the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
An updated synthesis of ocean total alkalinity and dissolved inorganic carbon measurements from 1993 to 2023: the SNAPO-CO2-v2 dataset
Abstract. Total alkalinity (AT) and dissolved inorganic carbon (CT) in the oceans are important properties to understand the ocean carbon cycle and its link with global change (ocean carbon sinks and sources, ocean acidification) and ultimately find carbon based solutions or mitigation procedures (marine carbon removal). We present an extended database (SNAPO-CO2, Metzl et al, 2024d) with 24700 new additional data for the period 2002 to 2023. The full database now includes more than 67000 AT and CT observations along with basic ancillary data (time and space location, depth, temperature and salinity) in various oceanic regions obtained since 1993 mainly in the frame of French research projects. This includes both surface and water columns data acquired in open oceans, coastal zones, rivers and in the Mediterranean Sea and either from time-series or punctual cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ± 4 µmol kg-1 for both AT and CT. The same technique was used onboard for underway measurements during cruises conducted in the Southern Indian and Southern Oceans. The AT and CT data from these cruises are also added in this synthesis. The data are provided in one dataset for the global ocean (https://doi.org/10.17882/102337) that offers a direct use for regional or global purposes, e.g. AT/Salinity relationships, long-term CT estimates, constraint and validation of diagnostics CT and AT reconstructed fields or ocean carbon and coupled climate/carbon models simulations, as well as data derived from Biogeochemical-Argo (BGC-Argo) floats. These data can also be used to calculate pH, fugacity of CO2 (fCO2) and other carbon system properties to derive ocean acidification rates or air-sea CO2 fluxes.
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Status: open (until 17 Jan 2025)
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RC1: 'Comment on essd-2024-464', Kim Currie, 11 Dec 2024
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General Comments
Marine carbon data that are of high and known quality, have appropriate metadata and are FAIR are vital in assessing the role of the ocean in the changing carbon cycle. The SNAPO-CO2- v2 alkalinity and DIC dataset described in this manuscript is an important contribution as the data have wide spatio and temporal coverage and are of very high quality. The v2 described in this manuscript builds on the initial dataset published earlier this year. The manuscript details the data sources, the QC processing, and then the data assemblage and synthesis. Five use-cases are then presented, with the spatial distribution and trend analysis of the five cases assessed.
The data are available via the SEANOE website, and via the doi number presented in the manuscript.
The paper is very well written, the tables and diagrams are clear, except where noted below, and the supplementary material is useful and suitable.
This is a very good manuscript, I'm sure the dataset will be well used by the marine carbon community and that the manuscript will be well cited. I recommend publication and have only a few minor comments outlined below.
Specific Comments
Line 433 missing word: “…precision was based on triplicate analyses was estimated.…”
Figure 7, shows the surface distribution of alkalinity and DIC in the Mediterranean Sea over the decade 2014 – 2023. Given that the trend in DIC over that decade was 0.72 umol kg-1 yr-1 , the changes in the graphed value could be up to 7.2 umol kg-1 for the decade. I suggest either normalizing the data to a particular year, or to describe the effect in the text.
Several of the Case studies in Section 5 involve time series analysis, however the methodology for doing this is not described or referenced. Sutton et al (2022) is a useful reference for this:
Sutton, A.J., Battisti, R., Carter, B., Evans, W., Newton, J., Alin, S., Bates, N.R., Cai, W.-J., Currie, K., Feely, R.A., Sabine, C., Tanhua, T., Tilbrook, B., Wanninkhof, R. (2022) Advancing best practices for assessing trends of ocean acidification time series. Frontiers in Marine Science, 9: 1045667. doi: 10.3389/fmars.2022.1045667
Line 930 This dataset is indeed a useful complement to other data compilations such as GLODAP and SOCAT. Are these SNAPO-CO2 data included in SOCAT and / or GLODAP (as appropriate)?
Figure 14. The colour scale is not necessary, and is not used in the Figure data. There are only two time periods identified by the colours, and these are indicated in the caption.
Fig S3 The caption is incorrect, the colour coding in the Figure is not as described in the caption. This needs to be corrected.
Fig S4 The concept of this Figure is good, and it relates well to the text (line 339), however the map insert is difficult to read, and it is difficult to relate the colour coding of the triangle symbols in the vertical profiles with the day scale on the map. This should be clarified, - if the time component is important the colours or symbols should be the same, otherwise the day scale is not necessary.
Citation: https://doi.org/10.5194/essd-2024-464-RC1
Data sets
An updated synthesis of ocean total alkalinity and dissolved inorganic carbon measurements from 1993 to 2023: the SNAPO-CO2-v2 dataset Nicolas Metzl et al. https://doi.org/10.17882/102337
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