the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Oct 2025
Two decades of pHT measurements along the GO-SHIP A25 section
Abstract. The North Atlantic (NA) GO-SHIP A25 OVIDE-BOCATS section is a long-term repeat hydrographic transect extending from Portugal to Greenland. Since 2002, physical and biogeochemical measurements have been carried out biennially along the OVIDE-BOCATS section, contributing to a better understanding of water mass properties, mixing, circulation, carbon storage, and climate change impacts such as ocean acidification (OA) in the NA. In particular, the high-precision pH measurements on the total hydrogen ion scale (pHT) from the OVIDE-BOCATS program represent a key milestone in monitoring OA in this particularly climate sensitive region. The method used for pHT determination relies on adding meta-cresol purple (mCP) dye to the seawater sample and spectrophotometrically measuring its absorbances at specific wavelengths. The OVIDE-BOCATS program has used unpurified mCP dye, which impurities have been proven to bias pHT values. Here we quantified the bias induced by these impurities in pHT measurements. We found that measurements carried out using the unpurified mCP dye tend to be, on average, 0.011 ± 0.002 pHT units higher than those obtained using the purified mCP dye, with this difference slightly decreasing at higher pHT values. Moreover, we tested independent methods to correct the effect of impurities in both the historical and recent OVIDE-BOCATS pHT data, demonstrating that the correction is consistent across methods. The long-term pHT dataset has been updated to include newly acquired data and absorbance measurements, and to standardize corrections for mCP dye impurities. This effort results in a twenty-year dataset of pHT corrected for mCP dye impurities, that demonstrates the possibility of a global effort to improve the reliability and coherency of spectrophotometric pHT measurements made with unpurified mCP dye. The corrections applied to our pHT dataset have negligible implications for the OA rates previously reported, but they do affect the depth of the aragonite saturation horizon, implying a shoaling of approximately 150 m.
Competing interests: A. Velo is editor for ESSD-ocean
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Status: open (until 26 Nov 2025)
- RC1: 'Comment on essd-2025-476', Anonymous Referee #1, 26 Oct 2025 reply
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Two decades of pHT measurements along the GO-SHIP A25 section F. F. Pérez et al. https://zenodo.org/records/17141184
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General comment:
The North Atlantic Ocean is an important CO2 sink (Takahashi et al. 2009) and this region contains high concentrations of anthropogenic CO2 in the water column (Khatiwala et al., 2013; Steinfeldt et al, 2024). Thanks to numerous observations in this basin since the seventies, it is now well known that simulated carbonates systems properties, including CO2 fluxes, OA and Cant, present significant bias. Quoting Perez et al., 2024: “The largest disagreement in the CO2 flux between GOBMs and pCO2 products is found north of 50°N”. Bias between models and data-based estimates are also observed for the Cant inventories in the North Atlantic (Perez et al, 2024, their figure 7). This calls for new analysis based on series of cruises to investigate the Cant variability, from seasonal to multi-decadal, such as recently presented by Bajon et al (2025). In this context, it is also important to extend and/or revise the data when observational bias are suggested or identified (e.g. Wang et al , 2025 for oxygen). Here the authors suggest that a correction of their pH data obtained along the OVIDE-BOCATS sections should be applied (by 0.011 on average). As an example, the comparison of data presented in the NEADW is convincing. Although this has apparently no impact on the pH trends, they show that the correction leads to a shift for the ASD. This new dataset (including 2021 and 2023 cruises) represents an important product, not only to re-investigate the drivers of ASD (Lauvset et al, 2020) but also for comparisons of pH data from BGC-Argo floats in this region (Wimart-Rousseau et al, 2024) as well as for model validation. The dataset includes 23500 corrected pH data from 11 cruises that will be probably revisited in the next GLODAP version. I wondered why authors did not include their AT data in the file that would help to calculate CT and Cant as well.
The document is well structured, figures and tables adapted. I recommend publication after minor revision. Below are listed specific comments.
;;;;;;; Specific comments:
C-01: Title: “Two decades of pHT measurements along the GO-SHIP A25 section”. For readers not familiar with GO-SHIP and cruises numbers, maybe specify this is in the North Atlantic: Two decades of pHT measurements along the GO-SHIP A25 section in the North Atlantic.
C-02: Line 59: Not sure that Ishii et al (2025) is a correct reference for OA and BGC-Argo data.
C-03: Line 139: “as well as the ocean's capacity to absorb, store, and transport CO2 (Pérez et al., 2013; Zunino et al., 2015).” You can add reference to Bajon et al, (2025) when published.
C-04: Line 141: “and understanding the SPNA's response to climate change (Rodgers et al., 2023).” Is it the correct reference for the response to climate change ? DeVries et al, (2023) would be more appropriate.
C-05: Figure 1: I guess one of the cruise in 2014 (GEOVIDE) extended to the west off Greenland (stations south of Labrador Sea). Are these stations included in the new dataset ? If yes, this should be shown in figure 1.
C-06: Line 176: “…should be adjusted by +0.0047 pHT units (Lee et al., 2000).” I think the reference should be DelValls and Dickson (1998). Lee et al used this correction to revise the dissociation constants.
C-07: Line 188: “(see Fig. S1 in Álvarez et al., submitted).” The paper by Álvarez et al. is not available at that stage (but would be happy to read it).
C-08: Line 387: “This fit, based on 6,910 samples from the 2018, 2021, and 2023 cruises (Supporting Information Fig. 3)”. In Figure S3, N= 2673. Is the fit based on 6910 or 2673 samples ?
C-09: Line 530: “While corrections related to the mCP dye addition effect were included in the data published in GLODAPv2.2023 (Lauvset et al., 2024), the 488A-based correction described in Sect. 2.2.3 had not yet been incorporated”. This is an important information for those who used and will use GLODAPv2.2023. Maybe also indicate here if the most recent cruises (2021 and 2023) have been submitted to GLODAP for the next version ?
C-10: Line 536: “We present a new database comprising 23,535 seawater samples with spectrophotometric pHT values,…”. I wondered why authors did not include their AT data.
C-11: Line 597: “To assess this impact, aragonite saturation horizons were recalculated using in situ temperature, salinity, AT, and pHT values,…”. Interesting sensitivity test, but the AT data are not in the files (correct ?).
C-12: Line 600: “This reevaluation reveals a more pronounced reduction in aragonite saturation at the surface (from -0.040 to -0.065),…”. I suspect this is reduction from pre-industrial period. Please clarify.
C-13: Line 622: “Notably, a persistent pHT minimum appears in the Iceland Basin between 500 m and 1,000 m, associated with intermediate waters with high Apparent Oxygen Utilization”. On this topic, the impact of biological processes on pH distribution was quantified by Lauvset et al (2020).
C-14: Line 651: “The highest OA rates (< -0.002 pHT yr-1) are observed in the surface layers (0—500 m) due to direct air-sea CO2 exchange.” Interestingly, such rate was also deduced in the NASPG from other surface data (-0.0021 pHT yr-1, Reverdin et al, 2018).
C-15: Line 652: “These upper layers also exhibit high interannual pHT variability (Fig. 10), which correlates negatively with AOU (Supporting Information Fig. 6b).” Figure 10 does not show the interannual variability. Maybe refer to Figure 9 here or present another figure for surface layer (in Supp Mat ?).
C-16: Line 673: “NEADW originates from Antarctic Bottom Water, formed in the Vema Fracture Zone, and is largely devoid of Cant (Steinfeldt et al., 2024).” See also Mercier and Morin (1997) who first investigate the AABW in the Atlantic Fracture Zones.
C-17: “6. Data availability” I wanted to explore the files but unfortunately, no access. On “zenodo” the message is: The record is publicly accessible, but files are restricted to users with access.
;;;;;;;;;;;;;;;;;;;;; Reference added in this review, not listed in the MS:
Bajon, R., Carracedo, L. I., Mercier, H., Asselot, R., and Pérez, F. F.: Seasonal to long-term variability of natural and anthropogenic carbon concentrations and transports in the subpolar North Atlantic Ocean, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-4425, 2025.
DeVries, T., Yamamoto, K., Wanninkhof, R., Gruber, N., Hauck, J., Müller, J. D., et al. (2023). Magnitude, trends, and variability of the global ocean carbon sink from 1985 to 2018. Global Biogeochemical Cycles, 37, e2023GB007780. https://doi.org/10.1029/2023GB007780
Khatiwala, S., Tanhua, T., Mikaloff Fletcher, S., Gerber, M., Doney, S. C., Graven, H. D., et al. (2013). Global ocean storage of anthropogenic carbon. Biogeosciences, 10(4), 2169–2191. https://doi.org/10.5194/bg‐10‐2169‐2013
Lauvset, S. K., Carter, B. R., Perez, F. F., Jiang, L.‐Q., Feely, R. A., Velo, A., & Olsen, A. (2020). Processes driving global interior ocean pH distribution. Global Biogeochemical Cycles, 34, e2019GB006229. https://doi.org/ 10.1029/2019GB006229
Mercier, H and Morin, P: Hydrography of the Romanche and Chain Fracture Zones, 1997 JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS, VL 102, 10373, DI 10.1029/97JC00229
Reverdin, G., Metzl, N., Olafsdottir, S., Racapé, V., Takahashi, T., Benetti, M., Valdimarsson, H., Benoit-Cattin, A., Danielsen, M., Fin, J., Naamar, A., Pierrot, D., Sullivan, K., Bringas, F., and Goni, G.: SURATLANT: a 1993–2017 surface sampling in the central part of the North Atlantic subpolar gyre, Earth Syst. Sci. Data, 10, 1901-1924, https://doi.org/10.5194/essd-10-1901-2018, 2018.
Takahashi, T., et al, 2009. Climatological Mean and Decadal Change in Surface Ocean pCO2, and Net Sea-air CO2 Flux over the Global Oceans. Deep-Sea Res II, doi:10.1016/j.dsr2.2008.12.009
Wang, Z., et al, 2025. Bias Evaluation for Sensor-Based Dissolved Oxygen from CTD and Profiling Floats in the World Ocean Database JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY, 42, DOI: 10.1175/JTECH-D-25-0027.1
Wimart-Rousseau, C., Steinhoff, T., Klein, B., Bittig, H., and Körtzinger, A.: Technical note: Assessment of float pH data quality control methods – a case study in the subpolar northwest Atlantic Ocean, Biogeosciences, 21, 1191–1211, https://doi.org/10.5194/bg-21-1191-2024, 2024.
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