bedford institute of oceanography 2001 in review - Région des ...

ScienceActivitiesGeological Survey of Canada (Atlantic)Fossil Dinoflagellates Make a Difference to the Dating Game- Rob Fensome and Graham WilliamsA popular educational website poses the question, “How do wedetermine the age of a rock?” The answer according to the websiteis: “By its minerals!” It continues: “Geologists first used fossils todetermine the relative ages of rocks. Thanks to the discovery ofradioactivity and advances in technology, it is now possible to assignan absolute age to a rock.” Although this answer is not strictly incorrect,it is misleading. The statement implies that the use of fossils isold-fashioned and no longer important for determining the ages ofrocks and that number-crunching radiometric dating techniquesare the modern way to go. Yet in reality, radiometric-dating techniquesare expensive, time-consuming, and only applicable forcertain rock types. Fossils are still the primary tool for dating rocksless than half a billion years old. Moreover, fossils, especially microscopicfossils, are vital tools in understanding the geology of oiland-natural-gas-pronesedimentary sequences such as those inoffshore eastern Canada.The study of the nature, origin and relationships of rocksequences is known as stratigraphy, and the dating of rocks throughtheir fossil content is called biostratigraphy. Geological periods (suchas the Jurassic) are defined by their biostratigraphy, not by theirabsolute age in millions of years. The best-known biostratigraphicevent is the Cretaceous-Tertiary (K-T) boundary - the extinction ofthe dinosaurs. If a rock contains Tyrannosaurus rex, it is Cretaceous,not Tertiary. No Tertiary rock contains fossils of T. rex. On the basisof radiometric dating, the K-T boundary is believed to be 65 millionyears old. But if new radiometric analyses were to revise the age to 60million years, this would be only a numerical re-adjustment. The K-T would not change physically since it would still be defined by fossiloccurrences. Numerical readjustments of important stratigraphicboundaries occur more commonly than might be suspected. Forexample, the Devonian-Carboniferous boundary was changedrecently from 354 to 362 million years ago.No dinosaurs have been found in the rock cores and cuttings (smallrock fragments) from the oil and gas wells of offshore eastern Canada.However, other “dinos”, fossil dinoflagellates that occur in the millions,Wetzelielloid specimens – photos by Lew Stover and Dan Beju.now constitute the principal biostratigraphic tool in the region fordetermining the age of marine rocks less than 200 million years old.Dinoflagellates are among the most common plankton found in today’soceans. They are single-celled organisms ranging from 10-200 µm insize, and have both plant-like characteristics (many contain chloroplasts)and animal-like features (most have an active, motile stage intheir life cycle). Some dinoflagellates cause red tides and some producepotent toxins. Fossilizable dinoflagellate cysts (dinocysts) are about thesame size as their motile counterparts, and always have a characteristicexcystment opening, or archeopyle, through which the next generation’smotile cell can escape.Rock samples analyzed for fossilized dinocysts must be treatedwith hydrochloric and hydrofluoric acids, plus other chemical andfractionation techniques to release the organic-walled dinocysts.When extracted from their stony tombs, dinocysts have been found tobe abundant. This profusion, plus their variable morphology andoften rapid evolution, make dinocysts ideal for biostratigraphy. Just asthe extinction of the dinosaurs represents an event that marks the endof the Cretaceous Period, so the origination and extinction of individualdinocyst species define particular stratigraphic events. As thedefinition of dinocyst species and the timing of their first and lastoccurrences become more refined, so does our understanding of the12 / BIO-2001 IN REVIEW