We examined whether the existence of authority claims signifies one’s rationales in online communication content, potentially contributing to the research on rationale identification and rationale generation. Authority claims are statements that reveal the writer’s intention to bolster the writer’s credibility. In open online communications, the anonymity and the dynamic participation make it challenging to establish the credibility of their viewpoints and reasoning. Therefore, we hypothesize these online participants will tend to use authority claims to bolster their credibility when presenting their justifications. We annotated authority claims in 271 text segments that contain online users’ rationales. These text segments are adapted from the open access corpora provided by Rutgers’ Argument Mining group. Contrary to our hypothesis, we found that in our dataset the users scarcely attempted to bolster their credibility when presenting their reasoning to the others in these activities. We call for more investigations to explore the role of activity context affects participants’ use of authority claims in their reasoning traces. We further state that the effects of communication medium on individuals’ cognitive and meta-cognitive processes are important to consider in argument mining research.