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

Abstract

Planktonic foraminifera and coccoliths were analyzed from six gravity cores obtained on a deep terrace located on the upper continental slope of the Gulf of Cadiz, between 400 and 700 m water depth. The lithology of these cores consists mainly of muds with some interbedded sandy or silty foraminifer-rich layers that have been reported as contourites originated by the action of the Mediterranean Outflowing Waters (MOW) sweeping the sea-bottom today. After the Last Glacial, characterized by muddy sedimentation, sandy contourites start to deposit in the Gulf of Cadiz during the Bølling–Allerød (14–11 14C kyr BP) climatic optimum. This trend was broken during the Younger Dryas (11–10 14C 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 contourite layer formed during the first deglaciation stage (Bølling–Allerød time) and a sandy contourite interval that initiates immediately after the end of the Younger Dryas, during the second stage of the deglaciation and continues during the Holocene. The stratigraphy and climatic reconstruction was 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 Bølling–Allerød time. This is also related to a prominent peak of the subtropical species ( G. sacculifer and G. ruber). The Younger Dryas is identified by a reduction in abundance of the subtropical species, that again increase just after this period. The second step of deglaciation is marked by a sharp decrease in the relative abundance of dextrally coiled N. pachyderma. In previous papers the sequence of sandy contourites 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 sand layer episodes are condensed layers originating during times of rapid warming and relative sea-level rise within the last deglaciation. During these times the coastline migrated more rapidly landward and the terrigenous input decreased as it began to be trapped on land and on the shelf, resulting 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 therefore the energy of currents would act longer, producing a more efficient winnowing of the sediment. At the same time more generations of benthic organisms would rework the mixed layer, favoring this winnowing.