| Graptolite species from the Ordovician and Silurian periods are abundant, geographically widespread and evolved relatively rapidly. Thus, their geochemical properties have high potential for identifying and carrying out in-depth studies on short intervals of geological time. At present there is no consensus on where in the water column graptolites lived, what they fed on, or how far from the shelf they resided. The isotopic composition of each graptolite has the potential to be used ultimately to start to answer these questions. To date, there has been no large-scale taxon-specific delta13C org and delta 15Norg study of graptolites. The few previous studies undertaken concentrated on demonstrating that such a study was possible. This thesis is a taxon specific study; rhabdosomes of eighteen species of graptolite were analyzed separately from a succession of samples, to yield both normalized species results and interspecies relationships.;My results show that despite strong differences in morphology, there are no significant or consistent differences between the isotopic compositions of co-occurring graptolite species, except in one sample that shows clear evidence of long-distance lateral transport of specimens. In all other cases, graptolites species from the same sample fall within the range of intraspecific variation of +/- 0.35‰ of less. In addition, the narrow range interspecific of variation isotopic composition indicates that all eighteen graptolite species studied represent the same trophic level. This suggests that graptolite body type/shape had less to do with trophic level and more to do with other factors, such as hydrodynamics. Furthermore, graptolite periderm delta 13Cgrap and delta15Ngrap seems not to be influenced by astogenetic stage of development or gross rhabdosome type, indicating no change in environment or trophic level through the life span of these species. I show that the carbon and nitrogen isotope composition of the periderm (delta13Cgrap and delta 15Ngrap) provides a signal that is consistently 0-2‰ heavier then surrounding whole-rock samples. Furthermore, I suggest that in situations (e.g. sea level fluctuations) where terrestrial organic matter may have been incorporated into the sediment, graptolite geochemistry may more faithfully reflect bulk marine organic matter. The results, therefore, support the LaPorte et al. (2009) hypothesis, that graptolites can be used for chemostratigraphic studies despite differences in species composition throughout a succession unless there is taphonomic evidence for significant lateral transport.;Keywords: Graptolites; Silurian; Carbon isotopes; Nitrogen isotopes; Chemostratigraphy.;Here I use graptolites from the lower Silurian of Cornwallis Island, Nunuvut, for chemostratigraphic study to test two theories that can be inferred from previous research. A paper by LaPorte et al. (2009) assumed that there is no isotopic difference between graptolite species, so graptolite periderm could be used for chemostratigraphic study without concern for variation between species. On the other hand, a paper by Finney et al. (2007) suggested that different species lived in different water masses, thus implying that each species could be imprinted with its own unique isotopic compositions. In modern zooplankton: carbon and nitrogen isotopes become heavier towards from shore, with increasing trophic level and with increasing depth, do to recycling through trophic levels. |
