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:Concentrations of cosmogenic nu-
clides in river-born sand (Safran et al. 2005) and suspended loads of sediments (Bourges et al. 1990; Aalto et al. 2006) yield erosion rates between 0.04 and 1.35 mm/yr on millennial to decadal timescales.
Category: