Foraminifera in marine sediments

The area of Kvarner Bay is part of a marine shelf. Today’s shelves are relatively young because the current level of the sea is the result of transgression (rising sea levels) that began more than 19,000 years ago and lasted until about 6,000 years ago with the melting of ice sheets after the Last Glacial Maximum of the Ice Age. The shelf is at a depth of about 200 metres, to which sunlight can reach, thus fostering biological diversity. Due to a large volume of bio-production, the shelf is characterised by biogenic sediment deposits, in addition to considerable terrigenous deposits. Most of the biogenic sediments in the sea are composed of carbonates, although there are organisms that build skeletons of opal (radiolarians, the spicules of sponges). The parts of organisms that can be found in the sediments include the shells of bivalves and snails, and parts of bryozoans and red algae, as well as the remains of echinoderms, annelids, arthropods and sponge spicules. In addition to these well-known groups, a lot of foraminifera can also be found in the sediments.

Foraminifera are single-celled organisms that live in the sea. They consist of an external shell with perforations through which pseudopods emerge for capturing food and for locomotion. Although foraminifera still live in our seas and oceans, they find their most important role in geology in helping to identify paleo-environments (Photo 3). They are good environmental indicators because certain species live only at specific depths and at specific temperatures. The large number of known species of foraminifera, which have appeared since more than 500 million years ago and up to the present, provides a wide range of data. More than 400 planktonic foraminifera (of which 40 species are still alive today) and about 27,000 benthic foraminifera (4,000 species still living) are known.

NINA TRINAJSTIĆ, MSc of Geology

Photo 2. Shells of planktonic and benthic foraminifera, size ranging from 0.25 to 0.5 mm. (Micro photo by Nina Trinjastić)

Photo 3. Foraminiferal limestone from the ornithological reserve Kuntrep on Krk Island. An example of the fossil remains of foraminifera shells. (Photo by Nina Trinajstić)