiatuses, sedimentation patterns, and biogeographic distribution of planktic organisms. The opening of the Drake Passage in the latest Oligocene to early Miocene (25-20 Ma) resulted in the establishment of the deep circumpolar current, which led to thermal isolation of Antarctica and increased global cooling. This development was associated with a major turnover in planktic organisms, resulting in the evolution of Neogene assemblages and the eventual extinction of Paleogene assemblages. The erosive patterns of two widespread hiatuses (PH, 23.0-22.5 Ma; and NH 1, 20-18 Ma) indicate that a deep circumequatorial circulation existed at this time, characterized by a broad band of carbonate-ooze deposition. Siliceous sedimentation was restricted to the North Atlantic and a narrow band around Antarctica. A major reorganization in deep-sea sedimentation and hiatus distribution patterns occurred near the early/middle Miocene boundary, apparently resulting from changes in oceanic circulation. Beginning at this time, deep-sea erosion occurred throughout the Caribbean (hiatus NH 2, 16-15 Ma), suggesting disruption of the deep circumequatorial circulation and northward deflection of deep currents, and/or intensification of the Gulf Stream. Sediment distribution patterns changed dramatically with the sudden appearance of siliceous-ooze deposition in the marginal and east equatorial North Pacific by 16.0 to 15.5 Ma, coincident with the decline of siliceous sedimentation in the North Atlantic. This silica switch may have been caused by the introduction of Norwegian Overflow Water into the North Atlantic acting as a barrier to outcropping of silica-rich Antarctic Bottom Water. The main aspects of the present oceanic circulation system and sediment distribution pattern were established by 13.5 to 12.5 Ma (hiatus NH 3), coincident with the establishment of a major East Antarctic ice cap. Antarctic glaciation resulted in a broadening belt of siliceous-ooze deposition around Antarctica, increased siliceous sedimentation in the marginal and east equatorial North Pacific and Indian Oceans, and further northward restriction of siliceous sediments in the North Atlantic. Periodic cool climatic events were accompanied by lower eustatic sea levels and widespread deep-sea erosion at 12 to 11 Ma (NH 4), 10 to 9 Ma (NH 5), 7.5 to 6.2 Ma (NH 6), and 5.2 to 4.7 Ma (NH 7).

(Table 1) Activity of strontium-90 in waters of the Northeast Atlantic at the end of 1965 and the beginning of 1966

In 1965-1966 R/V Mikhail Lomonosov conducted studies on concentrations of artificial radioactive products in the Northeast Atlantic. Concentration of strontium-90 at the end of 1965 and the beginning of 1966 was higher than the average level for the ocean and reached about 53 dpm/100 l in the surface layer. The most intense transport of artificial radioactive products out of the Irish Sea was detected in the northern and northeastern directions along the Hebrides and the Orkneys. In addition to radioactive fission products from nuclear weapons tests, radioactive wastes of atomic industrial facilities discharged into the ocean are an important source of radioactive contamination of some regions of the world ocean.

Paleomagnetic of DSDP Leg 90

Sediments recovered during Leg 90 (Sites 587-594, plus Site 586 cored during Leg 89) are, in general, extremely weakly magnetized carbonate oozes and chalks with NRM intensities seldom greater than 0.05 µG. The quality of the paleomagnetic records deteriorates with increasing depth caused by the combined effects of removal of primary magnetic oxides by sulfate reduction processes and the dispersal of magnetic grains during compaction. Magnetic reversal sequences are generally recognizable back to the Gilbert, 3.4 to 5.35 m.y., except at equatorial Site 586 where only the Brunhes/Matuyama boundary could be identified. Longer reversal records were obtained at Site 588 (to Chron 13, about 13 m.y.) and Site 594 (base of Chron 5, about 5.9 m.y.). Sediments are characterized by extremely high calcium carbonate contents (90-100%) with almost no biosiliceous components. Blebs and streaks of pyrite are common, and the presence of iron sulfides with poor magnetic stabilities is suspected, although not yet positively identified. Viscous components of magnetization are common, sometimes to the extent of dominating the primary remanence, and there is evidence to suggest that a magnetic remanence is imparted during core recovery. Siliceous carbonate oozes provide better paleomagnetic records than pure carbonate oozes.