Sedimentology and micropalaeontology of gravity cores from the N.E. Atlantic continental slope south west of Ireland

Master Thesis


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University of Cape Town

Eleven gravity cores from the continental margin off Eire and Land's End (SW England) were examined and found to document the major trends of the Late Pleistocene climate. Several stratigraphic indicators; - carbonate content, sediment texture, grain size, composition, nature of terrigenous components, ice-rafted debris and foraminiferal diversity were examined and show that the glacial history of the study area can be closely correlated with the palaeoclimatic evolution of the adjacent European shelf. Sediments deposited during Late Pleistocene glacial conditions show the following characteristics when compared to the surface sediments deposited under Holocene interglacial conditions: an increase in the quantity of ice-rafted debris and percentage of mica, and a notable increase in the degree of frosting and pitting of the quartz grains. Overall grain size was finer resulting in a silty sediment package. Sedimentologically the cores fall into two groups (1 and 2). The major difference being that Group 1 (located on the Pendragon Escarpment) received increased quantities of fine silts from a 'shelf spill-over' mechanism operating on the Fastnet and Western Approaches Basins, during glacial regressions. All sediment samples displayed polymodal characteristics reflecting the interaction of several different physical processes e.g. ice-rafting, contour currents etc. Striking variations in the populations of planktonic foraminifera were noted, alternating between Arctic and Sub-Arctic assemblages, reflecting the waxing and waning of glacial activity. The coccolith-carbonate minima correlate with the Arctic-fauna maxima and the ¹⁸O/¹⁶O maxima of the oxygen-isotope curves. Foraminiferal-test analysis (ratio of whole foraminifera fragmented foraminifera) revealed that no correlation existed with any of the other parameters analysed. However, the cores were severely affected by the presence of bottom currents which were strong enough to remove the fragmented tests. Parallellaminated contourites and evidence of erosion were noted in all cores. Ten cores penetrated sediments deposited during the last glacial maximum of 20,000 B.P - 18,000 B.P. near the 75cm depth mark (Core 1865 was too short to reach such sediments). However sediments reflecting the 11,000 B.P glacial readvance, detected at around the 25cm mark, were not as clearly represented. Bioturbation has smoothed the climatic record throughout the lengths of these cores and has also suppressed the high-frequency oscillations (<10³ B.P).

Bibliography: pages 45-55.