Mahmood Alam

Ph. D. Thesis

The Effect of Pleistocene Climatic Changes on the Sediments Around the Grand Banks.

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Piston and gravity cores were collected from four different areas around the Grand Banks. Cores from the Grand Banks slope and Flemish Pass were examined to establish glacial and interglacial sedimentation patterns on the slope and to investigate the paleoceanography of the area. Cores from the Fogo Seamounts and the Newfoundland Basin were examined for the climatic signatures and sedimentation records extending back to 500,000 B.P. The cores were divided into ten lithofacies, ten stratigraphic units and twelve foraminiferal stages. Cores were correlated by time dependent lithological variables, micropaleontological extinction and by C¹⁴ and paleomagnetic dates.

Foraminiferal evidence was used to investigate the paleoclimatology of the area. The paleoclimatic records obtained by using microfossils were calibrated by oxygen isotope analyses. The result suggests that the area around the Grand Banks was at least as warm as at present on four other occasions. Major warm events, which may be correlated with deglaciation, took place at around 11,000 B.P., 110,000 B.P., 220,000 B.P., 330,000 B.P. and 440,000 B.P. Faunal evidence from the Wisconsinan sediments indicates that the surface water was slightly warm on three occasions at around 22,000 B.P., 45,000 B.P. and 65,000 B.P., which probably indicate interstadial conditions. Late Holocene surface water temperatures on the eastern side of the Grand Banks indicate progressive cooling, whereas on the western side they show progressive warming. These two different types of records indicate the influence of two different currents on the two sides, the Gulf Stream and the Labrador Current. The warm Greenland component of the Labrador Current played an important role in sedimentation and biogenic productivity during interstadials and the early stages of interglacials. The cold Arctic component of the Labrador Current is important for the Late Interglacial stages.

Sedimentation on the Grand Banks margin during interglacial and interstadial stages is dominated by pelagic and hemipelagic deposition. During the glacial stage turbidite, nepheloid and plume sedimentation predominate. The input of detrital sediment on the outer continental margin increased very sharply during the glacial stages. Large amounts of sand were deposited on the Grand Banks slope by turbidity currents during these cold stages. The Fogo Seamounts and the Newfoundland Basin cores show sharp increases in mud input during cold periods. Under the present climatic and oceanographic conditions, very little sediment is transported from the Grand Banks to the outer margin. Most of the sediments that escape from the Grand Banks during interglacial and interstadial stages appears to be wave suspended and are drifted towards the southwest by the Ladrador Current, resulting in a higher rate of sedimentation on the western slope. Sediments deposited during glacial stages are rich in illite, chlorite and kaolinite, whereas the sediments deposited during interglacial and interstadial stages are rich in montmorillonite and mixed-lay clay and contain ice-rafted carbonate pebbles.

The red sediments injected into the North Atlantic through the Laurentian Channel appear to have been caught by the Western Boundary Undercurrent and the deep circulation of the Gulf Stream. It appears that the sediments were transported by these currents up to the Bermuda Rise and the Newfoundland Basin. The effect of the Western Boundary Undercurrent and deep circulation of the Gulf Stream is probably reflected in the shape of the Newfoundland Ridge.

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Pages: 426
Supervisor: D. J. W. Piper / H. B. S. Cooke