Holocene calcareous nannofossils and planktonic foraminifera assemblages in the Western Magellan Strait (Chile)

Two gravity cores, MB91/40 and MB91/54R, were recovered in the deeper and more inner basinal zone of the western sector of the Magellan Strait. Coccolithophores and planktonic foraminifera assemblages, sedimentological data and radiocarbon dating (AMS) were obtained. The aim of this work is to define the development of the coccolithophores and planktonic foraminifera assemblages and the sedimentological evolution of this sector of the Strait during the Late Pleistocene - Holocene, in order to contribute to the reconstruction of the paleoenvironmental and paleoceanographic features and to recognise the establishment of the current oceanographic characteristics. The data of this work allow us to distinguish, in both cores, paleoclimatic and paleoceanographic changes which occurred during the Late Pleistocene - Holocene. Siliciclastic sediments which are barren or characterised by rare occurrences of calcareous plankton, testify glacial-deglacial conditions of the Late Pleistocene - Holocene. The carbonate sedimentation is time-transgressive - starting at 8,900 yr BP (MB91/40 core) and at about 7,740 yr BP (MB91/54R core) - and is related to a climatic improvement and to the progressive melting of the glaciers. At about 5,863 yr BP in MB91/40, in the coccolithophore assemblage, there occurs a reversal in dominance of Gephyrocapsa muellerae and Gephyrocapsa caribbeanica. Moreover, in the record of the planktonic foraminifera assemblages, an increase of the warm-temperate water species Globorotalia truncatulinoides and a decrease of Globigerina quinqueloba (cold-water species) occur. In core MB91/54R, which is located in the inner area of this sector of the Strait, at about 3,420 yr BP, there only occur a significant decrease of G. muellerae and an increase of G. carribeanica. Moreover, a decrease of the cold form G. quinqueloba and the rare occurrence of G. truncatulinoides were recorded. In the same interval, a slight decrease of the total abundance of the coccolithophores and planktonic foraminifera species has been recorded. These changes, which occur in the coccolitophores and planktonic foraminifera assemblages of both cores, indicate another improvement of the climatic conditions and the establishment of the inverse stratification of the water masses and oligotrophic conditions. Statistical analyses confirm these trends and show that the late climatic improvement takes place through a phase of climatic instability.