GRAPTOLITE DIVERSITY AND COMMUNITY CHANGES SURROUNDING THE LATE ORDOVICIAN MASS EXTINCTION: HIGH RESOLUTION DATA FROM THE BLACKSTONE RIVER, YUKON

Abstract

The Late Ordovician Extinction Event (LOME), the first of the “Big Five” extinction events, decimated marine communities just prior to the end of the period (ca. 444 Ma). Graptolites, the principal index taxon during the interval, and a key proxy for studies of environmental and evolutionary change, were particularly strongly impacted. Studying the extinction event is problematic. A disconformity or abrupt facies change occurs at nearly all global sections, caused by the same mechanism believed to be responsible for the extinction: glacially-induced changes in sea level, ocean temperature, and ocean circulation. Until recently, only South China possessed a well-sampled graptolite record of the LOME and its aftermath. Interpreting Chinese data was complicated by the lack of an equivalently rich and well-sampled comparison section, which made it impossible to distinguish regionally unique patterns from sampling artefacts. This thesis describes a new global reference section, located along the Blackstone River, which holds North America’s only continuous graptolite biozonal record through the interval of extinction and recovery (ornatus to atavus zones). Intense sampling of a 50 m section identified 106 species of graptolites (described and illustrated with nearly 1000 photos and line drawings), including a number of taxa identified for the first time in Canada. This allowed highly detailed biostratigraphic correlation with China and other regions in the paleotropics. In addition to presence-absence diversity data, all pre- and extinction interval samples were counted (>20,000 specimens). This provided a test of sampling completeness in the present study, as well as quantifying the impacts of sampling on apparent diversity. Abundances also illuminated graptolite population structure, and demonstrated that community-level changes preceded the interval of peak extinction loss, providing support for models of habitat loss and food-web based models of graptolite extinction during the LOME. Within the post-extinction recovery interval, the Blackstone River section boasts the world’s highest levels of graptolite diversity. These post-extinction communities were highly unstable, with dominance fluctuating and high levels of species turnover. In addition to background instability, a second extinction event, previously observed in China, but otherwise very poorly known, occurs at the base of the Silurian, demonstrating its global scale. Geochemical and lithological data tie this second event to sea level fall.