The opal contents of recent marine sediments are ascertained by conversion of the opal to crystobalite by heating and by a subsequent quantitative x-ray diffraction analysis of the crystobalite. An opal stratigraphy is evident in Bering Sea deposits. Further, the opal concentrations appear to be covariant with barium, an element previously suggested as indicative of biological productivity in overlying water masses.
Goldberg, Edward D.. 1958. "Determination of opal in marine sediments." Journal of Marine Research 17, (1). https://elischolar.library.yale.edu/journal_of_marine_research/913