Improved bathymetry leads to >4000 new seamount predictions in the global ocean – but beware of phantom seamounts!

This study produced a global seamount census using Becker et al. (2009)’s SRTM30_PLUS bathymetric model, and reported a very interesting finding, inappropriate use of the sparsely distributed seafloor depths in historical admiralty charts may lead to erroneous presence of seamounts in global bathymetric models. However, the two stories are not well combined whereas usually only one theme is expected in a paper. This is the largest problem of this manuscript, as evidenced by the disordered structure.

The introduction of this paper has a nice start, highlighting the importance of seamount census in fishery management/research. But, the motivation of this study is not clearly presented. What is the gap in our current knowledge? What are the objectives of this paper? For example, you could say improved global bathymetric models have been released and an updated seamounts chart need to be produced. The updated chart may benefit the fishery by ***. Or, you could focus on another topic by saying that digitized historical nautical charts are used to expand the data coverage, but may lead to erroneous presence of seamounts in bathymetric models. The mechanism needs to be identified and false seamounts need to be removed from the current census. But remember to stick to one theme in a paper.

The findings reported in the “BIOT Seamount Survey” section is very interesting, but its connection with the last and next paragraphs is not well established, making it seems abrupt and standalone. How does these findings contribute to your seamount census? You can produce two seamount censuses, one produced with the “no bottom soundings” and one without, and then find a way to evaluate the improvement obtained by removing the “no bottom soundings”.

The method section is too simple, lacking of formulas and step-by-step description of how you produce the seamount census. This makes readers hard to duplicate your results. Besides, some sentences (Lines 120-122) in this section should belong to a separate section named “Data”.

You could consider adding more text to the discussion section, e.g., histogram of the seamount heights, evaluation of the reliability of results, known insufficiency in the present method that needs to be improved in the future, etc.; please refer to Wessel (2001) and Harris et al. (2014) for ways of extending the discussion. Besides, adding something specific about how your seamount census improves fishery will be helpful.

Minor problems I noticed are listed in the following.

Line 1: 4000 is not compatible with the number in the abstract.

Line 51, Line 67: Do you mean SRTM30_PLUS? Note that SRTM30 and SRTM_PLUS are two different models. Only the land and ice topography of SRTM30_PLUS comes from SRTM30.

Line 82: GEBCO 2020 has been released. Why do not you use the latest version?

Line 84: Add the text “Great Chagos Bank” onto Figure 1 to improve the readability.

Lines 117-118: This sentence is hard to understand. Please consider rewriting it.

Line 135: In the legend, the marker for “New Seamounts” is hardly visible.

Line 139: Define the EEZ shown in the legend.

Line 147: I did not find (Costello et al., 2010) in the reference list.

Line 152: Do you mean SRTM15+? Please use the name given by the author.

Line 162: Substitute “that be” by “it is”.

Seamount information, including position, depth of the summit, is of great importance for different kinds of applications. The manuscript predicted new seamounts using newer bathymetry grids, i.e. SRTM30 global bathymetry version 11. The findings are important and meaningful for the applications of current bathymetry models. For example, they found some seamounts predicted by the old bathymetry grids are not detected by sounding data. This means some phantom seamounts exist in the old version of bathymetry grids. Also, some seamounts would not be detected by the old bathymetry grids. In general, I think the result of this manuscript is very important. Here are my comments.

1. Although this study found new seamounts, however, there are also phantom seamounts. Hence, I think the author should point out that some erroneous predictions may exist in the new predictions. If the new results can be verified by sounding data, it would be better. However, the sounding data may be not enough. Maybe the authors can select some local regions to do an evaluation. For example, in the areas near South China Sea and the Philippines, as there is high densification of seamounts according to Figure 3.
2. It would be more helpful for the international research community if the authors could use mathematical equations to break down the method used in addition to the content of lines 115 to 119. It can be added as an appendix.
3. Line 64, give the geographic extent (corner coordinates) of BIOT Seamount Survey.
4. Line 117, insert ‘where’ after ‘terminate at the point’.
5. Line 118, “km2”->”km ² ”
6. It would be better to give the units of the data in the inset table of Figure 4.
7. Line 162, change ‘removed’ to ‘resolved’.

This is a short paper to inform readers about an updated seamount prediction database now available online. The new database was created using an “old” method (Yesson et al. 2011)  to analyze new and improved bathymetric data (false data points removed, higher resolution). There are two main findings:  an increased number of predicted seamounts and the identification of phantom seamounts. Seamount models are incredibly useful for marine spatial planning, and it is fantastic that the authors update model outputs as better bathymetry data become available.

Flow: The main story of the title and the discussion is “new seamounts identified,” which is different than the “phantom seamounts” focus of the abstract, methods, and results.

Line 11: While “hotpots” made me chuckle, I think it’d be better to change the word to “hotspots.”

Line 13: In large numbers, help the reader by using commas. “10,000 to more than 60,000”—consider changing throughout the article (used sometimes).

Line 13-14: Provide more detail, “Seamount locations can be estimated by extracting conical shaped features [that meet other criteria (e.g., elevation)] from bathymetry grids.”

Line 41: Mention deep-sea mining here. This data was used at the first ISA REMP workshop for deep-sea mining on seamounts (a marine spatial planning meeting for the International Seabed Authority; report still in progress) to start to inventory seamounts in the North Pacific Area, to identify seamounts that are (i) contracted for exploration and (ii) could/should be considered for protection. Please consider mentioning mining or that fishing isn’t the only threat (e.g., climate change impacts too).

Line 49: If complete list, add: Kitchingman, A., and Lai, S. 2004. Inferences on potential seamount locations from mid-resolution bathymetric data. Seamounts: biodiversity and fisheries 12: 7-12.

Line 54: 1.5km diameter, right? Or height?

Figure 1 / Line 82-87: Is the grey “sub-figure profile” line meant to show on the map where the inset profile data is from? The lines on the map are so close, and the colour difference so subtle that I can’t tell—poor quality. Extract profile and present separately to avoid cluttered and help with figure readability.

Figure 1 / Line 82-87: Not sure what is meant by “Chart symbol: No bottom detected at 183 m.” Delete since the line below clearly states, “No bottom detected on 2016 survey.”

Lines 83-87 (Figure 1): Include/explain figure labels A & B when describing the location of each site in the caption: “...NW of the Great Chagos Bank (site A)…Area 40km north of this (site B)…”

Lines 89-91 (Figure 2): Text jumps between “Seamount A…B” and “site A…B”. Go with one.

Line 95: “were” not “where”

Line 119: I’m happy to see the authors mention that new bases can encompass multiple “old dataset” peaks, but it makes it sound like the new dataset doesn’t suffer the same issue of individually identifying multiple peaks on the same seamount—in reviewing the shapefiles I see this is still the case. Please see the comment for lines 144-155 below.

Figure 4 / Line 139: Remove table inset and present separately to avoid clutter. I am having a really hard time with readibility of the figures (poor quality).

Lines 144-155: In my experience, these models are incredibly helpful in marine spatial planning--especially when assessed altogether--but I have witnessed the pitfall/danger in counting the predictions as the “number of seamounts” instead of the number of peaks (e.g., justification for allowing harmful activities on dozens of seamounts because models illustrate there are supposedly hundreds within the region--when in actual fact more than half of the predicted points are just peaks on the same seamount). It’d be beneficial for the authors to provide this word of warning regarding peaks vs. counts. I don’t think the high number of replicate predictions is unique to my study regions, but if the authors want to review overlapping bases and replicate predictions, I would suggest the NW and NE Pacific seamounts.

We discussed this paper as a group in our weekly lab meeting on March 8, 2021.  Three of us have provided comments below.

David Sandwell

This is an interesting paper that has two main conclusions.  The first is that some data were incorrectly harvested from old Admiralty charts and then used in global grids.  The second is an improved analysis of global seamounts from the SRTM30 V11 global grid which is based on predicted depth and measured depth.  For each of these grids there is a matching source identification (SID) grid that enables one to determine the source of every sounding.

I have a few comments about versions of SRTM and GEBCO grids.  First the, latest SRTM and GEBCO grids have moved to 15 arcseconds.  The latest SRTM 15 V2.1 grid is described in a recent publication (Tozer, B., Sandwell, D.T., Smith, W.H., Olson, C., Beale, J.R. and Wessel, P., 2019. Global bathymetry and topography at 15 arc sec: SRTM15+. Earth and Space Science, 6(10), pp.1847-1864).  Table 1 of that paper explains that the GEBCO grids are a copy of the SRTM grids with soundings added.  It is interesting that the GEBCO 2014 grid has two false seamounts while the SRTM V11 has only one.  Seamount A is in all versions of SRTM30 V7, 8, 9, 10, and 11.  However Seamount B is not in any of these versions.  The archive of all the old versions is at ftp://topex.ucsd.edu/pub/archive/srtm30 . Look in the topo30 folder for the SID grid in netcdf format.  Therefore, this B data point was somehow added to the GEBCO 2014.  I also see the B-pont it is not in the latest GEBCO 2020 grid which is mostly a copy of SRTM15_V2.0.

This incorrect sounding at location A came from the following source.  This is a dataset from the NGA which was harvested from digital nautical charts (DNC).  It has a depth of -140 m.

16393 shallow7.cm private DNC https://www1.nga.mil/ProductsServices/NauticalHydrographicBathymetricProduct/Pages/default.aspx YEAR NGA

Basically I completely agree with this part of the paper and the analysis of the phantom seamounts.  The authors should add the locations of the 14 additional “phantom seamounts” so they can be removed from the next versions of the SRTM and then the GEBCO data sets.

Regarding the claim of the discovery of 4000 new seamounts, this is probably also correct although I think the better way to locate seamounts is to use the vertical gravity gradient data following the Kim and Wessel 2011 study.   I see that none of the phantom seamounts described in this paper on Figure 4 were identified in the VGG by Kim and Wessel (2011).

Here are some general comments:

1. The data file on the locations and characteristics of the old and new seamounts needs to be available.

2. There should be a more complete discussion about the overlaps of the various seamount data sets.  Also in areas of rough topography, associated with abyssal hills and fracture zones, how do the authors discriminate between features created by seafloor spreading and off-axis volcanoes.

3. The quality of Figures 1, 3, and 4 is very poor.  The latitude/longitude labels on Figure 1 should be decimal degrees to match the text.

#20210308: From Katherine -- I mainly read this with an eye for editorial stuff, so I will just list some of the issues I had (I want to underline David’s point #3 above about the figures being quite poor too). Please feel free to rewrite/include whatever though!

Line 12 - 13: Delete “barely”, change to  “...vary from anywhere between 10000…”; Also, delete extraneous parentheses after “60000”

Line 13 - 14: If they are going to say “can be difficult” say/explain why (e.g. locating them can be difficult because of x, y, z reasons..)

Line 18 - 21: The second sentence of this paragraph needs a little help -- it’s very choppy/confusing. Why not instead say, from sentence one of that paragraph: “based on the most recent SRTM30 (is that the most recent?) global bathymetry, which was prompted by a recent (what year?) survey in the …. Two of the seamounts visited did not display any echosounder readings, despite having been previously identified as seamounts in earlier seamount maps/databases.”

Line 21: I’m not in this field and do not know what Admiralty charts are -- but maybe the audience of this paper does know what they are?

Line 23: “perhaps 15” features? Did they not just do an analysis to identify these? Maybe clarify, or don’t use the word perhaps (are they sure or not sure about their analysis?)

Line 26: Specify which “older bathymetry grid” they are comparing to.

Line 44 - 45: Does this not contradict the sentence in the abstract where they specify a seamount number estimate of 10000 - 60000 ?

Line 49 - 50: Probably should define those acronyms for completeness sake (GEBCO and SRTM)

Line 104 - 105: I feel like this sentence should have been stated earlier than now. This is the clearest thing I have read so far that describes what they are doing/trying to do, and I like it.

Line 108: First time telling the reader what SRTM is, but they don’t mention the acronym (stay consistent if possible)

Line 141 - 143: They mention that datasets vary, in particular with how they identify any seamounts along ridges, but the authors don’t appear to specify what they do when it comes to ridges and possible seamount features there?

Line 162: I think they mean “invalid” instead of “in-valid”?

Julie Gevorgian Review: I mostly looked for grammatical errors and the overall flow of the paper.

Line 50: Remove “themselves”

Line 51: Remove the dash between “Satellite- altimetry”

Line 54: Change to “these are utilized to constrain” - the word “used” is used twice in the same sentence, might be better to avoid repetition.

Line 59: remove “so”

Line 59-60: Might be worth rewording this sentence to make it flow better: “Since sounding data is limited, it is valuable to make use of all available data. Historical soundings based on weighted lines have been extracted from nautical charts to expand the data (Becker et al.,2009).”

Line 72: change “were” to “was” - important to be consistent with past/present tense

Line 115: change to “can and oftentimes do encompass”

Line 163: change to “phantom seamounts.” - the period always goes inside the quotation