Radiolaria – Siliceous plankton through time

Abstract

Preface SIII Radiolaria are a very diverse marine siliceous microplankton group that have existed at least since the Cambrian to the Recent. Fossil Radiolaria were known since the middle of the 19th Century and studied by many scientists at the turn of the 19th/20th Century, in part as a consequence of the discoveries made in samples collected be the HMS Challenger expeditions. Intense biostratigraphic radiolarian research was spurted again in the early 1970’s by the Deep Sea Drilling Project. Biochronologic zonations based on samples recovered by Ocean Drilling were first proposed for the Tertiary (Riedel & Sanfilippo 1978) and the Cretaceous (Foreman 1973, 1975). However, the radiolarites, cherty oceanic sediments associated with ophiolites, had so far only been studied in thin sections. This changed when radiolarian workers discovered the use of hydrofluoric acid to extract Radiolaria from chert and other siliceous rocks (Dumitrica 1970, Pessagno & Newport 1972) The new method allowed to work on land samples from mountain ranges that had undergone burial diagenesis or even metamorphosis. As a consequence, radiolarian biochronology was rapidly extended into the Jurassic, Triassic and the Palaeozoic (Pessagno 1977, Pessagno et al. 1979, Yao et al. 1980, Nakaseko & Nishimura 1979, Holdsworth & Jones 1980). A major obstacle to the elaboration of detailed biochronozones is the discontinuous nature of the radiolarian record, due to spotty preservation. Biogenous, opaline silica is unstable in the ocean and in the diagenetic environment. Large amounts of the more delicate radiolarian tests become dissolved already during their descent in the water column and in the bottom sediments (Takahashi & Honjo 1981). During early diagenesis, opaline silica dissolves and radiolarians become preserved either as casts of the mould that leaves the dissolved test, or as replacement by quartz, pyrite or other minerals. In Palaeozoic and Mesozoic samples, usually only robust forms are preserved. However, exceptionally well preserved samples from special diagenetic environments, such as Middle Jurassic manganese carbonate nodules (Yao 1997), or euxinic environments like the Late Jurassic Solnhofen Limestone (Zugel 1997, Dumitrica & Zugel 2002) show several hundred morphotypes, a diversity that rivals with the diversity of living radiolarians. These samples teach us, that the average radiolarian assemblage extracted by harsh chemical treatments from rocks is a poor residue of dissolution resistant forms. The consequence of this unavoidable fact is the discontinuous record of radiolarian ranges, which greatly hampers biochronologic correlation. A major