Northern peatlands represent important stocks of organic carbon (C). Peatland C dynamics have the potential to influence atmospheric greenhouse gas concentrations and are therefore of interest concerning future climate change. Quantification of Holocene variations in peat C accumulation rates is often based on a single, deep core. However, deep cores may overestimate accumulation rates when extrapolated to the ecosystem scale. We propose a reconstruction of C sequestration patterns based on multiple cores from three ombrotrophic peatlands in boreal Quebec, Canada. Both total C accumulation and temporal variations herein were quantified. Radiocarbon-dated stratigraphies from different sections resulted in peatland-specific age-depth models. Peatland initiation started rapidly after deglaciation around 7500 cal BP. Vertical accumulation slowed down in the course of the Holocene, whereas lateral expansion was rapid in the early stages but slowed down near mid-Holocene. Total C accumulation showed maximum rates between 5250 and 3500 cal BP with a regional mean Holocene apparent rate of 16.2 g m -2 yr -1. The Eastmain peatlands have been modest sinks of organic C compared to those of Alaska, western Canada, and western Siberia, although differences in calculation methods hamper direct comparisons. Considering within-peatland dynamics, maximum total C sequestration coincided with a period of slowing down in both lateral expansion and vertical accumulation. Late-Holocene diminishing peatland C sink functions have been attributed to autogenic as well as allogenic factors. Height-induced surface drying and/or neoglacial cooling effects may have forced the slowing down of C sequestration in the studied bogs. Results further show that, in order to obtain an accurate quantification of past C sequestration, reconstructions of peatland expansion are essential.