North Atlantic Subtropical Mode Water (NASTMW) serves as a major conduit for dissolved carbon to penetrate into the ocean interior by its wintertime outcropping events. Prior research on NASTMW has concentrated on its physical formation and destruction, as well as Lagrangian pathways and timescales of water into and out of NASTMW. In this study, we examine how dissolved inorganic carbon (DIC) concentrations are modified along Lagrangian pathways of NASTMW on subannual timescales. We introduce Lagrangian parcels into a physical-biogeochemical model and release these parcels annually over two decades. For different pathways into, out of, and within NASTMW, we calculate changes in DIC concentrations along the path (ΔDIC), distinguishing contributions from vertical mixing and biological processes. Subduction leaves the most distinctive fingerprint on DIC concentrations (+101 µmol/L in one year), followed by export out of NASTMW due to densification (+10 µmol/L). Most DIC enrichment and depletion regimes span timescales of less than ~30 days, related to algal blooms. However, varying physical and biological processes often oppose one another at short timescales, so the largest net DIC changes occur at timescales of more than 30 days. While the mean ΔDIC for parcels that persist within NASTMW in one year is relatively small at +6 µmol/L, this masks underlying complexity: individual parcels undergo interspersed DIC depletion and enrichment, spanning several timescales and magnitudes. Since biological and physical processes both strongly influence DIC concentrations in NASTMW, refining process understanding and models of both domains is important for accurate projections of carbon cycling and sequestration.