Dr. David Kidder
The Geological Sciences Colloquium Series presents Dr. David Kidder on “Changing influences on chert formation through the Paleozoic and beyond” on Friday, Feb. 17 at 3:05 p.m. to 4:00 p.m. in Clippinger 205.
Kidder is Associate Professor of Geological Sciences at Ohio University where he conducts research and teaches courses on Oceanography, Historical Geology, and Earth Systems Evolution.
Abstract: The depositional setting in which chert accumulates has been a moving target through the Phanerozoic. The first major step in this movement was an Early Paleozoic retreat of chert from shallow water (peritidal) settings to shelf and deeper facies. This retreat was initially proposed as a prolonged Cambro-Ordovician transition. A later major step was a retreat of cherty facies from the shelf to the deep sea. This step was probably caused by the Cenozoic rise in importance of ocean diatoms which were collectively so good at extracting dissolved silica (DSi) from shallow waters that insufficient DSi remained for siliceous sponges to manufacture sufficiently robust skeletons to flourish at shelf and shallower depths. Siliceous sponges retreated to the deep sea at this time. Siliceous zooplankton (radiolarians) underwent significant reductions in skeletal mass as the diatoms “stole” some of their needed DSi in the Cenozoic. The Early Paleozoic retreat of chert from shallow-water (peritidal) facies in the Early Paleozoic is thought to have resulted from increasingly important radiolarian biotas “stealing” shallow-water DSi from siliceous sponges, driving them and the cherts they helped produce into shelf and deeper waters.
Research by a team of Ohio University students reveals two points in which the Paleozoic story of chert needs revision. First, the Cambro-Ordovician cherty retreat appears to have been mostly with in the Mid-Upper Ordovician rather than a slower transition. Second, the Ordovician retreat of shallow-water cherts to deeper settings was not sustained. Cherty facies returned to shallow-water settings in the Devonian, Mississippian, and Pennsylvanian strata. This talk considers possible reasons for these unexpected developments as well as a few other aspects of the long-term evolution of the oceanic silica cycle.
Upcoming Colloquia
David Jeffery of Marietta College on March 24 at 3:05 p.m. in Clippinger 205.
Cathy Busby of the University of California at Davis on “Anatomy of a long-lived oceanic arc: Synthesis of three IODP expeditions in the Izu-Bonin-Marianas Arc” on March 31 at 3:05 p.m. in Clippinger 205.
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