Opal content in the Ría de Vigo and Galician continental shelf: biogenic silica in the muddy fraction as an accurate paleoproductivity proxy

Opal analyses were performed for the total and <63 μm fractions of both ria and core sediment samples. For the core samples, there is no correlation between opal content in the fine and bulk fractions, but opal percentage in the muddy fraction is an useful parameter to standardize results and to apply as a paleoproductivity proxy. For the ria surface sediments there is a good correlation between BSi content in both fractions (R2=0.90). This fact suggests that the information provided by total and fine fraction analysis is similar; as a result, the opal content analysis in the fine fraction does not supply any new information concerning diatom productivity.