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Radiolaria are single-celled planktonic marine organisms that construct skeletons of opaline silica. They are common throughout the world's oceans today, especially in areas of high oceanic productivity. Radiolarians have a rich evolutionary history dating back 500 million years.

Studies of Cretaceous-Cenozoic radiolarians

Current research on radiolarians at GNS is focussed on fossil assemblages from marine sedimentary strata of Late Cretaceous and early Cenozoic age (80 - 40 million years ago). Dr Chris Hollis uses evolutionary changes, such as speciation and extinction events, to correlate strata both within and beyond New Zealand. Other trends within the fossil assemblages over time can be related to changes in the ocean environment. Dr Hollis is especially interested in learning how oceanic changes may be associated with major crises in the geological record, such as the Cretaceous/Tertiary Boundary mass extinction and the Paleocene/Eocene Thermal Maximum.

Sites in the Southwest Pacific with documented occurrences of radiolarian-rich sediments or sedimentary strata of Late Cretaceous and early Cenozoic age.

Some radiolarian fossils from Late Cretaceous drill-core from eastern margin of the Campbell Plateau (Deep Sea Drilling Project site 275)

Some radiolarian fossils from Paleocene sedimentary strata on the Chatham Islands, New Zealand (basal Tutuiri Greensand)

Cretaceous species in Paleocene assemblages (reworked or Cretaceous survivors?)

A spumellarian radiolarian in a thin section of the basal Tutuiri Greensand siliceous unit.

Studies of Paleozoic-Mesozoic Radiolaria
Early studies of Paleozoic and Mesozoic radiolarians were hampered by strong recrystallisation of tests making traditional identifications based on transmitted light microscope examination of internal features very difficult. Since the discovery in the early 1970s of the hydrofluoric acid method of extraction of radiolarians from hard rocks such as cherts and siliceous mudstones and the development of the scanning electron microscope for detailed identifications based on external magnification, radiolarians have become a major tool for age determinations in New Zealand's basement rocks. Numerous studies, especially those by Yoshiaki Aita and John Simes have provided well-constrained radiolarian age determinations for previously undated or very poorly dated basement blocks extending over tens of kilometres.

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