PALEOGEOGRAPHIC, PALEOCEANOGRAPHIC, AND TECTONIC CONTROLS ON EARLY LATE ORDOVICIAN GRAPTOLITE DIVERSITY PATTERNS

The early Late Ordovician (Middle Caradoc) was an interval of significant decline in marine biodiversity, but whether this decline was a real phenomenon or an artifact of the relatively few studies devoted to this interval requires further research. We examined of the pattern of graptolite faunal changes across the Climacograptus bicornis - Corynoides americanus graptolite zonal boundary in North America and compared the diversity changes to published data from sections that span this interval on other continents. A sharp decline in species diversity took place in Laurentia but not on all other continents. In the Appalachian Basin, diversity drops from 18 genera and 38 species in the C . bicornis Zone to 10 genera and 13 species in the overlying C . americanus Zone. Only 11% of the species present in the C . bicornis Zone carry over into the C . americanus Zone. A similar pattern occurs in central Oklahoma. Rocks coeval with the C . americanus Zone along the Cordilleran margin are either entirely absent or yield a very sparse and low diversity fauna. High and middle latitude regions like Wales and Baltoscandia also exhibit a sharp decline in graptolite diversity. In other low latitude regions such as Australasia and Scotland, however, diversity remains fairly constant across this interval (although the percentage of carryover taxa remains low). Several geologic and paleo-environmental events have been suggested as possible causes for the biotic changes in Laurentia during the Caradoc. These include changes in basin circulation associated with the Taconic Orogeny, extensive explosive volcanism and ash deposition, and a drop in seawater temperature. Among these, seawater temperature changes or other physical and chemical changes that might accompany temperature fluctuation may provide an explanation for the similarity between Laurentian and higher latitude diversity patterns. Flooding of the Laurentian craton through the Sebree Trough by cool, sub-polar Iapetus seawater may have adversely affected graptolite diversity there. Higher latitude regions may have experienced seawater cooling associated with mid-Caradoc climate deterioration. Thus seawater cooling, albeit driven by different mechanisms, may have produced similar diversity patterns at different paleo-latitudes.