Geology Reference
In-Depth Information
Fig. 10.29. Morphology of spumellarian (A, B) and nassellarian (C-F) radiolarians. A - Archaeodictyomitra sixi Yang
(Tithonian: Longing Gap, Antarctic Peninsula; height 200 m), B - Mirifusus minor Baumgartner (Berriasian: Jebel Buwaydah,
Oman Mountains; height 500 m), C - Higumastra sp. (same locality as A; maximum diameter 500 m), D - Triactoma
kellumi Pessagno and Yang (Tithonian: Southern Alps, Italy; diameter of cortical shell 200 m), E - Acastea diaphorogona
(Foreman) (same locality as B; diameter of cortical shell 130 m), F - Leugeo hexacubica (Baumgartner) (Callovian: South-
ern Alps, Italy; pyritic specimen; diameter 175 m). Courtesy of W. Kiessling (Berlin).
shapes derive from geometrically simple ringlike spi-
cules from which latticed covers are added to form
chambers.
species occur in tropical and subtropical surface wa-
ters and in deeper waters (> 200 m). Fewer species live
in high latitudes. Homogeneity in radiolarian assem-
blages parallel to the latitudes, and heterogeneity across
the latitudes indicate that water temperature influences
the distributional patterns. Variations in the composi-
tion of assemblages depend on the difference of physi-
cal, chemical and biological parameters within water
masses, e.g. oceanic currents. The composition of ra-
diolarian assemblages and their distributional patterns,
therefore, are excellent proxies for paleoclimatic and
paleooceanographic interpretations of Meso- and Ceno-
zoic oceans.
Methods: Radiolaria are usually studied by separa-
tion techniques, but also in thin sections. The fossils
are investigated under the light microscope and in SEM.
Cathodoluminescence microscopy enhances thin-sec-
tion criteria.
Radiolaria in thin sections: Radiolarians are com-
mon to abundant in thin sections of fine-grained open-
marine pelagic limestones and radiolarites. Sections
through spherical and ellipsoidal tests of spumellarians
result in circular figures of different sizes (Pl. 76/1).
Circular figures can also be cross sections of nassel-
larians. Differentiating these two groups in thin sec-
tions is in most cases not possible apart from rare ex-
ceptions (Pl. 76/4). Calcitized radiolarian tests exhibit
some similarity with calcispheres (Sect. 10.2.1.9), but
can be differentiated by a toothed appearance of the
peripheral zone of the shell (Pl. 76/2). Unlike from ra-
diolarian tests, the sections of calcispheres commonly
show defined calcareous walls. Sections of calpionel-
lids are considerably smaller than those of radiolarians
(Pl. 76/8).
Distribution: First records are known from the Cam-
brian. By the Silurian deep-water forms are believed to
have evolved. All Early Paleozoic radiolarians are
spumellarians or belong to extinct subgroups. Meso-
zoic and Cenozoic assemblages are characterized by
various groups of spumellarians and nassellarians. A
rapid diversification took place at the end of the Juras-
sic. Warm-water faunas have existed since the Cam-
brian. Cold-water assemblages appear in the Oligocene.
Diagenesis and preservation: The fossil radiolarian
record is strongly controlled by diagenetic factors. Dis-
solution of radiolarian tests takes place in the water
column, and on and within the upper centimeters of the
sea bottom. It depends on the degree of silica under-
saturation of pore waters, the intensity of bioturbation
and the accumulation rate. High sedimentation rates
favor good preservation.
The transformation of siliceous muds into siliceous
rocks, connected with the maturation of the sediments
corresponds to a sequence starting with opal A, fol-
lowed by opal-CT (porcelanite) and ending with chal-
cedony or cryptocrystalline quartz (chert). Mesozoic
Biology and ecology: Radiolarians are a part of the
zooplankton and capture food from the water column.
They consume various zooplankton and phytoplank-
ton as well as bacteria and organic detritus. Some poly-
cystine radiolarians host photosymbiotic dinoflagellate
symbionts within their cells. Modern radiolarians are
widely distributed in the oceans and occur in the photic
surface zone of the ocean as well as in depths of hun-
dreds of meters down to water depths of 4000 m and
more. About the same number of modern radiolarian
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