Late glacial to recent paleoenvironmental changes in the Gulf of Cadiz andformation of sandy contourite layers

Planktonic foraminifera and coccoliths were analyzed from six gravity cores obtained on a deep terrace located on the upper continental slope of the G ulf of Cadiz, between 400 and 700 m water depth. The lithology of these cor es consists mainly of muds with some interbedded sandy or silty foraminifer -rich layers that have been reported as contourites originated by the actio n of the Mediterranean Outflowing Waters (MOW) sweeping the sea-bottom toda y. After the Last Glacial, characterized by muddy sedimentation, sandy cont ourites start to deposit in the Gulf of Cadiz during the Bolling-Allerod (1 4-11 C-14 kyr BP) climatic optimum. This trend was broken during the Younge r Dryas (11-10 C-14 kyr BP) and started again after the end of the Younger Dryas. This pattern is recorded in most of the cores by a single sandy cont ourite layer formed during the first deglaciation stage (Bolling-Allerod ti me) and a sandy contourite interval that initiates immediately after the en d of the Younger Dryas, during the second stage of the deglaciation and con tinues during the Holocene. The stratigraphy and climatic reconstruction wa s based on the evolution of the calcareous plankton assemblages during the last climatic transition, from the Last Glacial to the recent Holocene. The sharp reduction of sinistral N, pachyderma along with the reduction of G. quinqueloba mark the base of Bolling-Allerod time. This is also related to a prominent peak of the subtropical species (G. sacculifer and G. ruber). T he Younger Dryas is identified by a reduction in abundance of the subtropic al species, that again increase just after this period. The second step of deglaciation is marked by a sharp decrease in the relative abundance of dex trally coiled N. pachyderma. In previous papers the sequence of sandy conto urites has been related to sea level rise, greater Gibraltar sill depth and enhanced energy of the MOW. In this work, we suggest that the individual s and layer episodes are condensed layers originating during times of rapid w arming and relative sea-level rise within the last deglaciation. During the se times the coastline migrated more rapidly landward and the terrigenous i nput decreased as it began to be trapped on land and on the shelf, resultin g in a major drop in sedimentation rate on the upper continental slope. In times of low sedimentation rate the particles would have a longer residence time within the upper mixed layer of the near surface sediment and therefo re the energy of currents would act longer, producing a more efficient winn owing of the sediment. At the same time more generations of benthic organis ms would rework the mixed layer, favoring this winnowing. (C) 1999 Elsevier Science B.V. All rights reserved.