Herman Zimmerman

History of Plio-Pleistocene Climate in the Northeastern Atlantic, Deep Sea Drilling Project Hole 552A

DSDP Hole 552A, cored with the HPC on Hatton Drift, represents an almost complete and undisturbed sediment section spanning the late Neogene and Quaternary. Lithologic, faunal, isotopic, and paleomagnetic analyses indicate that the section represents the most complete deep sea record of climatic evolution hitherto recovered at high latitudes in the northern hemisphere. A glacial record of remarkable resolution for the late Pliocene and Pleistocene is provided by oxygen and carbon isotope ratios in benthic foraminifers. In the upper part of the section, the whole of the standard oxygen isotope record of the past million years is well preserved. The onset of ice-rafting and glacial-interglacial alternations occurs at about 2.4 m.y. ago.

Sediments of the New England Continental Rise

Sediment from short cores taken on the continental rise off Georges Banks show two facies which are best distinguished by clay mineral ratios. The green-gray material in the upper, Holocene section of the cores is carbonate and amphibole rich and has a montmonllonite/illite peak-area ratio which typically is greater than one. These characteristics serve to distinguish the upper facies from the red-brown Pleistocene sediment of the lower core section in which illite is the dominant clay mineral. The clay mineralogy of the Pleistocene facies is consistent with a source in the Permo-Carboniferous red-bed area of the Canadian Maritime Provinces. This red lutite, which acts as a tracer for bottom current movement, was entrained on the continental rise by the Western Boundary Undercurrent and transported southwestward along the regional contours. The coarse fraction of this facies probably originated in down-slope processes along the entire continental margin under a regime of lower sea level and as ice-rafted debris. The upper, Holocene facies cannot be linked o t any specific continental source, but it appears to have the sediment of the Labrador Sea as its immediate source. The montmorillonite-rich lutite is transported to the south parallel to the regional contours by the Western Boundary Undercurrent. The carbonate component consists of coarse Foraminifera which originate in the warm Gull Stream. The character of the sediment and measurement of the predominant direction and speed of the bottom currents indicate that the Western Boundary Undercurrent is a significant factor in sediment transport and deposition on the east coast continental rise. A dispersal system parallel to the regional slope is confirmed to be operative for the fine sediment.

Hydraulic piston coring of late Neogene and Quaternary sections in the Caribbean and equatorial Pacific: Preliminary results of Deep Sea Drilling Project Leg 68

Leg 68 of the Deep Sea Drilling Project used the newly developed Hydraulic Piston Corer (HPC) to recover two virtually continuous, undisturbed sections of late Neogene and Quaternary sediment. The sites are located in the western Caribbean (Site 502, 4 holes) and in the eastern equatorial Pacific (Site 503, 2 holes). The sediment of Site 502 is primarily forambearing nanno marl which accumulated at about 3 to 4 cm/thousand yr. The bottom of Site 502 (228.7 m) is ~ 8 m.y. old. The sediment of Site 503 is primarily siliceous calcareous ooze which accumulated at about 2 to 3 cm/thousand yr. The bottom of Site 503 (235.0 m) is ~ 8 m.y. old. The magnetostratigraphy of both sites was determined on the R.V. Glomar Challenger with a long-core spinner magnetometer. All paleomagnetic boundaries through the Gilbert were identified in Site 502; most of them were identified in Site 503. The sediment at both sites shows a distinct cyclicity of calcium carbonate content. These relatively high accumulation rate, continuous, undisturbed HPC cores will enable a wide variety of high-resolution biostratigraphic, paleoclimatic, and p a l e o c e a n o g r a p h ic studies heretofore not feasible.

Acoustic horizons in the Argentine Basin, southwestern Atlantic Ocean: New evidence from deep-sea drilling

Acoustic horizons previously described in the Argentine Basin, southwestern Atlantic Ocean, have been correlated with the stratigraphic sequence drilled at Deep Sea Drilling Project site 358. Horizon B is associated with the transition from basal marly chalks to siliceous mudstones in the middle Eocene at 730 m subbottom. Horizon A, at 460 m subbottom, occurs in the upper Oligocene section and is correlated with a diagenetic boundary between diatom-radiolarian mudstones above and siliceous mudstones below in which biogenic opal has been converted to amorphous silica. Similarly, horizon F at 155 m subbottom correlates with a middle Miocene change from diatom-radiolarian mud above to siliceous mud below. Horizon A at site 358 in the Argentine Basin differs in age from a similar horizon in the Malvinas Outer Basin (site 328) and differs in age and character from horizon A in the North Atlantic. It is possible that silica diagenesis responsible for acoustic horizons A and F at site 358 occurred at or near the sediment-water interface during times of relatively slow sedimentation or that the degree of diagenesis is controlled by the composition of the host sediment.