Interactive comment on “ Observed trends of anthropogenic acidification in North Atlantic water masses ”

The lack of observational pH data has made difficult assessing recent rates of ocean acidification, particularly in the high latitudes. Here we present a time series that spans over 27 years (1981-2008) of high-quality carbon system measurements in the North Atlantic that comprise fourteen cruises and cover the important water mass formation areas of the Irminger and Iceland basins. We provide direct quantification of acidification rates in upper and intermediate North Atlantic waters. The highest rates were associated with surface waters and with Labrador Seawater (LSW). The Subarctic Intermediate and Subpolar Mode waters (SAIW and SPMW) showed acidification rates of -0.0019±0.0001 and -0.0012±0.0002 yr, respectively. The deep convection activity in the North Atlantic Subpolar Gyre injects surface waters loaded with anthropogenic CO2 into lower layers, provoking the remarkable acidification rate observed for LSW in the Iceland basin of 0.0016±0.0002 yr. An extrapolation of the observed acidification linear trends suggests that the pH of LSW could drop 0.45 units with respect to pre-industrial levels by the time atmospheric CO2 concentrations reach ∼775 ppm. Under similar circulation conditions and evolution of the CO2 emission rates to the ones during the last three decades, the cLSW in the Iceland basin could become undersaturated in dissolved aragonite earlier than the surface SPMW, by the time atmospheric CO2 reaches 550 ppm.

Dear reviewer, Thanks a lot for your input.Your comments and suggestions have greatly helped improving this version of the manuscript (attached at the end of this document), which has changed substantially with respect to the previous one.The main modifications include the following: 1) Methods section: Section 2.2 where the averaging and basin normalization of data is described is more concise now and makes clear that such data treatment does not aim at separating the anthropogenic and the natural acidification signals (this was not explained well enough in the previous version of the manuscript), but rather calculate small pH corrections to the measurements (∆pH SWS25-BA ) to avoid potential sampling biases by centering / normalising the data from single cruises to the average basin conditions.
2) Figure 2 has been updated and now all panels include the isopycnals that separate the studied water masses in each basin, as listed in Table 2 and described in section 2.2 and Fig. 1b.
3) We updated the uncertainty of pH measurements in Table 1.Table 2 (included as Supplementary information in the previous manuscript version) now includes the pH averages per water mass and basin obtained from a) direct measurements (pH SWS25 ); b) basin-and-layer normalization (correction) element (∆pH SWS25-BA ); and c) basinnormalized pH averages (pH SWS25-BA ), after the expression in equation 1 (pH SWS25-BA =pH SWS25 + ΔpH SWS25-BA ).This way it is made explicitly clear what the orders of magnitude of the normalization of data are in each case, and how the measured and normalized pH data compare to each other.
4) Discussion section: Comparison of results with data from time series stations ESTOC and BATS; Calculation of expected rates of acidification due to C ant entry (i.e., anthropogenic acidification) from the C ant storage rates in Pérez et al. (2010) and comparison of those acidification rates with the ones here obtained (Fig. 3) from direct measurements.
After all these major modifications motivated by your comments, the reply to most of the suggestions made in your original review letter are answered by making reference to the revised manuscript version, rather than point-by-point.This is the reason why we have included it at the end of this letter.
We hope you find the changes we made in this new version respond satisfactorily to your detailed review and critic of the original document.
With our best regards.

Marcos Vázquez-Rodríguez and co-authors
In this paper the authors use a large collection of data for the subpolar North Atlantic (SPNA) in order to quantify the change in interior ocean pH over about 3 decades.The SPNA is well suited for this as large amounts of anthropogenic carbon (Cant) is taken up and stored in this region, and this region is relatively well sampled in comparison to other ocean regions.The SPNA is also a very active area with significant changes in water mass composition and properties over the last decades and the observational record (probably for much longer than that, but without observations to verify this).The authors show trends of observed pH trends in the SPNA over 3 decades, and attempt to project these trends linearly into the next 50 or so years.Rather than into the next 50 years, over higher atmospheric xCO 2 values (that is what was on the x-axis of Fig. S1 -now Fig. 4).Motivated by this type of confusion, we have deleted the references to the two CO 2 emission scenarios we mentioned to avoid complicating the main message.Our projections are based on xCO 2 values, not on time, or on how fast those xCO 2 values are reached (that depends on the emission rate / scenario).The reason we included the emission scenarios was to provide some context, but this seemed to blur the main message, so we decided to omit these references.The theme of the manuscript is well suited for publication in BG, but both the methods and the result/discussion sections is in the need of significant improvements.
You will find that those sections have changed significantly in the revised version of the manuscript (see general comments above).They are more concise and clearer thanks to your comments and suggestions

Main concerns:
The methods section of this manuscript needs considerable improvements.GENERAL COMMENT ON THIS SECTION: This whole section (now 2.2.1) has been restructured and rewritten aiming for clarity and conciseness.I do not understand why the authors only use density layers that vary between basins to determine water masses.The authors have access to several other parameters that can help distinguish different water masses with similar densities.One of the reasons to select those water masses and density layers was to be able to use the results obtained in Pérez et al. 2010.The C ant storage rates provided by Pérez et al. 2010 have been used to calculate the expected acidification rates due to human activities and we compared these expected rates with our observation-based results, where no separation between the anthropogenic and natural acidification signals has been done (see the second paragraph in Section 4, "Discussion").This way, it is possible to evaluate how much of the observed acidification in the different water masses is human-induced and how much is due to natural variability.This can potentially lead to problems with some water masses such as Mediterranean Water.For the particular case of MW, we used the recommendations in Ríos et al. 1992 to select the isopycnal limits.A more serious problem is probably the temporal variability aspect of this problem.Changes in water mass properties in the SPNA are well documented, and the authors refer to some of these studies.By following isopycnals from year to year (they are the same in all panels in Fig. 2 or in new Table 2) and not average properties of the water masses (which do change indeed, as reflected by the values on new Table 2 -former supplementary tables) we minimise the risk of misplacing a given water mass.The authors write: "The main factors that modulate the natural variability of ocean pH on decadal timescales need to be removed from observations in order to isolate and evaluate the anthropogenic forcing and its effects in ocean acidification", and then sets out to remove the "natural variability".It is not clear to me why one should take out decadal variability for a study concerned with decadal changes; it seems to defeat the purpose of the study.Please, see point 1) in the general comments on the first page of this letter.This was understood wrongly in the previous version, probably because we did not express well enough what was intended.The methodology does not aim at separating the anthropogenic and the natural acidification signals; it is just a normalization of the average pH (per layer and basin) to the average basin conditions to avoid sampling biases due to either the time or position of sampling with respect to the basin averages.Thanks a lot for this comment.Section 2.2.2 (now 2.2.1) was indeed the hardest part to follow in the manuscript and it has changed substantially motivated by your comments and from the rest of the reviewers.It is also not clear that the variability they apparently remove is natural.The authors present no evidence for that.If the variability is a result of anthropogenic changes effecting circulation, temperature, oxygen etc. would it not be a very bad idea to remove that signal?The section describing the normalization and averaging of pH data is very difficult to understand.Idem as in the above comment.I have tried hard, and I think I know what the authors did, but it is not clear to me why they did it this particular way; or why they did it at all, for that matter.It occurs to me as a strange way of normalizing the pH values.We believe this has improved significantly on the revised version.New section 2.2.1 has been restructured and rewritten aiming for clarity and conciseness.Please, refer to that section and to the answers to related comments in this letter.It is not clear to me that the factors (a) in equation 1 and 2 can be directly compared as the authors do, since equation 2 has one more term.The a i coefficients obtained from new equation (3) (former equation ( 2) on the original version of the manuscript) are the ones needed in equation ( 2) to obtain ∆pH SWS25-BA .As a matter of fact, that is the only purpose of equation ( 3), i.e., obtaining the a i coefficients from equation (2).NB: All equation numbers refer to the equations in the revised version of the manuscript (which are the same as in the previous version.They have only been reorganised).The authors state that the Delta-pHc term is spatially and seasonally detrended.We apologize for the misunderstanding and poor explanation of the methodology.We thought it was straightforward to follow, but it turned it wasn't so, so thank you for all this feedback.There's no seasonal detrending.Please, refer to new section 2.2.1.Firstly, is this not a normalization to climatological conditions for a water mass, rather than a spatial normalization?Secondly, how does the seasonal signal transports to the interior ocean?Is this even worth doing, except for the surface waters?Also, coming back to my previous concerns, why "reference to average climatological conditions" at all?The first sentence in new section 2.2.1 states exactly this: "…The average pH SWS25 was obtained for each layer at each year and for the three basins, following the averaging and "basin-referencing" methodology that Pérez et al. (2008Pérez et al. ( , 2010) ) and Ríos et al. (2012) used for C ant ".The basis of this averaging methodology has been fully described in the works by Pérez et al. (2008;2010) and Ríos et al. 2012, to which we make a reference now to simplify this section and explain only the specifics to our case (pH) compared to theirs (the were dealing with C ant instead).
Looking at the values in tables S1 to S3 there seems to be a clear decreasing trend for most water masses without any normalization to climatological conditions.Why not report on the trend of the averaged observed pH for each water mass?I think that is the very least that should be done in order for the reader to understand how large influence the "normalization" process has on the trends.The DeltapHc term is not reported on in any of the tables, which I think it should be (if at all used).Please, se point 3) to the general comments on the first page of this letter and new Table 2 (former supplementary tables).On Table 2 we give a full list of the values of a) direct measurements (pH SWS25 ); b) basin-and-layer normalization (correction) element (∆pH SWS25-BA ); and c) basin-normalized pH averages (pH SWS25-BA ), after the expression in equation 1 (pH SWS25-BA =pH SWS25 + ΔpH SWS25-BA ).This way it is made explicitly clear what the orders of magnitude of the normalization of data are in each case, and how the measured and normalized pH data compare to each other.Further the comment that the DeltapHc term is in the order of _10-3, and therefore not so important, might not be true.You are reporting of trends in pH in that order of magnitude.In summary, a lot of confusing calculations are presented to come up with an adjustment (DeltapHc) that, according to the authors, has "a very small weight in pHc" and to me seems to be poorly justified scientifically in the first place.We hope to have cleared this up.
In the Result section I found it interesting that cLSW has experienced large acidification rates in spite of low ventilation the last decade or so.It would be interesting to read a few lines of thought on why that is.Besides the newly introduced comparison between our results and those from time series stations (ESTOC and BATS), there are several spots in the revised version where this aspect you highlight is discussed more fully (third paragragh on the discussion section): "…The pH SWS25-BA decrease of the layers cLSW, uNADW and DSOW (Irminger basin), and SPMW and uNADW (Iceland basin) do follow the expected trend due to C ant entry.However, there are some deviations from this pattern in the rest of the considered water masses.
In the layers of uLSW (Irminger and Iceland basins) and cLSW (Iceland basin) there is a component (∼50%) of the observed acidification trends that is not explained by the uptake of C ant and is attributed organic matter remineralization.The SAIW layer in the Irminger basin presents an intermediate case compared to the previous ones: ~75% of the pH SWS25-BA decrease comes from the influence of C ant .In contrast with the observed in the Irminger basin, in the upper layer of the ENA basin the acidification due to the increase of C ant is partially compensated by the increase in ventilation (and, consequently, higher CO 2 removal via the enhanced photosynthetic activity) of the eastern NACW (ENACW) that produces lower acidification rates than expected...".It seems to suggest that the "acidification" due to respiration is more important than the uptake of Cant from the surface.In many instances these two effects are confused in the manuscript, and in the end I wonder if a fast ventilated water mass should be more affected by acidification, or a water mass whose ventilation rate has decreased.It the end it is a matter of balance between basically C ant input, organic matter remineralization and photosynthetic activity.Such balance is specific to each water mass, and this is another of the reasons why we decided to opt following the main water masses in the NASPG over the years.For instance, in the beginning of the discussion the authors state that " pH normally decreases with depth" (presumable due to respiration of organic matter -correct), and a few lines later they state that "surface water with lower average pH" is injected to depth explaining a mid-depth minima in pH.This was poorly expressed.This is how it is written now (first paragraph on the Results section): "…The general pattern of pH SWS25 follows the natural distribution expected, with higher pH values at the surface and lower pH in deep waters: The high values of pH SWS25 above the seasonal thermocline, in the photic layer (uppermost ∼400 m), respond to the photosynthetic activity of primary producers that withdraw dissolved CO 2 from seawater.The deep and less ventilated NADW has low pH SWS25 .The NADW is located generally below 2500 dbar (σ 2 >37.00 kg m -3 ; Fig. 1b) mainly in the deep ENA basin and shows weak signs of acidification over the last two decades, although there exist slight differences between the upper and lower NADW branches (uNADW and lNADW).The branch of uNADW that spreads westward into the Iceland basin mixes with LSW (Yashayaev etl al., 2008) forming a pH gradient that shows decreasing pH values over time.The influence of LSW in the uNADW is also revealed by the imprint of LSW in the AOU and Si(OH) 4 values of the uNADW, which are lower those in the lNADW layer (Table 2c).In the Irminger basin the decreasing trends of pH SWS25 are clearly visible in the most recently ventilated waters like the uLSW and DSOW (Fig. 2).The latter shows low pH SWS25 in 2004 and 2008 and higher values in 2006 due to the different NAO conditions (Pérez et al., 2010).The most evident sign of acidification is detected between 1000 and 2000 meters depth, where the volume of water with pH values below 7.725 thickens over time.".Similarly, on page 3016 the authors state "The hampered ventilation from increased surface ocean stratification is expected to bring about a decrease in dissolved oxygen concentrations and pH levels, amongst other things because Cant would not be as effectively transported toward the ocean interior via deep convection and water mass formation processes (Perez et al., 2010).".Why should pH decrease if LESS Cant is injected?Because surface stratification would facilitate the accumulation of more organic matter in the upper layers, and its oxidation would cause a drop of pH due to degradation of organic matter.There are basically three main components that affect the acidity of water masses: mixing with waters of different pH; synthesis-respiration of organic matter; and the influence of C ant .The first two factors are the main causes of the natural variability of pH, while the C ant influence belongs to the human induced variability.On the revised version we explore further the different contributions of these components to the observed acidification.As stated above, the C ant storage rates provided by Pérez et al. 2010 have been used on the revised manuscript version to calculate the expected acidification rates due to human activities and compare these expected rates with our observation-based results, where no separation between the anthropogenic and natural acidification signals has been done (see the second paragraph in Section 4, "Discussion").This way, it is possible to evaluate how much of the observed acidification in the different water masses is human-induced and how much is due to natural variability.This is what's on the third paragraph of the Discussion section in the revised manuscript: "…The pH SWS25-BA decrease of the layers cLSW, uNADW and DSOW (Irminger basin), and SPMW and uNADW (Iceland basin) do follow the expected trend due to C ant entry.However, there are some deviations from this pattern in the rest of the considered water masses.In the layers of uLSW (Irminger and Iceland basins) and cLSW (Iceland basin) there is a component (∼50%) of the observed acidification trends that is not explained by the uptake of C ant and is attributed organic matter remineralization.The SAIW layer in the Irminger basin presents an intermediate case compared to the previous ones: ~75% of the pH SWS25-BA decrease comes from the influence of C ant .In contrast with the observed in the Irminger basin, in the upper layer of the ENA basin the acidification due to the increase of C ant is partially compensated by the increase in ventilation (and, consequently, higher CO 2 removal via the enhanced photosynthetic activity) of the eastern NACW (ENACW) that produces lower acidification rates than expected.".I don't question the statement, but the driver of the change has to something else, such as respiration.
Prediction of future pH values: Since such a big part of the discussion is devoted to this subject, why is the figure contained in the supplementary material?Thank you for this remark.We have done as you suggested and now Fig. S1 is an integral part of the main text (new Fig. 4).We have also deleted the references to the two CO 2 emission scenarios we mentioned to avoid complicating the main message.Our projections are based on xCO 2 values, not on time, or on how fast those xCO 2 values are reached (that depends on the emission rate / scenario).The reason we included the emission scenarios was to provide some context, but this seemed to blur the main message, so we decided to omit these references.Is a decrease in pH for cLSW of 0.45 units consistent with the thermodynamics of the carbonate system for a pCO2 of 775 ppm?Considering only the thermodynamics in such hypothetical (future) acidification process would not be appropriate, more so in the case of cLSW in the Iceland basin, where according to our results only 50% of the observed acidification rate is caused by C ant entry.However, this case still seems to be consistent with the thermodynamics: if one calculates the expected C T that cLSW would have under an atmosphere with 775 ppm of xCO 2 , then one obtains an increase of about 100 µmol kg -1 in C T with respect to the average C T of this water for the 2000s (∼2150 µmol kg -1 ).That increase in C T can be expected to reduce pH by roughly 0.20 units (and this would be only the anthropogenic component of the acidification, i.e., about 50% according to our results).Can it be that the high trend for cLSW "acidification" is a result of low ventilation recently, and that if the cLSW becomes better ventilated in the future, waters with higher pH will dominate this water mass, so that your linear trends are wrong?One of the advantages of the time period studied here is that, in terms of NAO-driven ventilation (more ventilation during high-NAO index years, and vice versa; Pérez et al. 2010), it has undergone several different situations, and since our linear acidification trends come directly from the observations during those climatic conditions, we can have a high degree of confidence that the obtained trends (which turned out to be linear) are representative of different water mass ventilation scenarios.
Adding to this, we provide the following evidence: In the early 90s (1989)(1990)(1991)(1992)(1993)(1994)(1995) the 5-year mean ± standard deviation of this index was 3.3±0.8indicating a high phase of the NAO.A low NAO phase period followed during the years 2002-2006, when the index value dropped to −0.1±0.6.Year 1996 is characterized by negative NAO, and 1997 to 2000 by moderate positive NAO.However, in spite of the variations of the NAO index during the study period, NAO was close to neutral (above and below, i.e., positive and negative indexes), and for our purposes the important point is exactly that: "…the fact that the NAO phase was close to neutral both in the 1980s and 2000s should minimise potential biases in the proposed linear projections of pH, which are based on observations from the results here obtained (Fig. 3).", as argued in the text.It seems to me that the high level of pH decrease in cLSW is due to respiration, which can change if the ventilation increases again.As mentioned in the above comment, acidification is a generally the result of the combined effects of C ant entry, water mass mixing and of organic matter respiration.According to our analysis, in the case of cLSW it seems that the observed acidification is mostly due to the influence of C ant , i.e., human-induced.

Minor / technical comments:
GENERAL COMMENT: Given that the manuscript has changed substantially (motivated by the reviewers' comments), a lot of these comments no longer apply since the sentences to which they refer have been fully removed or changed.However, we will answer all of them here to make easier for the reviewer keeping track of all of the changes in the new version (attached at the end of this reply letter).
• Page 3004, line 26: What is excess anthropogenic CO2".When referring to a percentage, make very sure if you talk about fossil fuel emissions only, or if you include land use change, and also for which time frame you are presenting these numbers.They are all different.You are right.We meant simply anthropogenic CO 2 , i.e., all CO 2 derived from human activities since the Industrial Revolution.The sentence has now changed: "Roughly 20-35% of the anthropogenic CO 2 (C ant ) emissions are absorbed by the oceans (Khatiwala et al., 2009) mitigating the global warming.".
• Page 3007, line 17.Do the authors mean "higher" precision rather than "lower"?Thanks.We meant higher precisions (better measurements).The sentence now is as follows: "…Periodical checks of pH measurement precision with Certified Reference Material (CRM) during the FOUREX and OVIDE cruises indicated a precision better than the 0.002 pH units reported by Clayton and Byrne (1993) and Millero (2007).".
• Page 3008, line 18: What is "timely date"?We meant "convenient", because those two cruises were conducted in 1991 and filled the time gap between the TYRO (1990) and the OACES (1993).The sentence has been modified and now is as follows: "…The AR7E and A01E cruises (Fig. 1a) had comprehensive amount of C T measurements yet very few potentiometric A T data.Given the scarcity of A T data, the equation A T =S/35•(2294.7+1.37 [Si(OH) 4 ]) (R 2 =0.97; [Si(OH) 4 ] refers to silicate concentration) given by Pérez et al. (2010) was applied to the AR7E and A01E datasets to generate A T values at the sampling depths of measured C T .The pH was then calculated from C T and A T data as mentioned above.".
• The normalization of alkalinity on page 3008 assumes a zero intercept, which has been shown by (Friis et al., 2003)  • Page 3009, line 3: Please avoid words like "exceptionally".Done.The authors seem to forget the southern ocean is the region where most of the Cant is taken up.Please note that this entire sub-section has been removed in the revised version.However you are right.
When one looks at the C ant budgets (the balance between atmospheric uptake, lateral advection and storage of C ant ) of the major ocean basins, the Southern Ocean is where most C ant is taken up, but in the NASPG is where most C ant is stored (Khatiwala et al., 2009;Sabine et al., 2004).
• Page 3010, line 10: AOU does not "accurately" trace ventilation.You are right.This whole paragraph has been removed in the revised version of Section 2.2.2 (now 2.2.1).Strictly AOU traces respiration, and an assumption of constant respiration has to be made to convert AOU into ventilation.You are right.
• Page 3011, line 8. "Selected meteorological stations".State which ones.We replaced "selected" by "NASPG".We used the stations located in our study region (the NASPG), which can be easily identified on the map that's on the home page of the NOAA Carbon Cycle Greenhouse Gas group (provided on the text).The sentence now is as follows: "…The xCO 2 atm records were obtained from time series from meteorological stations in the NASPG (Storhofdi (Iceland); CIBA (Spain); Mace Head (Ireland); Ocean Station C (U.S.); Pico-Azores (Portugal); and Terceira Island-Azores (Portugal)), that are part of the global cooperative air-sampling network managed and operated by the National Oceanic and Atmospheric Administration (NOAA) Carbon Cycle Greenhouse Gas group (http://www.esrl.noaa.gov/gmd/ccgg/flask.html)…".Why you not use the well-known average CO2 concentration for the northern hemisphere.We wanted these data to be as local and accurate as possible.
• Table 1: The abbreviation "n.a." is often used for "not available", not for "no adjustment" as presented here.I suggest use "0" for no adjustment, and "n.a." for those cases where this parameter was not measured, or considered due to low quality.Done.
• Page 3012, line 15:"more acidic"??The pH is about 7.7 in this water mass.This is well above neutral pH.More correct would be "less basic".You are right.Both ways of putting it are in fact right."Less basic" is certainly more appropriate if one takes pH=7 as the reference for neutral pH, which is true for pure distilled water at 25 ºC and 1 atm.However if we consider as the reference or "baseline" the average ocean pH, which is around 8.1, then the "more acidic" tag is actually quite appropriate.This applies to a number of instances in this paper, for instance a few lines down on the same page; "NADW is natural acidic" -that is simply not true.
• Figure 2. Is it necessary to present pH sections for all the OVIDE lines?The advantage of the OVIDE section in this case is that it follows the very same track (which, on the other hand, gives representative coverage of the NASPG) over almost a decade, which is quite convenient in the context of the main objective in our study: to study the evolution of pH in the main water masses of the NASPG.You are hard pressed to visually see any differences, and you still don't know if differences you might see (such as in the deep Irminger Basin are due to variability in water masses rather than acidification.The new version of Fig. 2 now includes the isopycnals that delimit the studied water masses (also presented in Fig. 1b, in new Table 2 and in the main text).We hope this facilitates the visual follow of pH evolution, as you suggest.Also, Fig. 3 condenses all of that information in fewer points to give precisely that, a quantitative approximation to acidification rates.The difference between 1991 and 2008 is striking though.Yes indeed!
• Page 3014, line 5: The sentence starting with "Any of. .." is unclear.What is the "maximum acidification rates achievable during 1981 to 2008".The sentence has been removed.FYI, it referred to the expected acidification rate that the water mass would have had due to C ant entry and to its natural variability.
• Page 3014, lne 8: Iberian basin? Figure S1: Normally is the Omega value not given as percent.You are right, but we decided to express it as a percentage to make it easier for non-specialist readers to follow.Also, please notice that former Fig. S1 is now an integral part of the paper (new Fig. 4), since a good deal of the discussion section was based on it.
• Table 1: Which version of the TTO data is used?There is an updated version of the TTO data available, see (Tanhua and Wallace, 2005).We used the updated version that is publicly available in the CARINA dataset site: http://store.pangaea.de/Projects/CARBOOCEAN/carina/index.htm.That version of the TTO data was provided to the CARINA administrators / coordinators by the corresponding authors / P.I. of the cruise.
• Figure 3.In a recent publication by one of the co-authors of this study, the trend (in that case of Cant) was better fitted vs. the atmospheric perturbation of CO2 than vs. time.Why is that not done in this paper?That is what we did in former Fig. S1 (now Fig. 4).We have also deleted the references to the two CO 2 emission scenarios we mentioned, to avoid complicating the main message.Our projections are indeed based on xCO 2 values, not on time, or on how fast those xCO 2 values are reached (that depends on the emission rate / scenario).The reason we included the emission scenarios in the former version was to provide some context, but this seemed to blur the main message, so we decided to delete these references.It would at least provide some confidence check on the results, i.e. if the trends extrapolated to zero atmospheric perturbation is very different from a deltapH of zero, there might be a problem.Please, notice that the "baseline" (zero atmospheric perturbation) for such hypothetical "confidence check" would correspond to 280 ppm of xCO 2 , not zero.Also, the y-axis in our graph shows pH, not a ∆pH, so the y-axis intercept would not be zero.
It would be rather the preindustrial value of pH for cLSW and SPMW in the Iceland basin.

Introduction
The ocean acidification due to the increasing atmospheric CO 2 is well known (Doney et al., 2009;Raven et al., 2005) but the direct pH observations are sparse (Byrne et al., 2010;Tittensor et al., 2010;Wootton et al., 2008).Roughly 20-35% of the anthropogenic CO 2 (C ant ) emissions are absorbed by the oceans (Khatiwala et al., 2009) mitigating the global warming.Since the beginning of the Industrial Revolution the sea-surface has seen a 30% increase in hydrogen ion concentrations and Wickett, 2005;Raven et al., 2005).The current acidification episode is occurring ∼100 times faster than any other acidity change in the last 50 million years of Earth's history (Pelejero et al., 2010), and is thought to be the onset for a number of cascading effects throughout marine ecosystems that may leave no time for adaptation of many organisms (Feely et al., 2008;Doney et al., 2009).Ocean acidification causes a combination of contrasted impacts on the marine environment (Doney et al., 2009), from reproductive larval survivorship and growth-related issues in several taxa to the reduction of seawater's sound absorption coefficient (Ilyina et al., 2009).
The North Atlantic Subpolar Gyre (NASPG) is an important area of mode waters formation.
These waters formed in deep winter mixed layers are identified by nearly uniform properties in the vertical near the top of the permanent pycnocline (Thierry et al., 2008).The process of transformation of the warm, saline subtropical waters into intermediate and deep waters in the NASPG (McCartney and Talley, 1982;Read, 2001) results in several varieties of Subpolar Mode Water (SPMW) distributed around the gyre.The Labrador Sea Water (LSW), the densest variety of SPMW, is one of the thickest water masses in the NA and one of the main components of the lower limb of the Meridional Overturning Circulation (Thierry et al., 2008).The LSW has high contents of chlorofluorocarbons (CFCs) and anthropogenic carbon due to the ventilation processes (Azetsu-Scott et al., 2003;Pérez et al., 2010).Thus, it is expected that those water masses will suffer changes in [H + ].
There are relatively few places where the carbon system has been surveyed thoroughly enough to generate a comprehensive database that can be used in the assessment of ocean acidification and its environmental impacts (Wootton et al., 2008).Several past and future pH projections have been proposed from Ocean General Circulation Models (GCMs) and model data (Orr et al., 2005), but in situ measurements documenting the evolution of ocean pH over time are limited (Wootton et al., 2008).The present work examines the temporal variability of pH in the main water masses of the North Atlantic from direct observations.Here we have gathered the available high-quality carbon system data covering the NASPG between 1981 and 2008 (Fig. 1a) to study the decadal acidification rates of the main North Atlantic water masses (Fig. 1b) during that time period.

Dataset
A total of fourteen cruises with high-quality carbon system measurements were selected to follow the temporal evolution of pH in the North Atlantic (Fig. Over time, different analytical procedures were used to measure pH and so different adjustments and corrections were applied to the raw data to create the pH dataset used in this study (Table 1).The pH measurements in the database were determined either potentiometrically (using pH electrodes; Dickson, 1993) or, more commonly, with a spectrophotometric method that used mcresol purple as a pH indicator in either scanning or diode array spectrophotometers (Clayton and Byrne, 1993).The spectrophotometric pH determination has typical reported precision limits of 0.002 pH units (Clayton and Byrne, 1993;Millero, 2007).Periodical checks of pH measurement precision with Certified Reference Material (CRM) during the FOUREX and OVIDE cruises indicated a precision better than the 0.002 pH units reported by Clayton and Byrne (1993) and Millero (2007).All pH measurements that had not been originally reported in the seawater scale (pH SWS ;Millero, 2007) were converted to it from either the total or the NBS pH scale using the corresponding acid dissociation constants (Dickson and Millero, 1987), following the CARINA database second quality control recommendations for pH data scale unification and cruise adjustments (Velo et al., 2010).The acid dissociation constants for HF or HSO 4 - (Millero, 2007) were used to convert pH values originally reported in the total scale (those measured spectrophotometrically; Table 1) to the SWS scale.The pHs measured potentiometrically were all reported on the NBS scale and were converted to the SWS scale as specified in Pérez and Fraga (1987).Some of the cruises listed in Table 1 did not perform direct pH measurements but obtained total alkalinity (A T ) and dissolved inorganic carbon (C T ) data.In such cases the pH values were calculated in the SWS scale from A T and C T data using the thermodynamic equations of the carbon system (Dickson et al., 2007) and a set of CO 2 dissociation constants (Dickson and Millero, 1987).
The pH estimated accuracy ranges from ±0.002 to ±0.008 depending of the used methodology at each cruise (Table 1).
During the A16N cruise, pH was determined spectrophotometrically, but the spatial resolution was not as good as than for C T and A T , so we used pH values calculated from C T and A T for this cruise instead.The AR7E and A01E cruises (Fig. 1a) had comprehensive amount of C T measurements yet very few potentiometric A T data.Given the scarcity of A T data, the equation A T

pH data analysis
The dataset spans 27 years  with a wide spatial coverage of the study area (Fig. 1a; Table 1) that was divided in three basins: Irminger, Iceland and East North Atlantic (ENA).
These three basins and their geographical boundaries were defined by Pérez et al. (2010).So for the Irminger basin, the boundaries are defined by the main axis of the Reykjanes Ridge and the east coast of Greenland.The Iceland basin was defined as the region bounded between the Reykjanes Ridge axis and the line joining the Eriador Seamount and the Faroe Islands.The ENA basin extends south from Eriador-Faroe line over the Rockall through, the Porcupine bank, and the Biscay and Iberian basins (Fig. 1).
In order to evaluate the temporal variability of the pH in the water masses of the North Atlantic, the water column was divided in five layers by potential density (σ θ ) intervals for each region (Fig. 1b).To determine the isopycnals boundaries of the North Atlantic Deep Water

Basin normalization of average pH SWS25
The average pH SWS25 was obtained for each layer at each year and for the three basins, following the averaging and "basin-referencing" methodology that Pérez et al. (2008Pérez et al. ( , 2010) ) and Ríos et al. (2012) used for C ant .The spatial coverage of each year is variable and this can cause significant biases in the observed average layer properties in each year.These small differences can potentially introduce spatial biases in the average pH SWS25 due to different ventilation stages and rates of each water mass.Therefore, for each basin the pH SWS25 were normalized to better represent the pH SWS25 in each considered layer of the basin (Fig. 1) by adding a new term named as ΔpH SWS25- BA .This term represents the deviation of the pH SWS25 (average from cruise data) from the pH SWS25-BA basin average (BA).
The ΔpH SWS25-BA term was computed from cruise data and expressed as individual correction elements for each cruise and layer in the three basins as follows: ( ) Where "c" stands for "cruise" and subscript "i" denotes "property" (1=Si(OH) 4 ; 2=AOU; 3=θ; 4=S).The " !X i c " and " !X i WOA 05 " terms are the average magnitudes of the "ith" properties from direct observations along the cruise track and from WOA05 data in the whole basin, respectively (Table 2).The "a i " factors are the regression coefficients that were calculated in each basin for each layer from a multiple linear regression (MLR) fit (Equation 3) of the pH SWS25 averages vs. the averaged "i" properties using data from the fourteen cruises (Table 2).The obtained "a i " regression coefficients are listed in Table 3.
All terms and scripts in equation ( 3) have the same meaning as in equation ( 2).Also, the X i c terms for i=1 through 4 are the same as in equation ( 2).The same is true for the a i coefficients in equation ( 2).Actually, the purpose of equation ( 3) is obtaining those a i values to be used in equation ( 2).The X 5 = xCO 2 atm values used as input parameters in equation ( 3 3) in equation ( 3) is not used in equation ( 2) since the ΔpH SWS25-BA term should only include the effect of variables with spatial variation.Such xCO 2 atm terms are required when calculating the "a i " coefficients (equation 3, Table 3), since xCO 2 atm has co-variation with pH 25SWS .By including "a 5 " in equation ( 3) we remove from the rest of "a i " factors the transient influences that co-vary with pH SWS25 .Considering that pH varies with the time because of the xCO 2 change, the inclusion of this variable in the eq. 3 assures that coefficients of the other properties that change mostly spatially are more consistent than if the xCO 2 atm is not included.

Results
The vertical distributions of pH SWS25 measured along the section between the Iberian Peninsula and Greenland are shown in Figure 2, providing a first look at the evolution of pH over the last two decades.The general pattern of pH SWS25 follows the natural distribution expected, with higher pH values at the surface and lower pH in deep waters: The high values of pH SWS25 above the seasonal thermocline, in the photic layer (uppermost ∼400 m), respond to the photosynthetic activity of primary producers that withdraw dissolved CO 2 from seawater.The deep and less ventilated NADW has low pH SWS25 .The NADW is located generally below 2500 dbar (σ 2 >37.00 kg m -3 ; Fig. 1b) mainly in the deep ENA basin and shows weak signs of acidification over the last two decades, although there exist slight differences between the upper and lower NADW branches (uNADW and lNADW).The branch of uNADW that spreads westward into the Iceland basin mixes with LSW (Yashayaev etl al., 2008) forming a pH gradient that shows decreasing pH values over time.The influence of LSW in the uNADW is also revealed by the imprint of LSW in the AOU and Si(OH) 4 values of the uNADW, which are lower those in the lNADW layer (Table 2c).In the Irminger basin the decreasing trends of pH SWS25 are clearly visible in the most recently ventilated waters like the uLSW and DSOW (Fig. 2).The latter shows low pH SWS25 in 2004 and 2008 and higher values in 2006 due to the different NAO conditions (Pérez et al., 2010).The most evident sign of acidification is detected between 1000 and 2000 meters depth, where the volume of water with pH values below 7.725 thickens over time.
To estimate the acidification rates of the water masses we normalised the discrete in situ pH SWS25 data to basin-average conditions (pH SWS25-BA ), as described on section 2.2.The correction applied (ΔpH SWS25-BA ) is, on average, 0.003±0.009 in the studied region (Table 2).On average, the largest corrections correspond to the Irminger basin (0.007± 0.009), while in the Iceland and ENA basins they are smaller (0.003±0.009 and 0.002±0.010,respectively).In the Irminger basin no correction was applied to the uNADW and DSOW layers (Table 2a).The highest average corrections on this basin were applied to the uLSW (0.014±0.008) and cLSW (0.012±0.005) layers, and the highest individual correction (0.027±0.003) corresponds to the SAIW in 1997.The smallest average pH SWS25-BA corrections in the Iceland basin correspond to the uLSW (0.000 ± 0.003) and the largest to the SPMW layer (0.008 ± 0.014), to which also the highest individual correction was applied (0.003±0.005) corresponding to the 1991 A01E cruise.In the ENA basin the smallest average corrections correspond to LSW and NACW layers (0.0012±0.004 and 0.0045±0.004,respectively) and the largest to the MW (0.014±0.002),where the highest individual corrections were also applied (0.023±0.002), in 1998 and 2003, to the cruises conducted along 20ºW.In general, we can see a trend of decreasing pH over time for both pH SWS25 and pH 25SWS-BA in all basins and layers (Table 2).These decreasing pH SWS25 trends tend to be more pronounced in the Irminger and Iceland basins and less marked in the ENA basin (Table 2).The SAIW and uLSW layers in the the uLSW (-0.0017±0.00004yr -1 ) (both in the Irminger basin), and the SPMW (-0.0012±0.0002yr - 1 ) in the Iceland basin.The pH SWS25-BA of cLSW in the Iceland basin presents a remarkable average decrease of -0.0016±0.0002yr -1 , unlike in the Irminger and ENA basins (-0.00089±0.00004and -0.0008±0.0001yr -1 , respectively).The layer of uNADW shows decreasing pH SWS25-BA vs. time trends from the Irminger (-0.0010±0.0001yr -1 ) to the Iceland basin (-0.0008±0.0002yr -1 ) due to the influence of ISOW and to the mixing with LSW.Overall, the lNADW and uNADW in the ENA basin are the least acidified water masses over time, with low pH SWS25-BA vs. time slopes.These latter two regression fits are, in addition, statistically non-significant (both p-values >0.2) and show low pH-time correlation: 0.0002±0.0002yr -1 (R 2 =0.15; p-value = 0.57) and -0.0003±0.0001yr -1 (R 2 =0.28; p-value = 0.47) for lNADW and uNADW, respectively.The MW in the ENA basin showed a moderate acidification rate (-0.0006±0.0001yr -1 ) due to its known capacity for C ant drawdown by entrainment from surface layers (Ríos et al., 2001;Álvarez et al., 2005).

Discussion
The acidification of the upper layer NASPG waters here assessed from in situ pH measurements spanning the last three decades (1981 to 2008) shows very similar tendencies of pH decline to those observed in the time series stations ESTOC (29º10'N, 15º30'W) and BATS (31º43'N, 64º10'W), in the Subtropical Atlantic.At the Irminger basin, the observed values of pH SWS25-BA decrease rates for SAIW and uLSW are -0.0019±0.0002and -0.0017±0.0001yr -1 , respectively, similar to those obtained by Olafsson et al. (2009) for surface waters during the winter (0.0024 yr -1 ).The slight difference with the values reported by Olafsson et al. (2009) likely comes from the fact that the surface isopycnals here considered include thick layers of mode waters with lower interannual variations.The acidification rates here obtained for SAIW and uLSW in the Irminger basin are also comparable to those reported in the Subtropical North Atlantic at the ESTOC site in surface waters and in the mixed layer (-0.0017 yr -1 ) during the decade 1995-2004(Santana-Casiano et al., 2007;González-Dávila et al., 2010), and at the BATS site in surface waters (-0.0016 yr -1 ) from 1983 to 2011 (Bates et al., 2012).In the ENA basin, the pH SWS25-BA decreasing rate of the NACW (-0.0009 ± 0.0001 yr -1 ) is similar to the rates computed at the ESTOC site at 300 and 600 m (-0.0010 ± 0.0004 and -0.0008 ± 0.0003 yr -1 , respectively) for the decade 1995-2004(González-Dávila et al., 2010).At 3500 m, the pH SWS25-BA rate of decrease for lNADW here obtained (0.0002 ± 0.0002 yr -1 ) has a very low pH vs. time correlation coefficient (r 2 =0.15 ; Fig. 3c) and is therefore not significant, yet similar to that given by González-Dávila et al. ( 2010) (-0.0002±0.0002yr -1 ) for the same water mass between 1995 and 2004.On the contrary, at the layer where the MW spreads around 1000 m, González Dávila et al. ( 2010) reported a pH decreasing rate (-0.0008 ± 0.0003 yr -1 ) slightly higher (considering the associated uncertainties) than our pH rate (-0.0006±0.0001yr -1 ) for this water mass.The difference could be due to the way MW is defined in Ocean uptake and chemical equilibration of C ant with seawater results in a gradual reduction of seawater pH and saturation rates (Ω) for calcium carbonate (CaCO 3 ) minerals in a process termed ocean acidification (Bates et al., 2012).However, other contributions to these pH reductions such as ventilation of the water masses or remineralization of organic matter exist.We have checked if the here obtained pH SWS25-BA decrease rates follow the expected trends due mainly to C ant uptake using the C ant rates given by Pérez et al. (2010).The necessary pHs to obtain such rates were calculated using the expression !pH / !t , where !pH / !t ( ) ANT is the expected variation over time of human-induced pH (due to C ant ); !C ANT / !t ( ) is the corresponding C ant storage rate (from Pérez et al., 2010); and !pH / !C T ( ) (S, A T ) is the variation with respect to C T of the pH calculated from the thermodynamic equations of the marine inorganic carbon system (as described in section 2), using the available A T data and salinity measurements.
The pH SWS25-BA decrease of the layers cLSW, uNADW and DSOW (Irminger basin), and SPMW and uNADW (Iceland basin) do follow the expected trend due to C ant entry.However, there are some deviations from this pattern in the rest of the considered water masses.In the layers of uLSW (Irminger and Iceland basins) and cLSW (Iceland basin) there is a component (∼50%) of the observed acidification trends that is not explained by the uptake of C ant and is attributed organic matter remineralization.The SAIW layer in the Irminger basin presents an intermediate case compared to the previous ones: ~75% of the pH SWS25-BA decrease comes from the influence of C ant .
In contrast with the observed in the Irminger basin, in the upper layer of the ENA basin the acidification due to the increase of C ant is partially compensated by the increase in ventilation (and, consequently, higher CO 2 removal via the enhanced photosynthetic activity) of the eastern NACW (ENACW) that produces lower acidification rates than expected.
From our set of pH SWS25-BA observations we have made projections of future pH levels (Fig. 4).The Iceland basin is particularly suitable for extrapolating the pH trends from Fig. 3b into the future given the good coverage of measurements available in this region, as this would confer added robustness to the projected acidification trends.The SPMW and cLSW are selected for such projections, because they are some of the most susceptible of the considered water masses to human-induced acidification and also have strong pH vs time fits (Fig. 3b).The projections are calculated under the assumption that the observed acidification trends shown in Fig. 3 and the ocean's general circulation for the rest of the 21 st century remain similar to those observed during the last three decades.
The strength and phase of the NAO index affect water mass ventilation and C ant uptake rates (Pérez et al., 2010).However, the fact that the NAO phase was close to neutral both in the 1980s and 2000s should minimise potential biases in the proposed linear projections of pH, which are based on observations from the results here obtained (Fig. 3).Although such linear extrapolation is not constrained, several works have demonstrated that the decline of carbon system parameters like ] is almost linear for predictions made between 2000 and 2050 (Zeebe and Wolf-Gladrow, 2001;Hauck et al., 2010).The buffering effect of carbonate minerals and biogenic CaCO 3 dissolution can be disregarded since these processes tend to occur in deep waters over timescales that are at least one order of magnitude larger than the one here considered.We therefore assume analogous pH evolution to the one here observed in surface (SPMW) and intermediate (cLSW waters on decadal timescales (our observational time span), which is the time frame in which the atmospheric CO 2 concentration range considered in Fig. 4 is expected to be reached under a business-as-usual CO 2 emission scenario.
Concerning the assumption of general circulation there is the caveat that the increased stratification of surface layers expected in the future (Friedlingstein and Prentice, 2010) can hamper water mass ventilation processes and potentially bring about a decrease of pH (acidification), because C ant would not be as effectively transported toward the ocean interior via deep convection and water mass formation processes (Pérez et al., 2010).Therefore, if such increased stratification According to the obtained pH projections in Fig. 4, the pH of surface waters in the Iceland basin could drop ∼0.35 units with respect to the pre-industrial era by the time atmospheric CO 2 reaches 800 ppm, which is consistent with outputs from coupled climate/carbon-cycle models (Caldeira and Wickett, 2005;Orr et al., 2005).In the case of cLSW, the linear projection predicts a pH decrease of more than 0.45 units with respect to pre-industrial pH values by the time atmospheric xCO 2 reaches ∼775 ppm (about twice the present atmospheric CO 2 concentration).This result is 0.25 pH units lower than the values predicted by the well-known climate-carbon coupled model in Caldeira and Wickett (2003) for the same xCO 2 and ocean region.The difference between our observation-based prediction and the latter model (Caldeira and Wickett, 2003) could be due to the fact that our data is largely extrapolated and also that it is still difficult for General Circulation Models (GCMs) to model accurately the Meridional Overturning Circulation (MOC), its NAOrelated variability (Danabasoglu et al., 2012) and the deep winter convection of the NASPG.The NAO-related MOC variability has a strong influence on C ant storage in the NASPG (Pérez et al., 2010) and it is therefore expected that this will affect the long-term variability of pH too, in a way models cannot quite account for yet.In this sense, our results are a good complement to model outputs.On the other hand, it has also been reported that ocean acidification might be proceeding more rapidly than models have predicted (Wootton et al., 2008), as the contemporary CO 2 emissions are actually exceeding future scenarios based on business-as-usual emission rates (Canadell et al., 2007;Raupach et al., 2007).Such reports are consistent with the lower pH predictions we obtained compared to Caldeira and Wickett (2003).
The aragonite saturation state is defined as is the apparent solubility product of aragonite (Mucci, 1983).Because [ Ca 2+ ] is highly and positively correlated with salinity, Ω arag is largely determined by variations in [ CO 3 2-].This characteristic makes Ω arag an optimum indicator for environmental availability of dissolved carbonate ions.
From the measured pH data and our pH projections (Fig. 4) we calculated the Ω arag of the SPMW and cLSW in the Iceland basin for atmospheric xCO 2 values of 380 (present day), 500 and 750 ppm (see insets in Fig. 4).The results suggest that cLSW would actually reach aragonite undersaturation (Ω arag <1) by the time atmospheric CO 2 reaches ∼550 ppm and not 900 ppm, as suggested by the model predictions in Orr et al., 2005.The high-NAO enhanced ventilation that occurred towards the mid-late 1980s fostered the fast formation of a massive cLSW vintage (Kieke et al., 2007;Yashayaev et al., 2008).The rapid subduction of this newly formed cLSW injected C ant from surface to intermediate waters, transporting C ant much faster than via downward diffusion alone, thus causing a faster acidification rate in the cLSW (where organic matter remineralization also contributes significantly to the pH lowering) than in the SPMW, where C ant influence is the main contributor to acidification.Depending on the future CO 2 emission rates the 550 ppm threshold at which, according to our projections, cLSW would face aragonite undersaturation, could be trespassed in 2050, or before (Nakicenovic et al., 2000;Caldeira and Wickett, 2005;Feely et al., 2009).Guinotte et al. (2006) have in fact pointed out that some deep-sea cold-water corals may experience undersaturated waters as early as 2020 under an IPCC "business-as-usual" CO 2 emission pathway, which is in good agreement with our observation-based results for the Iceland and Irminger basins.
The data analysis also showed that the aragonite saturation depth (or lysocline = isopleth where Ω arag = 1) has shoaled at a rate of 7 and 4 m yr -1 between 1981 and 2008 in the Irminger and Iceland basins, respectively.The latter is in agreement with previous local studies (Olafsson et al., 2009).The fast rate of lysocline shoaling in the Irminger basin is promoted by the intense NAOenhanced deep convection that injects ventilated, CO 2 -rich waters into deeper layers (Messias et al., 2008), as mentioned previously.For comparison sake, the shoaling rates of the lysocline were estimated to be ∼0.2 m yr -1 during the Paleocene-Eocene Thermal Maximum (55 million years ago), when a massive natural release of CO 2 into the atmosphere caused global temperatures to raise more than 5 ºC in less than 10,000 years (Pelejero et al., 2010).

Conclusions
The

Table captions:
Table 1 List of selected North Atlantic cruises (Fig. 1a).Acronyms denote: P.I.= principal investigator; S= variable measured with spectrophotometric techniques; P= variable measured with potentiometric techniques; Calc= pH calculated from C T and A T using the thermodynamic equations of the carbon system (Dickson et al., 2007) and a set of carbon dioxide dissociation constants (Dickson and Millero, 1987).Uncert.= Analytical uncertainties of spectrophotometric, potentiometric, and calculated pH.Adjustments from a posteriori crossover analysis are listed in µmol kg -1 for C T and A T .
Table 2 Temporal evolution   Table 3 List of coefficients obtained for equation (1) using the expression in equation ( 2) in each water mass and basin.Between brackets are the properties associated to each "a i " coefficient and the corresponding units.All "a i " coefficients have been scaled up by a factor of 10 3 , except for the salinity ones ("a 5 ").The "n.s." ("not significant") variables explained very little of the pH variability and weakened the overall MLR fit so they were therefore rejected according to a stepwise method of MLR solving."c", "u" and "l" denote "classical", "upper" and "lower", respectively.(1) C T analysed with SOMMA (Johnson et al., 1993) and calibrated with CRMs, except in 1981 TTO cruise that was determined potentiometrically (Bradshaw et al., 1981) without CRMs.Analytical accuracy ±2 µmol•kg -1 .

(
NADW) we followedLherminier et al. (2010), who established different layers by potential density intervals on the basis of the hydrographic properties and circulation of the different water masses along the OVIDE section.They discriminate between the two components of NADW: the lower NADW (lNADW) spreading from the bottom to σ θ = 45.84 kg m -3 and the upper NADW (uNADW) spreading in the density range 36.94 < σ θ < 45.84 kg m -3 .We took the density range 37 < σ θ < 45.84 kg m -3 , which is almost identical, because the isopycnal σ θ = 37 seemed to delimit better the deepest boundary of the cLSW core (coincident with the uNADW upper density limit) in the Iceland and ENA basins.For the spreading of LSW in the ENA basin, the density range selected (32.35 < σ θ < 37 kg m -3 ) is very close to theLherminier et al. (2010).FollowingRíos et al. (1992) the Mediterranean Water (MW) layer is delimited by 27.2 < σ θ < 32.35 kg m -3 and the North Atlantic Central Water (NACW) layer is established from surface to σ θ < 32.35 kg m -3 according to the spreading of these water masses in the zone.For the Irminger and Iceland basins, the potential density limits were established followingKieke et al. (2007) andYashayaev et al. (2008).So, for the Iceland basin the layer of Sub Polar Mode Water (SPMW) is found between 100 m and σ θ = 27.6 kg m -3 .The upper and classical LSW (uLSW and cLSW) spread in the density ranges of 27.68 < σ θ < 27.76 kg m −3 and 27.76 < σ θ < 27.81 kg m −3 , respectively.For the Irminger basin the Sub Arctic Intermediate Water (SAIW) spreads from 100 m to 27.68 kg m −3 , the uLSW and cLSW are found between 27.68 < σ θ < 27.76 kgm −3 ; between 27.76 < σ θ < 27.81 kgm −3 , respectively.The North Atlantic Deep Water (NADW, which includes the ISOW contributions) is delimited by 27.81 < σ θ < 27.88 kg m −3 , and the Denmark Strait Overflow Water (DSOW) by σ θ >27.88 kg m −3 (Fig.1b).
) are the averages for the year of the corresponding cruise "c".The xCO 2 atm records were obtained from time series from meteorological stations in the NASPG (Storhofdi (Iceland); CIBA (Spain); Mace Head (Ireland); Ocean Station C (U.S.); Pico-Azores (Portugal); and Terceira Island-Azores (Portugal)), that are part of the global cooperative air-sampling network managed and operated by the National Oceanic and Atmospheric Administration (NOAA) Carbon Cycle Greenhouse Gas group (http://www.esrl.noaa.gov/gmd/ccgg/flask.html).The a 5 term associated with the xCO 2 atm variable (Table Irminger basin show a strong decreasing pH SWS25 trends in the period 1981 to 1997 (positive NAO index) and less pronounced ones from 2002 to 2008.In the deepest layers (cLSW, uNADW and DSOW) the pH SWS25 trends are lower although there is also a minimum value in 1997 when the NAO phase changes from positive to neutral/negative.Similar pH SWS25 trends are observed in the Iceland basin with a noticeable decrease from 1981 to 1997 during the high NAO followed of a slow decreasing pH SWS25 values.Differently, in the ENA basin the lowering pH SWS25 shows a more continuous trend with a maximum during 1981 and the minimum in 2006 in the NACW and LSW layers.Also at the ENA basin, the uNADW and lNADW show rather constant pH SWS25 values, with no clear trends.The pH SWS25 signal in the MW layer is noisier due to the important variations in salinity caused by the mixing between MW and other water masses, and as a consequence of the change in cruise tracks throughout the considered time period.The evolution of the average pH SWS25-BA between 1981 and 2008 in each layer and basin is plotted in Figure 3.The error bars on the graph represent the error of the mean and the uncertainty due to the normalization of the data.The general pattern is that the acidification rates tend to decrease with depth in all basins.The lowest slopes are found in the ENA basin, and the fastest acidification rates correspond to recently ventilated waters like the SAIW (-0.0019±0.0001yr -1 ) and our work compared to González-Dávila et al. (2010), were they consider MW as the mix of at least three different water types (including MW, Antarctic Intermediate Water and NACW) at the east North Atlantic (González-Dávila et al., 2010).
prediction holds true in the future, assuming a steady state for the general circulation can potentially cause overestimates in the pH values of the linear projections for surface and intermediate waters from Fig.4.Nevertheless, this process of slowing acidification due to less C ant entry could be counterbalanced by the increased remineralization of organic matter in the upper and intermediate ocean layers that would develop in a scenario of increased stratification.
progressive acidification of North Atlantic waters has been assessed from direct observations of pH spanning the last three decades.The increasing atmospheric CO 2 concentrations have largely affected the pH of surface and intermediate waters in the three studied North Atlantic regions, at varying extents.Most importantly, the LSW has shown very high acidification rates that are amongst the highest in the NASPG.In the Irminger basin, the acidification rate of cLSW responds to that expected from the influence of C ant , while in the Iceland basin only about 50% of the observed pH change in the cLSW is anthropic.The SAIW has the fastest acidification rate observed (-0.0019±0.0002yr -1 ), and 75% of this pH decrease is anthropogenic.In contrast, the C ant contribution to the acidification rates in the ENACW is partially compensated by the ventilation of this water mass thus explaining the moderate acidification rates observed in the upper layers of the ENA basin (compared to the Iceland and Irminger basins).Predictions from an observation-based extrapolation of the current acidification trends and rates are in agreement with model results(Caldeira and Wickett, 2005;Orr et al., 2005) in surface layers.However, our results indicate that the intermediate waters of the North Atlantic (LSW in particular) are getting acidified more rapidly than what some models predicted.

Figure 1
Figure 1 Fig. 1a shows the study area and selected cruises.The black straight lines delimit the

Figure 2
Figure 2 Evolution of measured pH SWS25 distributions in the NASPG from 1991 to 2008.The transect distances (km) from the southernmost tip of Greenland towards the ENA basin

Figure 3
Figure 3 Trends and rates of acidification between 1981 and 2008 of the studied water masses in

Figure 4
Figure 4 Extrapolation of the observed linear trends of acidification for the SPMW and cLSW in Pérez et al. 2010ggest changing this.The normalization to a common S=35 removes the A T -S co-variation, compensates for freshwater balance effects and brings all surface waters close to conditions in lower layers.You are right in that this normalization scheme ("zero alkalinity at zero salinity") comes with certain caveats (according toFriis et al. 2003).However, the recommendations of alternative alkalinity normalizations in the work byFriis et al. 2003refer to surface waters (upper 50 meter) rather than to the entire water column, which is where this equation was mostly applied in the case of the AR7E and A01E cruises, like inPérez et al. 2010, where they use this same A T expression (A T=S/35•(2294.7+1.37 [Si(OH)4 ])).

Table 1 .
Cruises and pH measurementsAdjustments