Techniques of study

Phase 1: Field detection

Radiolarians are no bigger than 0.1 - 0.2 mm in average and are not easily detectable in the field. By applying micropaleontological observation techniques, the rate of recovery is improved (radiolarian ribbon chert, limestone, siliceous shale, mudstone, and siltstone). Within suture zones, radiolarian biostratigraphy is locally the sole tool available for dating siliceous marine sedimentary rocks.

Close-up of grey radiolarian ribbon chert from the Cache Creek complex, Teslin plateau, Yukon, Canada.

Phase 2: Radiolarian Chemical Extraction

Polycystines radiolarian shells are made of silica (post-mortem recrystallization Opal A - Opal CT - Quartz). The surrounding matrix of chert, siliceous shale, mudstone, or siltstone, also contains silica with variable amount of clays. Chemical processing uses hydrofluoric acid (HF). Techniques related to radiolarian extraction from siliceous rocks have been discovered at the end of the 60s (Hayashi, 1969; Dumitrica, 1970; Pessagno and Newport, 1972). More recently, these processing techniques have been applied in the field in relation to geological mapping (Cordey and Krauss, 1990).
Left: red radiolarian chert bed surface after 24 hours of HF processing. Right: detail (scale 1 cm).

Left: radiolarian shale after HF processing. Silica shells partly extracted, internal cell structures locally visible. Right: Scanning Electron Microscope picture (scale 1 mm).

Stub SEMAfter chemical extraction, the best radiolarian shells are selected and aligned on a small aluminium support (stub) in order to be observed under a scanning electron microscope (SEM). Scale: 1 cent euro coin

Phase 3: Age determination and biostratigraphy

Radiolarians have a 500 million years age range, making them a powerful biostratigraphic tool. Selection of adequate and reliable radiolarian associations is based on picking techniques aiming at faunal assemblages. Morphological identification is performed through optical and/or Scanning Electron microscopes.

Radiolarians species of Late Devonian, Early Permian, Late Triassic, Middle and Late Jurassic in age, from left to right respectively.

Present and Future Developments

In addition to stratigraphic investigations relative to geological mapping for government agencies and mining/oil companies, new directions in radiolarian fossil research include study of faunal differentiation in relation to paleogeography, paleoclimates and paleoenvironments of ocean basins (Panthalassa, Tethys). In this context, radiolarian-bearing geological units from Canada, the US, Europe and Asia are under scrutiny.