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Abstract. The incomplete spatial coverage of CO 2 partial pressure (pCO 2 ) measurements across estuary types represents a significant knowledge gap in current regional-and global-scale estimates of estuarine CO 2 emissions. Given the limited research on CO 2 dynamics in large estuaries and bay systems, as well as the sources of error in the calculation of pCO 2 (carbonic acid dissociation constants, organic alkalinity), estimates of air-sea CO 2 fluxes in estuaries are subject to large uncertainties. The Estuary and Gulf of St. Lawrence (EGSL) at the lower limit of the subarctic region in eastern Canada is the world's largest estuarine system, and is characterized by an exceptional richness in environmental diversity. It is among the world's most intensively studied estuaries, yet there are no published data on its surface-water pCO 2 distribution. To fill this data gap, a comprehensive dataset was compiled from direct and indirect measurements of carbonate system parameters in the surface waters of the EGSL during the spring or summer of 2003-2016. The calculated surface-water pCO 2 ranged from 435 to 765 µatm in the shallow partially mixed upper estuary, 139-578 µatm in the deep stratified lower estuary, and 207-478 µatm along the Laurentian Channel in the Gulf of St. Lawrence. Overall, at the time of sampling, the St. Lawrence Estuary served as a very weak source of CO 2 to the atmosphere, with an area-averaged CO 2 degassing flux of 0.98 to 2.02 mmol C m −2 d −1 (0.36 to 0.74 mol C m −2 yr −1 ). A preliminary analysis revealed that respiration (upper estuary), photosynthesis (lower estuary), and temperature (Gulf of St. Lawrence) controlled the spatial variability in surface-water pCO 2 . Whereas we used the dissociation constants of Cai and Wang (1998)

Abstract. Despite intense focus on the ∼190 ‰ drop in atmospheric Δ14C during Heinrich Stadial 1 at ∼17.4–14.6 ka, the specific mechanisms responsible for the apparent Δ14C excess in the glacial atmosphere have received considerably less attention. The computationally efficient Bern3D Earth system model of intermediate complexity, designed for long-term climate simulations, allows us to address a very fundamental but still elusive question concerning the atmospheric Δ14C record: how can we explain the persistence of relatively high Δ14C values during the millennia after the Laschamp event? Large uncertainties in the pre-Holocene 14C production rate, as well as in the older portion of the Δ14C record, complicate our qualitative and quantitative interpretation of the glacial Δ14C elevation. Here we begin with sensitivity experiments that investigate the controls on atmospheric Δ14C in idealized settings. We show that the interaction with the ocean sediments may be much more important to the simulation of Δ14C than had been previously thought. In order to provide a bounded estimate of glacial Δ14C change, the Bern3D model was integrated with five available estimates of the 14C production rate as well as reconstructed and hypothetical paleoclimate forcing. Model results demonstrate that none of the available reconstructions of past changes in 14C production can reproduce the elevated Δ14C levels during the last glacial. In order to increase atmospheric Δ14C to glacial levels, a drastic reduction of air–sea exchange efficiency in the polar regions must be assumed, though discrepancies remain for the portion of the record younger than ∼33 ka. We end with an illustration of how the 14C production rate would have had to evolve to be consistent with the Δ14C record by combining an atmospheric radiocarbon budget with the Bern3D model. The overall conclusion is that the remaining discrepancies with respect to glacial Δ14C may be linked to an underestimation of 14C production and/or a biased-high reconstruction of Δ14C over the time period of interest. Alternatively, we appear to still be missing an important carbon cycle process for atmospheric Δ14C.