Along the west coast of North America, climate and marine productivity is affected by seasonal to interannual changes in coastal upwelling. Our understanding of upwelling variability in the past is limited by the short duration of instrumental records. Changes in upwelling intensity are expected to affect the 13C/12C (δ13C) and radiocarbon (∆14C) content of marine dissolved inorganic carbon (DIC) due to variable mixing of old, upwelled seawater into surface waters. If seasonal variations in the δ13C of DIC are recorded in marine bivalve shells, they may provide valuable information about the extent of upwelling in the past. Comparison of modern Mytilus californianus shells from South California with a 5 year time series of coastal seawater ∆14C and δ13C allows an assessment of the suitability of Mytilus shell ∆14C and δ13C as proxies of upwelling intensity. We show that both absolute values and the seasonal range in seawater ∆14C are preserved in shell ∆14C, allowing its use as an indicator of upwelling intensity. Interpretation of shell δ13C is more problematic, with the δ13C of shell carbonate lower than seawater DIC by variable amounts (ranging from 0.5‰ to 1.5‰) due to the incorporation of metabolic carbon. The spatial and temporal variability observed in specimens that grew during the severe El Niño event of 1997–1998 demonstrates how a transect of shells spanning the western North American coastline can be used to reconstruct large-scale patterns of seawater ∆14C variability for specific intervals of interest, such as those associated with El Niño Southern Oscillation–type phenomena.