Late Quaternary sedimentation on the Portuguese continental margin: climate-related processes and products

Abstract

The late Quaternary sedimentary history of the continental margin off Portugal was reconstructed from sediment gravity cores. Hemipelagic sedimentation (lithofacies A) was dominant during glacial times. It was interrupted periodically by deposition of shelf- and upper-slope-derived silty and sandy terrigenous material by dilute turbidity currents (lithofacies B and C), ice-rafted debris during distinct periods of breakdown of North Atlantic ice sheets (Heinrich events, lithofacies D) and large amounts of pteropods (lithofacies F). Settling of biogenic particulate material (lithofacies E) prevailed during the Holocene, when sea level and sea surface temperatures were high and terrigenous shelf-input was low. Downslope transport was dominant on the northern part of the Portuguese margin, culminating in frequent turbidity current transport 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 along the entire margin. The base of these layers have been dated at 13.6–15.9 14C ka, 21.0–22.0 14C ka, 33.8 14C 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 mineralogical study of the IRD within Heinrich layers suggests that most icebergs were derived from the Laurentide Ice Sheet in the Hudson Strait and Hudson Bay area through the Labrador Current and the Canary Current and flowed in a southward direction along the margin. IRD from European ice sheets may have been mixed in during Heinrich event 6. On their way along the margin the icebergs lost much of their sediment load due to melting of the ice in a progressively warmer climate. The southernmost latitude studied (37°N) may be close to the southeastern extension of iceberg transport during Heinrich events.