MESOSCALE DISTRIBUTION PATTERNS OF DIATOMS IN SURFACE SEDIMENTS AS TRACERS OF COASTAL UPWELLING OF THE GALICIAN SHELF (NW IBERIAN PENINSULA)

Diatom distribution on continental shelf sediments from the upwelling area off Galicia (NW Iberian Peninsula) has been determined in 78 surf ace sediment samples. Three well defined biofacies with a close relati onship to upwelling influence along the coast have been distinguished from both the absolute diatom abundances in the sediments and assembla ge diatom composition based on multivariate statistics: Rias Baixas an d the shelf areas south and north to Cape Finisterre. Chaetoceros rest ing spores as well as highest absolute diatom content in the sediments are recorded in the highly productive Rias Baixas where most of the p rimary production is due to intense upwelling phenomena. The western s helf, south of Cape Finisterre, is characterized by Thalassionema nitz schioides and Thalassiosira cf. leptopus. Both taxa reflect a minor in fluence of upwelling conditions and productivity related to more persi stent nutrient input due to coastal outwelling rather than upwelling. Minor and patchy upwelling conditions as well as lower productivity in the northern shelf are characterized by the lowest absolute diatom co ntent in the sediments and the important increase in relative abundanc e of the resistant Paralia sulcata. Caution must be taken in the inter pretation of Paralia sulcata dominated biofacies in downcore studies s ince a juxtaposition of discontinuous upwelling conditions and dissolu tion effects may be the factors responsible for the relative increase on the taxon in the sediments. The obtained results show the extent an d limitations of fossil diatom distribution in surface sediments as tr acers of regional coastal upwelling conditions in continental shelves where the existence of extensive areas of relict sediments and transpo rted diatoms may mask the present-day upwelling signal of the photic z one. (C) 1997 Elsevier Science B.V.