Surface ocean pH variations since 1689 CE and recent ocean acidification in the tropical South Pacific

Increasing atmospheric CO2 from man-made climate change is reducing surface ocean pH. Due to limited instrumental measurements and historical pH records in the world’s oceans, seawater pH variability at the decadal and centennial scale remains largely unknown and requires documentation. Here we present evidence of striking secular trends of decreasing pH since the late nineteenth century with pronounced interannual to decadal–interdecadal pH variability in the South Pacific Ocean from 1689 to 2011 CE. High-amplitude oceanic pH changes, likely related to atmospheric CO2 uptake and seawater dissolved inorganic carbon fluctuations, reveal a coupled relationship to sea surface temperature variations and highlight the marked influence of El Niño/Southern Oscillation and Interdecadal Pacific Oscillation. We suggest changing surface winds strength and zonal advection processes as the main drivers responsible for regional pH variability up to 1881 CE, followed by the prominent role of anthropogenic CO2 in accelerating the process of ocean acidification.

The data are very well presented and the paper is written in a logic way. Uncertainties of measurements are reported and appropriate tests for reproducibility of the Boron isotope proxy were added. Comparisons top other studies from the Pacific were added that help to constrain the finding of the present study.
To me the data and their interpretation are very convincing and of high interest to a broad readership. Records of ocean acidification are still scarce and this record adds an important dataset from a very unique ocean setting. The Supplement provides very important additional information that helps the reader to grasp the ideas of the manuscript.
I have a few minor suggestions that might improve the value of the paper. First of all, the authors show a comparison of the Nino3.4 index with the coral Boron record. What I would like to see more in detail is the actual signatures during major El Nino and La Nina years. What is the direction of change in Boron during a strong El Nino or La Nina? Is there a consistent relationship and does it follow the expected pH change during a El Nino or La Nina at New Caledonia? I can see a few prominent years or series of years where Boron shows a strong shift to more positive or negative values, does this coincide with a particular phase of ENSO? For instance 1790's and early 1800's, between 1890 and 1920, the latter a period of strong ENSO variability.
I also see anti-phasing between d18O and Boron, is it an indication for a link between mean SST and pH on interannual and decadal cycles?
The last remark is on the secular trend in Boron. There is a clear shift in Boron after the 1890's to lower values with almost 100 years before at high mean values, and again low Boron before 1800. While the recent shift towards the 20th century can be related to anthropogenic ocean acidification, the early period pre-1800 of low Boron or pH cant be. What could be the reason for the low Boron pre-1800? Is it the cooler mean SST at that time which led to naturally higher sink for CO2 in the SW Pacific? Normally colder water is more susceptible to CO2 uptake. Can the authors provide a mechanism or confirm SST as a potential link?
A last question js regarding the conversion of internal pH to seawater pH since I am not an expert in Boron isotope chemistry. Is the conversion chosen by the authors an accepted way of relating internal pH to seawater pH? I ask because we know by now that Boron gives us a valuable record of internal pH regulation by the coral. It means that the Boron helps us to decipher the calcification process of the coral driven by SST. How well do we know the offset between internal pH and seawater pH? The authors mention that they wanted to focus on relative changes in pH, yet a absolute value reconstruction is provided in Figs 3 and 5. Is that generally accepted in the Boron proxy community?
Overall, I recommend publication after addressing the comments.
Review: Acidification of the south Pacific surface ocean from anthropogenic CO2 uptake over the last 3 centuries Wu et al.
The authors present d11B, d13C and d18O data measured in coral core material representing 300 years of coral growth. These data are used to reconstruct respectively the pH, d13C of dissolved inorganic carbon and temperature of surface seawater at this South Pacific site. They find a decline in d11B (decrease in pH) since ~1880 that is accompanied by a decrease in d13C. Covariance between d11B and d13C suggest a common causal link between the drop in pH and decrease in surface ocean DIC; the incursion of anthropogenic carbon. Variability in these isotopic records shows a coupling to reconstructed sea surface temperature (d18O) and this variation occurs on a periodicity similar to that of the ENSO cycle. This suggests that changes in surface temperature and wind strength are key drivers of carbon uptake and seawater pH at this site.
I enjoyed reading this manuscript. Reconstructing surface water pH at high resolution, beyond current instrumental observations (going back only ~30yrs) is of clear importance to our understanding of the rates of ocean acidification and the impact on marine calcifiers. As such, the content should appeal to the wide readership of Nature Comms. I found many positives in the work that mean this has the potential to be a significant contribution to the field of coral geochemistry and ocean acidification research. These include: 1) The authors' choice of coral is a good one. Diploastrea heliopora grows at around half the rate of large Porites corals typically used for this purpose. This has allowed them to obtain an impressive 300-year sample section. Large polyps in this species are also beneficial during sampling. 2) The choice of a site that is currently a sink for atmospheric CO2 is a sound choice when trying to reconstruct past changes in OA 3) As far as I am aware, before now this species of coral had not been measured for is boron isotopic composition. These new d11B data demonstrate that pH upregulation is occurring in this taxon similar to other shallow water corals and that it too confers a degree of resilience to changes in external pH 4) The samples have been thoroughly checked for their preservation giving confidence to the primary nature of d11B data. 5) The weight of d11B data and 300 year duration of the record are impressive, particularly in comparison to previous coral d11B records (Fig 5) 6) Replicate overlapping sections of down core d11B are a nice addition and show close agreement. Again, this gives confidence that trends are not an artefact of sampling and chemically heterogeneous parts of the coral have been avoided. 7) The link of coral d11B to ENSO pacing is compelling and an important finding.
Unfortunately, parts of the discussion were unclear and I found the reasoning a little stretched in places, therefore I cannot recommend the paper for publication in its current form. I do however strongly urge the authors make some changes to the discussion and resubmit to Nature Comms. I do feel that these well-produced records are worthy of a place in a high impact journal for the reasons given above.

Joe Stewart
Points to address: Coral d11B here is used as a proxy for seawater pH, however, as stated in the manuscript, coral d11B actually records the pH within the coral. While there are examples where this internal pH is strongly affected by external pH it is important to point out that seawater pH is being inferred by proxy. I think therefore the wording in places is perhaps a little too strong (e.g. Line 194). There have been many examples now where external pH is far from the only factor controlling internal pH (e.g. Comeau et al. 2017, McCulloch et al.2016 While I like the use of a new species for this d11B-pH proxy work. This does present some challenges when converting d11B to seawater pH as there is no direct calibration of this species. On line 196 it is stated that "conversion based on several aragonite coral genera" however it looks like only one coral species (P. cylindrica) has been chosen from the McCulloch paper. A better argument would be that this is a conservative choice from the McCulloch calibrations available; one that gives the minimum observed pH change because of the slope. Or that this one has been chosen as it is another massive coral that occupies a similar niche to your species here (i.e. it makes sense that you would avoid the branching acropora calibrations).
Calculations of pH from d11B data appear correct based on the assumptions made here, however without a data table listing the isotopic results it if difficult to properly check this working. A lot of work has clearly gone into generating these data therefore I strongly encourage the authors to show these d11B, d13C and d18O results in a supplementary table.
Some coherency is shown between d18O and temperature, but this relationship (R2=0.4) is not that strong. The problem comes from calibrating across such a narrow range of temperatures (2 deg C). The δ18O-SST sensitivity of -0.25 ‰ per degree C that is used however sounds sensible based on studies in other carbonates. The coral geochem community has been slowly moving away from this proxy for the salinity complications mentioned in the manuscript. If the authors have Sr/Ca or Li/Mg temperature data for this site I would encourage them to share it, however I understand that generating these data will take more work that is perhaps beyond the scope of this study focusing on OA.
I have some concerns over the interpretation of the pH trends. The authors are keen to highlight newsworthy declines in pH (e.g. decrease in d11B observed since 1880), but there is little acknowledgement or discussion of the causes of large pH rises that are also documented in the down core record. For example, the rise in d11B up until 1880 is as large in magnitude as the decline in pH towards the modern that has been dubbed OA. What might be the cause of this elevation in pH at this site? Most of the drop in pH recorded in this core occurs in a large step around 1890. I notice from the core images that this is mighty close to a dark horizon in the core (just below the core break) that might represent a mortality event or an interval of reduced growth. It is difficult to see from the figures, but can the authors make a convincing case that this dark horizon is not influencing the d11B trends we see here? If it is a major stress event in the core, then the down core d11B could be interpreted as an increase up until 1880 then an interval of little pH change throughout the last century (see below)… …While I think the authors are probably correct in their interpretation of the d11B trends. I think it is important to remove the possibility that this upper core section may represent a different phase of coral growth. Similarly, the most recent 20 year decline in pH is noted (red arrow below), but this d11B fall is as large as the d11B rise from ~1975 to ~1990 (blue arrow). If OA is responsible for the downward trend over the last 20 years, what is responsible for the preceeding rise of equally "striking" magnitude and duration? Might this just suggest that ENSO modulation is dominant driver on these timescales? I am not sure of the answer to this myself, particularly as the red arrow is in excellent agreement with the recorded pH decline at the Hawaii HOTs site.
More minor points: Line58 with links Line59 timescales Line101 Diploastrea heliopora can be abbreviated to D. heliopora from here onwards (e.g. line112) Line167 Suggest change "observe" to "reconstruct" Line170 This reads as if the fractionation factor is pH dependent too. This needs rewording.
Line185: Suggest rewording here. Could perhaps be read as pH becoming higher. You mean a more consistent, homogeneous sampling protocol was achieved. Line206 Delete "coefficient" Line208 I do not follow the link here. This assumes that there should be a direct link between d11B and temperature. Strong relationships between d13C and d11B make sense as they are both measuring a component of the carbon cycle. However, I'm not sure such a direct link can be made between coral d11B and rates of ocean warming at this site. Both OA and SST warming are changing at different rates, with much spatial heterogeneity in the oceans. It seems strange to attack the fidelity of your ocean temperature record again when an a-priori link between these proxy records is unclear. Line 219 section needs rewording. No need to say both "depleted in 13C" or "enriched in 12C"; one is implied by the other. Perhaps just state that The oceans are the major sink of anthropogenic CO2 and incursion of isotopically light carbon (sourced from burning of fossil fuels) into the oceans has caused a decrease in the d13C of DIC, termed the seuss effect. Line 228 Delete "derived proxies" Line228 Suggest "cannot be attributed to growth rate as vertical extension rates remained constant throughout the interval of study" Try to avoid the word "since" when you mean "because", especially in this context as "since" has a dative connotation. Line 237 Delete "comparison" Line 239 Sounds as if the coral is acting as a CO2 sink for anthropogenic emissions. Rather this shows that the coral is recording d13C of DIC Line 240 agreement with Line 241 Can the porites record referred to in the text be made more visible in Fig 4? Line 242 I'm not sure the d13C itself "confirms the modern OA crisis" (whereas the d11B record if controlled by external pH certainly does). All this d13C record confirms is that the source of carbon is likely atmospheric. On its own, it says nothing about the pH of the ocean or the response of marine calcifiers to the pH changes (i.e. the OA crisis) Line 248 "...show pronounced low to high pH reversals and vice versa". Unclear what the authors are trying to say here. pH goes down and up, and up and down? Suggest rephrase Line 256 This sentence is very unclear. It is strange to talk in orders of magnitude when pH is a log scale measurement. Do you mean simply the swings in pH are almost half that recorded over G-IG cycles? Line 262 incredible is a strange choice of word. The pH fluctuations are large, but perfectly credible. Line 278 I am not entirely convinced by the reasoning here. How is the veracity of this new coral pH record strengthened by it being at odds with existing geochemical coral data sets (albeit from restricted sites) and model data? I feel this sentence is a step too far.
Line 298 and 300 Delete. I think this is a given. Data were generated a world-leading boron isotope lab. One would not expect sample wash out to be a considerable influence and the reader would not expect the authors to be interpreting trends that are within analytical error. Line 375 replace "due to" with "caused by" Line 393 What does DPU stand for? Line 457 Unless this in-house standard has been measured elsewhere and has certified values the averages obtained here mean little to the reader and give no further indication of accuracy. Suggest leaving these results as precision only. Line 479 "in-house" repeated Line 501 Too many details in this section. Uptake rate of the particular neb used for example is of no use to a reader looking to replicate these results in their lab. I presume most of these details (e.g. cones) are given in the established methods of ref 57 and can therefore be cut here. Line 517 Was carry over a concern. How big were the blanks compared to samples?

Response to reviewer comments
We sincerely thank all reviewers for the detailed and constructive comments that they have provided on our manuscript. These comments and suggestions have been very instructive and constructive in assisting us to improve the manuscript.
Below are our detailed responses to the reviewers' comments in 'blue' with the associated line numbers and text changes highlighted.

Reviewers' comments:
Reviewer #1 (Remarks to the Author): The authors claim to have an excellent record of surface ocean pH, SST and CO2 uptake in the southwest Pacific. The analysis is based on state of the art geochemistry of coral cores. Three proxies were used that track surface ocean properties or internal pH of the calcifying fluid in coral skeletons. The analysis was done at annual resolution for a period of 323 years on a slow-growing coral Diploastrea.
The data are very well presented and the paper is written in a logical way.
Uncertainties of measurements are reported and appropriate tests for reproducibility of the Boron isotope proxy were added. Comparisons to other studies from the Pacific were added that help to constrain the finding of the present study.
To me the data and their interpretation are very convincing and of high interest to a broad readership. Records of ocean acidification are still scarce and this record adds an important dataset from a very unique ocean setting. The Supplement provides very important additional information that helps the reader to grasp the ideas of the  Table 2). In addition, periods of more 'active' ENSO activity 55 (1890s-1910s) are recorded in our coral as a series of major depletions indicating decreases in pH in New Caledonia. The fluctuations in our coral-based δ 11 B-pH thus reflect the highly dependent nature of ocean CO 2 uptake across the air-sea surface interface following the pacing of ENSO." The new Figure 5 with caption and Table 2 in the revised manuscript are shown below in this Reply Comment.  For instance 1790's and early 1800's, between 1890 and 1920, the latter a period of strong ENSO variability.
We agree with this reviewer's assessment and based on historical ENSO events recorded in multiple proxy records [Gergis and Fowler, 2009], there are certainly more 'active' ENSO periods than others in the past. During time periods of higher ENSO activity (e.g. 1890s-1910s) with many high magnitude EN events recorded [Gergis and Fowler, 2009], our New Caledonia δ 11 B record exhibits major depletions indicating decreases in pH at New Caledonia.
It is likely that these series of years with extended high magnitude EN events resulted in the shifts in δ 11 B-pH that was pointed out by Reviewer 2 with the larger interannual oscillation of coral δ 11 B-pH. The recent events are also coincident to the large-scale Interdecadal Pacific Oscillation (IPO) phase shifts that occurred in 1945, 1977, and 1999[Henley et al., 2015. The timing of these IPO shifts based on SST is coincident to our δ 11 B-pH record. We have inserted additional statements in this revision to specifically indicate and better explain these shifts in our record. The sentence can now be found on lines 349-354: "Severe LN events are consistent with enrichment of δ 11 B indicating an increase in pH ( Fig. 5; Table 2). In addition, periods of more 'active' ENSO activity 55 (1890s-1910s) are recorded in our coral as a series of major depletions indicating decreases in pH in New Caledonia. The fluctuations in our coral-based δ 11 B-pH thus reflect the highly dependent nature of ocean CO 2 uptake across the air-sea surface interface following the pacing of ENSO." I also see anti-phasing between d18O and Boron, is it an indication for a link between mean SST and pH on interannual and decadal cycles?
Yes, there is definitely a direct link between the two isotope records (δ 18 O and δ 11 B) where the colder water are more acidified (contain more CO 2 ) than warmer water and follows the principles of seawater carbonate chemistry in terms of temperature [Zeebe and Wolf-Gladrow, 2001]. Because of the partial temperature component in coral After which our coral δ 11 B-pH is dominated by the effect of anthropogenic CO 2 as reflected in both δ 13 C and δ 11 B. Our original wording attempted to reflect this possibility but was perhaps not clear enough for the reader. We have re-written the sentences accordingly on lines 314-327: "Before this onset of modern anthropogenicdriven OA, a coral δ 11 B-pH maximum was reached in the late 1790s (Fig. 2). The increase in δ 11 B at the centennial-scale from the 16th to the 17th century appears to be decoupled from coral δ 13 C trend as the seawater DIC signature remained relatively level. We observed that the progressive increase in δ 11 B-pH during this time period (1701-1761 CE) coincided with changes in temperature most likely linked to the termination of the LIA, a period that was documented to be 1.4 ºC cooler at New Caledonia 27,51 . The end of this period was however characterized by a maximum δ 11 B-pH that is contrary to the recorded maximum δ 18 O enrichment (cooler and/or more saline conditions; Fig. 2). It is possible that as a consequence to the termination of the LIA, a redistribution of water masses occurred near New Caledonia, which experienced an intrusion of cooler and/or more saline water to the region from the enhanced subtropical countercurrent 52 (Fig. S10). These connections point to the substantial linked behaviour of pH, temperature and salinity, at the longer-term centennial timescales." A last question is regarding the conversion of internal pH to seawater pH since I am not an expert in Boron isotope chemistry. Is the conversion chosen by the authors an accepted way of relating internal pH to seawater pH?
The conversion method used in this study is based on the seminal works of [Trotter et al., 2011;McCulloch et al., 2012aMcCulloch et al., , 2017 and is currently the state-of-the-art method for the conversion of coral skeletal δ 11 B signature to coral internal and seawater pH without a species-specific calibration. In the previously published coral δ 11 B studies in Fig. 4, the results are derived from the same calibration methodology as our study if δ 11 B is converted to pH. For this particular genus and species of coral, a pH reconstruction has never been published before.
I ask because we know by now that Boron gives us a valuable record of internal pH regulation by the coral. It means that the Boron helps us to decipher the calcification process of the coral driven by SST. How well do we know the offset between internal pH and seawater pH?
The offset between coral internal calcifying and external seawater pH is highly linear as diagrammed in Figure 4 of [Trotter et al., 2011] for both temperate shallow water and tropical corals. Based on these relationships and other state-of-the-art studies on Nevertheless, if an offset from our reconstructed pH versus the actual seawater surface pH may be expected, again the trend and interannual to decadal acidification variability observed along the core should in no way be affected. Therefore, we believe that the focus on relative change in pH values is more important than the absolute pH values in this study. This is also the reason why the Figures presented in our manuscript along with most interpretations and discussions of the results are based on the coral δ 11 B dataset and we cautiously only discuss the changes in terms of relative pH and not absolute pH changes.
See the above reply comment for the revised sentences.
Overall, I recommend publication after addressing the comments.
We thank this Reviewer for the thoughtful comments and recommendations that improved our manuscript.

Reviewer #2 (Remarks to the Author):
The authors present d11B, d13C and d18O data measured in coral core material representing 300 years of coral growth. These data are used to reconstruct respectively the pH, d13C of dissolved inorganic carbon and temperature of surface seawater at this South Pacific site. They find a decline in d11B (decrease in pH) since Comms. I found many positives in the work that mean this has the potential to be a significant contribution to the field of coral geochemistry and ocean acidification research. These include: 1) The authors' choice of coral is a good one. Diploastrea heliopora grows at around half the rate of large Porites corals typically used for this purpose. This has allowed them to obtain an impressive 300-year sample section. Large polyps in this species are also beneficial during sampling.
2) The choice of a site that is currently a sink for atmospheric CO2 is a sound choice when trying to reconstruct past changes in OA 3) As far as I am aware, before now this species of coral had not been measured for is boron isotopic composition. These new d11B data demonstrate that pH upregulation is occurring in this taxon similar to other shallow water corals and that it too confers a degree of resilience to changes in external pH 4) The samples have been thoroughly checked for their preservation giving confidence to the primary nature of d11B data.
5) The weight of d11B data and 300 year duration of the record are impressive, particularly in comparison to previous coral d11B records (Fig 5) 6) Replicate overlapping sections of downcore d11B are a nice addition and show close agreement. Again, this gives confidence that trends are not an artefact of sampling and chemically heterogeneous parts of the coral have been avoided.
7) The link of coral d11B to ENSO pacing is compelling and an important finding.
Unfortunately, parts of the discussion were unclear and I found the reasoning a little stretched in places, therefore I cannot recommend the paper for publication in its current form. I do however strongly urge the authors make some changes to the discussion and resubmit to Nature Comms. I do feel that these well-produced records are worthy of a place in a high impact journal for the reasons given above.

Joe Stewart
Points to address: Coral d11B here is used as a proxy for seawater pH, however, as stated in the manuscript, coral d11B actually records the pH within the coral. While there are examples where this internal pH is strongly affected by external pH it is important to point out that seawater pH is being inferred by proxy. I think therefore the wording in places is perhaps a little too strong (e.g. Line 194). There have been many examples now where external pH is far from the only factor controlling internal pH (e.g. Comeau et al. 2017, McCulloch et al. 2016).
Yes, we absolutely agree with this comment that some other factors can influence internal pH and some appear to be environmental (e.g. warming) or even some species-specific related effect as demonstrated in [Comeau et al., 2017;McCulloch et al., 2017]. It is also true that both reconstructed pH records are inferred from the same coral proxy (pH of calcifying space and pH of seawater). The reviewer is thus correct that there might be bias in the pH reconstruction at each step: first when we reconstruct the internal pH at the coral calcifying space and second when we add an additional source of bias when reconstructing seawater pH. However, in our case where we reconstruct pH along the same coral core, the trend of the reconstructed internal pH values within our core can be considered as reliable as whatever bias applies to the absolute pH value, the same bias is applied along the whole core and does not interfere with the reconstructed pH trend. Nevertheless, it must be said that any use of climate proxy, and especially as they have a biological component, we must "convert" the geochemical data into physical quantity with great humility. We have tempered down these emphasized points in the revised manuscript and especially this paragraph that can be found on lines 189-207.  We have re-written the sentences accordingly: "Before this onset of modern anthropogenic-driven OA, a coral δ 11 B-pH maximum was reached in the late 1790s (Fig. 2). The increase in δ 11 B at the centennial-scale from the 16th to the 17th century appears to be decoupled from coral δ 13 C trend as the seawater DIC signature remained relatively level. We observed that the progressive increase in δ 11 B-pH during this time period (1701-1761 CE) coincided with changes in temperature most likely linked to the termination of the LIA, a period that was documented to be 1.4 ºC cooler at New Caledonia 27,51 . The end of this period was however characterized by a maximum δ 11 B-pH that is contrary to the recorded maximum δ 18 O enrichment (cooler and/or more saline conditions; Fig. 2). It is possible that as a consequence to the termination of the LIA, a redistribution of water masses occurred near New Caledonia, which experienced an intrusion of cooler and/or more saline water to the region from the enhanced subtropical countercurrent 52 (Fig.  S10). These connections point to the substantial linked behaviour of pH, temperature and salinity, at the longer-term centennial timescales." Most of the drop in pH recorded in this core occurs in a large step around 1890. I notice from the core images that this is mighty close to a dark horizon in the core (just below the core break) that might represent a mortality event or an interval of reduced growth. It is difficult to see from the figures, but can the authors make a convincing case that this dark horizon is not influencing the d11B trends we see here?
We agree with this reviewer that there appears to be 'darker' banding in certain areas of the coral core but these areas are not particularly severe as the ones witnessed in massive coral mortality events (e.g. 1997-1998El Niño [Cantin and Lough, 2014).
This particular 'darker' area (depth horizon) was also the region where skeletal SEM and microstructure analyses were conducted adjacent. These samples were not found to be anomalous when compared to the samples from the older section of coral core.
Moreover, this particular 'darker' banding occurred after the 1900 CE growth year and was not during the period in question ~1890. Most importantly we do not observe a reduction in linear extension or a growth hiatus years within our core after these darkened areas, which would likely be observed if a major stress event had occurred.
If it is a major stress event in the core, then the down core d11B could be interpreted as an increase up until 1880 then an interval of little pH change throughout the last century (see below)...
...While I think the authors are probably correct in their interpretation of the d11B trends. I think it is important to remove the possibility that this upper core section may represent a different phase of coral growth.
Possible stress event such as bleaching may lead to a depletion in coral δ 11 B [Dishon et al., 2015] but these stress events remain inconclusive as other experiments revealed no bleaching impact on coral δ 11 B [Schoepf et al., 2014]. The bleaching analysis of [Dishon et al., 2015] revealed a drop of -5.1‰ in coral δ 11 B, which is far greater in magnitude than the total analytical range of this study. We agree with this reviewer that a large 'shift' occurred around 1890, however, this change is nearly the same as the change in 1800. The plotting of these 2 simple linear trend lines is somewhat misleading as it does not take into account the nuanced long term secular trends change as indicated in our revised Fig. 2 and original Fig. 3 (now removed). In our previous responses, we postulate that these large changes likely occurred during periods of more 'active' ENSO or during time periods of higher ENSO activity (e.g. 1890s-1910s) with many high magnitude EN events recorded [Gergis and Fowler, 2009]. It is probable that these series of years with extended high magnitude EN events resulted in these larger interannual oscillation of coral δ 11 B-pH. Finally, the recent 'shifts' are also coincident to the large-scale Interdecadal Pacific Oscillation (IPO) phase shifts that occurred in 1945, 1977, and 1999[Henley et al., 2015 and supports our argument that it is not due to growth related bias.
Similarly, the most recent 20 year decline in pH is noted (red arrow below), but this d11B fall is as large as the d11B rise from ~1975 to ~1990 (blue arrow). If OA is responsible for the downward trend over the last 20 years, what is responsible for the preceeding rise of equally "striking" magnitude and duration?
Please refer to our response below the next comment.
Might this just suggest that ENSO modulation is dominant driver on these timescales?
I am not sure of the answer to this myself, particularly as the red arrow is in excellent agreement with the recorded pH decline at the Hawaii HOTs site.
During time periods of higher ENSO activity (e.g. 1890s-1910s) with many recorded high magnitude EN events [Gergis and Fowler, 2009], our New Caledonia δ 11 B record exhibit major depletions indicating decreases in pH at New Caledonia.
The recent events as pointed by this reviewer are also coincident to the large-scale Interdecadal Pacific Oscillation (IPO) phase shifts that occurred in 1945, 1977, and 1999[Henley et al., 2015. The timing of these IPO phase shifts based on SST is coincident to our δ 11 B-pH record. It is likely that these series of years are also years with extended high magnitude EN events and resulted in the large interannual oscillation of coral δ 11 B-pH. We have added additional sentences in the revised manuscript communicating these coherences. The new sentences are found on lines 338-354: "During most El Niño (EN) events, the δ 11 B signature decreases from the preceding year translating into a surface seawater decrease in pH, and the opposite occurs during La Niña (LN) events with an increase in δ 11 B signature from the preceding year (increase in surface pH; Fig. 5; Table 2).
The most recent ENSO events indicate possible changes of up to ±0.35‰ in coral δ 11 B ( Fig. 5;  Table 2). In addition, periods of more 'active' ENSO activity 55 (1890s-1910s) are recorded in our coral as a series of major depletions indicating decreases in pH in New Caledonia. The fluctuations in our coral-based δ 11 B-pH thus reflect the highly dependent nature of ocean CO 2 uptake across the air-sea surface interface following the pacing of ENSO." More minor points: Line58 with links Revised.
Line 101 Diploastrea heliopora can be abbreviated to D. heliopora from here onwards (e.g. line112) The coral genus is now abbreviated after the first mention in the main text.
Line 167 Suggest change "observe" to "reconstruct" The word "observe" has now been removed and is replaced with "reconstruct".
Line 170 This reads as if the fractionation factor is pH dependent too. This needs rewording.
Yes, this reviewer is absolutely correct in that the fractionation factor at equilibrium (α[B 3 -B 4 ], see [Klochko et al., 2006]) is independent of the pH by definition. We have revised the awkward wording in this sentence. This revised sentence can now be found on lines 169-172: "In seawater, the relative abundance of the two aqueous boron species (boric acid and borate) as well as their isotopic composition are pH dependent 34 with a constant fractionation factor between the two aqueous boron species 35 ." Line 185: Suggest rewording here. Could perhaps be read as pH becoming higher.
You mean a more consistent, homogeneous sampling protocol was achieved.
We agree with the reviewer that the awkward wording in this sentence may be misinterpreted or misunderstood. We have revised this entire sentence and it can now be found on lines 182-185: "The large amount of coral skeletal material used for δ 11 B analysis (~50 mg) compared to traditional δ 18 O and δ 13 C analyses makes it statistically likely that the sample comprises all different skeletal structures (columellar and septal) and is thus representative of the whole coral skeleton." Line 206 Delete "coefficient" Removed from revised manuscript.
Line 208 I do not follow the link here. This assumes that there should be a direct link between d11B and temperature. Strong relationships between d13C and d11B make sense as they are both measuring a component of the carbon cycle. However, I'm not sure such a direct link can be made between coral d11B and rates of ocean warming at this site. Both OA and SST warming are changing at different rates, with much spatial heterogeneity in the oceans. It seems strange to attack the fidelity of your ocean temperature record again when an a-priori link between these proxy records is unclear.
We want to point out that there is a link between the two isotope records (δ 18 O and δ 11 B) where colder waters are more acidified (contain more CO 2 ) than warmer waters [Zeebe and Wolf-Gladrow, 2001]. We welcome this critical comment on our Line 219 section needs rewording. No need to say both "depleted in 13C" or "enriched in 12C"; one is implied by the other. Perhaps just state that The oceans are the major sink of anthropogenic CO2 and incursion of isotopically light carbon (sourced from burning of fossil fuels) into the oceans has caused a decrease in the d13C of DIC, termed the seuss effect.
We have revised the original two sentences in the Discussion describing the 13 C Suess effect based on this reviewer comment. The new sentence can now be found on lines 221-224: "As the ocean is one of the major global sinks of anthropogenic CO 2 emission, the incursion of isotopically light carbon ( 12 C) from the burning of fossil fuel into the ocean has caused considerable decrease in the δ 13 C of seawater DIC, known as the 13 C Suess effect." Line 228 Delete "derived proxies" Removed from revised manuscript.
Line 228 Suggest "cannot be attributed to growth rate as vertical extension rates remained constant throughout the interval of study" Try to avoid the word "since" when you mean "because", especially in this context as "since" has a dative connotation.
Based on this reviewer suggestion, we edited the sentences to state that the δ 11 B changes are not related to the linear vertical extension growth with careful selection of our vocabulary to avoid any confusion. The new sentences can now be found on lines 230-233: "The geochemical variations observed in our coral cannot be attributed to the linear growth rate because the vertical extension rates remained relatively constant (2.68 ± 0.64 mm year -1 ) throughout the interval of this study." Line 237 Delete "comparison" Removed from revised manuscript.
Line 239 Sounds as if the coral is acting as a CO2 sink for anthropogenic emissions.
Rather this shows that the coral is recording d13C of DIC The original sentence has been revised for clarification based on this reviewer comment. The new sentence can now be found on lines 239-243: "These consistent secular trends from instrumental measurements and the rapid rate of coral δ 13 C depletion found at New Caledonia (-0.024‰ yr -1 ; Table S3) recording the δ 13 C of seawater DIC over the same period (1978-2011; Fig. 3 and Table S3) demonstrate the significant absorption of anthropogenic CO 2 emission by the oceans." Line 240 agreement with Line 241 Can the Porites record referred to in the text be made more visible in Fig 4? The Porites lutea δ 13 C record from New Caledonia [Quinn et al., 1998]  Line 242 I'm not sure the d13C itself "confirms the modern OA crisis" (whereas the d11B record if controlled by external pH certainly does). All this d13C record confirms is that the source of carbon is likely atmospheric. On its own, it says nothing about the pH of the ocean or the response of marine calcifiers to the pH changes (i.e.

the OA crisis)
We have attempted our best to tone down the manuscript following this reviewer comment and have removed this phrase, "modern OA crisis" in this revised sentence.  (Fig. 2)." Line 256 This sentence is very unclear. It is strange to talk in orders of magnitude when pH is a log scale measurement. Do you mean simply the swings in pH are almost half that recorded over G-IG cycles?
We now recognize our mistake and following this reviewer comment; the sentence has been revised to clarify the statement on the magnitude of pH change recorded in We now refer to the reconstructed pH fluctuations from the Great Barrier Reef [Wei et al., 2009] and the South China Sea [Wei et al., 2015] as "large" from this reviewer suggestion, which may be due to local reef effect or coastal riverine input. The new sentences can now be found on lines 264-270: "Moreover, studies from the South China Sea 14 and the GBR 15 recorded large interannual to interdecadal fluctuations (0.6-0.7 pH units) that are even greater than the 0.4 pH units predicted for the end-ofthe-century (Fig. 4) Line 298 and 300 Delete. I think this is a given. Data were generated a world-leading boron isotope lab. One would not expect sample wash out to be a considerable influence and the reader would not expect the authors to be interpreting trends that are within analytical error.
We have deleted the above sentences as suggested by this reviewer.
Line 375 replace "due to" with "caused by" Line 393 What does DPU stand for?
Original Line 375 words "due to" has been replaced with "caused by" as suggested by this Reviewer.
Original Line 393, the reference to the name DPU in the manuscript has now been removed. DPU is the internal name we gave for this coral colony based on the genus of Diploastrea (DP) and the location Uitoé (U).
Line 457 Unless this in-house standard has been measured elsewhere and has certified values the averages obtained here mean little to the reader and give no further indication of accuracy. Suggest leaving these results as precision only.
In this revised manuscript, the mean values of the in-house carbonate standard for δ 18 O and δ 13 C ratios have now been removed. The Reviewer is indeed correct that the values are not certified. The sentence stating this analytical precision can now be found on Line 478-480: "Long-term analytical precision based on repeated measurements of an in-house marble carbonate standard were ±0.04‰ (±1σ standard deviation, SD; n = 116) for δ 18 O and ± 0.02‰ (±1σ SD; n = 116) for δ 13 C verified against NBS 19." Line 479 "in-house" repeated This in-house standard for boron isotope analysis has not been certified in other analytical laboratories. This mention has now been removed from the revised manuscript and is only referred to in terms of its precision.
Line 501 Too many details in this section. Uptake rate of the particular neb used for example is of no use to a reader looking to replicate these results in their lab. I presume most of these details (e.g. cones) are given in the established methods of ref 57 and can therefore be cut here.
We agree that there is a lot of information contained in the Methods section of this manuscript. Based on the suggestion of the reviewer, we have removed particular sentences on the specific analytical procedure of the MC-ICP-MS.
Line 517 Was carry over a concern. How big were the blanks compared to samples?
Was ammonia or HF used to help wash out?
We are absolutely certain that there was no carry-over (memory effect) from sample to sample during the analysis or from the extraction chemistry that was completed in batches. This procedure of randomizing the sample analysis was a precautionary measure, perhaps overcautious and unnecessary, but it was the easiest and most thorough way to completely avoid and eliminate any possible bias. In our bracketing "standard -sample" protocol, the rinse solution was composed of 0.1N HNO 3 and 0.04N HF and was used in between every measurement, this is stated in the Methods on original line 506. For extended information, we have revised the Methods on line 523 to state: "typical blank contribution is < ~0.5%." Line 521 If JCt values are not quoted here they perhaps no need to mention them above. JCp is of course the more relevant standard here anyway.
The reference to the powder standard JCt-1 has now been removed and edited from the manuscript because the values were not reported.
Line 551 I fail to see the importance of using this fractionation factor calculation from the 1970s? Are the authors suggesting that the resultant low internal pH estimates are feasible? i.e. no pH upregulation at all.
Following this suggestion, we have modified the sentences in the Methods section.
For clarification, the fractionation factor proposed by [Kakihana et al., 1977] is now removed as the Reviewer is correct in that the isotopic fractionation factor proposed by [Klochko et al., 2006] is probably the closest to that expected and also because the pH values derived from the use of this isotopic fractionation factor seems to be the most consistent with observations. The revised text can now be found on lines 565- This error has now been corrected and to be more precise for the reader, we have now clarified that it is not only "similar to Fig. 2" (new Fig. 2), it is in fact the "trend shown in Fig. 2". This typo has now been corrected.
We thank this Reviewer for listing the positives of our work at the top of this review and noting the possible meaningful contribution to the field of coral-based geochemistry and ocean acidification research, as well as the thoughtful comments and recommendations that improved our manuscript.