We use 40Ar-39Ar incremental heating to fingerprint bulk fluvial sediment entering the northeast Pacific Ocean, with the long-term intent of tracking sediment source and transport changes from the terrestrial system to the marine environment through time. We show reproducible age spectra from individual rivers accounting for the majority of sediment delivered to the Pacific margin. Two tests are performed to confirm the validity of the bulk sediment 40Ar-39Ar incremental heating measurements and to address why polymineralic sediment might yield concordant age steps. The first model tests, in light of bulk mineralogy and diffusion of Ar from silicates, whether measured K/Ca spectra (measured from 39Ar and 37Ar, respectively) are consistent with typical values for K- and Ca-bearing minerals. Calculations show that the bulk mineralogy is reflected in the outgassing K/Ca spectra and identify plagioclase as the dominant mineral contributing to the plateau-defining portion of the age spectra. A second model predicts bulk sediment ages from integrated bedrock cooling age-area estimates in order to examine whether bulk sediment plateau ages are representative of the average cooling age of rocks from a given river basin. Calculated and observed ages are notably similar in three river basins when topographic and lithologic effects are accounted for. Overall, this technique shows considerable promise, not only in tracking individual terrigenous sources in the marine realm but also for understanding processes such as erosion and sediment transport in terrestrial systems.