the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Balancing water column and sedimentary 234Th 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 underlaying marine sediments. This finding reveals that on the timescale of ~100 days, 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 7.2 ± 0.7 mol C m-2 yr-1, within a factor of 2 to 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.
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RC1: 'Comment on egusphere-2025-2493', Anonymous Referee #1, 29 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2493/egusphere-2025-2493-RC1-supplement.pdf
- AC2: 'Reply on RC1', Madeline Healey, 31 Aug 2025
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RC2: 'Comment on egusphere-2025-2493', Anonymous Referee #2, 01 Aug 2025
The study by Healey et al., investigates the water column 234Th – 238U disequilibrium and surficial sediment inventory of excess 234Th to constrain the particulate organic carbon (POC) export and burial in Bedford Basin. The integration of water column and sediment data to quantify the POC fluxes is appreciable.
Authors conducted quasi-season sampling from 2021 – 2024, capturing the seasonal dynamics over three-year period. The water column and sedimentary 234Th fluxes and POC fluxes along with bottom sediment depositional POC fluxes are in close agreement which verifies the estimates of POC fluxes in water column and mostly contributed by the vertical setting of particles. The study is comprehensive with proper methodology.
Though the Bedford Basin is relatively small, the data from a single central station might not capture the lateral heterogeneity or edge effects. In the center, as shown by the data, the POC contribution is mostly from the primary production or vertical setting of particles, however, the lateral contribution of the particles cannot be ignored, especially in this shallow fjord. It can be discussed in the manuscript in general regarding the overall basin for wider implications.
The hydrological parameters of the basin show similar patterns for different years and seasonal variations. The mixing and stratification are seasonal and the importance of absence of winter mixing and spring blooms in relation with steady state 1D Model needs to be further discussed. The limited direct measurements of nepheloid layers and primary production weakened mechanistic interpretation of persistent 234Th deficits. Although 1-D steady state model is assumed in this study, the system shows several seasonal dynamic features, especially intrusion and spring bloom. The authors have included a paragraph of discussion about the NSS contribution for <3%, but it doesn’t justify this datasheet. The authors should describe how the absence of NSS might impact the fluxes especially during different seasons and spring bloom for this study.
I appreciate the authors for nice graphs and figures; however, the authors should include the 234Th profile data along with errors even if it’s in supplementary data.
Fig 6 a. There is no need to include the R2 value as this is cumulative fluxes and R2 values doesn’t have any meaningful essence.
Overall, the manuscript is valuable, relevant and methodologically sound. Incorporating these suggestions, addressing spatial heterogeneity would provide clarity and enhance the impact of this manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-2493-RC2 - AC1: 'Reply on RC2', Madeline Healey, 31 Aug 2025
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