Publication Date:
2022-07-18
Description:
Olive-gray mud covers the continental slope off southern Portugal; at the same time sandy, reddishbrown mud was deposited off Morocco. Biogenie carbonate is a main constituent in both sediment types, off Portugal contributing about 25% and off Morocco more than 50%. The sediment cores from the Portuguese and Moroccan continental slope can be correlated by sedimentological and micropaleontological methods (figs. 10, 15). The climatic variations during the late Pleistocene and Holocene are weil documented by the content of ice-rafted debris (figs. 8, 13) and by faunal compos1t1on of planktonic foraminifera (THIEDE 1971 ). Radiocarbon ages indicate that these variations coincide with variations in the climatic record of the Iberian Peninsula based on pollen analysis (fig. 33). The calculation of sedimentation rates allows an estimate of the amount of terrigenous supply and biogenic carbonate production during the last 15.000 years (table 5). Planctonic organisms produced about 400 gram pro 100 cm2 per 1.000 yrs, while the contribution of benthos is relatively low (2.5 gram). The sedimentation rate of planctonic foraminifera increases from the open ocean to the region of the upper continental slope (fig. 27). This trend, caused by the better food supply near the continental influx and upwelling area, appears both off Portugal and Morocco. The sedimentation rate of benthonic remains decreases by a factor of 100 from the upper continental slope to the deep-sea plain (fig. 28). lt can be shown that this decrease is caused mainly by the reduction of available food with increasing water depth (figs. 29, and 30). In contrast to the biogenic sediment supply, which off Portugal and Morocco is not quantitatively different, the terrigenous supply from the weakly humid Iberian Peninsula is 3-4 times greater than the supply from the semiarid Moroccan mainland (fig. 31 ). This difference influences diagenetic processes. Small manganese concretions and reddish staining iron oxides, which remain stable under the oxygenated conditions in the sediments off Morocco, were replaced by pyrite during the destruction of organic matter in the sediments off Portugal through anaero bic bacteria. During the late Pleistocene a maximum of terrigenous and biogenic sediment (fig. 33) was deposited together with additional, mainly coarse sediment brought in by drifting icebergs. Mediterranean outflow water reduces the sedimentation of silt- and smaller sized particles on the Portuguese continental slope between water depths of 500 and 1.500 m. As a result this sediment is mainly deposited in water depths of 2.000 and 3.000 m (fig. 32). This form of sediment distribution and the high sedimentation rate of silt-sized particles under the southern part of the outflow water (core 8066 B) are characteristic of the time at least since the late Pleistocene. This would indicate that since then the outflow pattern of the Mediterranean water has not changed.
Type:
Article
,
PeerReviewed
Format:
text
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