A method of determining proximity of marginal seas to an ocean

On any area of continental shelf the foraminiferal assemblage in the bottom sediment is likely to be composed of an indigenous benthic component and an allochthonous planktonic component sedimented from the water column above. Planktonic forminiferids are essentially oceanic and it seems clear that their presence in shelf sediments is often due to transport from an oceanic source. Evidence from the Celtic Sea, northwestern Gulf of Mexico, Persian Gulf and Andaman Sea shows that with passage away from the ocean there is a reduction in the planktonic—benthic ratio in the bottom sediment, a progressive decrease in size of the planktonic tests and a reduction in diversity of the planktonic assemblage. Thus a study of the relationships between the planktonic and benthic foraminiferids in these areas gives a measure of proximity to or isolation from the ocean. It is suggested that these relationships are useful tools in the interpretation of palaeoecology and in the reconstruction of former patterns of water circulation.