LATE QUATERNARY SEDIMENTATION ON THE PORTUGUESE CONTINENTAL-MARGIN - CLIMATE-RELATED PROCESSES AND PRODUCTS

The late Quaternary sedimentary history of the continental margin off Portugal was reconstructed from sediment gravity cores. Hemipelagic se dimentation (lithofacies A) was dominant during glacial times. It was interrupted periodically by deposition of shelf- and upper-slope-deriv ed silty and sandy terrigenous material by dilute turbidity currents ( lithofacies B and C), ice-rafted debris during distinct periods of bre akdown of North Atlantic ice sheets (Heinrich events. lithofacies D) a nd large amounts of pteropods (lithofacies F). Settling of biogenic pa rticulate material (lithofacies E) prevailed during the Holocene. when sea level and sea surface temperatures were high and terrigenous shel f-input was low. Downslope transport was dominant on the northern part of the Portuguese margin. culminating in frequent turbidity current t ransport between 35 and 70 ka. This may be due to a humid climate and a high fluvial input. Pteropod muds are confined to cores south of 41 N, Prominent peaks in pteropod concentration were radiocarbon dated at 17.8 and 24.6 ka. Layers rich in ice-rafted debris (IRD) were found a long the entire margin. The base of these layers have been dated at 13 .6-15.9 C-14 ka. 21.0-22.0 C-14 ka, 33.8 C-14 ka and +/-64.5 ka. which correspond well with the ages of Heinrich events 1. 2. 4 and 6 in the central North Atlantic. Heinrich events 0 (10.5 ka). 3 (27 ka) and 5( 50 ka) rarely influenced sedimentation on the Portuguese slope. A mine ralogical study of the IRD within Heinrich layers suggests that most i cebergs were derived from the Laurentide Ice Sheet in the Hudson Strai t and Hudson Bay area through the Labrador Current and the Canary Curr ent and flowed in a southward direction along the margin, IRD from Eur opean ice sheets may have been mixed in during Heinrich event 6. On th eir way along the marin the icebergs lost much of their sediment load due to melting of the ice in a progressively warmer climate. The south ernmost latitude studied (37 degrees N) may be close ro the southeaste rn extension of iceberg transport during Heinrich events. (C) Elsevier Science B.V.