R. Bao et al., MESOSCALE DISTRIBUTION PATTERNS OF DIATOMS IN SURFACE SEDIMENTS AS TRACERS OF COASTAL UPWELLING OF THE GALICIAN SHELF (NW IBERIAN PENINSULA), Marine geology, 144(1-3), 1997, pp. 117-130
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.