The purpose of this paper is to examine university oceanography students use of evidence in writing. Drawing from rhetorical studies of science writing and studies of argumentation in science education a model for assessing students arguments is proposed that considers the relative epistemic status of propositions comprising students written texts. The study was conducted in an introductory university oceanography course in a large public university that utilized an interactive CD-ROM that provided geological data sets for student exploration of scientific questions. Student arguments were analyzed through a process of sorting propositions by epistemic level and identifying the explicit links within and across levels.