Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Evidence for more prevalent chemolithotrophic associated activities, including ammonia oxidation, sulfur oxidation and CO2 fixation have all been reported in the mesopelagic at the NPSG (Karl et al., 1984; Ingalls et al., 2006; Hansman et al., 2009; Swan et al., 2011; Giovannoni and Vergin, 2012).
Category: