the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The macronutrient and micronutrient (iron and manganese) signature of icebergs
Abstract. Ice calved from the Antarctic and Greenland Ice Sheets or tidewater glaciers ultimately melts in the ocean contributing to sea-level rise. Icebergs have also been described as biological hotspots due to their potential roles as platforms for marine mammals and birds, and as micronutrient fertilizing agents. Icebergs may be especially important in the Southern Ocean where availability of the micronutrients iron and manganese extensively limits marine primary production. Whilst icebergs have long been described as a source of iron to the ocean, their nutrient signature is poorly constrained and it is unclear if there are regional differences. Here we show that 589 ice fragments collected from floating ice in contrasting regions spanning the Antarctic Peninsula, Greenland, and smaller tidewater systems in Svalbard, Patagonia and Iceland have similar characteristic (micro)nutrient signatures with limited or no significant differences between regions. Icebergs are a minor or negligible source of macronutrients to the ocean with low concentrations of NOx (NO3 + NO2, median 0.51 µM), PO4 (median 0.04 µM), and dissolved Si (dSi, median 0.02 µM). In contrast, icebergs deliver elevated concentrations of dissolved Fe (dFe; mean 82 nM, median 12 nM) and Mn (dMn; mean 26 nM, median 2.6 nM). A tight correlation between total dissolvable Fe and Mn (R2 = 0.95) and a Mn:Fe ratio of 0.024 suggested a lithogenic origin for the majority of sediment present in ice. Total dissolvable Fe and Mn retained a strong relationship with sediment load (both R2 = 0.43, p<0.001), whereas weaker relationships were observed for dFe, dMn and dSi. Sediment load for Antarctic ice (median 9 mg L-1, n=144) was low compared to prior reported values for the Arctic. A particularly curious incidental finding was that melting samples of ice were observed to rapidly lose their sediment load, even when sediment layers were embedded within the ice and stored in the dark. Our results demonstrated that the nutrient signature of icebergs is consistent with an atmospheric source of NOx and PO4. Conversely, high Fe and Mn, and modest dSi concentrations, are associated with englacial sediment, which experiences limited biogeochemical processing prior to release into the ocean.
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RC1: 'Comment on egusphere-2023-2991', Anonymous Referee #1, 19 Apr 2024
The authors have compiled a unique dataset of 589 iceberg samples, including 367 new samples. The authors present a suite of macro and micronutrient concentrations for these samples. The manuscript represents progress beyond current data availability and some new insight into the relationship between nutrient concentration and particulate load/ice melt rate.
Scientific quality (rigour):
The purpose of the work clearly articulated. The possible sources of nutrients in the glacier should be more clearly listed and explained in the introduction. The methodology needs more indepth discussion of models used to test nutrient source. Interpretation is underpinned by data presented. The reviewer is not an expert on the models and cannot comment on their validity – but would appreciate more detail on how the modelling methods can achieve the aim (alongside previous examples of their use). Results are presented in a robust way. Possible bias as a result of possible sea ice sampling is discussed in detail and lab methods are implemented to reduced possibility of sea water supply of nutrients.
Significance (impact):
The manuscript provides a step towards understanding nutrient supply to the ocean by icebergs. The strength of the manuscript is the large ice dataset and multiple nutrients considered. No substantial conclusion is reached. The authors conclude that micronutrient content from icebergs (e.g., Fe and Mn) is similar geographically. However, to provide weight to this argument requires i) more detail presentation of the tools used to reach this conclusion e.g., how the models work and previous use of models. In addition, it is limiting to use just iceberg concentration data to infer glacial source processes that drive the initial concentration changes. For this mineralogical characterisation, nutrient – organic carbon associations characterisation, isotope geochemistry to detect sediment source. Suggestion also to deconvolve ice nutrient concentration into: what processes drive initial concentration in the glacier; what processes could change nutrient concentration when iceberg is calved, what processes could change nutrient concentration when the iceberg is transported (e.g., cryoconite and rate of melting). The distinction between these points is discussed in the paper, but could be presented more logically so that the reader has a logical picture of the sequential reasons why nutrients to be of a certain concentration in the ice. The paper would have more impact if presented as ‘what can possibly affect the iceberg concentrations from source to sampling’. If the reasons are weighted towards iceberg melt/ice berg geochemical processes then this would put more weight on insitu and reduce weight on geographical source being governing factor. By framing in a 'source to sink' approach it links with the papers motivation to understand if there are geographical distinctions in nutrient source.Presentation quality:
Scientific results and conclusions are presented in a clear, concise, and well-structured way.Additional comments:
What is meant by micronutrient signature in the abstract?
Please explain in more detail why you would have expected a geographical difference in nutrient concentration – is this the result of variability in sediment supply and geochemical processes in ice sheet glaciers globally? If so, please state this evidence.
Unclear what is meant between dissolved and total dissolved in these two sentences: Total dissolvable Fe and Mn retained a strong relationship with sediment load (both R2 30 = 0.43) whereas weaker relationships were observed for dFe, dMn and dSi.
Suggest to better link the 'sediment load loss' discussion - to one part of the continuum from sediment nutrient supply in the glacier to iceberg sampling. This continuum could be presented in the introduction and again at the start of the discussion.
A very comprehensive introduction. One point: please clarify what is meant by sediment supply of micronutrients. Does a portion of the sediment supply refer to the supraglacial sediment deposits? Do these deposits interact with supraglacial meltwater and transport micronutrients to englacial systems? Also are cryoconite holes another possible source of micronutrients? Suggest that you provide a clear summary list of possible sources of micronutrients, supported by literature.
In the introduction, please make a clearer link between ‘sediment interaction’ and tidewater glaciers.
In the introduction, what is meant by sediment-rich peripheral layers?
Line 154: what is meant by ‘total dissolvable’ compared to dissolvable? Does this include a portion of particulate?
Please provide more information on how PERMANOVA works? Are there examples of its use for similar questions in other studies?
Please provide more information on how MDS works?
In Figure 1 caption, please provide information on what is meant by MDS1 and MDS2. Please explain what an nMDS ordination analysis is.
Line 273 – 274: ‘despite the potential for dSi to be released from sedimentary phases via similar mechanisms to Fe and Mn, neither trace metal correlated well with dSi.’ – what do you mean by ‘release by sedimentary phases via similar mechanisms’? Suggestion to include sentences on these mechanisms in the introduction. Please make the possible sources and mechanisms for sediment to dissolved phase transfer clear for all nutrients in the introduction.
Very good section on inshore to offshore concentrations in ice and possible contribution of elements from sea ice.
Line 426 – 429: ‘This is consistent with the expectation that englacial sediment drives a direct enrichment in TdFe and TdMn, which increase proportionately with sediment load, whereas the enrichment of dFe, dMn and dSi is more variable and depends on the specific conditions that sediment and ice experience between englacial sediment incorporation and sample collection.’ We are not introduced to the expectation that englacial sediment drives enrichment of Fe and Mn. Suggest to explain the possible mechanisms of sediment micronutrient enrichment in the introduction and support with literature.
Suggestion to graphically show the relationship between melting rate and sediment release rate.
Citation: https://doi.org/10.5194/egusphere-2023-2991-RC1 -
RC2: 'Comment on egusphere-2023-2991', Anonymous Referee #2, 17 Jun 2024
Review Krause et al. 07/06/2024
Krause et al. accumulated an impressively large and unique dataset of nearly 370 (not 600 as expected from the abstract) ice samples from both hemispheres. These were then analysed for macronutrients, several micronutrients of which mostly Fe and Mn (dissolved and total dissolvable in some samples) concentrations were shown. These were compared to the sediment load of the sample. These concentrations were methodically statistically analysed and for the td fraction a correlation was found in contrast to the dissolved fraction and local processes seemed to be more important than region, climate or geology.
The English in this ms is fine and the dataset is a wonderful collection which contains a lot of work and the authors must have put a lot of thought into the statistical methods. Also, I enjoyed reading the discussion.
Now to my concerns in regard to the ms though and I hope I don’t come across too harsh as I know how much work and brain power and time must have gone into this ms before submission - especially for the first author. Therefore, I have put a fair bit of my time into it to improve it as I think it could certainly be improved with more structure and focus and possibly a clear message. And the ms needs to be narrowed down. There is a lot of information about a variety of polar and marine topics that the reader can get lost which I don’t think helps and I think by restructuring the ms, it will be more read and especially cited. Sometimes less can be more even though it is very hard to cross out your own sections that have been written with a lot of detail and thought, but you might be able to reuse them for another ms or so in the future.
So, my suggestion would be to restructure the ms with potentially a new title (maybe rather the findings? But I think this question will be answered at the end of the revision), clear subheadings and structured paragraphs and leading the reader with your figures and first of all a global map in which you please show your sampling sites and a simplified sampling site name or starting with a table in the intro in which you name and classify the sampling sites (Hemisphere, region, climate, geology, which parameters were sampled, how many samples taken,…). At the moment I find very confusing to understand which of the different sampling sites each section is referring to (I know the person who sampled is usually very familiar with the names and sites, but a reader might not be and in this case not even be sure sometimes which hemisphere you are referring to and this needs to be clarified, please, it might be helpful to put an S or N or something into brackets) and the ms is jumping between topics and regions - the Arctic and the Antarctic. Both marine systems need to be better introduced, differences, similarities and the importance of ice, ice formation, nutrient input and trace metals in each. Then you can refer to these better in the discussion and might get a clearer message/conclusion in the end as well if all these are clearly laid out.
Also, it needs to be clarified if these 600 samples and their corresponding data and their publication are completely novel (this is what I thought in the beginning, but whilst reading the ms, I wasn’t sure any longer) or whether this paper is rather a collection of all the ice samples published until now or whether it is a mixture of both? Not clear to me, sorry. and that this paper is rather their statistical evaluation? No matter what, it is necessary to clarify this and somehow create an overview table, I know some information is in the tables in the SI, but still I think this information and an overview table with the sampling sites (the above-named regions, classifications, geography, sample types taken, year, season, number of samples,… and please feel free to add) is necessary in the main paper. As you should be able to understand the main messages of a paper when skipping through it and reading the title, possibly the abstract and checking out the figures and their figure captions and the possibly the discussion, but certainly the conclusions. In contrast possibly you could think about the long and detailed methods section and whether parts of this section could go into the SI instead?
I think a pretty good example and paper that I was thinking about which might help in terms of structure is D. Lannuzel et al., Elementa Sci Anth, 2016
Iron in sea ice: Review and new insightsIron in sea ice: Review and new insights
When I was looking for an example to give to you, I came across this one (and no, I am not an author of it😉) and it actually has a clear message title, two different introductory sentences for the abstract and the intro, setting the scene for each, but differently. Then Table 1 to give the reader an overview of the sampling sites shown in figure 1 (you could do a Southern hemisphere and a Northern hemisphere global map as panel A and B for example) and then Table 2 for the parameters used (not all of them, even if you measured a lot more, might be necessary for the story)
Details:
L1: The title led me to expect a ratio or several (signature) of the measured nutrients and especially Fe and Mn and how to use them to identify their sources, the prominence of the ice and to learn more about their input into seawater, possibly elevating the (co-)limitation in the SO and how it could be compared to the Earth/sediment ratios. Potentially add something like “fixed” or “static” to show in the title that the signature is in a small range? At the same time you had some samples which you excluded from the stats due to their values…. As mentioned above whether the title is completely fitting will be answered after a detail revision. L24 main message in regards to macronutrients- are these necessary in the title?
L15: is the first sentence in line with the title? It should be setting up what the reader expects of this ms. Potentially rather an introductory sentence for the introduction.
L25-30 lots of numbers, are both medians and means necessary, but there are no errors, ranges, which might be easier to grasp? some with stats, R2, but missing the p-value in L28. Please try to present this in a more uniform way. Jumping between dissolved and tD elements.
L27 suddenly total dissolvable, please try to lead over from the dissolved phase, maybe with a “in contrast to…”
L25 and throughout the text: chemically I would prefer the writing NO3- for nitrate and NO2- for nitrite as NO3would be rather a nitrate radical and NO2 laughing gas (nitrogen dioxide).
L32 please add a number behind the Arctic to make it comparable.
L33 what does this mean losing their sediment load? Message of this finding? Very interesting and curious
L35 correct to PO43-
L35 d or td?
L42 nutrient availability? Not clear what is limiting and where, potentially easier to explain if the regions are introduced first?
L50 Reg needed after ice
L52 Ref necessary
L70 what are the biogeochemical consequences?
L78 new topic so potentially a new paragraph?
L87 does it matter in which region, hemisphere we are here? Sorry, lost me
L113: if the nutrient signature of icebergs hast been published, maybe give some numbers/ratios, introduce them for the different regions, settings – if easier with a table?
L116 to the polar ocean
L128 if this is all about the signature an how to use it, how it varies, the signature needs to be introduced better and what it could be used for? Or do you simply mean with signature that the introduces a number of macro- and especially micronutrients?
L135 and throughout the intro, you are using d, td and p fractions and I think it is necessary to introduce them very briefly
L128 and 140 you have two clearly formulated hypotheses. Maybe number them and use these to build the ms clearly around these? And please try to answer them and it might help with the golden thread of this story.
L149: add subheadings, please. Start with sampling and sampling sites, sampling methods, sediment load experiment, analysis (nutrient and TM)….
L151 why were the samples directly thawed and not kept frozen?
L153: pre-cleaned filters and syringes for TM analysis? GEOTRACES protocol?
L155: new paragraph, potentially this needs a map/figure for Disko Bay or potentially a table to show what for and where these samples were taken, as mentioned above – or move L151-155 as sampling below all the sampling sites, to sort this in a better way.
L167 as written above this would better lead over to this section above sampling. Not clear to me were the wet sediment-sub-samples come from as they haven’t been mentioned before. Possibly change the order og the sentences here as well to clarify this?
Very interesting experiment, not sure why the sediment was decanted each time? And which in which phases what (d and td was sampled in the seawater or in the mixed seawater thawing ice samples, which parameters were taken? And how?) potentially a table or figure could help to explain these experiments? It feels like there is sooo much information and interesting samples, sites and a sediment load experiment in this ms, but they can get lost if not clarified or possibly split into two ms that could be accompanying each other?
Also here L178: better to have a complete paragraph about one analysis, then the next, very convoluted.
L179 and 181 which grade?
L182 why where the samples standing upright for more than six months? Because otherwise and addition of HF would have been necessary? Please add a sentence and a Reference
L183 sample volume?
Where all the TM samples (p, d, td) measured in the same way? Usually different procedures and preconcentration-7dilution etc processes necessary, not clear at all. Restructure and clarify please. What about blanks, repeat seawater or or samples and errors and detection limits? Methods and references to these?
L189 first time the additional TM Ni, Cu and Co were mentioned (UV? Of the samples?)
L198: varied by how much?
L200 in my opinion this needs to be clarified before as! As you usually write about your 367 samples, their analysis etc and then use the other compiled data in the discussion to compared and, yes, sure for stats and modelling, but potentially think about separating this and the question is whether it might clarify your story…. Were all these samples from the Arctic? Is the Antarctic necessary or only then in the discussion? This way the title might be more adequate as statistical analysis of…
L208 it is necessary in this section of stats to explain with a sentence why which was chosen and what it is used for
L220 if removed these samples need to be discussed and if the TM ratio were still the same?
L224 eplain this in 2-3 sentences
L226 below DL, has this been mentioned in the methods? If the treatment mad a difference, how did you treat them and what happened, what were the different results and please add Ref
L241:td has been introduced before
L244 a range or rather a concentration with plus and minus might be easier to grasp?
L249 add number to show this, overall not clear where all this is heading to. A lot of information but what about the order and common thread?
L254 at catchment level- this is what I meant rather with the table and sorting to show which subset you are referring to. Possibly add this with a colour code or a symbol or so in your figures
L259 I don’t think that every reader will understand this figure with a MDS versus the other. In cases of concentrations okay, but these haven’t been presented until here at all and you are jumping straight into this which is not leading the reader. Not clear what can be read from this. This needs to be better explained in the text. Also it is necessary to clarify which data is novel and which from the literature (then refer to them correctly, please) and honestly I found the tables and figure in the SI more enlightening and interesting than this one. Maybe with more context…
L266 if it would skew the relationship, please explain how and why before simply excluding these samples
L269, 270 rather discussion
L274 add Ref.
L285 the grey area needs to be explained and mentioned. Add the relationship in numbers of the correlation, please. I would also keep the symbols and colours the same in all the plots. And why not start at 0 with the axis and finish accordingly?
Also if all the units per L-1 then the question is, why you us M and not mol L-1 then it would be all more uniform and more obvious?
L291 which subset now and why was it chosen?
L297 if only used 1 per site or a median or mean for each site/category what would happen then?
L301 and 307discussion mixed in
L313 add Ref.
L320 not sure why these samples (additional not new ones) where chosen for this compilation? Are these all the samples available? Or because they were from the same group of authors? Then even more a good discussion and statistical evaluation with other samples would be important to put this dataset into perspective.
L330 add Ref.
L331 unprecise: a few samples were collected. n=?
L333 range, unclear
L338-342 good and interesting but rather discussion? add Ref.
L344 add Ref.
L345 if this is a critical difference what does it mean for the data, any idea about the differences?
L350 nice pictures, but still no map and not clear where exactly these were taken. Not exact enough. Global or regional importance? Not clear enough and getting lost here what the importance of the paragraph for Disko Bay is just trying to follow this in the text. Therefore new figures are necessary for this.
L413 discussion
L421 discussion
L446 Supp Fig. 1 very interesting, I would prefer this in the ms, why SI?
L448 add numbers to compare the load and the content (and possibly a picture😉)
L449 necessary for what?
L458 add numbers
L460 by how much? Discussion?
L466 can or could? Are some ways more common than others?
L470 add Ref
L473 more sensitive?
L478 only Greenland again – all the samples from there? Discuss regions separately? Or by catchment etc, jumping
L482 how sshown in this figure?
L483 add Ref
L486 interesting! And compared to the floating ones if possibleà use info to discuss
L506 and L514how to be seen?
L519 add Ref
L521 add Ref
L527 have been
L531 very interesting!
Add references to this paragraph
L539 more easily observed- why and how?
L544 where are these 3 bays? And 3 Bays but only two numbers…?
L553 not sure how to see this in Fig 5
L572 add Ref
L577 ice fragments- just from parts discharge or the whole floating ice?
L577 pieces of ice
L578 how rapidly washed out (give numbers and discuss the possible processes
L587 any evidence? Ref. to be discussed
L588 part of the conclusion rather read like a new or further discussion, not concluding and closing this whole story. Paragraph L 618 feels thrown in and rather discussion as Table 1 needs to be explained for example
Citation: https://doi.org/10.5194/egusphere-2023-2991-RC2
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