Balancing water column and sedimentary ²³⁴ Th fluxes to quantify coastal marine carbon export

Abstract. Quantitative estimates of particulate organic carbon (POC) flux and burial in coastal systems are critical for constraining coastal carbon budgets and understanding their role in regional and global carbon cycling. In this study, POC export fluxes were quantified in the Bedford Basin, a coastal inlet in the Northwest Atlantic, based on measurements of 234Th/238U disequilibria and the POC : 234Th ratio on small (1–51 µm) and large (>51 µm) particles. These water column export fluxes were compared to sediment accumulation fluxes of 234Th and POC to refine the carbon budget in the Bedford Basin. The coupled water column-surficial sediment sampling approach, which is relatively new, was applied quasi-seasonally throughout 2021–2024, including for the first time in boreal winter. Total 234Th activities reveal persistent deficits with respect to 238U throughout the water column, likely due to extensive particle scavenging, which is also indicated by high 234Th activities on particles. Here, we find that the removal of 234Th in the water column is typically balanced, within uncertainties, by the inventory of excess 234Th in underlying marine sediments. This finding reveals that on the timescale of ∼100 d, the 234Th budget in the Bedford Basin is in balance, and no major particle loss is occurring. Using the POC : 234Th ratio on sinking particles and integrated 234Th water column fluxes, we report a mean (± s.d.) depositional flux of 20±14 mmol C m−2 d−1 (range 3.6 to 44.5 mmol C m−2 d−1) to the seafloor. This mean flux translates to an annual molar flux of 7.3±5.1 mol C m−2 yr−1, within a factor of 2 of model estimates and previous sediment trap results at this site. Our findings contribute to ongoing research efforts in the Bedford Basin, and aid in the evaluation of coastal regions in local carbon budgets.