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The Upper Devonian Chagrin and Cleveland Shales contain a diverse and abundant organic-walled microplankton assemblage of acritarchs and leiospheres with associated spores, whereas the Lower Mississippian Bedford Shale phytoplankton assemblage is greatly reduced. Ten-gram samples were examined at intervals of 10 ft or less for the entire 573 cores through the 3 formations, to determine microplankton abundance and diversity.
In these samples, spores are the most abundant element, followed by leiospheres and then acritarchs. More than 50 species of acritarchs and leiospheres, mostly new, have been identified in 62 samples of the Upper Devonian section, whereas one Gorgonosphaeridium species occurs abundantly in all Upper Devonian samples and is present in most Lower Mississippian samples.
General acritarch diversity decreases slightly up-section in the Chagrin Shale, and increases slightly in the basal Cleveland Shale; the decrease is more marked in the upper Cleveland Shale, and is most notable in the Bedford Shale. Acritarch abundance also lessens up-section through the Upper Devonian formations, with minima in the upper fourth of the Chagrin Shale and the upper half of the Cleveland Shale. Acritarchs are very scarce in the Bedford Shale.
As total phytoplankton abundance shows a marked decrease up-section, from the middle of the Chagrin Shale to the top of the Bedford Shale, a drop in net primary productivity is indicated for the Late Devonian and Early Mississippian of Ohio.
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