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
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on essd-2025-476', Anonymous Referee #1, 26 Oct 2025
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RC2: 'Comment on essd-2025-476', Anonymous Referee #2, 14 Nov 2025
General Comments
I really enjoyed reading this paper. It is well written, thoughtfully laid out, and has incorporated many rigorous experiments explained in significant detail. I liked the background information provided for the A25 section and felt it gave good context to the remainder of the manuscript. This large pHT dataset will be a great resource for researchers. I particularly commend the authors for including the absorbance values in their pHT dataset, which the authors correctly point out will be useful in the future if pHT characterizations are updated. I also appreciate that the authors put these pHT revisions in the context of the larger scale OA trends. I recommend acceptance of the manuscript after some minor revisions.
Specific Comments
Lines 74-75: pHT calculated from AT and CT is known to have potential issues, such as the pH-dependent pH offset and larger uncertainty. I would argue that this sentence should be rephrased to convey that both (1) pHT reported with no additional internal consistency corrections AND (2) pHT calculated from AT and CT could both affect the reliability of OA analyses. The key is having directly measured pHT that can be “trusted”.
Lines 78-79: The pH method was detailed previous to the 1990s (Robert-Baldo et al. 1985; Byrne and Breland, 1989), using mCP as the indicator dye began in the 1990s.
Line 100: Was this the same lot of dye used for all 11 cruises? Or were different batches of dye made over time, just with repeat purchases from the same brand?
Eq. 1: Need to define what pK2 is for this equation. You should also make it clear that this pK2 is different from the pK2 defined in Clayton and Byrne (1993). They list their K1 and K2 as formation constants, whereas the K2 in this manuscript is a dissociation constant. Their K2 describes the formation of H2I, whereas the K2 in this manuscript describes the dissociation to form H+ and I2-.
Lines 171-172: Again, make clear that the pK2 shown here is actually labeled as log K1 (so –pK1) in the CB’93 paper. I understand why you are showing yours as dissociation constants, but since you are referencing CB’93, you need to make it clear that they are not the same.
Lines 171-173: This sentence is almost identical to a sentence in Lee et al. (2000) and should be properly cited, not just citing the last sentence of the paragraph.
Line 178: Would be useful to mention here that the mCP in CB’93 was also made in DI water. Which is different than what is done in this study (and described in detail later).
Line 185: I think this value should be 0.018 from L’11.
Line 205: Since you’re referencing eq. (11) of DB’17, you should note that your “Runpur” is noted as “Robs” in their paper.
Fig. 2: It needs to be made clear that these 434Aimp values are specific to one lot from each of these vendors, which are listed in the CB’93 paper. These 434Aimp values aren’t generic for all lots of impure mCP with the same vendor name.
Fig. 2: What is the purified line showing? Is this using the same Runpur and comparing the CB’93 and L’11 parameterizations? It’s difficult to follow the text in lines 220-222 describing this.
Line 219: List the concentration for the mCP dyes determined in this study.
Line 240: Clarify that the blank measurement were recorded at the three target wavelengths (434, 578, 730 nm) like you do for the dye-addition measurements (line 246).
Lines 249-252: It should be clarified that measurements of R are insensitive to slight temperature changes (which is what Byrne and Breland 1989 noted). Also, Byrne and Breland were using cresol red, not mCP. However, pH is quite sensitive to temperature so accurate T measurements are necessary.
Line 264: Specify that an R of 1 corresponds to a pHT of 7.67 at what salinity and temperature?
Line 279: Can authors justify why their double-dye additions used 50 uL increments instead of 75 uL like their experiments? The authors account for the difference correctly in their equation 6, but I think readers will wonder the reasoning.
Line 297: Might be useful to add a clarifying sentence here to explain that at pHT y=0, the R (pH) of the original sample and the R (pH) of the indicator are the same, which is why no change is observed.
Line 307: For a couple of the cruises, normalizing the delta R with the isosbestic point reduced the R2. Can the authors explain why that would be?
Line 339: R=1 (or pHT=7.65) at what temperature and salinity?
Fig. 3: Also need to specify which temperature condition
Lines 368-369: How did the researchers determine when the volume deviated by more than 20%?
Lines 395-396: Authors should note that some uncertainty is introduced by collecting duplicates on 2 different Niskin bottles, rather than 2 samples from the same Niskin. (i.e., small leaking in one bottle, biological activity, delay in closing between Niskins, etc.)
Fig. 4: The grey shading is useful but hard to see. Can the authors make it slightly darker? Min and max values are also very small and hard to read.
Line 425: Would be useful to include the final dye concentration in the sample cell for the 2 dye types for a direct comparison, since the experiments used different volumes and stock indicator concentrations.
Fig. 5: Might be useful to reorganize the figure so that the purified dye column is on the right-most side. The unpurified dye columns could then all be shown together as increasingly improving the results to most closely match the purified dye results.
Fig. 6: The “N=176” within the figure seems unnecessary, since it’s listed in the figure caption. Its placement is confusing since it makes it appear that the “N=176” is specifically referring to the middle column of the figure, rather than all of the data.
Lines 514-516: Mention this finding is reasonable since you are working with two different lots of dye. The phrasing now makes it seem like you are recommending a change to the DB’17 value, rather than that you have a different 434Aimp value because you have a different lot of dye.
Line 543: No pH values were flagged as 4? If you’re going to highlight the number of flag 3, you should include the number of other flags as well.
Lines 545-547: So, were the pHT data from 2002-2018 not corrected using the DB’17 adjustment and L’11 equation? Or this was what was in GLODAPv2 before and will be updated using DB’17 and L’11 for v3? It needs to be very clear which data is in each version. Same for the 2021 and 2023 cruises – which procedure was used and what is the final product? Spell it out here for readers who skim, since this is a very important point.
Line 603: What’s the uncertainty in these saturation depth changes? I would be curious what is the range in the depth that saturation = 1 based on the uncertainty in pH measurements themselves?
Fig. 9: Difficult to read pH scale numbers and numbers in each panel, also difficult to read the x and y axes labels and numbers.
Lines 649-651: What are the uncertainties on these pH rate changes per year?
Technical Corrections
Line 14: Provide the acronym meanings for GO-SHIP and OVIDE-BOCATS.
Line 97: Provide acronym meaning for CLIVAR.
Line 163: Should be CB’93.
Citation: https://doi.org/10.5194/essd-2025-476-RC2
Data sets
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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- 1
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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